Download - Electrical methods of detecting contaminated groundwater at the stringfellow waste disposal site, riverside county, California

Electrical Methods of Detecting Contaminated Groundwater at the Stringfellow Waste Disposal Site, Riverside County, California DONALD J. STIERMAN

Department of Earth Sciences

University of California

900 University Avenue

Riverside, California 92521

ABSTRACT / At the Stringfetlow Class I waste-disposal site near Riv-

erside, California, the influence of variations in groundwater chemis-

try and saturation on electrical measurements made from the surface

was tested Spontaneous potential, D C electrical sounding, dipole-

dipole resistivity profiles, and mise-a-la-masse measurements were

employed to investigate the sub-surface migration of the acidic fluids

deposited in this site between 1956 and 1972 Mise-a-la-masse ex-

ploration conducted at the downstream edge of the site detected a

major anomaly which, on excavation, proved to be the signature of a

previously unsuspected leak from the surface disposal ponds on the

site Downstream from the site, a dipole-dipole profile and electrical

soundings correlate well with electrical conductivity of groundwater

obtained from monitoring wells This study demonstrates that geo-

physical methods may be used to search for and map leaks from

toxic chemical waste dumps

I n t r o d u c t i o n

The Stringfellow Class I disposal site, located in the Jurupa Mountains near Riverside, California (Fig. 1), operated from 1956 through 1972. Approximately 140 million liters (32 million gallons) of liquid wastes, including industrial solvents, acids containing salts of heavy metals, and organic residues from pesticide manufacturers, were released into surface lagoons. Following the flooding of the site during heavy rains in 1969, and the discovery of contamination in a monitoring well 1,100 m downstream from the site (Fig. 2) in 1972, the operating permit for the site was revoked. Surface runoff escaped the site again during the wet winters of 1978 and 1980. Legislation and funding for clean-up and closure of the site followed. Currently, the site has been graded and covered to prevent additional surface escape of hazardous chemicals. The very expensive final clean-up awaits additional studies and further funding.

Discharge lagoons were constructed from the regolith and alluvial materials on the floor of Pyrite Canyon (Fig. 2). Igneous and metasedimentary rocks of the Southern California batholith underlying the site were classified as impermeable in an engineering report written by Bookman (1955), despite clear evidence of a spring, visible or air photos taken in 1953, that was flowing from these rocks near the head of the canyon. Water wells drilled into the bedrock of the Jurupa Mountains can produce modest quantities of water from fracture zones in the granite (3,000-5,000 gallons per day; personal communication from several property owners), evidence that, while these rocks are not a particularly rich water resource, they are indeed permeable. One 70-m-deep well is reported to exhibit seasonal fluctuations in water levels, evidence that fracture permeability

is sufficient to rapidly transport fluids. A debris flow forming the southeast flank of the site was characterized in the 1955 engineering report as a "natural dike or barrier--composed of well cemented gravels, cobbles, and boulders and appears to be impermeable" (Bookman 1955). It was also assumed that this conglomerate formed a relatively shallow cover over impermeable bedrock. A concrete barrier was set in the ravine draining the site to capture any leakage from the lagoons and a road was constructed across this barrier to provide access to ponds on the west flank of the canyon.

Air photos show a well-developed set of near-vertical, southeast-striking joints in granodiorite northwest of the site (Morton 1978). Excavations at the foot of the concrete barrier in the autumn of 1979 showed that contaminated fluids were flowing at rates of several liters per minute from fractures in the granodiorite, and that the so-called conglomerate was not cemented below the surface. Further exploration seemed neces- sary.

Our objectives in conducting geophysical studies in and around the Stringfellow site were, first, to gather information bearing on sub-surface structure, rock properties, and fluid properties--parameters that will control the extent and degree of sub-surface fluid migration. Our second objective was to study how a waste disposal site might be interrogated geophysi- rally, and thus contribute to exploration strategy around other waste disposal sites. The third objective was to correlate geophysical characteristics of the rocks to the fluid flow patterns in those rocks.

