ML20002E351
| ML20002E351 | |
| Person / Time | |
|---|---|
| Issue date: | 01/31/1981 |
| From: | Hart P, Rachel Johnson, Michaelson T NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS), NRC OFFICE OF STANDARDS DEVELOPMENT |
| To: | NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-7 NUDOCS 8101270528 | |
| Download: ML20002E351 (57) | |
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SURVEY OF SALT DOME INVESTIGATIONS
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Peter Hart Robert L. Johnson Thomas J. Nicholson Ellen Quinn Robert J. Wright i
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U. S. Nuclear Regulatory Comission January 1981 l
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TABLE OF CONTENTS PAGE Introduction.............................................................
1 Stratigraphy.............................................................
5 Structure................
7 Dome Growth..............................................................
9 Dome Internal Structure.................................................
12 Hydrology....................................
15 Dissolution...........................................................
19 Modeling.............................
23 Observations.............................................................
25 Appendix A -
Itinerary...................................................
33 Appendix B - List of Meetings............................................
35 Appendix C - Notes of Meetings...........................................
36 LIST OF FIGURES 1.
Gulf Interior Region Trip Route......................................
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b INTRODUCTION Purpose of Trip Between September 8 and 19, members of the NRC staff undertook a technical survey of the U.S. Department of Energy (00E) studies in the Gulf Interior Region (GIR).
Within the GIR, seven salt domes are under preliminary investiga-tion as possible sites for storage of high-level nuclear waste.
Two domes are in northwest Louisiana, two domes are in east Texas, and three are in Mississippi (Figure 1).
The trip is one of several to survey 00E investigations for the siting of a high-level waste repository.
This work is under way in a variety of rock types at a number of locations within the United States.
The trip was intended to serve several purposes:
to provide an opportunity for the NRC staff to become familiar with the salt domes and i. heir geologic setting; to establish contacts and consult with specialists familiar with salt dome geology; to survey the nature and thrust of DOE-sponsored investigations; and to develop a basis for refinement of various NRC documents now in preparation.
It should be noted that it was not the purpose of the trip to make any evaluation 1
of salt dome suitability for a repository or any comparison of the suitability of any of the seven domes presently under study.
Organization of Trip A general introduction to the investigations in the GIR was provided in Columbus, Ohio on September 9 by the Battelle Memorial Institute's Office of Nuclear
2 Waste Isolation (ONWI).
During the morning, representatives of DOE and ONWI discussed the program organization and the site screening and decision process.
In the afternoon, the geology of salt domes and of the GIR was presented by Law Engineering and Testing Company (LETC0) and consultants.
During the following ten days, field tours were undertaken to:
the seven domes, numerous Tertiary exposures, the Winnfield dome caorock quarry, the LETCO Hattiesburg, Mississippi field office, and a hydrologic testing opera-tion underway in Louisiana.
These visits were interspersed with conferences with salt dome and regional specialists in Jackson, Mississippi; Baton Rouge, Louisiana; Alexandria, Louisiana; and Austin, Texas.
In addition, a tour of the Avery Island salt mine in Louisiana provided a look at salt dome internal structure and the DOE in situ tests.
The trip route is shown on Figure 1 and the itinerary is presented in Appendix A.
NRC members who undertook the trip are:
Peter Hart (engineering geology),
Robert L. Johnson (stratigraphy), Thomas Nicholson (hydrogeology), Ellen Quinn (modeling) and Robert J. Wright (exploration geology).
Participating in the first week was F. Larry Doyle (hydrogeology).
Organization of Report This trip report is a summary of information gained during the trip.
It is divided into three major parts:
topical material, observations, and meeting notes.
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w LEGEND Mississippi Domes 1 Richton 2 Cypress Creek 3 ' *"' "
O study areas Louisiana Domes 4 Vacherie 5 Rayburn's Texas Domes 6 Keechi 7 Oakwood Fiqure 1 Gulf Interior Region Trip Route
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The topical material includes stratigraphy, structure, dome growth, internal structure of a dome, hydrology, dissolution, and modeling.
These sections give a brief overview of each topic as background for the observations.
The observations section discusses the major issues which the group feels should receive attention.
The meeting notes (Appendix A) briefly discuss the major topics discussed during each meeting.
5 STRATIGRAPHY This section summarizes the regional stratigraphic framework and identifies some technical problems encountered in Gulf Coast stratigraphy.
Tne focus is on the regional stratigraphy of the sedimentary units surrounding the salt domes, while sections titled Dome Internal Structure and Dome Growth focus on the salt domes themselves.
Salt domes of the GIR pierce through and are surrounded by a thick sequence (15 to 20,000 feet near the domes under study) of generally unlithified sedi-ments of Triassic, Jurassic, and Cretaceous (about 63 to 180 million years before present) and Tertiary and Quaternary (present to 63 million years before present) age.
Three units are of primary importance.
One, the upper Triassic and lower Jurassic Louann salt is a deeply buried deposit which is the source of salt for the domes.
Two, the Tertiary sediments (about 2 to 63 million year before present) surrounding the domes are the most relevant to
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l understanding the hydrogeologic system, dissolution, and dome growth.
- Three, the Quaternary sediments, Where they overlie the domes, are useful in detecting any uplift or subsidence.
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The sediments thicken towards the Gulf of Mexico and represent the Gulfward migration of ancient depositional environments through time and the downwarping of the Gulf area.
These sediments include alternating nonmarine clastics and marine clastics, carbonates, and evaporites.
The resulting high vertical and l
areal lithologic variability is due to repeated episodes of emergence and l
6 submergence causing shifting fluvial, deltaic, barrier island, shelf, and slopedepositionaienvironments.
This lithologic variability leads to aquifer heterogeneities such as high permeability sands interbedded with low permeability muds.
The dome study areas in Texas, Louisiana, and Mississippi are all located within the same complex sequence of sediment; therefore, many of the same formations are being studied in each state.
There is, however, some difference in that the units in the Mississippi study area are higher in the Tertiary section while those in Louisiana and Texas are lower in this section.
Technical problems exist in identifying, correlating, and mapping the complex Tertiary and Quaternary sedimentary units due to a) lack of fossils, b) lack of distinctive lithologic characteristics, c) lack of material for accurate age dating, and d) high degree of vertical and areal variability in lithofacies.
Only limited lithologic data exist from continuous cores of aquifer and confining units due to difficulties in core recovery within the generally unlith'ified Tertiary and Quaternary sediments.
Missing or limited extent of Quaternary deposits over the domes limits the ability to detect any uplift or subsidence which might have occurred during the Quaternary.
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1 7.
STRUCTURE This section focuses on regional and near-dome structure.
The three study areas lie within the East Texas Salt basin, the Northern Louisiana Salt basin, and the Mississippi Salt basin.
These are all contained within the broad Gulfward-dipping Mississippi Embayment.
The regional structures reflect a tectonically stable area.
The structures, whether of basement or salt move-ment origin, are generally old and not complex.
The low seismic activity in most of the region also suggests stabili.ty.
One of the structures currently being investigated is the Elkhart Graben-Mt. Enterprise fault system near the southern limit of the East Texas Salt basin.
This fault zone has a higher seismic activity than that of the surrounding region.
