ML20038A082
| ML20038A082 | |
| Person / Time | |
|---|---|
| Site: | National Bureau of Standards Reactor |
| Issue date: | 09/30/1981 |
| From: | NATIONAL INSTITUTE OF STANDARDS & TECHNOLOGY (FORMERL |
| To: | |
| Shared Package | |
| ML20010J303 | List: |
| References | |
| NUDOCS 8109300392 | |
| Download: ML20038A082 (50) | |
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I GEOLOGY, SEISMOLOGY AND HYDROLOGY OF THE NATIONAL BUREAU OF STANDARDS I
RESEARCH REACTOR SITE GAITHERSBURG, MARYLAND I
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I September 1981 I
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I Prepared by: Geotechnical Engineering Group National Bureau of Standards Center f Building Technology Structuri, and Materials Division I
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GEOLOGY, SEISMOLOGY AND HYDROLOGY NATIONAL BUREAU OF STANDARDS iE RESEARCH NUCLEAR REACTOR SITE jg GAITHERSBURG, MARYLAND l.
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1 SEPTEMBER 1981 i
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! I Prepared by: Geotechnical Engineering Group l
National Bureau of Standards Center for Building Technology I
Structures and Materials Division iI l
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E Table of Contents Page List of Tables.......................................................... iii List of Figures.........................................................
iv 1.0 Introduction........................................................
1 1.1 Executive Summary..............................................
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1.2 Purpose and Scope
3 1.3 Site Location..................................................
4 2.0 Regional Geology....................................................
5 2.1 Wissahickon Formation..........................................
6 2.2 Sykesville Formation...........................................
8 2.3 Saprolite......................................................
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2.4 Serpentinite...................................................
10 3.0 Site Geology........................................................
I 10 3.1 To pog ra phy and D ra inag e........................................
10 3.2 Geology........................................................
3.3 Subsurface Conditions.......................................... 10 13 4.0 Seismicity..........................................................
4.1 General........................................................
13 4.2 Tectonics...................................................... 13 I
14 4.3 History of the Piedmont........................................
4.3.1 General.................................................
14 4.3.2 Summary of Structural Features.......................... 16 4.3.2.1 Precambrian Structures......................... 17 4.3.2.2 Paleozoic Strcetures........................... 18 4.3.2.3 Late Paleozic (?) and Mesozoic Age Structures..
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4.3.2.4 Quaternary Structures.......................... 20 22 y
4.4 Seismic History................................................
4.5 Summary and Conclusions........................................ 25 28 5.0 Ground-Water Hydrology..............................................
5.1 General........................................................ 28 5.2 Regional Ground-Water Hydrology................................ 28 I
5.2.1 Hydrological Characteristics of Regional Geological.
Formations............................................ 28 5.2.2 Ground-Water Use....................................... 29 30 5.3 Site Hydrology.................................................
30 5.3.1 Ground-Water Movement...................................
5.3.2 Cation Exchange Capacity................................ 33 5.4 Ground-Water Monitoring Program................................ 33 34 5.5 Conclusions....................................................
37 6.0 References..........................................................
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I List of Tables Table No.
Title 4.1 Chronological List of Earthquakes for the State of Maryland 5.1 Field and Laboratory Permeability Data 5.2 Cation Exchange Data (meg /100 grams)
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Figure 1 Site Vicinitv : ap Figure 2 Regional Geology Figure 3 Site Topography Figure 4 Plot Plan Showing Locations of Borings and Geologic Cross-Sections Figure 5 Geologic Cross-Sections A-A' and B-B' Figure 6 Geologic Cross-Section C-C' Figure 7 Regional Map of Earthquake Epicenters Figure 8 Regional Tectonic Map Figure 9 Regional Seismotectonic Map Figure 10 Seismicity Map of the State of Maryland Figure 11 Ground-Water Table Contours at NBSR Site I
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REPORT GEOLOGY, SElSM0 LOGY AND HYDROLOGY NATIONAL BUREAU OF STANDARDS I
RESEARCH NUCLEAR REACTOR SITE GAITHERSBURG, MARYLAND
1.0 INTRODUCTION
1.1 Executive Summary I
This report presents a discussion of the geology, seismology and hydrology for the National Bureau of Standards Reactor (NBSR) site to provide an understanding of the earthquake exposure and probable ground-water migration path for the NBSR site. It is an up>-te of data presented in NBSR 7, Preliminary Hazards Summary Report dated January 1,1961, and NBSR 7C, Preliminary Hazards Summary Report, Supplement C dated August 1,1962 submitted in support of the original license application for the NBSR. The discussion is based on:
a) previous NBS submittals b) recent USGS geological and seismological maps c) a comprehensive site selection and evaluation study for sanitary landfill sites performed by Dames and Moore in 1978 in Montgomery County d) information from reports for the North inna Power Station.
The rocks underlying the NBSR site consist entirely of the Wissachickon Formation of Precambrian or early Cambrian Age [22]. The formation is derived from ancient sedimentary strata which had been greatly compressed and consists of a great thickness of schist. The upper portion of the basement rocks is weathered to saprolite, a residual so,il formed by the chemical decomposition of rocks in situ. Saprolite is composed generally of clay, silt, sand-size frag-ments of quartz and rock fragments. Textures range from silty sand to sandy clay.
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The NBSR site is located within the Piedmont belt defined by King [13]. With the exception of the Triassic rocks found in the region, the textural, mineral-ogical and structural conditions of most of the Piedmont rock today are the result of the major cycles of tectonic activity outlined in Section 4.3.1 of this report. The Piedmont belt is an area of low-level seismic activity with no strong earthquakes except one of Modified Mercalli intensity VII near the head of Delaware Bay [10]. Considering that few earthquakes in the eastern United States can be shown to be related to specific faults, as are many earth-quakes in the west [10], examination of the seismic history of an area in the eastern United States is important to enluate the earthquake exposure of a 5
site in the eastern United States. The earthquike history of Maryland as pre-l l
sented in table 4.1 and figure 10 indicates that the NBSR site is located in an i
i area which has experienced only a minor amount of earthquake activity. This conclusion is consistent with the recent report by Barstow et al. [1].
Recent work performed by Dames and Moore as part of the Montgomery County landfill site selection and evaluation study has providad much information on the surface water and ground-water hydrology of the NBSR site.
Based on Dames and Moore's work [5] it appears that topography rather than geologic structure controls ground-water movement. Drainage is to the south and to the west [22].
The estimated rate of ground-water movement is on the order of.3 ft/ day.
This estimated rate of movement correlated well with Otton's range of 0.1 to 1.0 foot per day presented in NBSR 7, Preliminary Hazrrds Summary Report dated January 1, 1961. Additional data on cation exchange capacities from the Dames and Moore study [5] indicate that the values at the NBS site are lower than those referred to in NBSR 7C, Preliminary Hazards Summary Report, Supplement C I
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dated August 1, 1962. Cation exchange capacities of 2.4 meq/100 grams for Zone A (surficial soils which are approximately five (5) to ten (10) feet thick) and l
l 3.6 meq/100 grams for Zone B (soils below Zone A with thicknesses which range from eleven (11) to sixty (60) feet) are considered reasonable based on the Dames and Moore study.
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l Because movement of ground-water appears to be slow and ground-water flow through the site can be monitored for anomalous radioactivity should an acci-dent occur, corrective measures should be possible to minimize the deleterious effects of a spill if an accidental release of contaminants necessitates such corrective action.
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1.2 Purpose and Scope
I The report herein presents a general discussion of the ge, logy, seismology and hydrology for the National Bureau of Standards Reactor (NBtR) Site to provide an understanding of the earthquake exposure and probable groundwater migration path for the NBS research reactor site. A summary of the geology, geohydrology and seismology of the site was prepared for NBS by the U.S.
Geological Survey in 1959 [22]. The text of the 1959 report was included as Appendix III of NBSR 7.
A second report concerning specifically the rate of movement of ground water in the vicinity of the NBSR Site was included as Appendix II of NBSR 7C
[4].
Because of plans by the National Bureau of Standards to increase the power level and renew the license of the NBS research reactor, the geology, seismology and hydrology of the site are being updated to reflect presently available informa-t 1_.
