ML20132A380

Selected Papers in Hydrologic Sciences,1985

ML20132A380
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Site:	Wood River Junction
Issue date:	05/31/1985
From:	INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
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Low-Level Radioactive Ground-Water Contamination From a Cold-Scrap Recovery Operation, Wood River Junction, Rhode Island By Barbara 1. Ryan and Kenneth L. Kipp, Jr.

Abstract INTRODUCTION guid w stes containing radionuclides and in 1981, the U.S. Geological Sursey began a 39 ear ther chenitcal solutes from an enriched uranium -

study of ground-water contamination at a uranium-cold-scrap recovery plant have leaked from polyethy-bearing cold-scrap recovery plant at Wood Reser lunction, Rhode Island. Liquid wastes from this industrial site were lene-and polysinylchloride (PVC)-lined ponds and discharged to the environment through esaporation trenches into a highly permeable sand and gravel aq-ponds from 1966 to 1980. Leakage from the uifer in southern Rhode Island. The resultant plume polyethylene-and polyvinylchionde lined ponds resulted of ground-water contamination extends about 2,300 in a plume of contaminated ground water that extends ft from the ponds and trenches to the Pawcatuck from the ponds northwestward to the Pawcatuck P.iver River and the contiguous swamp into which ground-

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through a highly' permeable sand and gravel aquifer of water discharge occurs. In 1981. the U.S. Geological

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lectri ! conductivity, determined by contamination at a plant at W.ood River Junction, electromagnetic methods, was used to delineate the Rhode Island (6g.1). The objectives of the study are plume areally before observation wells were installed.

to (1) identify constituents in the plume. (2) deter-These data, combmed with water-quality data from more than 100 observation wells, indicate that the plume is mine solute interaction with aquifer materials. (3) rpproximately 2,300 feet long and 300 feet wide and is model ground-water flow and solute transport in the confined to the upper 80 feet of saturated thickness study area, and (4) use the model to predict residence where sediments consist of medium to coarse sand and times in the aquifer and fate of contaminants in the gravel. No contamination has been detected in fine sands plume.

and silts underlying the coarser materials. Piezometric Contaminated ground water at this site moves head and water-quality data from wells screened at through a highly permeable glacial outwash aquifer multiple depths on both sides of the river indicate that that yields water readily to wells. The Rhode Island contaminants discharge to the river and to a swampy area Water Resources Board has conducted test drilling at the west edge of the river. Dilution precludes detection around Wood River Junction and has considered de-of contaminants once they have entered the river, which has an average flow of 193 cubic feet per second.

veloping ground water from the Meadow Brook Pond area for use both within and outside the basin.

Water-quality data collected from April 1981 to June 1983 indicate that strontium-90, technetium-99, boron, The possibility that supply wells developed in the nitrate, and potassium exceed background concentrations area might be contaminated as a result of migration by an order of magnitude in much of the plume.

of contaminated water beneath the Pawcatuck River Concentrations of gross beta emitters range from 5 to 500 is of concern to the Water Resources Board.

picoCuries per liter No gamma emitters above detection By October 1982, most of the data collection IIvels have been found. Electncal conductivity of the network for the investigation was in place, and rou-water ranges from 150 to 4,500 micrombos per centimeter tine water-level measurements, water-quality sam-at 25 degrees Celsius. Water-quality sampling shows pling, and precipitation measurements were begun.

zones of concentrated contaminants at both ends of the This paper describes geohydrologic conditions at the plume, separated by a zone of less contaminated water.

site, the source of ground-water con: amination, and Laboratory tests for exchangable cations indicate little capacity for uptake by the coarse sediments. In the presents preliminary Gndings based on data collected swamp, reducmg conditions may promote observable through June 1983. National Geodetic Vertical Da-solute interaction with sediments or organic material.

tum of 1929 is referred to as sea level in this report.

Low.tevel Radioactive Ground. Water Contamination from a Cold Scrap Re<overy Operation. Wood River lun(tion. Rhode island 21 i

and trenches mdicate that much of the hquid waste discharged to the ponds and trenches did not esapo-rate but rather percolated into unconsohdated depos-

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from 3 to 15 f t belew land surface. the bottoms of the

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From 1964 to 1966, liquid wastes were dis-

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charged to the Pawcatuck Riser through a buried drain 1,500 ft in length. Begmning in 1966, liquid g

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wastes were discharged into a pcnd approximately 5,000 ft in area and 6 ft in depth (fig. 2). Pond

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capacity or overBow problems due to precipitation r

4 and disposal now rates (estimated by plant oDicials to have averaged about 400 gal /d) led to periodic

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construction of additional ponds and trenches. In 1

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1967, a second pond (8.400 ft:in area and 4 ft in

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depth) was used as a replacement, and, in 1972, a new pond was constructed in the same area as the N

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original pond. A series of trenches were built to replace the first and second ponds in 1977 and were

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used until 1979. The liquid waste disposal ponds and 2

N NQ trenches encompassed approximately 25,000 ft.

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J of the contaminated liquid percolation to the water table. In 1979, a covered tank with a double poly-l ethylene liner was constructed 50 ft north of the orig-l Figure 1. Location of study area.

inal pond area (fig. 2) to hold the liquid waste during evaporation and concentration processing. To date.

no evidence exists for ground-water contamination from the covered tank storage area.

PLANT HISTORY AND CONTAMINATION Because data on chemical composition and SOURCE physical properties of the liquid wastes are limited and concentrations of chemical and radiochemical From 1964 to 1980, an enriched uranium cold-constituents in waste discharges changed with time, scrap recovery plant was operated (fig. 2) at Wood defining the actual source loadings is not possible. In River Junction, Rhode Island. Acid digestion with addition to hydrofluoric and nitric acids, tributyl hydro 0uoric and nitric acids and organic separation phosphate, and kerosene, the following chemicals with tributyl phosphate and kerosene were used in were used in the recovery process and were present in the process. Solid wastes from the process were the liquid wastes in varying concentrations: alumi-shipped offsite, and liquid wastes were discharged to num nitrate, calcium hydroxide, mercury, sodium the Pawcatuck River through a drain pipe from 1964 carbonate, sodium hydroxide,and potassium hydros-to 1966 and to uncovered " evaporation" ponds and ide. Although primarily nonirradiated fuel elements trenches from 1966 to 1980.

were processed from 1964 to 1980 slightly irradiated In southern Rhode Island, however, average an-fuel elements from test reactors were processed from nual precipitation is much greater than average annu-1967 to 1980. This could account for the strontium-al evaporation; for example, from 1950 to 1970, pre-90 and technetium-99 that are in the contaminated cipitation at the National Weather Station at water.