Five configurations of electrical exploration arrays were employed in attempts to characterize and map the plume of contaminated fluids in the alluvium and rock under and downstream from the site. Schematics of the electrode configu-

Environ Geol Water Sci Vol 6, No 1, 11-20 �9 1984 Springer-Verlag New York Inc

2 Donald J Stierman

k'4 s / CONTOUR 'NTERV2AL : 200 FKEMET ~ /

Figure 1. Index map showing the location of the Stringfetlow site in the Jurupa Mountains of southern California.

rations are shown in Figure 3. A more complete description of the theoretical bases for these procedures can be found in Parasnis (1975, Chapters 4 and 6) or Telford and others (1976, Chapter 8).

Instrumentation

A Soiltest R-60 D.C. Meter was used for all of the measurements except for the dipole-dipole profile downstream from the concrete barrier (Line BB', Fig. 2). For that dipole- dipole survey, we employed a Geotronics FT-20 transmitter as a current source because the R-60 would not provide sufficient current for a dipole-dipole configuration beyond N = 4 (dipole length = 100 ft) here. Problems in reading the ammeter in the R-60 system were overcome by employing a high-quality digital ammeter. Current electrodes were metal rods driven I 0 to 30 cm into the surface or lead plates buried 10 to 20 cm deep. Potential electrodes were ceramic porous pots containing a saturated copper sulfate solution as an electrolyte.

Electrical Measurements

Spontaneous Potential (SP)

When fluids of different electrochemical characteristics occur together in the earth, a voltage difference develops that

Figure 2. Map showing the locations of toxic waste lagoons, electrical survey lines, and concrete barrier in Pyrite Canyon.

becomes larger as the difference in electrochemical activities of the fluids increases. Although many other factors unrelated to chemical properties may influence SP, including fluid flow (streaming potential) and telluric currents, the influence of temporal variations in SP was minimized by establishing a recording millivoltmeter as a base station.

SP measurements were made along the west side of the site, extending upstream and downstream approximately 300 nieters from Pond 2 (Fig. 2, line QQ'), and also along an east-trending line (RR') crossing the canyon 100 m south of Pond 2. The SP measurements did not exhibit a pattern obviously related to surface lagoons containing acidic fluids. The signal was very noisy throughout the floor of the canyon, with stations separated by only a few meters sometimes exhibiting differences in SP of tens of millivolts (my). We have not determined the reason for the high SP noise level; perhaps the modifications introduced to the canyon by quarrying and other industrial activities have overwhelmed other SP signals. More likely, large differences in contact resistance between points on the surface distorted natural SP signals originating at

Detecting Contaminated Groundwater 13

TO , REFERENCE

( R O V I N G P R O B E )

SPONTANEOUS POTENTIAL (SP)

|

A M N []

WENNER ARRAY

A M N []

SCHLUMBERGER ARRAY

Na ~ DIPOLE-DIPOLE

(N = I, 2, 3 , 4 , - - )

TO REFERENCE

O0 CO r ,.s {~) , MISSE-A-LA-MASSE

FIXED ROVING (IN CONOUCTIVE BODY )

Figure 3. Schematics of electrode configurations for arrays employed in this study. I denotes current source, V denotes voltage measurement.

depth. Once it became clear that SP surveys did not contribute to our knowledge of the pollution pattern, we abandoned passive electrical measurements. SP was routinely determined as part of active electrical prospecting, but no further attempt has been made to interpret the values tabulated.

Direct Current Resistivity

The electrical resistivity of a fluid-saturated rock is generally dominated by the amount and chemistry of the fluids contained in the interconnected pores of that rock. Archie's Law (Keller 1982, p. 223), an empirical relationship between rock resistivity, fluid resistivity, and fluid porosity is generally written

p (rock) = p (solution) a P - m

where p = electrical resistivity, p = porosity of the rock, a = a constant usually approximately equal to 1, and rn, called the cementation factor, usually approximately 2. Since the electrical resistivity of water generally decreases as the concentration of dissolved ions in the watei" increases, increases in the

degree of saturation, greater interconnected porosity, or in the degree of pore water contamination should produce a measur- able decrease in resistivity patterns in the earth.