Th. eisc.ssion pr.s.nt.e h.r.in is bas.e _
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I a) previous NBS submittals.
b) recent USGS geological and seismological maps.
c) a comprehensive site selection and evaluation study for sanitary landfill sites performed by Dames and Moore in 1978 in Montgomery County [5).
d) information from reports for the North Anna Power Station.
1I 1.3 Site Location The site of the NBS research reactor is a 576 acre tract of land in upper Montg..nery County, Maryland, approximately one mile southwest of Gaithersburg, I
Maryland (figure 1).
The nearest population centert are Caithersburg and Rockville, Maryland. The site is located approximately twenty miles northwest I
of the center of the District of Columbia.
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2.0 REGIONAL GEOLOGY (excerpted from [5])
The NBS research reactor site is located within the lower Piedmont physiographic l
l province. Ne Piedmont terrain is characterized by gently sloping upland areas and broad, relatively shallow valleys [30]... Regionally, the Piedmont Province consists of metamorphic and igneous rocks that have a strong structural grain
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soil derived from chemical weathering of rock, covers most of these rocks; outcrops occur on only a minor portion of the land surface. Outcropping rocks in the region studied by Dames and Moore, for a site approximately four miles from the NBS site, consist predominately of quartz-chlorite schist, quartz-muscovite schist, and quartz-feldspar-mica gneiss.
In general, the saprolite is predomi-nately silty or sandy in texture with locally significant amounts of clay and abundant residual boulders of quartzite and other weathered rocks. The site area is included in the bedrock map of Montgomery County by Froelich [9]. The dominant lithologies, i.e.,
schist and gneiss have been described in a summary report by Hopson [12] as comprising the Wissahickon Formation (Western Sequence) and the SyKesville Formation, respectively. Contacts between the two lithologies are elongated along the regional structural trend, N. to N. 20* E., and form irregular stringers and lenses along the regional grain. A major contact between the two lithologies extends from the northeast, between Laytonsville and Brookuville, toward Rockville, the Wissahickon Formation lies to the west and the Sykesville Formation lies to the east.
South of Rockville, the country rock is predominantly Wissahickon, with smaller lenses of Sykesville lithologies treading east of Potomac to Cabin John and the Potomac River. These units were differentiated according to lithology on the map by Froelich [9] which depicts I
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the generalized geology of Montgomery County (figure 2) including the dip and strike of dominant foliation.
In the discussion that follows, the terms "metasiltstone" and "metasandstone" I
have been used to describe metamorphosed clastic rocks that are recrystallized to a relatively low degree, and " quartzite" to describe those that are either more highly recrystallized, exhibit strongly deformed grains, or both. The term "metasediments" includes "metasandstone" and "metasiltstone".
" Schist" is applied to those fine grained rocks in which no granular structure is discernible and which exhibit schistosity.
2.1 Wissahickon Formatica This unic extends from southeastern Pennsylvania through Maryland, and into I
Fairfax County, VA.
It is well exposed in the gorge of the Potomac River between Great talls and Glen Echo, forming steep cliffs.
Because the units within the Wissahickva Formation are steeply inclined to vertical, the width of the outcrop belt represents a great stratigraphic thickness.
From a study of the section exposed along the Potomac, Fisher [7] concluded that the thickness of the unit was considerably in excess of 14,000 feet.
The formatic n is composed of metasediments which, prior to metamorphism, were probably similar in composition throughout; variations in degree of metamorphism, however, have produced differences in fabric and mineralogy.
From Great Falls downstream, the rocks of the Wissahickon Formation show evidence of the highest degree of metamorphism, being composed of coarse-grained mica schist with quartz-and feldspar-rich lenses. Locally, pegmatite injections have altered the e.dja-
_ _ try roc m o mig.am e c,abe cs.
zn spe. - the e fe e s
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however, primary sedimentary structures are apparent. These include bedding, slump structures, and various structures caused by deformation of soft sediments prior to consolidation [12]. Rounded, clastic structures are easily visible in quartzose units, though considerable cementation and recrystallization has occurred in some units. Where outcrops exposing more than a fea feet of sec-tien occur, graded beddi.:q relationships between quartz-rich (and sauretimes quartz-and feldspar-rich) layers and mica-rich layers can be seen. This relationship suggests that the granular, quartz-feldspar-rich layers formed the basal beds of depositional sequences which became progressively finer with time.
Such a stratigraphic relationship is typical of cyclical deposition and also of turbidite deposits. The characteristics of the Wissahickon Formation in this regard are discussed in detail by Hopson [12]. Of immediate significance to the present discussion, however, is the likelihood that the original sedimentary layers were probably of considerable lateral extent and continuity. Thus, phys-ical characteristics of the former sedimentary layers, although some metamor-phosed to various degrees and structurally disrupted, can be expected to extend in all directions along the old bedding planes.
I Along the north-to north-northeast-trending outcrops belt described, the main foliation of the Wissahickon Formation, which was developed parallel to former sedimeat ry bedding planes, strikes generally parallel to the main trend and dips very steeply to vertically [9]. Thus, former bedding planes now stand nearly on edge and any continuity of physical characteristics inherited from these primary structures extend vertically and along the regional trend.
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I 2.2 Sykesville Formation Rocks attributed to the Sykerville Formation are described by Hopson [12] as extending in a broad belt from southeastern Carroll County into east-central Montgomery County just east of Rockville, where the belt narrows and pinches out.
Farther south, narrow oands appear along the Potomac in the vicinities of Cabin John and Chain Bridge. Hopson [12] described the Sykesville Formation in the area south of Rockville as a semi-continuous mass that extends to the Potomac River, having an intertonguing relationship with the adjacent Wissahickon Formation, and being divided into separate outcrop areas by mafic and quartz-dioritic intrusive bodies. Its apparent stratigraphic thickness in l!E northern Montgomery County is 15,000 feet.
I The lithelogy of the Sykesville Formation was described by Hopson [12] as pebble-and boulder-bearing, arenaceous to pelitic, metamorphic rocks, with various fabrics ranging between medium-grained, weakly gneissoid to strongly foliated gneiss and schist. Contacts among these lithologies are broadly gradational so that a massive, unstratified appearance is generally character-istic of the formation. Contacts between the Sykesville and Wissahickon For-mations are gradational, so that the change from one lithology to the other occurs over a zone that may be as much as hundreds of feet in width.
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2.3 Saprolite The upper portion of the crystalline rocks forming the basement is weathered to saprolite, a residual soil formed by the chemical decomposition of rocks in situ.
Saprolite is composed generally of clay, silt, sand-size fragments of quartz, and rock fragments. Textures range from silty sand to sandy clay.
Commonly, saprolite exhibits original fabrics inherited from the crystalline 8
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parent rock. Where reworking has occurred, however, such textures have been completely or partially obliterated. The contact between saprolite and weath-ered or fresh rock is gradational in nearly all instances. Therefore, the determination of such contacts is largely subjective.
Because saprolite is a product of weathering, its occurrence or absence in a given area is the result of 1, number of interacting factors, such as mineralogy of the rock, permeability of the rock to aqueous solutions, and movement of water through the rock. Thus, the thickness of saprolite in an area of rocks with non-uniform properties may cover a considerable range of values.
2.4 Serpentinite Numerous bodies of serpertinite are described from the outcrop belt of the Wissahickon Formation in central Montgomery County [12]. These bodies are typically in the form of sheets and thick lenses that are concordant with the structure ot the country rock. Several occur as an elongated swarm from Gaithersburg to approximately 8 miles to the north; two narrow and slightly sinuous bodies extend from Redlands to approximately 3 miles to the north; and an elongated body approximately 2 miles by 0.5 mile lies between Gaithersburg and Redlands [12).
The serpentinite bodies are considered to have formed during regional metamorphism by alteration from ultramafic rocks. Chlorite schist and tale schist are associated with the bodies and may form rims around the larger bodies.
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!E l3 3.0 SITE GEOLOGY l
3.1 Topography and Drainage (excerpted from [22])
The topograhy in the vicinity of the reactor site is undulating and the relief is moderate (figure 3).