In 1891, an earthquake of about MM 8 was recorded in the vicinity, but the significance of this earthquake is still disputed since the records are not clear.
During the last one hundred years several earthquakes of MM 4-5 have also been recorded, with epicenters in the Elkhart Graben-Mt. Enterprise system.
The only Quaternary faults observed in the GIR are in the Elkhart Graben-Mt. Enterprise System.
These three oblique slip faults, with a maximum throw of 119 cm, displace Quaternary deposits along the Trinity River.
They dip steeply and strike to the east-northeast.
Their location and orientation are consistent with the larger Elkhart Graben-Mt. Enterprise system.
Relative movement of 130 mm, in Quaternary terrace deposits along another section of this system, suggests that the fault system may be active.
8 Around each dome is a system of faults that reflect structural adjustments resulting from dome growth and dissolution.
These near-dome faults can provide either pathways or barriers to groundwater flow, depending on the level of permeability.
Faulting has been described around some of the seven domes (e.g., Keechi, Rayburn's, and Cypress Creek), but characterization has not been extensive.
The investigations include surface and subsurface mapping, drilling, and high resolution seismic surveys.
At all domes the interface between the salt and surrounding sediments is a fault zone due to the upward displacement of salt through the sediments.
This feature is described in the " anomalous zone" discussion in the Dome Internal Structure section.
Dome growth has often resulted in complex changes in the dip of surrounding beds.
These changes affect the local groundwater system, may affect the potential for dissolution (see Dissolution section), and are used for recon-structing dome growth histories (see the Dome Growth section).
9 00ME GROWTH According to present theory, the first step in dome formation was horizontal movement in the bedded Louann salt.
One of the mechanisms for this movement was differential loading by overlying sediments, where heavier clastics (sand) pushed the salt towards areas overlain by lighter clay or carbonates, forming salt ridges or " pillows." As the overlying sediments consolidated, they became denser than the salt, and this density difference led to further upward movement of the salt and piercicg of the overlying sediments.
The salt may also have been plastic, due to elevated temperature after burial, and the thermal effects would tend to increase the upward mobility.
As upward move-ment progressed, additional salt moved horizontally toward the dome, providing additional salt for growth, and creating a circular depression (" rim syncline")
around the dome.
Some domes may have become completely separated from the bedded salt source.
Several basic approaches are being used in assessing present and future growth potential for the domes.
a)
Regional - The salt domes under investigation are in the interior of the Gulf Coastal Plain.
These domes are geologically older and are in a stable geologic environment, compared with the coastal domes.
There is no currently active erosion or deposition to seriously alter the current equilibrium, and hence no driving forces for additional movement.
10 b)
Near-dome structure - Studies of stratigraphy and structure of sediments surrounding the domes can indicate the relative age, stage, and rate of movement.
In an early stage of development, a dome has a rim syncline f
that is relatively distant from the dome with thinning and steeply dipping 1
strata near the dome.
At a later stage the rim cyncline is closer to the dome, with strata thickening and dipping downward toward the dome.
Cylindrical and/or mushroom-shaped domes suggest that salt has been withdrawn from around the base and a source is lacking for additional salt movement.
Rim syncline analysis has indicated less than a meter of upward growth per 10,000 years for Oakwood dome.
Generally, it is believed that the domes under study stopped growing as long as 30 million years ago.
c)
Quaternary Deposits - The best evidence for Quaternary dome stability would be the presence of continuous, undisturbed Quaternary sediments across the top of a dome. Where present, the Quaternary deposits have been studied and they appear to be generally unaffected by uplift.
Unfortunately, this approach is of limited value for several reasons.
Many of the Quaternary and pre-Quaternary formations can be very similar in appearance and without distinctive fossil or marker zones.
Exposures are restricted.
Disturbances may be caused by dome dissolution and collapse.
At many of the dome locations, Quaternary formations are missing either due to erosion or lack of original deposition.
11 d)
Precise leveling and tiltmeters - Very sensitive tiltmeters have been used in Louisiana to measure possible current uplift.
Results to date indicate that very small movements are taking place, but these may be related to seasonal changes, groundwater withdrawals or earth tides.
From numerous lines of evidence it appears that the domes under study are not growing and are in a stable system.
However, it will be difficult to prove lack of present or future growth for each dome, due to lack of direct evidence.
12 DOME INTERNAL STRUCTURE Most of the current knowledge on internal dome structure comes from mines, and most of the mining experience has been in the coastal domes.
Six domes are now mined:
Avery Island, Weeks Island, Cote Blanche, and Belle Isle on the coast of Louisiana; Grand Saline, and Hockley in east Texas.
The Winnfield dome in north Louisiana was mined until it flooded, during the course of one day in November 1965, and was abandoned.
Jefferson Island dcme was mined until November 1980 when the mine was flooded following a penetration by an oil exploration drill hole.
The mines provide most of what is known about internal structures.
The mineralogy of salt domes is very simple, but the process of dome formation
~as created a very complex internal structure.
Generally the rock is composed of about 90 percent halite (salt, Nacl), with small amounts of intermixed and layered anhydrite (Ca 50 ), and trace amounts of shale, silica and carbonate.
4 The salt is generally white, or transparent, and is finely granular to coarsely crystalline.
Darker bands result from finely disseminated anhydrite within the salt.
These darker bands reveal the nearly vertical orientation of the l
complex folding.
Shearing, recrystallization, and discontinuities are also characterized by vertical orientation.
These features are all related to the dominantly upward, plastic movement of the salt dome.
The problems related to mining have mainly been in what Kupfer refers to as
" anomalous zones." The anomalous zones are also referred to in the literature
13 as " shear zones." Along the edges of each dome is an edge anomalous zone caused by shearing between the salt and surrounding sediments during dome growth.
In addition, a dome may have one or more central anomalous zones.
The central anomalous zones represent the contact between two " spines" of salt.
As a dome moves upward to the surface as a heated viscous material, it cools down and movement eventually ceases.
New source material finds an easier upward path along the edge of the older spine until it, too, stops.
i.. zone between two spines is the anomalous zone with sheared materials that include sediments and fluids that were in contact with the salt.
Several mining problems are related to the edge and central anomalous zones, and these zones are avoided, wherever possible, in mine development.
Present practice is to mine no closer than 300 feet to the edge of the salt.
The problems include:
a.
leaks into the openings, from internal and external sources, including brines, gases, and hydrocarbons; b.
joints and fractures; c.
slabbing of the roof and pillars; d.
gas blowouts and pressure pockets; and e.
shear zones.
The mines with problems are at depths of 450 meters or less.
The problems may be more acute if greater depths are anticipated at a repository.
14 Since the interior structure of the domes is near-vertical, exploring a dome's interior with vertical borings will provide only limited data.
The alternative of exploring with angle borings does not appear to be acceptable because of the potential for undesirable effects on containment and dissolution.
Only limited success has been achieved in testing dome interiors with underground geophysical methods such as radar and sonar.
Horizontal borings from mine workings have been used to distances as great as 1000 feet.
One possibility for underground repository exploration is to undertake horizontal tunneling in the salt at a level above a proposed repository.