Kingston, Rhode Island,9 mi northeast of the study Processing at the plant, which ended in August area, averaged 46.06 in/yr while estimated annual 1980, currently is being decommissioned. Material free water surface evaporation for the same period from the bottom of the ponds and trenches and sedi-was only 29 in/yr (National Oceanic and Atmospher-ment from below the ponds and trenches were re-ic Administration,1982). This and the fact that moved and combined with a cementtike mixture and highly permeable sediments occur beneath the ponds shipped offsite for burial. Sediments in the unsatu-22 selected Papers in the Hydrologic sciences L

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rated zone between the pond and trench bottoms and the plant and the river that ranged in depth from the water table were not sampled.

approximately 50 to 80 ft. Water-quality data ob-tained by the company from one of these wells indi-cate ground water of high conductivity (14,500 PREVIOUS SITE INVESTIGATIONS pmi o/cm at 25'C), high nitrate (2.200 mg/L). and significant gross beta emitters (1,518 pC/L) 200 ft From 1974 to 1977, the Rhode Island Water from the source area (Dickerman and Silva,1980, p.

Resources Board drilled approximately 20 test holes 177-178).

on the plant property to obtain lithologic and water.

In 1977, resistivity surveys were conducted by quality data for evaluating potential areas for David Huntley, University of Connecticut, and by ground-water development. Water-quality data ob-Daniel Urish, University of Rhode Island. Results of t:ined as part of the Water Resources Board investi-these surveys indicated a plume of ground water with gation indicate ground water of high conductivity high conductivity between the plant site and the Paw.

(5,500 pmho/cm at 25'C), high nitrate (225 mg/L),

catuck River. Adjacent to the source area, depth end significant gross alpha (43 pC/L) and gross beta below land surface of the highest conductance water (489 pC/L) emitters 1,100 ft from the source area was estimated to be 40 ft (David Huntley, written (Dickerman and Silva,1980, p.177-178). In 1977, commun.,1981). Maximum known extent of con-the company installed 10 observation wells between tamination at the start of the present study (October Low-Level Radioactive Ground-Water Contamination from a Cold Scrap Recovery Operation, Wood River lunction, Rhode Island 23 L-

1982) was approumately 1.200 f t trom the source Hydrology area.

The plant site. located withm the lower Paw-catuck Rner hasm. is approumately 2 mi cast of the junction of the Pawcatuck and Wood Rn ers. Iincon-sohdated deposits near the jur'ction of these two rn -

STUDY AREA DESCRIPTION ers comprne the most estensis e accumulation of sed.

iments in the lower Wood aquifer (Gonthier and Geology others,1974. p. 7). The aquifer is approsimately 8 mi in length and ranges from 2.000 to 8.000 ft in The study area is underlain by the Hope Valley width with the majority ofit estending north, north.

Alaskite Gneiss. a metamorphie rock unit of Late west, and west of the plant site. Saturated thickness Proterozoic age (570-900 million years old). The in the Ellis Flats area esceeds 290 ft. Swamp and till gneiss was an igneous rock unit that underwent one deposits form the southern and eastern limits of the and possibly two episodes of metamorphism (N1oore, aquifer, respectively.

1959). The bedrock crops out east, northeast, west.

The aquifer is uncontined with a water table and southwest of the study area, and unconsolidated that slopes westward from the plant site at an average glacial deposits of Pleistocene age (less than I million gradient of 28 ft/mi. The lower boundary of the years old) have been deposited on top of the bedrock, aquifer is the bedrock surface (fig. 4). Generally.

Glacial till deposits (poorly sorted clays, silts, ground-water movement in the aquifer is from the sands. gravels, and boulders) form a relatively thin lateral boundaries or till upland areas toward the (less than 20 ft) mantle oser the bedrock (LaSala and Pawcatuck River (fig. 5). Ground water discharges to

. Hahn,1960) and appear at land surface east of the the Pawcatuck River, which is the major surface plant site (fig. 3). Glacial outwash deposits (well.

water drainage from the study area. Ground-water sorted silts, sands, and gravels) were deposited in the.

potentials (fig. 6) show upward vertical stovement of

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bedrock valley (fig. 4) and range in thickness from 0 water into the Pawcatuck River and contiguous I

to 300 ft in some parts of the valley, swampy area west of the river.

l In the bedrock valley, the outwash deposits con-Water enters the ground-water system through sist of predominantly medium to coarse sands and infiltration of precipitation (rainfall or snowmelt).

gravels to a depth of about 80 ft below land surface Overland runoff and some ground-water flow from and mostly fine sands and silts below a depth of 80 ft adjacent till-covered bedrock areas also enter the aq-uifer. Based on annu~ l ave (age runoff of 27.51 in (fig. 4). A glacial terminal moraine (till with some a

stratified deposits) approximately 3 mi south of the from 1966 to 1980 upstream of the U.S. Geological study area may be responsible for the fine sands and Survey gage on the Pawcatuck River at Wood River silts at depth. Slow-moving glacial meltwater flowing Junction and a relation developed by hiazzaferro and into a lake behind the moraine apparently resulted in others (1978, p. 45), long-term average annual the deposition of the fine-grained sediments.

recharge to the aquifer is estimated to be 26 in/yr.

The fine sands and silts are cohesive in places; Assuming ground-water outflow is a conservative es-however, few clay-sized particles have been found to timate of the amount of natural recharge, hiazzaferro date. Clay-sized particles from two split spoon sam-and others (1978) related ground-water outflow to pies taken from the fine sand and silt unit were 2.94 the percentage of stratified drift in a drainage basin.

and 3.07 percent. Clay-sized particles taken from The relation developed by linear regression is de-seven split spoon samples from the coarse sand and scribed by the equation, gravel umt ranged from 0.12 to 7.6C percent, with an Y = 35

• 0.6X, average value of 1.53 percent and a median value of 0.38 percent. In two locations (one approximately where F equals ground-water outflow as a percent of 100 ft south of hieadow Brook Pond and one be-total runoff and X equals percentage of total basin tween the plant site and river) where test holes have area underlain by stratified drift. For this case, X -

exceeded 150 ft in depth, a zone (5-15 ft) of coarse 100.

sands has been encountered below the fine silts and Discharge of water from the aquifer occurs sands and above the bedrock surface. The mineraio-through ground-water runoff and evapotranspiration, gy of the outwash deposits is predominately quartz primarily where the water table is near the land sur-and feldspars; dark minerals (biotite and hornblende) face. Hydraulic conductivity of till is estimated to are generally more abundant in tfie finer sediments average about I ft/d as does the tillin the nearby (F. T. Ntanheim. wntten commun.1983).

upper Pawcatuck Riser basin ( Allen and others.