Altitudes range from 300 feet, in the valley of Muddy Branch, to 520 feet above mean sea level at Gaithersburg. Drainage is to the south and to the west.
Drainage to the south is by Muddy Branch and to the west by Long Draught Branch of Seneca Creek. Both streams are tributaries of the Potomac River. Muddy Branch, the easternmost of the tributaries, enters the Potomac near Katie Island at a point about 5.5 miles above Lor:k 20 at Great Falls.
I 3.2 Geology The rocks underlying the NBS reactor site consist entirely of the Wissahickon Formation of Precambrian or early Cambrian Age [22]. The formation is derived from ancient sedimentary strata which have been greatly compressed, resulting in substantial lithologic alteration [22]. A detailed discussion of the formation has been presented in Section 2.0 entitled, " Regional Geology."
B 3.3 Subsurface Conditions To aid in the understanding of the subsurface conditions at the NBS Reactor Site, three generalized profiles of subsurface materials have been prepared using the boring logs from a field investigation program consisting of eleven (11) borings performed in 1961. The orientations of the subsurface profiles are shown in figure 4.
These profiles consist of four idealized strata (zones A through D) which have been delineated using the descriptions on the boring logs and recent interpretations of subsurface conditions at sites in the vicinity of the NBS Reactor made by Dames and' Moore [5] as part of the Montgomery 10 I
I County site selection and evaluation study for sanitary landfills (figures 5 and 6).
The profiles consist of residual soils grading into relatively unweathered bedrcck. The material properties vary within each zone. The zones shown repre-sent an evaluation of the most probable subsurface conditions based upon pre-sently available data.
Some variations from the interpreted conditions I
described below should be expected.
ZONE A - The zone is characterized by a layer of top soil approximately one (1) foot thick overlying silty clay or clayey silt. This zone ranges from five (5) to ten (10) feet t' nick with the bottom of the strata being determined by the lowest layer of clay or clayey silt. A three (3) foot layer of sandy silt was encountered in this zone in Boring 10-A.
According to the 1961 Soil Conserva-tion Service Soil Map for Montgomery County [25], the site of the reactor is underlain by Glenelg and Manor soils (loam, silty clay loam, and silt loam).
I ZONE B - This zone is characterized by variable amounts of sand and consists of sandy silt, fine sand, silty sand, and shaley schist (some decomposed). The thickness of this zone ranges from eleven (11) to sixty (60) feet. The extent of this zone is from the bottom of Zone A to the depth in the boring where rock coring began (generally 14 to 63 feet belos 'he surface).
Standard Penetration resistance values at the base of Zone B range from 150 to 200 blows per foot.
Because the ground-water table generally lies within Zone B, both partially saturated and saturated materials are present.
In the borings drilled, the I
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I ground-water table was encountered 2 feet to 23 feet below the ground surface (Elev. 411 to elev. 403).*
ZONE C - This zone forms a transition zone in which the subsurface materials grade from the saprolite of Zone B to the relatively scund bedbrock of Zone D.
It consists of gray or yellow-brown shaley schist, often decomposed and often seamy. Zone C begins at the base of Zone B and has a maximum thicknees of twenty feet in the borings drilled. The principal criterion ueed to establish the extent of this zone was the zone where rock coring was done and a recovery ratio of less than seventy (70) percent was achieved.
ZONE D - This zone represents the relatively sound bedrock and is characterized by a gray-green shaley schist, often seamy. Bedrock was generally considered to be that material which was not sampled with conventional soil samplers, required rock coring equipment and the coring operation reeulted in a recovery ratio of 70 percent or greater. Not all borings were drilled deep enough to i
encounter relatively sound bedrock, but it appears that the top of Zone D lies at a depth of 35 to 74 feet below the ground surf ace.
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e 4.0 SEISMICITY 4.1 General, To provide an understanding of the earthquake exposure for the NBS site, the distribution of historic seismic activity in relation to geologic structures and tectonic provinces will be presented and any structures or regions that are characterized by consistent relations between seismic activity and structural features identified. The basic method of this study will be to compere, for an area within 200 miles of the NBS site, seismicity as shown by the location of epicenters of recorded earthquakes (figure 7), with lithologic, structural, and geophysical features (fiture 8) that may have served to generate or control the release of seismic energy. Results of this comparison, depicted as areas of varying amounts of seismic activity and varying degrees of inferred struct.iral centrol, will then be shown on the seismotectonic map (figure 9).
4.2 Tectonics The tectonic provinces of the eastern United States, as outlined and described by King [13], are as follows:
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Central stable region a) Laurent'.an shield (part of Precambrian Canadian t.hield)
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b) Interior lowlands (bordering platforms covered by younger rocks)
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Appalachian (orogenic) system or tectonic province a)
Fold belt (roughly the Valley and Ridge physiographic province) b)
Blue Ridge belt (Blue Ridge physiographic province) c) Piedmont belt (Piedmont physiographic province and part of Blue I
Ridge physiographic province)
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d) New England-Maritime belt (extension of belts II a, b, and c north of New Jersey and Pennsylvania)
III. Coastal Plains (postorogenic deposits overlapping the Appalachian system; of subordinate tectonic significance except in Mississippi embayment).
While it is clear from existing structural data, as well as from the distribution of earthquakes, that historic earthquake activity bears no consistent relation to the provinces and subprovinces defined above, this regional system of classi-fication allows discussion of the broad architecture of the upper part of the earth's crust in terms of the origin and historical evolution of regional struc-tural or deformational features. A summary of the structural features in the eastern United States have been discussed in detail by Hadley and Devine [10]
and those features which fall in an area within 200 miles of the NBS site are shown on figure 8.
The NBS research reactor is located within the Piedmont belt defined by King [13]. Therefore, the history of the Piedmont as presented in the 1969 preliminary report on seismology for the North Anna Power Station [30]
and by Hadley and Devine [10] is provided to document the origin of the structural features on figure 8.
4.3 History of the Piedmont 4.3.1 General (e.tcerpted from [30])
Geologic studies, aided by radiometric dating, have disclosed several marked periods of tectonic activity in the Appalachians. This activity occurred through the Paleozoic and into the Mesozoic Era, creating the pre.9 ant structural features of the Piedmont Province.
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The rocks of the Piedmont exhibit varying degrees of metamorphism depending upon their location in rela.: ion to the axes of major stresses. Generally the axis of major metamorphirm has been northeast-southwest. Prior to their metamorphism, the rocke are believed to have been composed of sediments deposited in a vast geosyncline. This geosyncline was probably fonned by the dc m-warping of the Precambrian basement rock.
The it 'tial down-warping and sediment accumulation is believed to have begun in the Precambrian, over one billion years ago.
The deposition is believed to have continued until Ordovician time, about 470 million years ago. At this time, deposition was interrupted by uplift and mountain building associated with the Taconic Orogeny. The aedimentary strata were folded, faulted, intruded by granitic masses, and metamorphosed on a regional scale.
Further sediment deposition has been postulated throughout the Paleozoic, between the periods of mountain building. This postulation remains unconfirmed, however, due to a lack of detailed stratigraphic studies.
Again, naar the end of Davonian time, as indicated by radiometric dating, further orogenic activity and/or intrusion of large igneous masses occurred.
I The ce-tral axis of the most intense deformation is believed to have been in the rocks of the Inner Piedmont as indicated by the high grade of metamorphism present in the area.
I The last period of major folding, faulting and metamorphism is believed to have occurred at the close of the Paleozoic. This stage is called the Appalachian Revolution and occurred from 230 to 280 million years ago. The discovery of post-Devonian granitic plutons, and a minor increase in metamorphism, suggest 15 I
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I that the central axis of intense deformation may have shifted farther to the southeast in the Piedmont Province during this cycle.
I With the exception of the Triassic rocks found in the region, the textural, mineralgical and structural conditions of most et the Piedmont rocks today are the result of these major cycles of tectonic activity.
The beginning of the Mesozoic Era marked the last period of major faulting affecting the rocks of the Piedmont. This was a period of extensional faulting rather than the compressional folding and faulting that characterized the earlier periods of tectonic activity.