Conditions at repository elevation could then be predicted by downward projection, since the geologic structures are mostly vertical.
15 HYDROLOGY The fieldwork for the regional hydrologic studies of Gulf Interior Salt Domes is divided as follows:
the U.S. Geological Survey (USGS) in Mississippi; Institute for Environmental Studies, Louisiana State University (IES/LSU);
the USGS in Louisiana; and the Bureau of Economic Geology (BEG) in Texas.
LETCO is performing the site specific studies that involve flood analysis and subsurface drilling and testing of wells at the domes under study.
The various state programs are being synthesized by LETC0 for the study area assessment of the various domes.
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The surface-water studies, which involve determination of flood histories, flood frequency analysis, and Probable Maximum Floods, are being done by LETCO for all of the seven domes.
The procedures followed are those outlined in l
NRC's regulatory guides (RG 1.59 and 1.70) for siting of nuclear power plants.
Surface groundwater relationships are also being studied in the vicinity of the domes.
The specific groundwater conditions are highly localized by the topography, structure, and stratigraphy.
However, in general the freshwater aquifers are the near-surface Quaternary and Tertiary units.
The first major confining unit is generally of lower Tertiary age:
in northern Louisiana and Texas the l
lower Tertiary Midway Clay is the regional aquiclude.
Individual overlying units may act as aquitards and affect surface groundwater relationships and the degree of upward leakage from deeper confined saline units.
Regional groundwater flow directions are generally to the Gulf of Mexico or the Mississippi i
16 River, with the intermediate and local groundwater flow to the surfte drainage network, to the deeper units, or to pumping operations.
The location of the fresh (less than 1,000 mg/l total dissolved solids (TDS)),
brackish (between 1000 and 3000 mg/l TDS), and saline (greater than 3000 mg/l TDS) interfaces is dependent upon (1) local groundwater flow conditions, (2) groundwater and/or petroleum pumping and injection operations, (3) faulting, and (4) proximity to the Gulf of Mexico.
However, the freshwater-saline boundary is generally in the lower Tertiary units below the regional confining unit.
The Cretaceous units are generally saline, and the upper Tertiery units can vary markedly from fresh to brackish depending upon local conditions.
The regional groundwater investigations involve identification of the principal freshwater aquifers, confining units, and saline-to-brackish units adjacent to the domes.
In each state, groundwater information is lacking for the units between the near-surface (to 1000 feet) fresh water aquifers used for domestic l
and municipal water wells, and the deep (more than 4000 feet) saline aquifers encountered in oil and natural gas development.
l The identification of the regional flow systems, including the recharge, discharge and leakage areas, and the regional hydraulics, has been defined in Texas and Louisiana, but not as yet in Mississippi.
The restricted study area and lack of historical records and previous groundwater investigations in deep aquifers has limited the work in Mississippi.
17 Using wells drilled and tested by LETCO, along with previous groundwater records, hydraulic potentials and gradients were determined in all three study areas.
Because of the presence of high TOS in the deeper aquifers, groundwater quality data were also gathered to calculate equivalent hydraulic heads for the more saline units.
One irregularity in the preparation of potentiometric surface maps in Mississippi arises from the fact that the USGS uses hydrogeologic units, while LETCO uses various horizons based upon groundwater quality.
The detection and mapping of salt plumes is an important consideration of salt dome stability and is being done in each area.
However, hydrochemical facies work is very preliminary.
Groundwater quality data is being collected in each area.
BEG has been using its groundwater chemistry data in assessing the carbonate dilution factor for carbon-14 dating studies, and validating ground-water recharge, leakage, and discharge zone delineations.
To ascertain hydraulic properties of the various hydrogeologic units, LETCO l
has performed single-well pumping tests using the modified Jacob Method to l
determine the hydraulic conductivities and storage coefficients.
Permeabilities 1
were determined by laboratory tests on side wall cores.
Vertical hydraulic conductivities and dispersivity data have not been collected.
For each area, the boundary conditions have been assessed.
Regional stresses such as large-scale groundwater withdrawals, waste injection, and oil and natural gas development have been identified and put in the context of the regional groundwater hydraulics.
Groundwater residence times have been f
18 investigated in Texas using C-14 studies.
Groundwater velocities and pathways have been studied in Texas on a limited basis but not in Mississippi or Louisiana.
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19 DISSOLUTION This section discusses investigations and factors important to understanding salt dome dissolution.
Dissolution is obviously one of the most significant processes having the potential for adversely affecting the hydrologic stability of a salt dome.
Investigators in each state are actively studying this problem, using various approaches, but this work is far from complete.
Dissolution is an obvious problem because the existence of several hundreds of feet of caprock (the residue of salt dissolution) is evidence that large amounts of salt dome have dissolved in the past.
Furthermore, salt domes at repository depth are now surrounded by a groundwater system which is unsatu-rated with respect to salt.
Nevertheless, salt domes exist today, which indicates some form of protaction from external groundwater.
Investigators have, to date, observed no solid evidence for significant dissolution occurring at any of the seven domes under study.
l Two hypotheses may be responsible for the apparent lack of significant salt I
dissolution.
One relies on dilution of high salinity water near the dome by local freshwater recharge.
The other assumes that some type of barrier exists i
which precludes or greatly reduces the exposure of salt to circulating l
l groundwater.
l Four possible barriers are proposed, each affecting a different part of a l
i dome:
caprock, clay sheath, upturned clay units, and less permeable l
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20 lithofacies in surrounding aquifers.
One, the caprock exists on the top of the dome and to some extent along the sides of the dome.
The anhydrite, gypsum, and limestone making up the caprock is far less soluble than salt and can be several hundreds of feet thick.
Two, a clay sheath has been detected at places on the sides of some coastal salt domes.
Very little evidence exists for its occurrence in interior domes.
However, it is likely that the edge anomalous zone may contain sheared clay material derived from adjoining sediments during upward movement of the salt.
Third, the Midway Clay (regional 1
aquiclude) has been upturned by the growth of Keechi, Rayburn's and Vacherie domes to a position where it appears to separate the salt from the surrounding aquifers.
Fourth, permeabilities in aquifers surrounding some domes may decrease toward the dome due to changes in lithofacies.
All of the above possible barriers could protect parts of the salt dome, depending on the extent, continuity, and degree of permeability of the barrier.
However, it may not be possible to determine the effectiveness of these barriers using available exploratory methods.
Even a number of drill holes would reveal little about the nature of a barrier and the degree of isolation of salt from groundwater.
l Investigations are also focusing on possible indications of both present and l
paleo dissolution.
Salinity anomalies have been detected in the groundwater near some of the seven domes by analyses of water chemistry and interpretation l
of electric logs.
Anomalies are seen as high-measured concentrations of sodium chloride ions; high TDS; low Ca/Na ratios; and a high base of the fresh water.
At the present state of knowledge, however, these anomalies could be 1
21 the result of any of the following:
- 1) dissolution of the salt dome,
- 2) dissolution of the anhydrite caprock, 3) upward flow of saline groundwater from underlying units, and 4) lack of complete flushing of either saline connate water or water from paleo dissolution from sand units having a higher clay content than surrounding sands.