The quantity measured in direct current exploration is the apparent resistivity. It is important to distinguish between this quantity (which represents, for a given geometry of electrodes, the response of a homogeneous, isotropic earth to direct electrical currents), and the true resistivity of the materials at a given point in the sub-surface. Variations in apparent resistivity data can, for some electrode configurations, be interpreted in terms of true resistivities and, therefore, quantitatively related to fluid resistivities and rock porosity. For other configurations, the relationship between apparent resistivity and true resistivity is less easily determined.

The geometry of the dipole-dipole electrical array, shown in Figure 3, is useful in detecting both lateral and vertical changes in electrical properties. Apparent resistivities are plotted in a pseudosection, resembling but not necessarily a true cross section. The pseudosections plotted in this report have been modified using depth adjustment factors published by Edwards (1977). This places the apparent resistivities at their interpreted effective depths, the sub-surface level that contributes most of the signal detected at the surface. One limitation on use of the dipole-dipole configuration is that large input currents are required to investigate deeply. The major disadvantage regarding the interpretation of the dipole-dipole array is the difficulty in determining true resistivities.

Electrical soundings were conducted using both the Wenner and Schlumberger array geometries (Fig. 3). In electrical sounding, increasing the distance between the current electrodes (A and B) increases the depth of geophysical interroga- tion. The Schlumberger array requires that potential electrode separation M N is less than 20% of the distance between current electrodes AB. This restriction can limit the usefulness of the Schlumberger configuration when investigating highly conductive strata, or when the instruments are limited by low current output or a relatively insensitive potentiometer. The Wenner configuration can often overcome these difficulties but has other drawbacks. In the conduct of Wenner soundings, the separation of the potential electrodes changes each time current electrode separation AB increases. Not only does this require additional field effort, but the Wenner array may fail to detect lateral inhomogeneities, a limitation to which the Schlum- berger array is tess susceptible. For both Schlumberger and Wenner soundings, apparent resistivity is plotted as a function of the distance between the current electrodes (Fig. 4).

Electrical soundings were interpreted by comparing the data with hypothetical curves generated using computer programs published by Campbell and Watts (1978). Implicit in calculating both apparent resistivity and hypothetical curves is the presumption that the earth consists of homogeneous,

14 Donald J Stierman

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isotropic, horizontal layers, assumptions that are usually vio- ,oDD! lated in some degree. Despite such limitations, electrical sound- L ing does provide an average image of the sub-surface electrical ~ ~~176 t behavior that can be related to true resistivities and thicknesses ~ 3oo of materials hidden underground. ~ 200

Mise-a-la-masse, which can be translated as "excitation of 100! the mass, is used by mining geophysicists to map the sub-

surface extent of a conductive body partially exposed on the ~ 8% surface or in a borehole. One current electrode is attached ~- ~o~ directly to the conductive body, with the other current pole at a ~ 3c[ a:: , large distance (effectively, infinity). The potential field around ~< ! the body will resemble a monopole except where departures ,o from a homogeneous earth occur. Equipotential lines tend to outline the conductive body buried in a less conductive earth. Parasnis (1975) describes the use of mise-a-ta-masse in mining exploration.

:/

3 I ; . . . . [0 2:0 3=O 1 5i 0 . . . . i ; 0 2 ;0 300 i 500 I i i'0; 0 A B / 2 m e t e r s

Figure 5. Sounding curve for Schlumberger sounding centered over resistivity low 600 ft south of the north end of AA' (Fig. 4). Dots indicate data, curves show fits to 3-layer models discussed in text.

L e a k D e t e c t i o n

Both lateral and vertical changes in apparent resistivity occur along dipole-dipole profile AA'. Values of 40-60 ohm- meters on the north end of the line reflect either rocks of lower porosity or containing fluids of lower electrical conductivity than the rocks underlying the center section of the line (apparent resistivities 13-20 ohm-meters). A resistivity low occurs about 200 m southwest of point A'. Apparent resistivities again increase south of this point, reflecting a shallowing of the relatively low-porosity bedrock or a decrease of ionized compo- nents. Because of the low apparent resistivities encountered along line AA', our greatest depth of penetration, correspond- ing to N = 4 and a = 30 m (100 ft), was about 40 m (130 ft).