It appears that the Inner Piedmont Zone of high metamorphism was arched in the Triassic, producing basins bordered by deep graben-type faults. The basins were formed on each side of the Inner Piedmont axis. Erosion of the uplifted Paleozoic metamorphic and igneous rocks resulted in large accumulations of terrestrial sediments in these basine.
Some
'aulting and widespread igneous activity persisted throughout the period of deposition. The faulting and igneous activity are believed to have ceased by the end of the Triassic, 180 million years ago.
The region was relatively dormant during Jurassic and Cretaceous times.
Generally, extensive erosion occurred during these periods and has continued to the present time. The eroded sediments were deposited farther to the east, in the present day Coastal Plain Province.
I 4.3.2 Summary of Structural Features (excerpted from [10])
In this section, the structural features broken down according to geologic age are discussed in more detail with particular emphasis on the Piedmont belt.
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I 4.3.2.1 Precambrian Structures Precambrian rocks underlying a relatively thin cover of Paleozoic rocks are the dominant supracrustal rocks throughout the interior of the United States and Canada. Although largely concealed within the United States, the lithology, age, and distribution of the Precambrian rocks are at least partly known from exposures, drill records, and aeromagnetic and gravity surveys.
Structures thus recognized include both local trends and regional features, such as the I
I.ake Superior syncline in Wisconsin, Minnesota, and Iowa and the so-called Grenville metamorphic or orogenic front that has been traced from eastern Canada southward into Michigan and Ohio.
I For the most part, major Precambrian structures of the continental interior are not specifically associated with current or historic seismicity.
It is not known whether earthquakes have resulted from movements on Precambrian faults or other structural discontinuities, especially in the central United States where scattered smaller earthquakes commonly cannc,0 be related to known features in the Paleozoic or younger rocks.
It is notable, however, that such structures seem to have played only a minor or local role in determining the release of 3
seismic energy [14].
I Rocks similar in age and Precambrian structural history to those in the eastern part of the continental interior occur also within the Appalachian system from New England to Georgia. Their structural trends have, however, been so strongly overprinted in most places by Paleozoic deformation that no distinctively Precambrian structures can be related to regional seismt activity.
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4.3.2.2 Paleozoic Structures The Appalachian Piedmont belt adjoining the Blue Ridge belt on the southeast is as wide as or wider than the Blue Ridge and is at least as complex structurally.
It is well known geologically and consists largely of unfossiliferous sedimen-tary and volcanic rocks moderately to strongly recrystallized and invaded by many different kinds of felsic and mafic intrusive rocks. The metamorphosed volcanic and sedimentary rocks are probably mostly of early Paleozoic age but may include rocks of late Precambrian age, as well as isolated bodies of older Precambrian rocks. Deformation, metamorphism, and intrusion are believed to have taken place at several times during the Paleozoic. The predominant features are those of supracrustal compression, resulting in folds and faults, regional metamorphism, and emplacement of various intrusive rocks. Rocks in the northeastward continuation of the Piedmont belt in New England range in age from early through middle and late Paleozoic, and their structural history has been at least as complex as that of the Piedmont rocks.
I The geologic boundary between the Blue Ridge and Green Mountain belts and the Piedmont belt to the southeast is not well defined.
In the southern Appalachians, the geologic boundary has been interpreted as a zone of combined recumbent fold-ing, overthrust faulting, and ore-Triassic high-angle or strike-slip shearing known as the Brevard fault zone [23, 3, 11, 14]. Farther northeast in the I
central and northern Appalachians, the boundary is even less well defined.
For the purposes of this report, the Blue Ridge and Piedmont belts and their equivalents in New England are considered as a single tectonic province.
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no structurally significant eastern boundary is known in the eastern United States. Evidence from drill holes and aeromagnetic surveys, however, suggests I
that the rocks of the Florida platform buried beneath the Coastal Plain are significantly different from those of the Piedmont in central Georgia, and therefore a major tectonic province boundary may exist between them.
I 4.3.2.3 Late Paleozoic (?) and Mesozoic Age Structures The orogenic events that formed the Appalachian system in the eastern United States were succeeded by a new tectonic regime both in the Appalachian orogenic belt and in the central stable region to the west.
This regime was at least partly extensional and, caused high-angle faulting of near-surface rocks, in contrast to the compressional and deep-seated effects of the earlier tectonism.
These latter features are divided into two broad groups, a series of fault-block basins of Triassic age entirely within the Appalachian orogenic system, and 1 group of rift-like fault zones within the central stable region.
Structures known to be Triassic arc <anfined essentially to the Appalachian Piedmont and its New England counterpart. They consist of block-Qulted basins in which continental eediments were deposited, in part contemporaneously with the faulting. The deformation was primarily extensional, resulting in tilted i
and down-dropped basins bordered by graben-type faults.
Invasions by basaltic rocks are associated vith these movements throughout the Piedmont belt.
Neither the sedimentary nor the igneous rocks have been extensively deformed or metamcr-phosed, as they were formed after Paleozoic deformation and regional metamorphism had ceased completely.
I 19 I
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l 5
4.3.2.4 Quaternary Structures The most recent structures, those of Quaternary age or younger, are of course particularly important in a study of the causes of historical seismicity.
Sev-eral lines of evidence attest to crustal movements during the Quaternary, although quantitative data are scattered and incomplete. One of the best docu-mented crustal movements is the record of upwarping throughout the northeastern part of the continent as a result of deglaciation and unloading of the crust during and after removal of the Labrador ice sheet. These movements were appar-ently limited to the region north of a line exterding roughly from central Minnesota southeastward through Milwaukee, Detr.)it, New York, and Long Island, and northeastward to Nova Scotia. The central part of this region, in the Hudson Bay area, appears to be still rising relative to sea level at about 7 mm/yr. Isobases cf equal uplift during the past 13,000 years and hinge lines marking successive northward shifts of the limit of warping trend approximately parallel to the outer glacial limit and concentric to the former ice cap.
I Similar crustal warping, apparently unrelated to glacial unloading, is suggested by recently completed leveling traverses throughout the eastern United States, as reported by Meade [16]. These results indicate crustal updomirg during the L]
period 1930-1970 at a rate of 7 mm/yr centered in northwestern Georgia, as well as subsidence along the Atlantic coast north of Cape Hatteras reaching 5 to l
l 6 mm/yr in New England.
Elsewhere in the eastern United States, including i
Florida, changes of elevation appear to be minimal. Regional tilts indicated by the level measurements show a maximum of 6 mm/yr/100 km in New England and 3.5 cxn/yr/100 km in northern Georgia and the Carolinas. In comparison, current I
i 20 I
5
I tilt rates due to glacial rebound in the Great Lakes region are estimated at only 1.5 mm/yr/100 km [8].
Very different evidence of Quarternary deformation is found in faults that offset surficial materials of late Cenozoic age. These include many examples of faults displacing geologically young features, such as alluvial deposits, till, loess, glaciated bedrock surfaces, and river terraces. Ilthough such J
features have been widely reported from localities in the Appalachian Piedmont and Coastal Plain, most have been observed only in limited exposures such as road cuts and river banks in unconsolidated materials which are subject to rapid erosion and overgrowth of vegetation. Little information has been available as to their regional extent or tectonic significance. A reported exception is that of the glaciated bedrock surfaces in the Champlain-Hudson valley region; these surfaces show systematic reverse-fault displacements of a millimeter or more on southeast-dipping regional cleavage [21].
In the 141ssissippi embayment region, faults are known in rocks of Pliocene-Pleistocene age [6, 19, 20]. Faults in loess and alluvium of Pleistocene age have been interpreted as occurring along topographic lineaments which may repre-sent more extensive faults and fractures in the Coastal Plain sediments [15].
For the most part, however, direct evidence indicating that faults are coinci-dent with these features is lacking. With the exception of the vicinity of New Madrid, Missouri, and Reelfoot Lake, Tennessee, no faults or other indications of recent faulting are known to be associated with any historic earthquake.
I I
21 I
4.4 Seismic History (excerpted from [10])
A summary of earthquake epicenters for areas within 200 miles of the NBS research reactor site is shown on figure 7.