Additional hydrochemical sampling and analyses will be needed before these indications could be taken as conclusive evidence for present dissolution.
Conclusive evidence for present dissolution was only shown for Palestine dome in Texas, where high sodium chloride concentrations were measured in a manmade lake over the dome.
This dissolution was initiated by a solution mining operation during the 1930s and has continued naturally since mining stopped.
The dissolution around boreholes is responsible for numerous surface collapses and formation of sink holes.
Based on this information and related surface effects and uncertainties, all studies and further consideration of the Palestine dome has ended.
Drilling encountered a permeable interface between the dome and caprock at some of the seven domes.
It is thought that a permeable interface is an indication that dissolution could occur.
As with the exploring for potential barriers discussed earlier, one or more coreholes would reveal little about the total extent of this interface or its permeability and connection with surrounding aquifers, unless extensive pumping tests are also used.
Suggestions of Quaternary or late Tertiary dissolution exist at most of the seven domes under study.
These include widening and thickening of late Tertiary
22 and Quaternary deposits in subsidence depr.:ssions over the domes, and the existence of subsidence deformation of Quaternary deposits, most importantly stream terraces.
However, these features may have been caused by activities other than salt dome dissolution, such as caprock dissolution, or structural readjustment (graben formation) as units were deformed over an uplifting dome.
The limited extent or lack of Quarternary deposits around some of the seven domes restricts Quaternary evidence of dome dissolution.
By combining indications of dissolution it may be possible to determine the domes which are relatively more or less susceptible to dissolution.
The model stable dome would have a clay sheath, upturned clay units around the flank, no saline plumes in groundwater, and no salt springs at the surface.
The model unstable dome would have very thick caprock, loose anhydrite sand at the caprock / salt dome interface, a salt plume downstream in the groundwater and surface salt springs at the surface.
i Although the studies to date have greatly advanced the understanding of the dissolution process, potential indications of dissolution, and methods of study, these are not presently sufficient to provide:
- 1) conclusive evidence l
for or against present dissolution at any of the seven domes, 2) unt eastanding of the complicated dissolution process from the past into the future, or
- 3) understanding of the possible adverse effects on a salt dome hosting a repository.
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t 23 MODELING This section summarizes the hydorgeologic and dome movement modeling studies discussed during the trip.
This does not represent a comprehensive survey of all modeling studies related to salt domes since the principal modelers at ONWI were not available for consultation at the time of the visit to Columbus.
Hydrogeologic The primary objective of the modeling studies is to picture the regional groundwater flow in both the fresh and saline aquifers.
The amount of data in the states varies greatly because of various levels of prior study in the different states.
In Mississippi the USGS and LETCO will be responsible for modeling the ground-water of the Mississippi study area.
Both plan to use the Trescott~, Pinder, Larson 3-0 Finite Difference Code (USGS Code).
The data base consists of 41 aquifer test holes completed during the exploratory drilling phase.
Estimates of the permeability of the units will be acquired from tests on sidewall cores.
Modeling of the region (basin) in Louisiana will be done by the USGS using the Trescott, Pinder, Larson 3-D Finite Difference Code.
Although the modeling has not yet begun, the study area is defined and a data base has been completed.
Much information exists about upper aquifers (30,000 points), but, at potential repository depths, little is known since the aquifers are saline.
1 24 In Texas, work is beginning on moceling water movement around Oakwood dome.
The most important parameters to define are the vertical conductivity, the aquifer geometry, and the variability in permeability.
The 3-D integrated difference model TERZAGI will be used for the evaluation.
The mesh will be generated by the OGRE program.
Model validation will be done by comparing known heads in the Wilcox against those generated by the program.
The :eview of modeling efforts shows the need for:
- 1) more data in many areas, particularly within saline aquifers, and 2) more information about confining units and vertical conductivities.
There is also a difference in the level of information available in the different states which impacts modeling attempts.
Dome Movement In Louisiana, modeling is planned to examine dome movement using SIMTRO, a finite element code.
A two-dimensional analysis of Vacherie dome has begun.
The analysis results will be compared to the amount of movement described by tiltmeter data, but several problems remain to be overcome.
The movement described by the highly sensitive tiltmeters is not always a reliable indicator of dome growth.
Outside disturbances such as local construction, traffic, or soil moisture can distort the results.
In addition, the mechanical properties of both the salt and surrounding beds are not well known.
The assumption has been made that salt doming is approximated by parabolic movement of a viscous fluid.
Laboratory tests are being done to investigate the salt properties.
Plans for the Rayburn's dome include modeling in three dimensions, as its asymetric geometry does not allow representation in two dimensions.
25 OBSERVATIONS To plan and coordinate the GIR program, so as to ensure that it provides results that can be linked with decision points in the national waste isola-tion effort, is a challenging task indeed.
The range of geoscience disciplines is broad.
The locations of the investigations are many.
The number and variety of contractors and subcontractors--federal, state, university, and private--is considerable.
Much excellent work by competent investigators was observed in the three states during the course of the trip.
The following observations deal with the thrust and methodology of investiga-tions and the relevance to requirements for information in licensing.
The observations do not concern the program organization and management.
As might be expected, the observations reflect special interest in the fields of knowledge represented by members of the group.
No comments are made in fields where the group has inadequate familiarity-notably geophysics and seismology.
It should be said, also, that time did not permit a complete and exhaustive survey of all aspects of DOE's geotechnical and earth science work.
The observations are summarized under the following headings:
o Regional and near-dome groundwater studies, o
Certain screening criteria.
26 o
Should the dome flanks be drilled?
o Studies of internal dome structure.
o Integration and coordination of technical studies.
Regional and Near-Dome Groundwater Studies An understanding of the regional and local groundwater is central to two matters that heavily influence the suitability of a dome:
(1) probable release pathways of radionuclides to the accessible environment and (2) protection of the salt from external groundwater.
To address these matters requires satisfactory characterization of the groundwater regime in which the dome is situated.
The screening process, that has to date yielded seven domes for detailed study, has made little use of groundwater hydrology.
The reasons are that c'.her considerations have been more important, and the hydrologic data have not been available.
However, the understanding of the groundwater picture will become more important during the more advanced stages of screening and selection, when greater attention is given to release scenarios.
The collection of hydrologic data--whether from borehole testing or from records--takes time.
Also, the analysis takes time.
Therefore, it is important now to identify the nature and form of the hydrologic data needed for input into decisionmaking; then, working back, the timing of contributory activities can be set.
Such an analysis would probably show that the hydrologic studies are off to a slow start and may not yield information in time for effective use in decisionmaking.
27 These are some of the specific issues:
1.
Around a dome the groundwater regime needs to be considered on a regional basis to cover recharge and discharge points of the relevant aquifers.
In Mississippi, the ONWI study area appears to be too limited for this purpose.
2.
In Mississippi some potentiometric surface work is done on fixed horizons, whereas other work is by hydrogeologic units.
This difference in approach should be evaluated and a common method sought.
3.
Attention needs to be given to the hydrologic properties of the confining units.
Only the aquifers appear to have been studied to date, but knowledge of both aquifers and confining units is needed for modeling the vertical and horizontal flow components.
The modified slug test may be useful here.
't 4.