F-igure 5 displays the data from the Schlumberger sounding centered over the resistivity low identified on the dipole-dipole profile east of Pond 2. Three electric layers are sufficient to fit these points. The upper layer of undersaturated near-surface material 3 m thick overlies a more conductive layer, interpreted

as being water-saturated, 18 to 22 m thick. The electrical half-space representing the basement is so resistive that its resistivity cannot be measured with the electrode spacing used; the slope for AB/2 of 80 m and more exceeds the theoretical maximum (45 ~ for a good conductor overlying a perfect electrical insulator. This is no doubt a lateral effect due to the narrowness of the canyon in the vicinity of the site; the electric current is not spreading uniformly into the granitic rocks forming the canyon walls, thus violating the assumption of infinite horizontal layers implicit i5 computing the apparent resistivity. Nevertheless, the data constrain both the thickness and the true electrical resistivity of the conductive middle layer. The base of the conductive layer lies at least 21 m (69 ft), and perhaps as much as 25 m (82 ft), below the surface at the east side of Pond 2.

At the time of this survey (December 1980), this interpretation was a bit disconcerting, as bedrock was exposed in a trench at a depth of about 8 m just west of line AA'. The mise-


TOPOGRAPHIC CONTOUR INTERVAL 20f t . POTENTIAL CONTOUR INTERVAL 5mY

el I OOl 2001 5?0 METERS t / 0 500 I000 FEET

I I N

Figure 6. Equipotential contour pattern from mise-a-la-masse survey no. 1. Star shows location of excitation electrode in Pond 2. Stippled pattern shows major lagoons or areas where waste ponded following leakage or flooding.

a-In-masse technique was used to further investigate the anomaly.

The excitation current electrode was placed in Pond 2, in saturated mud near the water line. The second current electrode was placed about 600 m down the canyon. The reference potential electrode was placed 300 m north of Pond 2. Poten- tials were normalized to an input current of 140 ma and contoured in 5 mv equipotential increments. Our first survey (Fig. 6) showed evidence of electrical distortion into material southeast of Pond 2. If the earth were a homogeneous, level, isotropic conductor, the equipotential lines would form concen- tric circles centered on the input electrode. Results of a second survey to investigate this distortion of the potential pattern toward the southeast are shown in Figure 7. The electrical distortion is apparently marked in a zone about 100 m southeast of the current electrode. This anomaly cannot be due to measurement errors; reoccupied stations consistently repro- duced values to within 0.1 my, well below the contour interval used in Figure 7.

Electric currents flow along the lines of least resistance, directions we can infer by recalling that currents flow at right

- - TOPOGRAPHIC CONTOUR INTERVAL 2 0 f t .

POTENTIAL CONTOUR INTERVAL 5mY

0 I00 200 500 METERS t I I t 1 0 500 I000 FEET

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Figure 7. Equipotential contour pattern for mise-a-la-masse survey no. 2 Distortion of pattern to the east indicates zone where subse- quent excavation encountered a thick lens containing previously unsuspected leakage.

angles to equipotential lines. These electric currents probably reflect, in this case, diversion of equipotential lines to follow more conductive and thus presumably contaminated groundwater. Flow of sub-surface fluids from Pond 2 is thus mapped in Figure 7. The resistivity pattern detected along dipole-dipole line AA' was consistent with this anomaly, and interpretation of the Schlumberger sounding suggested that this leak extended to a depth of 21 to 25 m (70 to 80 ft).

Excavation in October 1981 showed that the above interpretation was correct. Eastward extension of a trench, dug parallel to the concrete barrier to expose bedrock for grout injection, encountered an abrupt drop in the bedrock surface from 8 m just west of line AA' to 30 m east of AA'. Bedrock is overlain by unconsolidated, chaotic material containing boulders up to 2 m in diameter, probably from debris flows that blocked and diverted the drainage of the canyon from its most deeply eroded notch into a new channel to the west. Although this debris appears well cemented at the surface, it is loose and permeable when saturated. Fluids forming from the walls of the newly excavated trench were the coffee-colored acidic wastes from the

16 Donald J Stierman

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Concentration of chromium extracted from fractures, pores, and weathered rock found in cores, plotted as a function of depth. Arrow marked Control shows range of chromium extracted from granitic rocks collected elsewhere and thought to be uncontaminated. One contaminated sample collected from the shore of Pond 2 yielded 46.7 mg kg.- I.