The epicenter plot of figure 7 shows computer plotted locations of epicenters of earthquakes of Modified Mercalli (MM) III or greater intensity which occurred during the period 1800 - 1972.
The epicenter data used for this map were extracted from the working file of the Nattopal Earthquake Information Service and do not represent a final edited compilation of castern United States seist ' city. However, subsequent refinement and improvem.nt of locations or maximum intensities were expected to result in only minor changes in the seismic-frequency contours developed for the Hadley and Devine [10] study.
The locations and maximum epicentral intensities of earthquakes were recorded on magnetic tape and plotted from the tape. Locations were fixed to the nearest 0.1 degree of latitude and longitude; the symbol used represents the highest epicentral intensity located at the plotted point.
l The contours shewn on the epicenter plot were developed by counting the number I
of epicenters, including repeated events at the same plotted location, within a 4 2 circle representing 10 km at the original map compilation scale of 1:2,500,000.
I They were generated by tracing in a somewhat generalized fashion the position of the center of the counting circle while moving it so as to include up to but not more than the 4, 8, 16, 32, and 64 epicenters selected as contour intervals.
Because Hadley and Devine [10] wished to empaasize the areal distribution of seismic activity which might be related to geologic structure, rather than the 22 1
I
I I
total seismic energy released, the intensities of individual earthquakes were ignored in contouring seismic frequency.
A study of figure 7 shows that large areas of background or minimal seismic activity are distinguished from epicentral concentrations representing areas that are seismically more active. The upper limit of activity in the background areas is arbitrarily taken as less than 8 epicenters per 10 km2 (in most places 4
less than 4) and the absence of earthquakes with epicentral intensities greater than VI.
Not all such areas are wholly non-seismic, although some areas consid-4 2 erably larger than 10 km have generated no recorded earthquakes.
In fact, 4 2 could be outlined, but this distinction areas with no epicenters within 10 km is believed to be tectonically uninterpretable from a regional point of view.
For meaningful tectonic interpretation, one must turn to the more seismic areas and these, classified according to the level of seismic activity and degree of known structural control, are shown on figure 9.
In establishing the different areas of earthquake activity shown on the seismotectonic map, use is made of three parameters. One parameter is the areal density (seismic frequency) of epicenters, including separate events at the same location.
A second parameter is the maximum epicentral intensity, according to the Modified Mercalli scale [31], of any earthquake known to have occurred in the given area. Although magnitude determinations are given for many eatthquakes that have occurred since 1960, they are generally not avail-able for earlier events. Moreover, since it is expected that a principal use of the map will be te estimate potential earthquake damage, intensity values are considered to be more useful. The third parameter is the evidence of 23 I
I
I structural control on epicenter distribution as determined by the tectonic study. From there parameters, five levels of seismic activity and three categories of structural control were selected (see figure 9).
I The seismic areas described herein are intended to represent areas designated by notable seismicity and, in so far as possible, the structural features believed to be casually related to the seismicity.
In some areas, notable seismic activity is not associated with any known structural feature, but is distributed so as to indicate rather <.learly that the activity is localized along concealed or unidentified faults or other structures.
Seismicity, usually of low level, is stattered in many areas so that no localizing fault or other structare 1.
indicated.
In a few places, s...:h as central '<irginia, seismic activity of intermediate or even high levels cannot, in f.he light of present information, be considered localized along known structures.
In the southern Appalachian fold belt, the level of seismicity is more uniform, but is nowhere well enough defined to indicate individually active structures.
I It should be emphasized that the meismic areas show on figure 9 do not have I
precisely determintsle boundaries.
Few earthquakes in the eastern United States can be shown to be related to specific faults, as are many earthquakes in the west.
Moreover, scatter of eptcenters and lack of correlation with geologic structure may be due in part to the scarcity of instrumental data and the resulting inaccuracy in locating epicenters. Where a good correlation seems to exist betweem seismic activity and known structures, or where the distribution of epiccaters seems to outline a localizing fault zone or other structure, the seismotectonic map boundaries are dravn to conform rather closely to the geologic 24 I
I
I or seismological information available. Elsewhere, boundaries are more generalized and essentially indicate areas where more information is needed to determine the relations between seismic activity and structure.
4.5 Summary and conclusions [ modified from [10])
For seitsmotectonic purposes, the Blue Ridge and Piedmont form a single unit with complex and incompletely known tectonic features. Within this single unit there is much diversity of structure and of seismic activity but relatively little information on structural control. The most concentrated activity occurs in an east-trending area in central Virginia, within which the seismic frequency is 16 to 32 or higher. Most of the earthquakes in this area have been of HM intensity VI or less, although one near Richmond, Virginia, is reported to have been MM VI-VII [2].
I Another area of more than average seismicity in the Blue Ridge-Piedmont province covers parts of western North Carolina and northwestern South Carolina.
Struc-turally the area includes part of the Murphy syncline, much of the great allochthonous terrain of Blue Ridge rocks with its large negative gravity anomaly, and segments of the Brevard fault zone.
Northeast of central Virginia, the Piedmont belt is an area of low-level seismic activity with no strong earth-quakes except one of intensity MM VII near the head of Delaware Bay.
Considering that the NBS reactor site is located within the area between central Virginia and the head of the Delaware Bay, and the earthquakes for the state of Maryland listed in Table 4.1 and shown on figure 10, it can be said that the NBS reactor site is located in an area which has experienced only a minor amount of earthquake activity. This conclusion is consistent with the recent report by Barstow et al. [1].
25 I
I I
Table 4.1 Chronological Listing of Earthquakes for the State of Maryland (modified from [28])
I DATE ORIGIN TIME (UTC)
LAT.
LONG.
DEPTH HYP0 CENTER INTENSITY YEAR MONTH DAY H
M S
( N..'
(W.)
(KM)
QUAL MM I
1758 APR 25 02 30 38.9 76.5 H
I 1828 FEB 24 38.9 76.7*
H 1876 JAN 30 02 05 38.9 76.5*
G 1876 APR 10 38.5 76.6*
H III*
1877 SEP 01 16 38.7 76.8*
H III*
1883 MAR 11 23 57 39.5 76.4 H
IV*
1883 MAR 12 05 39.5 76.4 H
III*
1902 MAR 10 05 39.6 77.2 H
III*
I 1902 MAR 11 10 30 39.6 77 2 H
III*
1903 JAN 01 17 30 39.6 77.2 H
III*
1903 JAN 01 22 45 39.6 77.2 H
II*
1906 OCT 13 15 39.2 76.7*
H III 1910 JAN 24 02 20 39.6 77.0 H
II 1910 APR 24 02 39.2 76.7*
H III*
1911 APR 08 01 38.3 75.5 x H
IV I
1911 APR 08 04 11 38.3 75.5 x H
IV 1928 OCT 15 38.3 75.l*
G IV
- 1930 NOV 01 06 34 39.1 76.5*
G IV I
1930 NOV 01 07 02 39.1 76.5*
G III*
1939 JUN 22 23 10 39.5 76.6*
G III*
1939 NOV 18 02 33 39.5 76.6*
G IV
- 1939 NOV 26 05 20 39.5 76.6*
G V*
I 1962 SEP 04 23 40 39.5 77.7 x G
IV 1962 SEP 07 14 00 45.9 39.7 78.2 038 C
I NOTES:
1.
The data are listed chronologically in the following categories: date, origin time, N. latitude, W. longitude, depth, hypocenter quality, intensity (Modified Mercalli). Table 1 has some basic limitations in terms of the size of the earthquakes listed. Prior to 1965 all recorded felt earthquakes are listed, after 1965 only felt earthquakes or those with magnitudes above the 2.5-3.0 range are listed; the lower magnitude levels apply mostly to the eastern United States.
If no magnitude was computed and the earthquake was felt it was included B
in the earthquake list. The low magnitude events located in recent years with dense seismograph networks have not been included.
2.
Leaders (..) indicate information not available.
3.