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1966, p. 9), w hereas hydraulic conductivity of out-Uncontaminated ground and surface water in wash deposits at the plant, estimated from lithologic the study area generally meet U.S. Environmental logs is about 180 ft/d (Gonthier and others,1974 Protection Agency (1976) drinking water standards.

plates 2,4). Hydraulic conductivity determined from Specific cond actance, an indication of dissolved min-analyses of three aquifer tests made within a 1 mi erals in the water,is generally less than 100 pmho/cm radius of the site, including one on the plant supply at 25*C. Principal cations, sodium, calcium, potassi-well(fig. 2), ranged from 140 to 190 ft/d (D. C. Dick-um, and magnesium are present in concentrations of erman, oral commun.,1983). Hydraulic conductivity 14 mg/L or less, principal anions, sulfate, chloride, of the fine sands and silts at depth probably falls and nitrate are present in concentrations of 20 mg/L somewhere in between those of the tills and coarse or less. Some naturally occurring radionuclides, such outwash deposits. Fractures in the bedrock also yield as potassium-40 t I pC/L), radium 226 (3 pC/L), radi-water to wells but generally only enough for domestic um 228 (2 pC/L), and strontium-90 (3 pC/L), have supplies (5 gal / min or less)( Allen,1953, p. 26).

been detected in g ound and surface water in the Ground-water flow, which was calculated from a study area.

water-table gradient of 28 ft/mi, an estimated aquifer porosity of 0.38 (obtained from averaging porosity EXPLORATION TECHNIQUES values from six sediment samples), and hydraulic.

conductivity estimates that ranged from 140 to 190 Geophysical techniques and well drilling were

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,g g VERTICAL EX AGGERATION X10 Figure 6. Hydrogeologic section showing average ground water potential from July 1982 to lune 1983.

ination plume. Geophysical techniques included to locate areas within the aquifer containing water of seismic refraction to determme depth to bedrock, ge-high specific conductance.

ophysical well logging (gamma gamma, natural gam-More than i 35 observation wells ranging in ma, and neutron) to determine relatise hthologie dif-depth from 10 to 230 ft were installed during sis ferences within a given well and from well to well.

drilling phases using hollow-stem auger, mud rotary.

and electromagnetic (inductive) conductivity surveys and drive and-wash methods. Welk generally were 28 Selc<ted hpers in the Hydrologis sciences

l constructed of 1% to l'-:-m diameter flexible poly.

tered the river which has an aserage discharge of 193 ethylene or rigid PVC plastic pipe. Two wells were it'/s. The plume is approximately 300 ft in width and constructed with 5-in diameter rigid PVC pipe for is confined to the upper 80 ft of saturated thickness geophysical logging purposes, and two wells were (fig. 9) where sediments consist of medium to coarse constructed with 1%-in diameter galvanized steel for sand and gravel. The top of the contamination continuous water-level recording. Screened inter als plume is depressed below the water table, and its or well points ranged from 2 to 10 ft in length and depth increases as it mos es away from the source were either No. 10 (0.010-in) or No. 12 (0.012-in) area. The plume obtains a masimum depth (80 tt slot. The first drilling phases were used to install below land surface) between 1.400 and 1.500 ft from relatively shallow (less than 30-ft) observation wells the source area. Beneath the discharge area (river to determine the water-table contiguration. Later and adjacent swamp), the plume rises to land surface.

phases were devoted to the installation of wells rang-Specilie gravity of three samples of contaminat-ing in depth from 10 to 100 ft to locate the contami-ed ground water collected in 1981 ranged from 1.000 n tion plume horizontally and vertically. Ten split to 1.001 (Daniel Urish, written commun.1982). It spoon samples were taken for such sediment analyses is assumed, therefore, that freshwater recharge on top

,I as cation exchange capacities, mineralogic descrip-of the plume is probably responsible for increased tions, porosity tests. and sieve analues.

depth of the plume away from the pond area. Sea-sonal variations in hydrologic conditions may alTect CONTAMINATION PLUME dimensions of the plume; for example, high precipi-tation in the spring of 1983 depressed the contamina-i

~ The plume ofcontaminated ground water ex-tion plume below the water table at the river-swamp tends from the source area northwestward approxi-interface (fig.10).

mately 1,500 ft to the Pawcatuck River and south-Chemical and radiochemical constituents in the westward approximately 800 ft in a downstrea:Ti contaminated water include nitrate (5-600 mg/L),

' direction through the swampy area west of the river, boron (20-400 pg/L), potassium (3-25 mg/L), stron-a total distance of 2,300 ft (figs. 7,8). Dilution pre-tium-90 (4-250 pC/L), and technetium-99 (75-1,350 ciudes detection of contaminants once they have en-pC/L). Due to the expense of the analytical proce-

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e e une et hy.ro,.w.,.e sece a Figeste 7. Strontium-90 concentration in ground water at the plant site, October 1982.

tow level Radioactive Ground Water Contamination from a Cold Scrap Recovery Operation, Wood River lunction, Rhode Island 29 t

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.; w Figure 8. Nitrate concentration in ground water at the plant site, October 1982.

dure, only five water samples have been analyzed for processing material (1964-80). The zone near the technetium-99 (two by the U.S. Geological Survey source area apparently resulted from flushing of addi-and three by Oak Ridge Associated Universities). In tional contaminants from the unsaturated zone while these five samples, strontium-90 accounts for 10 to the sediment below the ponds and trenches was being 30 percent of the gross beta activity; the remainder is -

excavated for site-decommissioning.

attributed to technetium-99. The sums of strontium-Sediment-and water-quality analyses from j

90 and technetium-99 actually may exceed the gross sampling locations from the plant to the river indi-beta activity level for a given sample; this is most cate chemical and radiochemical constituents are not likely due to the fact that the separation and counting being sorbed by aquifer materials. Cation-exchange efliciency for individual radionuclide measurements capacities from five split spoon samples ranged from is greater than that of the gross beta counting appar-0.1 to 4.2 milliequivalent per 100 grams (meq/100 g).

atus. Concentrations of gross beta emitters range with a median value of 0.5 meq/100 g. Technetium-from 5 to 500 pC/L No gamma emitters above de-99 and strontium-90 have been detected in water tection levels have been found. Electrical conductivi-from observation wells that are 1,500 and 2.000 ft, ty of the water ranges from 150 to 4,500 pmho/cm at respectively, from the plant. In the swamp. however.