In developing potentiometric surface maps, the methods of correction for differences in salinity need rationalization.
At least two approaches are now in use.
5.
Maximum use has not been made, in all places, of knowledge which can be gained from available drill hole information.
Examples include the use of electric logs and litbofacies analysis to determine permeability distributions.
28 6.
Consideration should be given to the future use of observation wells to more accurately determine storage coefficients, leakance, and other data not obtainable by single-well pumping tests.
7.
More emphasis should be given to water chemistry studies, including isotopic chemical analyses, to determine hydrochemical facies, groundwater residence times, and present dissolution.
8.
The approach and selection of computer codes used for groundwater modeling by different investigators needs radionalization.
Otherwise, the comparison of results from different areas may be difficult.
At least two approaches are now in use.
Certain Screening Criteria Two documents present siting criteria applicable to salt domes:
Y/0WI/TM-48,1 and ONWI 33(2).2 We understand that updated criteria are in preparation; however, the impression is that the above documents express the standards used l
in work to date.
Improved criteria are needed for further screening and site selection.
The following imperfections are noted.
Numbered criteria are taken i
from Y/0WI/TM-48.
2G. D. Brunton, R. B. Langhon, and W. C. McClain, Office of Waste Isolation, Union Carbide, " Screening Specifications for Gulf Coast Salt Domes,"
Y/0WI/TM-48, February 1978.
I 20ffice of Nuclear Waste Isolation, "NWTS Criteria for the Geologic Disposal of Nuclear Wastes:
Site-Qualification Criteria," ONWI-33(2), January 1980.
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29 Shale envelope.
Criterion 7 calls for a " shale (mud) envelope" separating the dome and surrounding aquifers.
DOE /NE 00071 also suggests that the domes are " encapsulated in plastic clays or other impermeable minerals" (pages II-108 and B-5).
It is true that most, if not all, domes are probably surrounded by an envelope of sheared sediment (which may or may not contain clay) that helps to protect the salt from dissolution by groundwater (otherwise the salt would not be there).
However, a requirement to demonstrate the presence of a shale envelope that seals the sides of a dome presents an impossible task.
An irregularity that might provide a path for dissolution can be quite small, and an unrealistic amount of drilling would be required to prove the absence of such an imperfection.
Further, such a large amount of drilling might impair the basic integrity of the dome.
Quaternary Uplift and Subsidence.
The specification for Criterion 4 calls for a lack of uplift or subsidence during Quaternary time.
Whereas this is c ubtless a desirable condition, it may be unprovable at a particular doma if very little or no Quaternary deposits are present for observation.
l l
l Should the dome flanks be drilled?
Some thought is being given to the possibility of testing the flank of a dome by drilling.
The main objective would be to establish the presence of a shale i
envelope or other barrier between the salt and external groundwaters.
f l
^U.S. Department of Energy, " Statement of Position of the United States Department i
of Energy, In the Matter of Proposed Rulemaking on the Storage and Disposal of j
Nuclear Waste," 00E/NE-007, April 1980.
i i
30 This is a matter for caution and careful judgment.
While the presence (or absence) of a hydrologic barrier is clearly a matter of concern and interest, it is doubtful whether drilling can provide a definitive answer.
Consider that a suitably sized dome might have a diameter of two miles and a perimeter of more than six miles.
Even a small imperfection could be a threat to the integrity of the barrier.
It is unlikely that any drilling program can prove the absence of an imperfection in the entire perimeter.
The other part of the picture is that each borehole penetration could weaken the integrity of the reposi tory.
On balance, then, there appears to be greater potential losses than gains in such drilling.
Studies of Internal Dome Structure Little attention is now given to internal dome structure as an element in the screening and selection process.
The presence of a central anomalous zone in a selected dome will present a design problem and, perhaps, a construction and waste emplacement hazard.
Such a zone, if present, will also reduce the l
l usable area of the dome.
Evaluation of a dome interior should be considered as an integral part of site screening and selection.
However, present methods
[
to detect anomalies within tne salt are inadequate.
Studies to understand such features and to develop appropriate geophysical detection methods will be difficult, and long lead times are involved.
Therefore, these efforts should be started well in advance of the time when the information is needed for j
decision milestones.
31 Integration and Coordination of Technical Studies The salt dome investigation is a complex technical venture.
It represents a wide range of geoscience disciplines, study topics, investigation locations, and types of investigators--corporate, individual, and institutional.
Furthermore, programmatic needs change with time; e.g., the technical input for decisionmaking in site screening is somewhat different from that in detailed studies of a particular dome.
It is a challenging task to assure that the right studies are sequenced in the right way to provide the right kina of input at the right time for decisionmaking.
Some of the preceding paragraphs have' identified subjects where better coordina-tion or tigh'ter planning or more standardization are called for.
In hydrologic studies, for example, the approaches to computer modeling and developing potentiometric surface maps differ between investigators.
Ultimately, in assessing the suitability of several domes, comparisons will be made of the results from different investigators.
If the results are obtained by dif-ferent methods, questions may arise about the comparability and relative reliability of the data.
Cross pollination of ideas between work centers is a normal need of a program ofthismtgnit[de.
It would be surprising if some improvements along this line were not advisable.
An example is the wide application of stratigraphy by the Bureau of Economic Geology in Texas to studies of dome growth, regional hydrology, and dome dissolution.
Parallel studies in the other states could well be beneficial, particularly if results from all three states are integrated
=
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32 on a regional basis.
The Texas stratigraphic studies also show the value to be gained by applying to the preferred domes the results from studies of other domes.
Finally, the mix of study topics needs periodic review from the standpoint of changing current and future needs.
As examples, the usefulness of continued caprock and salt petrology studies might be reevaluated with respect to future decisionmaking needs.
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33 APPENDIX A a
ITINERARY Date AM PM Tuesday U.S. Department of Energy, NWTS Program Office-Columbus and Office September 9 of Nuclear Waste Isolation (ONWI), 505 King Avenue, Columbus, Ohio 43201.
Wednesday Alvin Bicker Charles A. Spiers September 10 Mississippi Bureau of Geology U.S. Geological Survey P.O. Box 5348 Water Resources Division Jackson, Mississippi 39216 100 W. Capital Street (601) 354-6228 Jackson, Mississippi 39201 (601) 960-4600 Thursday Mississippi study area and LETC0 field office in Hattiesburg.
September 11 Friday Joseph Martinez, Institute for Environmental Studies, Louisiana Septe~ ber 12 State University, Baton Rouge, Louisiana 70803 - (504) 388-8521.
m Donald H. Kupfer Chip Groat Department of Geology Louisiana State Geological Survey Louisiana State University Box G, University Station Baton Rouge, Louisiana 70803 Baton Rouge, Louisiana 70893 (504) 388-2975 (504) 342-6754 Monday Avery Island Mine Gary Ryals September 15 International Salt Company U.S. Geological Survey Avery Island, Louisiana Water Resources Division P.O. Eox 5402 Alexandria, Louisiana 71301 FTS 8-497-7992 Tuesday Louisiana study area and Winnfield dome caprock quarry.