Latitude and longitude are listed to a hundredth of a degree if they have been pubished with that degree of accuracy, or greater; however, most historical I
26 I
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I I
TABLE 4.1 (CONTINUE 9)
I events have been published only to the nearest degree or tenth of a degree and are therefore listed at this accuracy. An asterisk (*) to the right of the longitude indicates that the latitude and longitude were not given in the source reference, but were assigned lv the compilers of the data file. An (x)
I to the right of the longitude indicates that the event is an explosion, a suspected explosion, tockburst, or a nontectonic event; these have not been plotted on the map.
4.
The letter code in the HYP0 CENTER, QUAL column is defined below I
a.
Determination of the instrumental hypecenters are estimated to be accurate within the ranges of latitude and longitude listed below; each range is letter coded as indicated:
A 0.0*-0.1*
B 0.1*-0.2*
C 0.2*-0.5*
D 0.5'-1.0*
E 1.0* or larger b.
Determination of noniastrumental epicenters from felt data are estimated I
to be accurate within the ranges of latitude and longitude listed below; each range is letter coded as indicated:
I F
0.0*-0.5*
G 0.5'-1.0*
H 1.0*-2.0*
I 2.0* or larger 5.
An asterisk (*) in the INTENSITY, MM column indicates that the intensity was assigned by the compiler on the basis of the available data at the time the I
catalog was compiled.
I
, I
!I I
2' l3 I
I 5.0 GROUND-WATER HYDROLOGY 5.1 General This statement on the hydrology of the NBS reactor site has been prepared to update a report by Alfred Clebach, Jr., entitled, " Rate of Movement of Ground Water and Accidentially Released Radionuclides at a Proposed Reactor Site Near Gaithersburg, Maryland," [4] based on presently available data. The basis for this review was the site evaluation study for Montgomery County performed by Dames and Moore in 1978 [5]. The Dames and Moore study provides a comprehen-sive hydrologic study of the soils formed on the Wissahickon Formation on which the NBS reactor site is located. At the four sites studied, permeability tests (laboratory and field) were performed at varying depths and the results are reported based on location, depth and soil zone. The information from Site E-57 and Site S-135/271 was used because the site geology for these sites was considered representative of the geologic conditions at the NBS site.
5.2 Regional Ground-Water Hydrology (excerpted from [5])
5.2.L Hydrological Characteristics of Regional Geological Formations The NBS reactor site lies within the Lower Piedmont physiographic province, which is composed of metamorphic and igneous rocks. A mantle of saprolite covers most of these rocks, and rock outcrops constitute only a minor portion of the land surface. The exposed rocks consist predominantly of quartz-chlorite schist, quartz-muscovite schist, and quartz-feldspar-mica gneiss.
The sapro-lite is silty or sandy in texture, with locally significant amounts of clay and residual fragments of quartzite and other weathered rocks.
Ground water in the Piedmont occurs almost exclusively in the crystalline rocks and saprolite under unconfined (ground-water table) conditions. Recharge to 28
the ground-water table is derived from direct infiltration of precipitation.
Average annual precipitation in the Maryland Piedmont is approximately 40 inches, of which approximately 8 inches infiltrate to the ground-water table.
In genC al, ground water moves downgradient from topographic highs and eventwally discharges to local streams, seeps, and lakes.
The saprolite which typically mantles the crystalline rocks is primarily silty or sandy in texture; therefore, the porosity (defined as the percent of the tota. rolu=e occupied by voids) of the saprolite is intergranular (or primary).
The primary porosity of the unaltered, unfractured crystalline roci. is usually very low, seldom exceeding 1 percent [18].
Ground water in the unaltered t acks occurs predominantly in fractures (secondary porosity) resulting from jointing and faulting of *.he rock.
Typically, the size and frequency of fractures decreases with depth below the ground surface.
Therefore, the greatest amount of ground water stored in the rocks of the Maryland Piedmont is in the saprolite and che upper few hundred feet of the bedrock.
5.t.2 Ground 'a'ater Use Grourd water in the Maryland Piedmont is used for farm, domestic, commercial, institutional, industrial, and public supplies [18).
Most rural homes and farms in Montgomery County rely on individual wells.
The mean yield of the wells used for domes *ic supplies is about 10 gal / min.
Most of these wells extend to depths of less than 300 feet; however, several have been drilled to depths greater than 500 feet [18].
29
I 5.3 Site Hydrology 5.3.1 Ground-Water Movement The direction and rate of ground-water movement is primarily dependent on local topography and the porosity and permeability of the subsurface materials [5].
A topographic map of the NBS site is presented on figure 3.
Infocation on the permeability of the subsurface materials has been obtained from the Damer and Moore site selection study [5].
Permeability values at Site E-57 (located 4 milea northeast of the NBS reactor site) range from 1 x 10-5 cm/see to 3.2 x 10-3 cm/sec, and increase gradually from Zone B to Zones C and D.
At this site, the zone of maximum permeability appears to be the zone of fractured, unweathered bedrock (top of Zone D) and the zone of weathered rock (Zone C) immediately above it.
At Site S-135/?.71, located approxime cely 9 miles south of the NBS reactor site, permeability values range from 1 x 10-5 cm/see to 5 x 10-4 cm/sec. The highest value occurred in unweathered bedrock (Zone D) and was assumed to be the jointed zone at the top of the bedbrock. A permeability value of less than 1 x 10-6 cm/sec was recorded below this point and was assumed to be unjointed bedrock.
(Since the joint spacing varies from a few inches to 2 or 3 feet, permeability tests run in jointed bedrock may vary in their results, depending on the number of joints crossed by the 2-foot test section.) At Site S-135/271, the permeability values do not show any steady trend from Zone B to Zone D.
Table 5.1 shows permeability values from the Dames and Moore tests on Sites E-57 and S-135/271. The results of both field and laboratory tests are shown. The permeability values given in the text above are from field tesus because their results are considered more 30 I
reliable than the laboratory tests. Laboratory results were frequently less than the field test results by approximately one order of magnitude.
The results of laboratory permeability tests performed by Dames and Moore on samples of soil from Zones A and B indicate that the soil in Zone A may be slightly more permeable than the soil in Zone B (see Table 5.1).
The results of laboratory tests given in the Soil Conservation Service report [26] and Otton in NBSR-7 [22] are also listed in Table 5.1.
The Dames and Moore values would seem to be the most represen.:ative of conditions at the NBS site for several reasons:
I
- 1) The results are based on a large number of tests at different depths and locations,
- 2) The two Dames and Moore sites are relatively near and geologically similar to the NBS site, while the permeability values given by Otton are for soil samples (developed on the Wissahickon Formation) in Baltimore County, and the value guen by the Soil Conservation Service is a general value for the Glenelg loam, wherever it occurs in Montgomery County, and
- 3) The Dames and Moore results represent the more recent work performed.
For Zones B, C, and D, the Dames and Moore results are what is available. The maximum average permeability value from the Dames and Moore study is 3.2 x 10-3 cm/sec (see Table 5.1). This represents a conservative (i.e., high permeabil-ity) value for ground-water movement, and was used in computing the rate of ground-water movement at the NBS reactor site. The permeability valuea for the soil, saprolite, and bedrock al?. fall within the low to medium permeability range as established by Terzaghi and Peck [27].
I 31 I
I
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" Water table contours based on measurements of water level made January 20, 1961, in foundation borings in the vicinity of the reactor building are shown on figure 11.
Bore holee 7-A, 8-A, 10-A, and 3-A are at the corners of the building. Depths to water ranged from 1.7 feet in hole 2-A to 23.0 feet in hole 3-A.
All the water levels were in subsoil or decomposed rock [4]."
Since the soil derives its structure from the in place weathering of the rock, one might expect it to have a higher permeability parallel to the foliation and former bedding planes, and for ground water to flow preferentially along the planen of foliation. However, the Dames and Moore study [5], based on four dif-ferent sites an. approximately 2200 acres mapped with ground-water table contours in Montgomery County, concluded that the ground-water table elevation was con-trolled by topography rather than by geologic structure. This implies that the permeability of the soil and rock does not vary with direction. The ground-water table contours at the NBS reactor site are more easily related to the topography than to the geologic structure, since the primary ground-water movement is to the west (to a perennial stream) and across the plane of foliatien.