Stringfellow disposal lagoons. The low electrical resistivities measured here were due to both the high porosity of this unconsolidated material and to the low resistivity of the contaminated fluid saturating the pores.

The resistivity contrast between the acid-saturated debris and underlying lower-porosity bedrock is so high that the true resistivity of the granite cannot be measured from the surface. Cores collected from six exploratory boreholes drilled by Woodward-Clyde Consultants along the southern shore of Pond 2 (Fig. 2) document a wide variety of conditions in the granitic bedrock. The granite ranges from highly weathered and friable to completely unweathered. While fracturing in general decreases with depth, cracks and alteration exist at 60 m depth.

Unweathered granitic rocks are characterized by low intrin- sic fluid permeabilities, but fractures and joints that occur in many granitic rocks can provide considerable secondary permeability. Samples of decomposed granite and alteration minerals were collected from 46 of the fracture surfaces found in the core and tested for contamination. Chromium was selected as the tracer because of its high concentration in the liquid acid wastes and because of its relatively low abundance in granitic rocks. Preliminary tests demonstrated that an acidic leaching fluid effectively extracted chromium from the rocks, so the samples were leached for about 72 hours in 0.14 N nitric acid. The mixtures were filtered and the leachate samples analyzed for

chromium on a Perkin-Elmer Model 5000 atomic absorption spectrophotometer (AAS). In addition to the 46 samples obtained from the cores, control samples were also treated with the leaching solution and tested for extracted chromium. Concentrations of chromium measured by the AAS were used to compute total chromium extracted. The results are displayed in Figure 8. Traces of extracted chromium exceeding 1 mg kg - t persists to depths of almost 50 m. This phase of the investigation points out one limitation of electrical exploration. The low porosity of the granite yields high electrical resistivity despite the presence of contamination in fractures. Wireline electrical logs in boreholes may be useful in mapping contaminated fractures in the granite but, because of the high resistivity contrast between the shallow material and the bedrock, surface electrical methods were unable to interrogate the bedrock for contamination.

D o w n s t r e a m Electr ical Studies

New monitoring wells drilled by James M. Montgomery, Consulting Engineers of Pasadena, California ( JMM) , in 1980 and 1981 found that contaminated groundwater in shallow (up to 30 m thick) alluvium extended to about 600 m south of the concrete barrier. Infiltration of contaminated runoff during the 1969, 1978, and 1980 floods is thought to be the source of this pollution plume, although the possibility that this contamination is due to sub-surface leakage has not been eliminated. Because of the high density of monitoring wells, Pyrite Canyon presents an opportunity to test the sensitivity of electrical geophysical methods in monitoring groundwater quality. Wenner array electrical soundings (Fig. 9) were conducted using J M M monitoring wells 7-11 as sounding points. Resistivity of the saturated alluvium between the water table and bedrock changed from 25-28 ohm-meters for sounding points 7, 8, and 9 to 80 ohm-meters for Well 11. This change is in excellent agreement with the change in fluid resistivities reported by Barto and others (1981) for these wells (Table 1). Only near the edge of the plume are fluid resistivities and interpretation of resistivities based on sounding curves in disagreement.

Dipole-dipole data are also displayed in modified psuedo- section in Figure 9. These also show the increase in apparent resistivity in shallow rocks near MW-10. Unfortunately, our Geotronics FT-20 transmitter failed and we were unable to extend the profile.

Discuss ion

The new monitoring wells provide data on the water table and the thickness of alluvium overlying bedrock in addition to


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Figure 9. Pseudosection (adjusted for effective depth) showing dipole-dipole apparent resistivities along line BB' Resistivity-depth information derived from Wenner soundings centered on monitoring wells (MW) 7-11 is superimposed (heavy lines). The arrows indicating the sounding points and well locations also show the 100 ohm-meters value for the resistivity-depth plot deduced from soundings.