September 16 l
l aFigure 1 is a map of the Gulf Coast showing the seven domes and their three study areas, the cities where meetings were held, and the general route taken on the trip.
t
34 Wednesday Texas study area.
September 17 Thursday William Fisher and Charles Kreitler, Texas Bureau of September 18 Economic Geology, 1300 Lavaca, University Station, Box X, Austin, Texas 78712 - (512) 471-7721.
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35 APPENDIX B LIST OF MEETINGS HELD DURING THE GULF INTERIOR SALT DOME TRIP State Organizations Mississippi Bureau of Geology, Jackson, Mississippi Louisiana State Geological Survey, Baton Rouge, Louisiana Bureau of Economic Geology, Austin, Texas Federal Organizations Department of Energy ind Office of Nuclear Waste Isolation, Columbus, Ohio U.S. Geological Survey, Jackson, Mississippi U.S. Geological Survey, Alexandria, Louisiana University Department of Geology, Lousiana State University, Baton Rouge, Louisiana Institute for Environmental Studies, Louisiana State University, Baton Rouge, Louisiana
36 APPENDIX C MEETING NOTES There follows, in chronological order, a brief summary of each meeting with groups other than 00E/0NWI.
l l
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l
37 Mississippi Bureau of Geoloay Jackson, Mississippi--September 10, 1980 Attendees:
Alvin R. Bicker (State Geologist), Mark Smith, Curtis Stover (Bureau of Geology); John W. Green, Ronald J. Forsythe (Energy and Transportation Board); L. Doyle, P. Hart, R. Johnson, T. Nicholson, E. Quinn, R. Wright (NRC).
Discussion Mr. Bicker is familiar with construction control by NRC, which he had seen in action at the Yellow Creek, Mississippi nuclear power plant, and hoped that the same principles would apply to repository construction.
He noted that not much had been seen of the work done by 00E.
A discussion followed, led by John Green, of the monitoring and studies being done at Tatum Dome, where nuclear weapons were detonated in 1964 and 1966.
Tritium is now being found in near surface and deeper (Cook Mt. Formation) aquifers, and (perhaps) surface soils.
The measured tritium concentrations and releases appear to be increasing.
Mr. Bicker feels monitoring of the Tatum-Dome tests was not adequate.
A bibliography was provided.
In Mississippi, four state agencies have responsibilities in connection with waste management:
Bureau of Geology (Jackson); State Board of Health--
Radiological Division (Jackson); Research and Development Center (Jackson);
and Mineral Resources Institute (Oxford).
38 U.S. Geological Survey, Water Resources Division Jackson, Mississippi--September 10, 1980 Attendees:
P. Dooley, C. Spiers (USGS); 0. Swanson (ONWI); L. Doyle, P. Hart, R. Johnson, T. Nicholson, E. Quinn, R. Wright (NRC).
Discussion The function of the USGS in the area studies in Mississippi is to act as a j
consultant to La,< Engineering.
The USGS advises on techniques used for hydro-logic testing, assists on aquifer tests and observes onsite drilling and testing.
The USGS also conducts an independent analyses of the data for DOE to determine the regional flow system within the study area.
Forty-one pump tests had been completed.
The data from twelve are considered invalid because of gas pressure and density changes; these are not used in regional analyses.
The head values are corrected from brine to fresh water and potentiometric surfaces are drawn for the various hydrologic units.
l Two gradients are seen in the area.
There is regional flow to the south, l
l There is also a vertical component of flow into areas between Richton and Cypress Creek domes.
A head level difference of 75 feet exists, but the amount of flow and the point of discharge is not known.
These values will be affected by the vertical conductivity of the confining units, which were not studied.
l The area is referred to as a " source sink" for the flow system.
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39 Chemical analysis are also being conducted and Ca/Na ratios determined.
The highest level of total dissolved solids was seen down-dip of the Richton dome in the Catahoola Formation.
The anomalous concentration does not appear to be the result of vertical gradients since it is centered in one formation.
The amount of data does not allow a complete evaluation.
A similar study needs to be done at Cypress Creek Dome.
40 Institute for Environmental Studies, Louisiana State University Baton Rouge, Louisiana--September 12, 1980 Attendees:
Joseph Martinez (Director, Institute for Environmental Studies),
Charles Kolb, Betty Lee, Edward Newchurch, John Sterling, Rooert Thoms, Ronald Wilcox, (LSU); Owen Swanson (ONWI); L. Doyle, P. Hart, R. Johnson, T. Nicholson, E. Quinn, R. Wright (NRC).
Discussion Studies being conducted at the Institute for Environmental Studies include geomechanics, geohydrology, regional geology, and Quaternary geology.
These studies are focusing on the tectonic (dome growth) and hydrologic stability (dissolution) of north Louisiana salt domes, the nature of salt in the domes, and hydrologic stability of mined openings in salt domes.
After a cursory review of Gulf Coast Salt Domes, Dr. Martinez outlined the various theories as to the origin and growth of both interior and coastal salt 1
domes.
The geology of Louisiana was then outlined, placing the salt domes in l
l a regional perspective.
Although it would appear that the geologic aspect of salt domes is well understood, the interior structure of salt domes has been l
l barely studied.
Hydrologic stability studies are concerned with present and the potential for l'
future dissolution of the salt dome.
Hydrologic stability was illustrated by two extremes.
The unstable model has a thick caprock above the dome, uncon-solidated anhydrite sand along the interface between the dome and caprock, and l
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41 an aquifer in connection with the salt, indicated by a salt plume down gradient from the dome and salt springs above the dome.
The stable model has a clay sheath around the dome, the dome is overlain by clay units and the aquifer is not in direct connection with the salt, as indicated by absence of salt plumes or springs.
All domes fall somewhere between these extremes.
Hydrologic stability studies are also concerned with brine leaks in salt domes.
The origin of this water and the mechanism of movement through the dome is of major interest.
Analysis of water samples from salt dome mine leaks indicates that the water is formation water rather than a residual brine.
One hypothesis suggests that cor'. ate water in the sediments surrounding the domes (formation water) migrates along a natural temperature gradie'nt from the dome edge to the interior of the dome, which has a higher temperature.
In geomechanics the principal objective is investigation of the tectonic stability of Vacherie and Rayburn's domes by monitoring any possible ongoing movement.
Tiltmeters have been located on the ridge tops surrounding the salt l
l domes, and precise leveling studies are being conducted to determine the rate of movement.
The tiltmeters and leveling studies are extremely accurate and j
sensitive to everyday meteorological events and groundwater conditions.
One tiltmeter location was abandoned on Vacherie Dome because of the local distur-I bances.
To assess these local perturbations, soil and air temperatures and rainfall data are also collected.
Additional studies include seismic and microseismic monitoring, corehole closure measurements, and numerical modeling of collected data.
Studies to date indicate that if there is any movement at l
all it is very small and can be ? asked by effects of other natural processes.
l t-
42 The Quaternary geologic studies focus on Quaternary deposits over and near the dome to detect any movement during Quaternary time due to dome growth or dissolution.
So far no movement has been indicated.
The terraces have been investigated by mapping and shallow borings have been done, but problems exist in correlating and dating deposits.
Quaternary deposits fill closed depressions over both Vacherie and Rayburns' domes.
Thickening of some of these units suggest a small amount of collapse due to dissolution over both domes.