25'C. Dissolved solids have been measured up to reducing conditions may promote observable solute 3,500 mg/L, and these concentrations interfere with interaction with sediments or organic material once the detection of alpha emitters. Concentrations of the plume rises to land surface. Additional sediment-chemical constituents in contaminated water at the and water-quality analyses are being conducted on plant site, and background concentrations are sum-materials from the swamp, marized in table 1.

i From 1982 to 1983, two zones of concentrated

SUMMARY

contaminants were present at both ends of the plume and were separated by a zone ofless contaminated Liquid wastes from an enriched uranium cold.

water. The zone near the Pawcatuck River resulted scrap recovery plant have leaked into a highly perme-from infiltration of contaminants while the plant was able sand and gravel aquifer in southern Rhode Is-30 Selected Papers in the Hydrologic Sciences

e s

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800 700 600 goo 00 300 200

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DISTANCE FROM SOURCE ARE A. lN MErERS l

VERT 6C AL ER AGGER ATION E,0 l

t Figure 9. Strontium-90 concentration ira ground water at the plant site, October 1982.

I:nd. The resultant plume of contamination extends basin, Rhode Island: U.S. Geological Survey Water-2,300 ft from the source area (evaporation ponds and Supply Paper 1821,66 p.

trenches) to the aquifer's discharge area (the Paw-Dickerman, D. C., and Silva, P. J.,1980, Geohydrologic catuck River and swampy area west of the river).

data for the Lower Wood River ground-water reser.

Dilution, however, precludes detection of contamt-voir, Rhode Island: Rhode Island Water Resources nants once they have entered the river. Chemical Board Water Information Series Report 4,193 p.

and radiochemical constituents in the plume include Duran, P. B., and fiaeni, F. P.,1982, The use of electro-nitrate, boron, potassium, strontium-90, and techne-magnetic conductivity techniques in the delineation of tium-99 Unconsolidated deposits comprising the ground-water contamination plumes: Proceedings of the Symposium on the Impact of Waste Storage and aquifer contain few clay-sized particles, and contami-Disp sal n Ground-Water Resources, sponsored by n"nts do not appear to be interacting significantly.

the U.S. Geological Survey, and Cornell University, with the' sediments. In the swamp, reducing condi-Ithaca, New York, June 28-July 1,1982,38 p.

tions may promote observable solute interaction with Gonthier, J. B., Johnston,11. E., and Malmberg, G. T.,

j sediments or organic material.

1974. Asailability of ground water in th: Lower Paw-catuck River basin, Rhode Island: U.S. Geological Survey Water-Supply Paper 2033,40 p.

SELECTED REFERENCES LaSala, A. M., Jr., and llahn, G. W.,1960, Ground-water map of the Carolina quadrangle, Rhode Island: U.S.

Allen, W. B.,1953, The ground-water resources of Rhode Geological Survey Ground-Water Map 9, scale Island: Rhode Island Development Council Geologi-1.24.000.

cal Bulletin No. 6,170 pi Manaferro. D. l.., llandman. E.11, and Thomat M. P.,

Allen, W. B., Flahn. G. W., and Brackley. R. A.,1966.

1978. Water rewurces insentory of Connecticut, Pan Availability of ground water, Upper Pawcatuck Riser N. Quinmpiac Riser basin: Connecticut Water Re-Low-Level Radioactive Ground. Water Contamination from a Cold sc rap Recovery Operation, Wood River lunttion, Rhode Island 31 L

U 1

5 3

i 7

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1 s

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e

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I

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I 1*

'I 7

I I

I 5"

I

'o

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~

r u.

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I I

y I

-*-- 20-.t......

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,,0

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I 1............

- 9.i.

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e

.00 r00 600

.00 400 300 200 000 0

e i

s i

i OI.T.feCE FROM OURCE AREA.tN hefrER.

vEmitC.L E E.GGER. TION E f 0 Figure 10. Nitrate concentration in ground water at the plant site, May 1983.

Table 1. Representative chemical analyses of water from observation wells near the middle of, the edg the contaminant plume

[Results in milhgrams per hier except as indicated)

Observation Obwevation Observation well outsede welli:1 meddle wellon edge of plume, of plume, of plume, Chemical constituent feb.17,1982 feb.3.1982 Dec. 23,1981 7

3 9

Alkahnity-CACO,. - - - -

50

< 10 230

<1 Boron (pg/L) 1 2

Cadmium (pg/L) 50 4.1 Calcium - - - - - - - - - -

720 5.0 180 9.2 Chloride - - - - - - - - - -

2 5

4

<.1

<1

<,1 Copper (pg/L)

Fluonde - - - - - - - - -

130 16 1,900 310 Hardness -- - ----

250 20 fron (pg/L) - - - - - - - - -

6 1

5 lead (pg/L) - - - - - - - -

1.5 1.4 23 Magnessum 67 1,600 600 Manganese (pg/L) - - - -

1 2

14 Nitrate (NO, + NOi) 580 37

.18 Nickel (pg/L) 5.6 5.7 5.6

<.01

<.01

<.01 (ortho as P) - - - - - - -

3. 4 2.5 21 6.9 i

Potassium ---- ----

<.1 11 4.4 Sihca -- --------

25 7.8 Sodium ----------

j Speofic conductance 376 77 4,260 2.9 l

(pmho/cm at 25*C) 222

6. 7 14 Strontium-90 fpC/L) - - -

14 Sulf ate - - - - - - -

50 10.5 12.0

, i 1.5 l

Water temperature (*C) 11 16 50 Zinc (pg/t) - - - - - - - -

I 32 Selected Papers in the Hydrologic Sciences L

4 sources Bulletin No. 27 MS p Moore, G. E., Jr.1959. Bedrosk geology of the Carolina and Quonchontaug quadrangles. Rhode Island: 11.S.

Geological sun cy Quadrangle Map I 17. scale 1:31.680.

National Oceanic and Atmospheric Administration.14S2.

Es aporation atin for the contiguous 48 L f nited States:

Nou Technical Report NWS 33. 27 p.

U S. Ensironmental 1*rotection \\ geno.1976. Quahty cn-teria for water U S. Gosernment l'rinting Omce.