Table 1. Compar ison of Format ion Resistivities with Fluid Resistivities Repor ted by Barto and Others (1981) Depth to Bedrock Also f rom Barto and others (1981).

Well

Fluid Formation Depth to Meters from resistivity resistivity bedrock pb

concrete barrier (ohm-meters) (ohm-meters) (meters) (%)

MW-7 320 2.3 25, 30 ~ >30 30 MW-8 442 1.3 28 >30 22 MW-9 610 2.4 25 >34 31 MW-10 701 5.4 25, 100 ~ 27 23 MW-1 l 853 5.3 80 17 26

aTwo layers needed to fit saturated zone. bPorosities (P) listed are calculated based on Archie's Law, assuming cementation factor rn = 2.

information regarding fluid resistivities. These data place important constraints on modeling the resistivity data, as illustrated by the Wenner sounding conducted on MW-10. Two significantly different resistivities are required in the saturated alluvium to fit the sounding curve. The Wenner sounding suggests a shallow zone of low-resistivity, contaminated fluid overlying alluvium containing normal groundwater. If this interpretation is correct, contaminated fluid may lie near to but remain undetected by MW-10 (Table 1). Either the water sample from M W - 1 0 was fluid from the deeper, uncontaminated section of the monitoring well, or lateral flow has diverted the pollution plume to the east of MW-10. As was demonstrated in the vicinity of the dumpsite leak, electrical measurements are sensitive to conductive material lying beside as well as directly beneath the electrode array. Lateral changes in resistivities underlying the electrodes can also yield data which, when interpreted, may not yield accurate results. Since the casing in Monitoring Well 10 is not perforated in the upper 40 ft (Barto and others 1981, p. 2-9), it is possible that this monitoring well does not sample contamination that is present there. As the water table in Pyrite Canyon drops due to

extraction of contaminated water by production wells, an increase in electrical conductivity in samples from M W - 1 0 would confirm the existence of a shallow zone of contaminated fluid when the lowered water table causes contaminated water to reach the perforated casing. This points out the important fact that, because a contamination plume is three'dimensional, water quality may vary with depth as well as horizontally. Wireline resistivity logs run prior to casing monitoring wells should provide information regarding variations in fluid conductivity with depth.

A second (and possibly more important) inference of the shallowing plume is that this pattern is consistent with the hypothesis that downstream pollution is dominated by infiltration of surface runoff rather than sub-surface escape of fluids from the lagoons.

Comparison of the resistivity-depth relationships deduced from soundings with a dipole-dipole pseudosection (Fig. 9) demonstrates how adjustment of the depth at which apparent resistivities are plotted (Edwards 1977) provides a cross section that resembles geologic conditions more accurately than do traditional dipole-dipole plots. Apparent resistivities in the shallowest section of the pseudosection (N = 1) increases from 14-15 ohm-meters in the vicinity of MW-8 , the well exhibiting the highest fluid conductivity (Table 1), to 32 ohm-meters near MW-10. The 32-ohm-meter contour (apparent, not necessarily true resistivity) rises to the surface at M W - 1 0 after having been drawn at a depth consistent with the alluvium- bedrock interface from M W - 7 to MW-9. As this apparent resistivity is still considerably less than the resistivity found for MW-11, we are probably seeing the front of the pollution plume as contaminated water mixes with normal groundwater in the vicinity of MW-10.

No single electrical method provided as complete an image of sub-surface as the integration of several methods. While mise-a-la-masse anomalies indicated the map location of sub- surface contamination, dipole-dipole and Schlumberger sound-

1 8 Donald J Stierman

ing arrays were needed to discover the depth, lateral extent, and formation resistivity of the leak in the southeast flank of the Stringfellow site.