I "The presence of a perennial stream between the reactor site and the three of f-site wells to the west (wells 37, 38, and 39 in Otton's report [22]) effec-tively eliminates the possibility that cones cf depression caused by pumping these wells could extend beneath the reactor building [4]."
The ground-water velocity can be computed from the Ljdraulic gradient and the permeability. The maximum hydraulic gradient at the reactor site (measured from the groundwater table contours on figure 11) is to the northwest and is 6'/170' or 0.035.
32 I
I
I 1
W The maximum average permeability value from Dames and Moore's field permeability tests on Sites E-57 and S-135/271 is 3.2 x 10-3 cm/sec (see Table 5.1).
The ground-water velocity, v, should be approximatelv:
v = ki = (3.2 x 10-3 cm/sec) (0.035)
= 1.1 x 10-4 cm/sec
= 0.3 ft/ day This correlates well with Otton's estimate of 0.1 to 1.0 foot per day.
5.3.2 Cation Exchange Capacities The cation exchange capacity of the soil is a particularly important characteristic indicating the degree to whig radionuclides will become adsorbed on or fixed in the solid phase of a soil-water system [4). Cation exchange capacities have not been determined for samples from the NBS site, but determina-tions on samples of the same geologic unit (soil and weathered rock of the Wissahickon Formation) from two Dames and Moore sites [5] and from the site of the Nacional Naval Medical Center Reactor [4] are available and should be reason-ably representative of conditions at the NBS site. The cation exchange values for these samples are listed in Table 5.2.
The Dames and Moore values differ from those obtained at the National Naval Medical Center site. The Damea and Moore values are recommended for use because they were obtained from a larger number of tests, the results are more consistent with each other than those obtained for the Naval Medical Center, and the Dames and Moore results are more conservative, i.e.,
the cation exchange values are lower.
5.4 Ground-Water Monitoring Program A routine sampling and analysis program has been in effect since November 1963, and is described in hTSR 9 [17).
33 I
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5.5 Conclusions Recent work performed by Dames and Moore as part of the Montgomery County landfill sita selection and evaluation study indicate permeability values for surficial soils (Zone A) which are similar to the values reported by Clebsch
[4). Based on the Dames and Moore work it appears that topography rather than geologic structure controls ground-water movement. The estimated rate of ground-water movement is expected to be on the order of.3 ft/ day using the maximum hydraulic gradient at the reactor site measured from the ground-water table con-tours on figure 11 and the maximum average permeability value from the Dames and Moore's field permeability tests. This rate of movement correlates well with the estimate from the earlier work performed by Otton [22] of 0.1 to 1.0 foot per day.
I Additional data on cation exchange capacities from the Dames and Moore study indicate that the values at the NBS site are lower than those referred to in Clebsch's report (4). Cation exchange capacities of 2.4 meq/100 grams for Zone A and 3.6 meq/100 grams for Zone B are considered reasonable based on the Dames and Moore study.
Because movement of ground-water appears to be slow and ground-water flows through the alte can be monitored for anomalous radioactivity should an acci-dent occur, corrective measures should be possible to minimize deleterious effects of a spill if an accidential release of contaminants necessitates such corrective action.
I 34 I
I
I Table 5.1 Field and Laboratory Permeability Data Dames and Moore Landfill Study [5]
Permeability in em/sec SITE E-57 SITE S-135/271 Lab Field Lab Field Zone A 8 x 10-5 (3) 1.3 x 10-4 (5)
I Zone B 1 x 10-5 to 2.6 x 10-4 (10) 3.3 x 10-5 (22) 5 x 10-4 7 x 10-5 Zone C 3.2 x 10-3 (3) 1.5 x 10-5 (1) 1 x 10-5 5 x 10-4 Zone D 1.2 x 10-3 (3) 1 x 10-4*
5 x 10-4* t0
< 1 x 10-6**
(n) - indicates an average of n values I
- - assumed zone of fractured bedrock
- - assumed zone of sound bedrock Soil Conservation Service [26]
Zcne A 4 x 10-4 to 1. '+ x 10-3 cm/sec (0.6 to 2 in/hr)
NBSR 7 [22]
l Zone A 6 x 10-4 to 6 x 10-3 cm/sec (12 to 93 gpd/f t2)
NOTE: See text for further explanation of these results I
I I
35 I
I
i.
Table 5.2 Cation Exchange Data (meq/100 grams)
Dames and Moore (5]
I 4
i Site E-57 Site S-135/271 Zone A 2.75 (1) 2.15 ( 1)
Zone B 3.60 (4) 3.65 (1)
- I National Naval Medical Center Reactor [4]
Depth Below Surface Total Exchange Capacity 2 '-6 " t o 4 '-0 "
7.6 l
19 '-6 " to 21'-0" 15.1 l
l 33'-6" to 33'-9" 5.7 NOTE:
(n) - indicates that the value given is the average of n values I
4I ilI
- I i
!I i
!I 36 I
I 6M REFERENCES 1.
Barstow, N. L., Brill, K. G., Nut?li, O. N., and Pomeroy, P. W. (1981),
I "An Approach to Seismic Zonation for Siting Nuclear Electric Power Generating Facilities in the Eastern United States," Final Technical Report:
NUREG/CR-1577, USNRC, 143 pp, and Appendices.
I 2.
Bollinger, G. A., and Hopper, M. G. (1971), Virginia's Two Largest Earthquakes--December 22, 1875, and May 31, 1897:
Seismol. Soc. America Bull, v. 61, p. 1033-1039.
3.
Burchfiel, B.
C., and Livingston, J. L. (1967), Brevard Zone Compared to Alpine Root Zones: American Jour. Sci., v. 265, p. 241-256.
4.
Clebsch, A. Jr. (1962), " Rate of Movement of Ground Water and Accidentally Released Radionuclides at a Proposed Reactor Site near Gaithersburg, I
Maryland," in Preliminary Hazards Summary Report, NBSR 7C, Supplement C, Appendix II:
National Bureau of Standards, 1962.
5.
Dames and Moore (1978), Site Selection and Evaluation Study for Sanitary I
randfills, Montgomery County, Maryland.
5.
Finch, W. I. (1966), Geologic Map of the Paducah West and part of the Metropolis Quadrangles, Kentecky-Illinois:
U. S. Geological Survey Geological Quad. Map GQ-557.
I 7.
Fisher, G. W. (1970), The Metamorphosed Sedimentary Rocks Along the Potomac River near Washington, D.C. in Fisher, G. W., and others, eds., Studies of Appalachian Geology - Central and Southern:
New York, Interscience.
8.
Flint, R. F. (1947), Glacial Geology and the Pleistocene Epoch:
New York, John Wiley and Sons, 589 p.
I 9.
Froelich, A. J. (1975), Bedrock Map of Montgomery County, Maryland:
U. S.
Geological Survey, Map I-920-D.
10.
Hadley, J. B. and Devine, J. F. (1974), Seismo*ectonic Map of the Eastern I
United States: United States Geological Survey.
11.
Hatcher, R. D. Jr. (1971), Stratigraphic, Petrologic, and Structural I
Evidence Favoring a Thrust Solution to the Brevard Problem:
Am. Jour.
Sci., v. 270, p. 177-202.
I 12.
Hopson, C. A. (1964), The Crystalline Rocks of Howard and Montgomery Counties in The Geology of Howard and Montgomery Counties: Maryland Geo b ' cal Survey.
13.
King, P. B. (1951), The Tectonics of Middle North America: Princeton, New Jersey, Princeton University Press, 203 p.
I 37 I
I
14.
Kisslinger, C. and Nuttli, O. W. (1965), The Earthquake of October 21, 1965 and Precambrian Structure in Missouri:
Earthquake Notes, v. 36,
- p. 32-36.
15.
Krinitzsky, E. L. (1950), Geologic Investigation of Faulting in the Lower Mississippi Valley:
U. S. Waterways Expt. Sta. Tech. Mem., No. 3-311.
16.
Meade, B. K. (1971), Report of the Subcommission on Recent Crustal Movements in North America: Natl. Ocean Survey Tech. Paper, 19 p.
17.
National Bureau of Standards (1966), Final Safety Analysis Report on the NBS Reactor, NBSR 9.
16.