Washington. D.C. 20402. Stock No. 055-001-01049-4. 256 p.

\\

Low tevel Radioactive Ground. Water contamination from a Cold 5< tap Recovery Operation, Wood River lunction, Rhode tiland 31

r An Electromagnetic Method for Delineating Ground-Water Contamination, Wood River Junction, Rhode Island By Paul M. Barlow and Barbara 1. Ryan INTRODUCTION Abstract Surf ace electromagnetic (EM) surven were conduct-Surface electromagnetic (Eht) surveys were con-ed in August 1981 to dehneate the areal and vertaat es-ducted in August 1981 to delineate the areal and tent of ground-water contamination at a site in Wood vertical extent of ground water contamination at a Riv r junction, Rhode Island. The surveys were conduct.

site in Wood River Junction, Rhode Island. The ed in conjunction with a 3-year study of low-level radio-surveys were conducted in conjunction with a 3-year active ground-water contammation from a cold scrap re-study oflow-level radioactive ground water contami-covery operation (Ryan and Kipp.1983).

nation from a cold-scrap recovery operation (Ryan Surf ace electromagnetic mduction techniques that musure terrane condwtmty were used in August 1981 at and Kipp,1983).

a low level radic oudid-waste site in Wood River lunc-Surface electromagnetic inductiori techniques tion, Rhode Island, to de!meate areal and vertical estent that measure terrane conductivity have been found of contamination 'r< a sand and gravel aquifer of glacial to be an efTective tool in the preliminary assessment origm. Data from the terretw-conductivity survey were Y

g consistent with values of 'recific conductance of ground municipal contamination s.ites (Kelly,1976, p. 7; wtier measured as pan of a 3 year study of ground-wate, Greenhouse and Slaine,1983, p. 49; hicNeill,1980b,

{

contamination at the site. Data from the terrane-and

p. I1). Electromagnetic induction techniques are a specific-conductance sursep mdicate that a plume of relatively inexpensive and reliable method of map-contaminated ground water estends from wastew.ter 1: goons and trenches at the plant site to the Pawcatuck ping contamination plumes and may,in the early River. Above background terrane conductivities are p'*P stages of a study of ground-water contamination, aid ent over an area that is 370 meters in length, ranges in in the placement of water quality observation wells width from 100 to 200 meters. and ranges in depth from (Greenhouse and Slaine,1983, p. 49).

lind surface to 25 meters below land surface.

The ability of earth materials to transmit an e

Electromagnetic data were contoured in knear and electrical current is related directly to the electrical in dimensionlessloganthmic units. Electromagnetic data conductivity of the interstitial pore fluid and, to a contoured in imear umts indicated high-conductivity lesser esitent, the rock type. Electrical conductivity of zones that suggested potential ground-water contamina, the interstitial pore fluid (water) is determined pri-tion. Linear contounng also depicted changes in conduc.

marily by ion concentrations in the solution. As the tivity with depth more clearly than did the loganthmic ion content of the pore fluid increases, the ability of I

contouring.

the fluid to conduct an eiectrical charge and the con-Logarithmic contouring of electromagnetic data was successful in masking background noise, thereby de.

ductivity of the earth material also increase.

f lineating boundaries of the contammation plume more The instrument used in the Eh! surveys consists clearly. Selection of background apparent-conductivity of an alternating-current transmitter coil that produc.

{

values at the site for the logarithmic (ontounng schemes es a time varying magnetic field (primary field),

i proved to be the greatest objection to Ihe logarithmic g

g g

gg method. Background values which were too high caused earth (fig.1). These currents produce a secondary an unreahstic reduction in the boundaries of the contami-magnetic field. which, together with the primary n: tion plume, whereas background apparent conductivi.

magnetic field, are intercepted by a recciser coil (htc-ty values that were too low allowed meerference from Neill,1980b, p. $; Evans.1982. p.105-108; Zohdy background none to bias the hydrogeologic and others,1974, p. $$). Ilecause the magnitude of interpretation.

An tie <tromannetic Method for Delineatina Ground Water contamination, Wood River lunction. Rhode 1, land is l

l m

6 u>une to the extent that linear contours of raw data

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on surwy results may be realized by the use of

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logarithmic values is objective except for the choice g

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of a background apparent-conductivity value.

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This report summarizes the use of ENt sursey s ll to delineate a ground water contamination plume at 4

y a low-level radioactive waste site in Wood River

!t :

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C Junction, Rhode Island (fig. 2). The objectives of jl this paper are to compare the results of EM surveys

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l to specific conductance measurements of ground water to evaluate the ability of such a survey to delin-x),

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MO' N X \\\\ N 7 eate ground water contamination and the advantages N\\

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ing of electromagnetie data.

In this report, the results of horizontal-and ver-Figure 1. Schematic model showirig theory of electromag-tical-dipole measurements at

• included. It has been netic instrumentation and terrane-conductivity measure-noted (McNeill,1980b, p. 6; Greenhouse and Slaine, ment. (A) Transmitter coil produces primary magnetic field 1983, p. 48) that data acquired from the vertical-di-(Hp);(S) Induced current loops produce a secondary mag-pole configuration are more commonly subject to netic field (Hs). Relationship to eddy currents not shown; cultural interferences than data acquired from hori-(O Current flow is achieved by the availabihty of charged ronta!-dipole configurations. Misalinement of coils particles in the sediments and pore fluids; and (D) Receiv-er coil senses pnmary and secondary magnetic helds.

in the vertical-dipole configuration and a pro-Conductivities are recorded.

nounced departure from linearity of response at high the currents induced by the transmitter is a function

"*w of hydrogeclogic conditions, the magnitude of the

" u-secondary EM field is linearly proponional to the

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terrane conductivity (relative case with which an 7}

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instrument (Evans,1982, p.108; McNeill,1980b, p.

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5). The relative contributions to the apparent con-j ductivity of each of these factors need not be deter-w,,,,'- #/

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mined to interpret electromagnetic data. Results of

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data collected from many traverses at a study site are

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terrane conductivity (Evans,1982, p.108).

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Greenhouse and Slaine (1983, p. 47-59) sus-

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gested that the presentation of electromagnetic data N

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ty values. The dimensionless ratios then are con-("N V

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toured to outline the zone of contamination. Green.

house and Slaine cited three advantages to N.,

logarithmic contouring of converted data over linear contouring of raw data: First, logarithmic contours of converted data do not cluster near the contaminant Figwee 2. Location of study area.