Evidence from such domes also suggests no uplift due to dome growth.
A continuous core through the caprock and upper 2500 feet of salt from both Vacherie and Rayburn's domes is being studied.
Portions of these cores were examined by the NRC group.
Various crystal textures, coloring, and inclusions were observed in the salt with the aid of a light box.
The salt from the Vacherie dome core contains a high percentage (5 to 10 percent) of anhydrite as deformed laminated layers.
The Rayburn's salt core is purer (less than 5 percent anhydrite) and more typical of representative dome salt.
43 Department of Geoloav, Louisiana State University Baton Rouge, Louisiana--September 12, 1980 Attendees:
Donald H. Kupfer (LSU); P. Hart and R. Wright (NRC).
Discussion Dr. Kupfer's primary interest in salt has been study of the interior of the domes, and much of his work had been related to mining of salt.
The problems related to mining have mainly been in what Kupfer refers to as "anomalaus zones." The anomalous zones are also referred to in the literature as " shear zones." Along the s'ide of each dome is an edge anomalous zone caused by shearing between the salt and surrounding sediments during dome growth.
In addition, a dome may have one or more central anomalous zones.
The central anomalous zones represent the contact between two " spines" of salt.
As a dome moves upward to_the surface as a heated viscous material it cools down and movement eventually ceases.
New source material finds an easier upward path along the edge of the older spine until it, too, stops.
The zone between two spines is the anomalous zone with sheared materials that include sediments and fluids that were in contact with the salt.
Mining problems, where encountered, are generally related to the anomalous zones:
~
44 1) leaks of fluids, such as brine, gas, oil;
- 2) joints, which can extend up to 100' away from openings;
- 3) slabbing of salt from the roof;
- 5) weak salt in shear zones from 20 to 200 feet wide.
The growth history of salt domes can be illustrated by examples from various areas.
Examples ranked according to increasing extent of upward growth are:
Spain, Germany, interior Louisiana domes, Mississippi domes, shoreline Louisiana domes, and offshore Louisiana domes.
Other specific points that Dr. Kupfer discussed are:
a) salt is only "self healing" at depth, say, over 2500 feet deep; b) shales in the anomalous zone can con:ain water under pressure; c) since salt dome geologic structure; are aligned vertically, features in a repository can be predicted by observations in excavations at higher levels; d) two schools of thought on design of openings in salt present opposite opinions--one says make the opening as large as possible to create a wide-stress envelope, the other says keep them as small as possible to minimize the stress; e) in Germany, tunnel headings are predrilled in advance of excavation.
45 Louisiana State Geological Survey Baton Rouge, Louisiana--September 12, 1980 Attend 9es:
Chip Groat (State Geologist), Judy Bond, Lori Eversull, John E. Johnston III, Harry L. Roland, Jr., (LGS); P. Hart, R. Wright (NRC).
Discussion The state has initiated a new program for monitoring and regulating disposal of wastes by injection, the Underground Injection Control (U.I.C.) program.
The state considers groundwater with salinity of less than 10,000 ppm as fresh water.
Fresh water is as deep as 3000 feet in the northern part of the state and becomes shallower to the south.
In northern Louisiana the Wilcox, Claiborne, and Jackson groups contain lignite which is now being considered as a potential economic resource.
Maps of the lignite have been prepared by the state.
Clay is also considered a resource in northern Louisiana.
The Cook Mt. Formation contains iron but is not now a resource due to the high phosphorous content.
The state survey has available numerous publications, geologic maps, and deep well logs including computerized data on 133,000 wells.
Some of the items available are:
surface map of lignite, clay resources map, oil and gas well map, groundwater recharge map, surface water supply, Quaternary map, and bulletins on origin of caprock.
46 It should be noted that there is a gap in the data coverage between depths of water wells (generally less than 1000 feet deep) and oil and gas wells (generally more than 4000 feet deep).
47 Avery Island Mine, International Salt Company Avery Island, Louisiana--September 13, 1980 Attendees:
Michael Gibbons, William Ubbes (ONWI); Donald Kupfer (LSU);
P. Hart, R. Johnson, T. Nicholson, E. Quinn, and R. Wright (NRC).
Discussion The tour of the Avery Island salt mine consisted of two parts:
- 1) observation of the in situ tests being conducted for DOE and 2) observation of the internal structure of the Avery Island salt dome.
The test location is in a mine room on the 500 foot level.
Electrical heater tests, accelerated borehole closure tests and brine migration tests are being conducted by RE/ SPEC, Inc. for DOE /0NWI.
The three heater tests which have been underway for about three years are presently in a steady-state condition and will be shut down in 0:tober.
The results concerning the near-field response of dome salt to a heat source will be compared to Project Salt Vault test results in bedded salt to determine if the bedded salt thermomechanical models are applicable to dome salt.
Four accelerated borehole closure tests, or flat jack tests, are presently measuring the rates of closure in test boreholes for four different pressure and temperature conditions.
48 The brine migration tests consist of 1) natural brine movecent under ambient-temperature conditions, 2) natural brine movement under elevated-temperature conditions, and 3) synthetic brine (deuterium-tagged) movement under elevated temperature conditions.
The synthetic brine test resulted in the collection of 17 ml. of brine in 300 days with an elevated temperature gradient of 1.5 C/cm.
No deuterium was detected, indicating that the brine collected is natural brine from within about two inches of the heater.
Some problems with these tests were noted.
One of the synthetic brine boreholes lost all of its brine overnight with no explanation yet known.
Also, it is uncertain how much of the brine collected is old mine water trapped in microfractures during a period of mine flooding.
Dr. Kupfer noted a variety of banding structures exposed in the ceilings and walls of the test facility room and adjacent rooms.
The near-vertical orientation of the banding allows for excellent correlation of major structures from one depth to another.
In another part of the mine a sandy salt zone was observed; it has been the source of brine leaks and may be the edge of an internal anomalous zone.
The development of the mine is strongly influenced by mining problems associated with these anomalous zones.
49 U.S. Geological Survey, Water Resources Division Alexandria, Louisiana--September 15, 1980 Attendees:
G. Ryals (USGS); M. Gibbons (0NWI); P. Hart, R. Johnson, T. Nicholson, E. Quinn, R. Wright (NRC),
Discussion The USGS is conducting a regional study of the hydrogeology in northwest Louisiana.
The area studied is 125 x 130 miles and contains Vacherie and Rayburn's domes.
The stratigraphic units are grouped into hydrogeologic units.
Much information (30,000 data points) exist for the Sparta Sand unit, since it is the principal freshwater aquifer i.: the area.
The flow in the Sparta Sand has been greatly affected by commercial pumping at Monroe and Jonesboro.
Historically the Sparta Sand had southeast flow, but the pumpirg has induced a northeast flow direction.
The time required for the flow to return to the predevelopment state, if the stress ceased, has not been determined.
Less information exists for the deeper saline units in the area, and potentiometric maps have not yet been drawn.
The USGS recommends more drilling in these formations since they occur at the proposed repository level near the domes.
The USGS also recommends that information be obtained about the vertical conductivity of the confining units.