Nutter, L. J. and Otton, E. G. (1969), Ground-water Occurrence in the Maryland Piedmont, Report of Investigation No. 10: Maryland Geological Survey.
19.
Olive, W. W. (1963), Geology of the Elva Quadrangle, Kentucky:
U. S.
Geological Survey Geological Quad. Map GQ-230.
20.
Olive, W. W. (1965), Geology of the Mico Quadrangle, Kentucky:
U. S.
Geological Survey Geological Quad. Map GQ-332.
21.
Oliver, J., Johnson, T., Dorman, J. (1970), Postglacial Faulting and Seismicity in New York and Quebec, in Symposium on Recent Crustal Move-ments, Ottawa, Canada, 1969, Papers: Canadian Jour. Earth Sci., vol. 7, no. 2, pt. 2, p. 579-590.
22.
Otton, E. G. (1959), "Geohydrology of a Proposed Nuclear Reactor Site near Gaithersburg, Maryland" in Preliminary Hazards Summary Report I
NBSR 7, Appendix III: National Bureau of Standards,1961.
23.
Reed, J. C. Jr., and Bryant, B. (1964), Evidence for Strike-Slip Faulting I
along the Brevard Zone in North Carolins: Geological Soc. American Bull.,
v. 7 5, p. 1177-1196.
24.
Roper, P. J. and Justus, P. S. (1973), Polytectonic Evolution of the I
Brevard Zone:
Am. Jour. Sci., v. 273-A (Copper Volume), p. 105-132.
25.
Soil Conservation Service (1961), Soil Survey, Montgomery County.
26.
Soil Conservation Service (1973), Soil Interpretations Guide for Urbanizing Areas, Montgomery Cow.ty, Maryland.
27.
Sowers and Sowers (1970), Introductory Soil Mechanics and Foundations:
Third Edition, The MacMillan Company, New York.
28.
Stover, C. W., Reagor, B. G., and Algermiesen, S. T. (1981), " Seismicity Map of the States of Delaware and Maryland, Miscellaneous Field Studies Map MF 1281.
38 I
i
! I 29.
U. S. Department of Commerce, National Bureau of Standards, " Preliminary Hazards Summary Report," NBSR 7, January 1,1961.
30.
Virginia Electric and Power Company (1969), Preliminary Report on l
Seismology, North Anna Power Station.
31.
Wood, H. O. and Neumann, F. (1931), Modified Mercalli Intensity Scale of i
193 1" Seismol. Soc. American Bull., v. 21, p. 277-283.
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Figure 4.
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NOTES
- 1. This map has been abstracted from Hadley, J.8 and Devine. J.F. (1974). B. Earthquake Epicenters. 1800-1972 Seistnotectonic Mapof the EasternUnited States: United States Geologecal Survey Map MF-620. Sheet 2 of 3.
- 2. The center of each triangular symbolindecates the epicentrallocation of one ce more seismic events, plotted tothe nearest 0.1 degros of latitude and longitude. The intensity shown is maximum Modified Me calli (MM)intensety in the epicentral area of the largest e-ent at the plotted locateon. Most locations are based on observatsons of intensity rather than on instrumental records.
Figure 7.
Regional map of earthquake epicenters, Seismic frequency contours =epresent the areal distribution of earthquake epicenters with spacentral intensity of MM lli and greater, as indicated by the totalnumber per 10%m during the period 1800-1972. Contourintervals 8
are 0-4. more than 4 but less than 8. more than 8 but less than 16. more than 16 but less than 32. more than 32 but less than 64. and more than 64. The contours are considerably generaleaed and are shown only as a guide 6 estimateng regional seismicity. They have no value for precise location of seesmic boundaries.
E E
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Hachures on downthrown side e nown; arrows show relative movement.
'5 8
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t NOTES
- 1. This map has been abstracted frorn Hadley. J.B. and Devine.J.F. (1974). A. Tectonic Map.
Seismotectonic Map of the Eastem United States: United States Geological Survey, MAP MF.620. Sheet 1 of 3.
- 2. Gerwralization inwohred in reducmg data from moos at scales of 1:62.500 or larger to the 1:2.500.000 scale 1
of the tectonic and basement maps has resulted in discrepencies in some locations amounting to several miles.
- "'"'""""*""""***""d'"****'""'"*'"**'"'"'"'
Figure 8.
Regional tectonic map,
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KEY:
I 4,,,,q on unc Tectonic province boundary Dashed where concealed by v unge deposits Very appromemate limrts of seesmsc actrvaty areas Areas e and lor) structuralty controlled areas of rec (
or indf 1
Figure 9.
Regional seismotectonic map,
(
I
- 1. This mq Seismod o-
STRUCTURAL CONTROL a
2> 2 A
B C
t>
C 3 A
B C
U 4
94 A
B C
lEe 55 A
C SEISMIC ACTIVITY LEVEL Leved1 l frequency en epecenters es less than 8 per 10*km Includes large areas m whech seismec frequency is 3
y areas of this level are endecated without pattern. because information about historical sessnucity is sent to make structural analyses posssble, Level 2 frequency is generally more than 8 but less than 32 and no earthquake en the area has a manemum atral entensity greater than MM VI Used locally for are s of sessmic frequency higher than 32 around and on areas whok epecerstral pattem endecates structur. control, Level 3 g
genera 4ty to areas where seesmg frequency is more than 8 but less than 32 and at least one eartnquake of tral mtensrty Vil or Vill is recorded Commonly restricted to areas where epicentral destribution or relation a structure endicates a limitmg structural factor Apphes also to some areas where seismec frequency x more of not epecenters of entensety greater than VI are recorded. notab 9 central Virginia and the adach St Lawrence area.
Level 4 frequency is 32 or more and earthquakes of mtensety Vil or Vill have been recorded Locally entended fault trends ento areas of somewhat lower seismic frequency, Level 5 here one or more epicenters of mtenssty IX or hegher are present end seismic frequency is more than 32 seesmic frequency drops buiow 32 along structure.' trends level 3 applies because both maximum
'ty and seismic frequency decrease No areas exist vvhere the seesmic frequency es less than 32 and earth-6 of mtenssty greater than Vill have been recorded.
STRUCTURAL CONTROL m
A w
which known faults are associated weth epecentral aimements or destributKm. m such a way as to indicate ovements on the known faults or closely related f aults have been the source of recorded earthquakes, B
zhech mator f aults are not known. but epicentral concentration and aimement mdecate that movements ecognued or concealed faults have been the source of recorded earthquakes, W
C whech maior f aults are known. but the epicentral dastribution does not mdecate that they are the source trded earthquakes Also. areas en whech maior faults or other seismically actrve structures are not known bated.
NOTE has been abstracted from Hadley J B, and Devine. J F (1974). C. Seesmotectome Map.
tonec Maip of tne Eastern Uneted States Umted States Gelogscal Survey. M AP MF 620. Sheet 3 of 3 h
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Seismicity map of the state of Maryland.
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_ _ T i
01 O
1 (l7-A f-A h Borehole ll l9.0
%' 6.3 1
Dep3 to Water j g l
i 1
I lI l
I 1
l Outline of Reactor Building t
I l~ ~ ~ ~ ] O9.9j lg i
e l
l 6-A I
408.4 12.0 410.2 408.0 g
g l
5-A l 1-A 11-A 14.0 4
17 4 l 13.7 l
- - _,08.4 l
l 4-A l
l 16.0 l l
l g
I g
I i
i I
g h
0 40 80 l
l i
i,
i N86000 +
407.2 407.7 Scale (Ft)
I S t_ _ __r- - 9 \\
Grid Coordinates 10-A 3-A (See NBSR-7) 19.8 23.0 I
NOTE: 1. Ground-Water Tab:e contours of NBS site taken from NBSR-7C, Preliminary Hazards Summary Report-Supplement C, dated August 1.1962.
NOTE: 2. Ground-Water Elevationa Refer to Mean Sea Level Datum.
NOTE: 3. Ground Water Table contours are based on water level measurements in borings on January 20, 1961.
Figure 11.
Ground-water table contours at NBSR site.
_ _ _ _ _ _ _. _ _ _ _ _ _ _ _ _ _ _ _. _. _