The use of the Wenner sounding array was for purposes of uniformity. Wenner soundings were used in order to simplify field routines and quick calculation of sounding curves. Fur- ther soundings, except for those made to repeat previous measurements, will take advantage of the relatively simpler to deploy Sehlumberger array. Zohdy (1974) recommends use of the Schlumberger configuration in most cases. Since we incor- porated a digital ammeter into the R-60 system, Schlumberger soundings have been used almost exclusively.

Care must be taken in the interpretation of resistivities. Geologic structures, changes in porosity, or changes in the clay content of an aquifer can influence the apparent resistivity. Electrical measurements must be integrated with other geological and geophysical information in order to limit the family of models one might use to interpret the electrical data. In applying electrical methods to studies at other waste-disposal sites, investigators should recognize that each site will exhibit unique characteristics. Resistivity values reported here for Pyrite Canyon may not be valid in other geological environ- ments. In addition, electrical methods will not prove useful in tracking dumpsite leakage unless the leachate exhibits an electrical resistivity significantly different from the resistivity of the underlying uncontaminated groundwater. Small amounts of certain organic pollutants, although dangerous to health, may go undetected by electrical methods. Cultural activity (pipelines, buried cables) can also render electrical methods difficult to employ. So, despite the success with which electrical measurements tracked sub-surface pollution in Pyrite Canyon, each new case will undoubtedly present site-specific chal- lenges.

Future studies in Pyrite Canyon will attempt to detect temporal changes in electrical resistivity as the pumping of the pollution plume continues to remove groundwater. Compari- son of electrical parameters with hydrologic parameters measured by hydrologists will allow correlation of empirical relationships between geophysical parameters and fluid permeability in situ. Additional resistivity work is needed to investigate other details of the dumpsite leak (now blocked by an earthfill dam, with some unknown amount of material still downstream of this dam) and the Pyrite Canyon pollution plume.

Electrical resistivity studies cannot completely replace chemical analysis of fluids from monitoring wells near chemical waste-disposal sites. This study has shown, however, how geophysics can aid in prospecting for anomalous fluids and contribute to monitoring electrical conductivity, from which water quality can be inferred.

Conclusions

A Schlumberger sounding, a dipole-dipole profile, and mise-a-la-masse survey detected the location of an unsuspected leak of liquid waste from lagoons at the Stringfellow Class I disposal site near Riverside, California. Electrical studies downstream in Pyrite Canyon outline two dimensions of the pollution plume. A combination of electrical soundings and dipole-dipole profiles proves more useful than either method used alone in mapping groundwater contamination in Pyrite Canyon. This case study, therefore, demonstrates the successful use of nonpenetrative geophysical methods to detect and map groundwater contamination. In those cases in which the contamination changes the electrical resistivity of the groundwater, electrical studies should be used as a prospecting tool prior to selection of sites for monitoring wells. Electrical measurements can be integrated with other geologic and hydrologic data as part of the exploration program needed to define and characterize sub-surface contamination prior to specifying clean-up strategy. Electrical measurements might also prove useful in monitoring temporal changes near toxic chemical waste dumps or testing the integrity of operating or abandoned dumps without exploratory drilling.

Acknowledgments

I am grateful to Ron Barto, of Montgomery Engineers, and James Anderson, Executive Director of the Santa Ana Regional Water Quality Control Board, for their cooperation and assistance. Graduate students Kenneth Dinger and Peter Aronstam assisted with field work. Jerry Ervin operated the atomic absorption spectrophotometer. This project was sup- ported by SCAR (Statewide Critical Applied Research) grant for Project No. 3659, Agricultural Experiment Station of the University of California.

References Cited

Barto, R. and others, 1981, Report on Phase 2 Work for Closure of Stringfellow Class I Hazardous Wastes Disposal Site: Hydrogeo- logic Evaluation. Report to the California State Water Resources Control Board: Pasadena, California, James M. Montgomery, Consulting Engineers, Inc.

Bookman, M., 1955, Proposed Industrial Dump Site, Jurupa Moun- tains, Riverside County: California Dept. Public Works, Division of Water Resources, Interdepartmental Communication.

Campbell, D. L., and R. D. Watts, 1978, Exploration geophysics calculator programs for use on Hewlett-Packard models 67 and 97 programmable calculators: U.S. Geological Survey Open-File Report 78-815.