M Selected Papees in the Hydrologic Sciences i

salues of terrane conductivity also may be causes for wells ranged from I 50 to 5.000 S/cm at 25'C, un-spurious readings in the vertical-dipole contaminated ground water at the site generally had a configuration.

specilic conductance ofless than 100 pS/cm at 25'C.

SpeciGe conductance data indicate that a plume of SITE DESCRIPTION contaminated ground water extends from the plant area to the Paweatuck Riser and adjacent swamp.

From 1966 to 1980, liquid wastes containing The plume is 520 m long and 100 m wide (fig. 4) and radionuclides and other chemical solutes were dis-s confined to the upper 25 m of saturated thickness, charged to kned lagoons at a cold-scrap uranium-where sediments consist of medium to coarse sand ruovery plant in Wood River Junction. Rhode Is-and gravel (fig. 5). The top of the contamination land (fig. 3). Leakage from the lagoons resulted in plume is about 10 m below the water table between contamination of a highly permeable sand and gravel the plant and river, whereas contamination is en-aquifer of g!rcial origin. Chemical constituents in countered at the water table within the swamp area.

the contaminated ground water included nitrate, po-tassium, strontium-90, and technetium 99. Concen-ELECTROMAGNETIC SURVEY trations ornitrate and calcium, both of which were present in plant efiluents, ranged from 3 to 600 mg/L Method and from 10 to 700 mg/L, respectively. Nitrate and calcium ions were the predominant constituents of The EM surveys were conducted in August the high dissolved-solids concentrations (as much as 1981 (Duran,1982) with a Geonics EM 34-3 induc-1,960 mg/L)in the contaminated ground water.

tive terrane-conductivity meter. Measurements j

Specific conductance of contaminated ground were obtained in both horizontal-and vertical-dipole water sampled from approximately 100 observation modes at 20-m intercoil separations, providing effec-e

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An Electromagnetic Method for Delineating Ground. Water contamination, Wood River lun(tion, Rhode Island 37 I

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Figure 4. Speci6c conductance of ground water, Wood River lunction, Rhode Island, April 1982.

tive depths of exploration on the order of 15 and 30 units. The first method (linear contouring) consisted m, respectively (fig. 6). A thorough explanation of of contouring apparent-conductivity values from the electrical conductivity of soils and rocks and of field measurements at 1.0 to 2.0 mS/m at 25'C con-the theory and operation of the EM 34-3 instrument tour intervals. Contouring of EM data in this man-has been given by McNeill(1980a, b).

ner did not necessitate the determination of a back-EM stations were located by pace and compass ground apparent-conductivity level. Therefore, no with the aid of aerial photographs. Data stations attempt was made to determine the level of back-were located midway between the transmitter and ground noise [ natural scatter of terrane-conductivity receiver coils (Duran,1982, p.106). Six traverses values caused by " topography, spatial or temporal (lines 3-8) were made approximately perpendicular variations in the depth to water table, observation to the direction of ground water flow between the accuracy, lateral changes oflithology, and cultural plant and the eastern bank of the Pawcatuck River interference from power lines, metal fences, etc.",

(fig. 7; table 1). On the western side of the river, one Greenhouse and Slaine (1983, p. 48)].

traverse (line 2) was made parallel to the river, this The second method of contouring follows the traverse then fermed the basis for several traverses recommendation of Greenhouse and Slaine (1983, p.

perpendio..ar to the river (line 1). Station spacings 49). They suggested the conversion of data to the were approximately 33 m, with the exception of those following logarithmic format:

located near the plant and in the swamp west of the 20 logm " (background)

Pawcatuck River, which were 15 m apart.

where a (x,y) equals apparent conductivity readings Linear and Logan.thmic Contour,mg at any location on a grid with x and y coordinates Contounng of the electromagnetic data was and o (background) equals the background apparent-done by two methods using linear and logarithmic conductivity value.

18 selected Papers in the Hydrologic sciences l

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150 - Une of equel specific conductance.

-- desned where soproximately located.

Intervet in microsiemens per centimeter et 2 5 'C 6e verleblo.

Hgure 5. Specific conductance of ground water, Wood River lunction, Rhode Island, April 1982, cross-sectional view.

The dimensionless ratios (decibel units) ob.

ties are present over an area 370 m in length by 100 tained from the logarithmic format for each station to 200 m in width; this is compatible with specific and the linear nondimensionless values then are con.

conductance results (fig.14).

toured. The zero logarithmic contour [0 decibel (db)]

Although difficult to quantify, the vertical ex-then separates contaminated from noncontaminated tent of the high-conductivity zone has been qualita-areas. Contour intervals of 4 db, which were used in tively identified with the aid of effective depths of this study, correspond to incremental changes of a penetration for the horizontal-and vertical-dipole factor of approximately 1.6 above background appar-configurations (fig. 6). Contours oflinear vertical-ent conductivity.

dipole data show local high-conductivity zones near the power substation and near the plant, where fences Results and transmission lines are concentrated (fig. I 1).

These elevated conductivity values may be the result Data from :nt EM surveys indicate a high-con-of electrical currents produced by the power substa-ductivity zone at the site in an area which extends tion and power lines that interfere with the electro-i from the plant to the Pawcatuck Riser and adjacent magnetic instrumentation or high ground-water con-swamp (figs. 8-13). Above background conductivi-ductivity between 6 and 12 m below land surface [the 6

An Electromagnetic Method for Derneating Ground Water Contarnination, Wood River function, Rhode Island 39 i

i

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Source area

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.c EXPL AN ATioN

-3000 Specific conductance values, Aprit.1982, dashed where interred.

t 20 -4 Interval in m6crosiemens per cent 6 meter at 25 C le variable.

Approximate zone of mastmum contribution to horizo'ntal-dipole Z'- '

configuration, mg Approximate zone of manimum contribution to vertical-dipole 10 --.

c on figur a tion.

i Fine sand and silt I

i 0

100 200 METERS s

?,',';

Dedrock Figure 6. Section showing effectwe depths of penetration and plot cornparing relative responses for horizontal and vertical dipoles.

interval of the aquifer that contributes most to the may occur within the 6-to 12-m intervalin this vertical-dipole con 0guration (fig. 6)].

swampy area.

Vertical-dipole vi. lues remain high (5 mS/m at 25*C) from the plant to within about 100 m of the On the contrary, however, horizontal-dipole Pawcatuck River and indicate that some ground-measurements (Og, 8) remain low (2--3 mS/m at water contamination has occurred of 6 to 12 m below 25'C) to the east of the Pawcatuck River but increase land surface. However, specific conductance values to between 4 and 8 mS/m at 25'C to the west of the indicate that the most contaminated round water is river in the swampy area. Because the greatest con.