Discharge appears to occur outside the study area but this cannot be verified until more information is obtained.
w e -
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s 50 Modeling of the system using the USGS finite difference model is planned.
Before modeling occurs, determination of head gradients in the Nacatoch Sand is recommended.
An additional element in the system is the presence of an inferred fault, 65 miles long with a 200-foot displacement, which cuts the top of the Sparta Sand.
The inferred fault, middle Eocene in age, occurs 4-5 miles from Vacherie dome and 2 miles from Rayburn's dome.
It is not known if this inferred fault acts as a vertical pathway for leakage.
m 51 Bureau of Economic Geology Austin, Texas--September 18, 1980 Attendees:
L. F. Brown, Jr., Eddie Collins, Owen Dix, E. Dow Davidson, Shirley Dutton, W. L. Fisher, Alice Giles, Tom Gustavson, Martin Jackson, Charles Kreitler, Mary McGowen, Alice Seni, Steve Seni, E. G. Wermund, Debra H. Wood, (BEG); Wayne Pennington (UT Department of Geology); Michael Gibbons (ONWI); P. Hart, 1
R. Johnson, T. Nicholson, E. Quinn, R. Wright (NRC).
Discussion BEG's function in the high-level waste program is to provide LETCO with basic data for characterizations.
While accomplishing that task, the study will look at the East Texas basin overall and determine the adequacy of using salt domes in a generic sense.
This regional approach will look at the variety of domes. The studies fall into four categories:
1.
Tectonic or geologic stability; 1
Hydrologic stability; 3.
Human intrusion; and l
4.
Quaternary studies.
l A number of detailed presentations were made by BEG staff covering the scope and results of their various regional and specialized studies.
Major points are summarized as follows:
L
6 52 Theories of salt dome crowth - Mechanisms of salt mobilization were discussed.
For initiation of salt movement the salt must behave plastically (in response to moisture, heat, or pressure) and have a lateral variation in thickness.
The driving force may result from bouyancy, differential loading, or. thermal convection.
Differential loading from sediments seems to be the major factor with bouyancy contributing to growth at depth.
The stages in dome development from Trueheim's model were discussed.
Studies in Texas are trying to refine this model and fit various domes to it.
Dome shape and the thickness and dip of surrounding units are used.
Gravity mapping of the Louann salt shows anticlines separated by synclines from which domes originated.
Geologic framework - Subsurface areas around fifteen domes were studied.
Only three of these are below the freshwater Tertiary aquifers.
Studies of dome tectonic stability use the sedimentary / stratigraphic framework around the domes and dome shape to determine growth histories of domes and the stage of development.
Domes along the margin of the East Texas basin appear to have advanced to a later stage of development than those in the basin center.
l Basin sedimentation - The upper Triassic and lower Cretaceous is characterized by alternating clastic (regressive) and carbonate (transgressive) sequences.
Stratigraphic studies are used to understand initial dome growth.
It is believed that depocenters controlled the initial salt mobilization which then controlled subsequent delta locations.
A thick carbonate unit only in the center of the basin may have limited salt movement by distributing the loading.
This resulted in domes now in the center of the basin appearing to have reached only an early stage of development while domes near the basin margin have reached a late stage.
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53 Sand deposition and salt movement - The control of sediment loading on dome growth was illustrated by sand lobes surrounding Hainesville dome and its rim syncline.
Other domes without rim synclines are not surrounded by sand lobes.
Basin faulting - The Mexia-Talco fault zone is crescentric in shape and forms a peripheral graben system to the north and west of the East Texas basin.
It corresponds with the limits of the Louann salt at depth, and was active from early Triassic to early Tertiary.
The Quaternary was not affected.
The Elkhart Graben-Mt. Enterprise fault system may be part of the same system.
Studies are being made to check on the current activity of this system.
Quaternary faulting - Quaternary faulting was observed in outcrop along the Trinity River following the Elkhart Graben trend.
The faults are oblique slip faults with normal displacement and 80 dip.
Leveling surveys in another area at the edge of the Mt. Enterprise showed changes in surface elevation of 130 mm (5 in.).
Earthquakes have been recorded along the same trend.
Seismicity studies - The GIR is characterized by a low seismicity.
Due to the higher historical earthquake activity near the Elkhart Graben-Mt. Enterprise fault system, a seismometer station has been there.
Many small events have been recorded, but the sources have not been defined.
Hydrology and domes - Most of the grondwater salinity data from the seven domes comes from electric logs, from which TDS is calculated.
In general domes show TOS increasing away from domes and higher in lower portions of the Wilcox.
This may indicate higher mud content in sands near the dome resulting in poorer
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54 flushing of old saline water, rather than dissolution.
The upturned Midway clay separates Keechi dome from high TDS zones.
A high TDS plume has been detected cround Oakwood dome.
The lack of solid evidence for dissolution might be explained by 1) dil.ution from a low dissolution rate combined with a high flushing rate and 2) the caprock acting as a barrier to dissolution.
Hydrology of the meteoric section - Studies are focusing on the Wilcor and Carizzo formations which act together as a confined to semiconfined aquifer.
Potentiometric maps showing flow lines were used to compare to topography, watershed features, and lithofacies (percent sand maps).
It appears that topography and rivers strongly influence the flow system.
At the scale of present study, little correlation is seen between lithofacies and the flow system.
Groundwater chemistry - Groundwater chemistry studies are used to evaluate groundwater flow paths, zones of recharge, and the evolution of water chemistry, and to i.dentify potential nuclide changes during transport.
Deeper groundwater chemistry is predominantly sodium bicarbonate, whereas near the outcrop it is calcium carbonate.
The pH rises with depth.
he chemistry is important to correct C dating procedures, since " dead" carbon (mostly from calcite caprock 14 dissolution) is being added to the system.
Carbon-14 dating of water in the Gulf Coast will be very difficult.
Hydrology of the saline section - Studies in the Woodbine formation (saline ulit at repository depth) have just started.
Pressure declines in the Wooobine have been high and rapid due to fluid withdrawal by oil and gas production.
55 The decline has spread indicating a lack of recharge to the system.
Presently, the Woodbine head is lower than that of the overlying Wilcox.
Leakage from saline units around domes or along fault zones such as the Mt. Enterprise is possible.
'! Clay sheaths" have not been seen surrounding Texas domes.
Hydrologic transport modeling - Modeling of water movement around oakwood dome is being done using the 3-D integrated finite difference code TERZAGI.
The mesh will be generated by the OGRE program.
Studies are being conducted on transport models regarding:
- 1) location of releases, 2) paths of releases,
- 3) dilution due to convective dispersion and sorption, and 4) thermal effects on flow.
Model boundaries are:
- 1) lateral flow in and out of the system,
- 2) vertical leakage, 3) aquifer geometry, and 4) variations in permeability.
At present, permeability of the aquitards is not known, but samples are being taken from cores.
Permeabilities are also being estimated by correlating grain size analysis results of lithofacies to pump tests and then to electric
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log interpretation.
Sediment samples have been sent to Battelle (PNL) for transport retardation studies.
Solute transport modeling is planned.
Other investigations reported on were:
Oakwood salt core studies, East Texas basin formation, lineament mapping, dissolution of Palestine dome, and caprock studies.