,?o ~?o 300 ~?o 500 F~. 50 I(~0 I~O M TOPOGRAPHIC CONTOURS : aO FEET

Figure 1. Spontaneous potentials measured during the first mise- a-la-masse investigation at the Stringfellow site. Two points incor- rectly plotted in Figure 6 of the original manuscript are correctly plotted here. This error did not significantly distort the previously published pattern.

Edwards, L. S., 1977, A modified pseudosection for resistivity and I.P.: Geophysics, v 42, p. 1020-1036.

Keller, G. V, 1982, Electrical properties of rocks and minerals, zn R. S. Carmichael, ed., Handbook of Physical Properties of Rocks: Boca Raton, FL, CRC Press, p. 217-293.

Morton, D. M., 1978, Geologic map of the Fontana Quadrangle, San Bernardino and Riverside Counties, California: U S. Geological Survey Open-File Report 78-19.

Parasnis, D. S., 1975, Mining Geophysics: New York, Elsevier.

Telford, W M., L. P. Geldart, R. E. Sheriff, and D A. Keys, 1979, Applied Geophysics: New York, Cambridge University Press.

Zohdy, A. A. R., G. P. Eaton, and D R. Mabey, 1974, Application of surface geophysics to ground water investigations, in Techniques of Water-Resources Investigations of the United States Geological Survey, chapter D1

Addendum

In the previous report, I state that spontaneous potential (SP) measurements did not contribute .to our knowledge of the

,?o 2,oo ~oo 4?~ ~oo~,. , i , ~ TOPOGRAPHIC CONTOURS : 20 FEET

0 50 Ioo 150 M

Figure 2. Spontaneous potentials measured during the second mise- a-la-masse survey. The point at the center of the anomaly has a value of - 8 0 mv with respect to the remote electrode

pollution pattern of subsurface groundwater contamination at the Stringfellow site. A review of SP values tabulated during the mise-a-la-masse ( M M ) survey revealed anomalies quite similar to the patterns generated by this active method. A review of field techniques and textbooks on mining geophysics has provided some insight into the reasons why, in contrast to the results reported here, my initial SP surveys at the Stringfel- low site were failures Changes of instrumentation and field procedure resulted in a serendipitous set of valid SP data.

Contour maps showing SP patterns collected during the M M survey are displayed in Figures 1 and 2. Points at which measurements were made are identical to those used to gener- ate Figures 6 and 7 of the previous report. These SP maps exhibit excellent correlation with the M M anomaly associated with the subsurface leak through the southeast flank of the site. The maximum SP anomaly in Figure 2 occurs where the M M survey showed maximum distortion.

During the M M survey, the voltmeters such as those used during early attempts at SP exploration were replaced by the sensitive potentiometer unit of the Soiltest R-60 system This

0 Donald J Stierman

instrument is unstable when the input circuit is open or when contact resistance between the electrodes and the ground is insufficient. It was only after using the R-60 for active (that is, controlled source) electrical exploration that the importance of contact resistance became clear. Care was taken during the M M survey to plant the electrode in soil moist enough to provide the needed electrical contact, a procedure that resulted in stable SP measurements.

A second important modification involves station separation and distribution. If survey points are too widely separated, isolated anomalies due to point contamination can not be identified. The single point anomaly near the southwest corner of the area surveyed in Figure 2 may be one such point. Single point anomalies are insignificant in comparison to anomalies defined by three or more points because a signal originating at depth will spread its influence over a surface area proportional to its depth. My initial SP reconnaissance efforts involved many stations too widely separated to discriminate signals originating at the water table from anomalous surface points.

I do not know if the SP anomaly is a result of electrochemical variations or of streaming potentials, or, if both mechanisms contribute. My initial conclusion that the SP survey was not providing useful information was based on inadequate under- standing of the technique in this challenging environment. Spontaneous potential should prove useful in exploration for contaminated groundwater near landfills or lagoons containing strong inorganic chemicals.

Download - Electrical methods of detecting contaminated groundwater at the stringfellow waste disposal site, riverside county, California

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