F present 13 to 20 m below land surface near the river tribution to horizontal-dipole measurements is from (fig. 5). Because the maximum con ribution to the near-surface electrical conditions (fig. 6), elevated vertical-dipole con 0guration occurs in the depth in-conductivity measurements in the swamp may result terval between 0.3 and 0.6 percent of the intercoil from (1) the absence of a resistive, unsaturated layer spacing, the vertical-dipole configuration has not in the area,(2) the variation in grain size from uncon-sensed fully this high-conductivity zone. A greater solidated sand and gravel cast of the river to sitt and intercoil spacing (such as 40 m), however, may have organic matter in the swamp, or(3) a rise in the sensed this zone. Vertical-dipole values increase from electrical conductivity of the ground water. Although 4 to 6 mS/m at 25'C on the western side of the the absence of a resistive, unsaturated layer in the Paweatuck River, which suggests that contamination swampy area probably adds to the overall increase in 40 selected Papers in the Hydrologic sciences W

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s conductivity levels there, elevated levels of ground-in a higher range of background apparec.t-conductivi-water contamination are suspected. Horizontal-and ty values than to the east of the river, which consists vertical-dipole linear contouring of EM data show of unconsolidated sands and gravels. To bring the clustering of contour lines in this area (figs. 8, i 1).

western and eastern areas of the site into a common Logarithmic contouring schemes (figs. 9 and range of decibel values, higher values were assigned 10.12 and 13) were obtained by first assigning back-to the western side of the river than to the eastern ground values to the logarithmic equation given by side.

Greenhouse and Slaine (1983). Background values at Differences in the ranges of apparent-conduc-the study site were obtained in the following ways:

tivity values for the horizontal-and vertical-dipole j

(1) By averaging the low apparent-conductivity val-modes also necessitated the use of two background ues for those stations which are believed to reflect apparent-conductivity values (one for the horizontal uncontaminated terrane conductivities and (2) by and one for the vertical configuration) in each area of subjectively setting a background apparent-conduc-the site to the east and to the west of the river.

tivity value below which contamination does not Table 2 summarizes background apparent-con-seem likely.

ductivity values used in this study.

Background values are determined more easily Contours oflogarithmic data reflect above if the lithology of the study site is known. In the background conductivities from the plant to the river present study, the presence of a conductive, saturat-in the vertical and horizontal modes. Contours of ed swampy zone to the west of the Pawcatuck River, horizontal-dipole values converted to averaged back-consisting of silt, clay, and organic material, resulted ground apparent-conductivity levels (fig. 6) show An Electromagnetic Method for Delineating Ground Water Contamination, Wood River lunction, Rhode Island 41 l-._______-._______

i Table 1. Electromagnetic data from honzontal and vertical dipoles at study area

[ Data in millisiemens per meter at 25'C.:. no measurementj Electromagnetic Dipole tie (tromagnetic Dipole station Horizontal Vertical stateon Horizontal Vertic al Line 4:

Iine I:

A 3.9 5.1 A

l.9 2.1 H

3.9 3.5 B

32 48 C

44 5.5 C

2.9 3.7 j

D 3.3 D

3.1 3.3 E

4.2 -

4.3 E

I.7 2.6 F

6. 3 4.3 F

2.6 G

4.9 6.4 Line 5:

H 3.5 3.7 A

l.8 2.3 1

3.9 3.3 B

1.8 1.7 J

4.1 4.1 C

3.0 2.4 K

4.6 6.3 D

2.8 3.4 L

8.4 4.8 E

3.0 5.0 F

3.5 4.8 G

2.0 2.6 Line 2; 2.6 A

2.3 3.4 H

B 2.3 32 Line 6:

C 2.6 3.3 A

l.9 2.6 D

2.4 3.0 B

1.9 3.4 E

2.4 2.5 C

2.2 3.2 F

2.9 3.0 D

2.9 5.4 G

4.6 6.0 E

3.2 5.0 H

6.6 4.9 F

l.9 2.6 I

9.0 6.0 Line 7; J

8.5 5.7 A

1.4 2.5 K

7.2 6.5 B

l.9 5.2 L

9.7 3.4 C

2.6 4.0 M

7.2 5.8 D

2.5 5.0 N

6.2 7.2 E

1.5 2.8 s

0 3.7 5.5 Line 8:

P 3.5 3.5 A

1.6 1.9 Q

3.4 3.4 B

2.8 2.7 C

l.0 1.8 D

3.3 5.0 Line 3:

A 1.8 2.0 E.

3.2 5.6 B

2.3 3.4 F

4.4 7.5 C

48 4.8 G

3.3 8.4 D

2.6 4.6 H

4.4 13.0 E

I.8 3.3 1

3.2 6.2 F

2.5 -

J I.8 2.6 higher decibellevels (8 db)in the swamp than to the DISCUSSION cast of the river (as they did in the linear-contouring scheme). Because lithologic variations have been The principal advantage of plotting terrane con-masked deliberately in the logarithmic ratios, higher ductivities in dimensionless logarithmic ratios is that decibellevels probably reflect increased ground-it is possible to mask the contribution of background water contamination, which was suggested by the noise to the survey results; that is, local lithologic clustering oflinear contours in the swamp. However, variations, difTerences in the apparent and terrane if contamination has occurred in the swamp, it has conductivities resulting from variations in depth to been masked slightly by the second scheme of con-the water table due to topographic variations, cultur-touring loganthmic ratios in which a subjectively as-alinterferences, and inaccurate.aeasurements. With signed background apparent-conductivity value was the data converted, identification of contaminated zones is easier, inasmuch as any increase in the deci-used (fig.10).

42 Selected Papers in the Hydrologic Sciences

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e m ne Figure 8. Linear contouring of horizontal-dipole electrornagnetic data.

s 3

bel level indicates an increase in the apparent con-Linear app arcat-conductivity values are spaced ductivity over and above what might be caused by more closely near zones of elevated contamination and s rggest that the plume is composed of a broad background noise.

\\

The selection of a background apparent-con.

zore of relatively lower contamination (2-3 mS/m at ductivity value for contours oflogarithmic ratios, 25'C in the horizontal-dipole configuration and 3-5 however,is quite subjective (Greenhouse and Slaine,

. mS/m at 25'C in the vertical-dipole configuration) 1983, p. 49) and presents the greatest drawback to with Unes of relatively higher contamination near this method of data presentation. Difficulties were the piam snd m tne swamp. Although linear contour-found with both techiques for determining back.

ingof apparent-conductivity values does not show a ground values used in this study. If too low a back-continuous zone of contamination as clearly as does ground value is used, local lithologic variations nicy the logarithmic contouring of apparent conductisity obscure the hydrogeologic interpretation (as with I ratios (due to local variations in lithology and cultur-contouring oflinear values). Conversely, too high e al interference), linear contours emphasize high background value will not give sufficient d5finition levels of contamination. thereby outlining areas of of the boundaries of the plume, thereby masking ar-i possible importance.

cas of potential contamination. The authors propom '.

Contours oflinear values also portray the dif-that both methods of determining background appa:N ' ferences in vertical and horizontal electromagnetic ent-conductivity values be used in the contouring of results rhore clearly than the logarithmic format. This logarithmic ratios. Low background values will then is especially true west of the Pawcatuck River where aid in the delineation of the boundaries of tha plume, linear contcers show high levels of conductivity in whereas high background values will shcw most the upper layershf the aquifer. Differences between clearly the core of the contamination plume.

horizontal and ertical-dipole results are not seen as An Electromagnetic Method for Delineating Ground. Water Contamination, Wood River fundion, Rhode leland 43 t

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clearly with contours oflogarithmic ratios because tion was helpful in qualitatively determining shallow difTerent background apparent conductivities were versus deep levels of contamination.

used purposefully with horizontal and vertical con-Contouring of the EM data by linear and loga-figurations to put all values at the site into a similar rithmic methods shows that advantages and disad-range. As a result. contours oflinear values are more vantages are associated with each method. Advan-helpful in determining the relative depth of contami-tages to the linear method of contouring include an nation than are the contours oflogarithmic ratios.

emphasis on high-conductivity zones that are poten-tial contamination source areas and better depiction CONCLUSIONS of differences in the response of the horizontal and Results of EM surveys at a low-level ra-vertical-dipole configuration with depth. However, dionuclide ground-water contamination site indicate interference from background noise and some uncer-that areas of high apparent conductivity coincide tainty in delineating boundaries between contaminat-with areas of high specific conductance. Measure-ed and uncontaminated areas of the site create ments in horizontal and vertical-dipole configura-problems in the hydrogeologic interpretation oflin-f tions indicate chemical stratification that is con-early plotted EM data.

.l firmed by specific conductance samples from wells The ability to mask the contribution of back-j screened at various depths in the aquifer. Specific ground noise to EM survey results, thereby outlining conductance results do show. however, that contami-more accurately areas of contamination, is the princi-nation has occurred at greater depth than has been pal advantage to fogarithmic contouring. The selec-sensed by the 20-m intercoil spacing of the vertical-tion of background apparent-conductivity values at dipole configuration. The vertical-dipole configura-the study site posed the greatest drawback to logarith-44 sele <ted Papers in the Hydrologic stiences r

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mically contoured data. Background values that Amencan Chemical Society Symposium Series, No.

were too high caused an unreal reduction in the 204 Risk Assessment at Hazardous Waste Sites, F. A.

boundaries of the contamination plume, whereas low long and G. E. Schweitzer, eds., p.93-115.

background apparent-conductivity values allowed in' Greenhouse, J. P., and Slaine, D. D.,1983, The use of terference from background noise to bias the reconnaissance electromagnetic methods to map con-hydrogeologic interpretation.

taminant migration: Ground Water Momtoring Re-view, v. 3, no. 2, p. 47-59.

SELECTED REFERENCES Kelly, W. E.1976, Geoclectric sounding for delineating E

Dickerman, D. C., and Silva, P. J.,1980, Geohydrologic n.I,p 6-W data for the Lower Wood River ground-water reser-McNeill, J. D.,1980a, Electrical conductivity of soils and voir, Rhode Island: Rhode Island Water Resources rocks: Geonics Limited Technical Note TN-5, Missis-Board Water Information Series Report 4,193 p.

Duran, P. B.,1982 The use of electromagnetic conductivi, sauga, Ontario,22 p.

1980b, Electromagnetic terrane conductivity mea-ty techniques in the delineation of ground water con.

surements at low induction numbers: Geonics Limit-tamination plumes,in The impact of waste storage and ed Technical Note TN-6, Mississauga, Ontario,15 p.

disposalin ground-water resources: Proceedings of the Northeast Conference, Ithaca, N.Y., June 28-July 1, Ryan, B. J., and Kipp, K. L,1983, Ground. water contam-1982, U.S. Geological Survey and Center for Environ-ination plume from low level radioactive wastes, mental Research, Cornell University, p. 8.4.1-8.4.33.

Wood River Junction, Rhode Island (abs): Transac.

Evans, R. B.,1982 Currently available geophysical meth-tions of the American Geophysical Union, v. 64, no.

ods for use in hasardous waste site investigations:

18,p.224 An tiedromagnetic Method for Delineating Ground Water Contamination, Wood River fundion, Rhode lifand 4s

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Slaine D. D.,1983, Predicting the response of mapping Zohdy, A. A. R., Eaton, G. P., and Mabey, D. R.,1974, subsurface contamination with inductive conductivity Application of surface geophysics to ground water in-techniques, m 1983 Technical Education Sessiom vestigations: Techniques of Water. Resources investi-Ground Water Technology Division, National Water sations of the U.S. Geological Survey, Book 2, Chapter Well Association,34 p.

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aw 1

Figure 13. Loganthmic contounng of vertical-depole electromagne tic data using assigned background values.

l P

t 41 Selested Papers in the Hydrologic Scientes e

I

,.. ~ - -

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One of equal apparent conductiv'ty. etasheo abere approximately g

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where approvenate'y incated interval n m<roszemens per cent meter at 25' C i

g

t stus

,s,ar,suse Figure 14. Comparison of specific conductance in ground water to hncar horizontal dipole electromagnetic data; April 1982.

Table 2. Background apparent conductivity values used to normahre the electromagnetic data

( All salues in mithsiemens per meter at 25'C]

Dipole onentation Vertical Honsontal Values East West Iast West 2.4 3.3 1.7 3.3 Averaged A ss i g n ed - - - - - - - - - - - - - - - - -

3.0 4.0 2.0 4.0 l

t a

An Uettromagnetic Method for Delineatina Ground. Water Contamination, Wood River lunction, Rhode island 49 l

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