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

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

Post on 10-Jul-2016




0 download

Embed Size (px)


<ul><li><p>Electrical Methods of Detecting Contaminated Groundwater at the Stringfellow Waste Disposal Site, Riverside County, California DONALD J. STIERMAN </p><p>Department of Earth Sciences </p><p>University of California 900 University Avenue </p><p>Riverside, California 92521 </p><p>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- </p><p>dipole resistivity profiles, and mise-a-la-masse measurements were </p><p>employed to investigate the sub-surface migration of the acidic fluids deposited in this site between 1956 and 1972 Mise-a-la-masse ex- </p><p>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 </p><p>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- </p><p>physical methods may be used to search for and map leaks from toxic chemical waste dumps </p><p>In t roduct ion </p><p>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contamination in a monitoring well 1,100 m downstream from the site (Fig. 2) in 1972, the operating permit for the site was revoked.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. </p><p>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 communica- tion 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 </p><p>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 imperme- able 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. </p><p>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. </p><p>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. </p><p>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- </p><p>Environ Geol Water Sci Vol 6, No 1, 11-20 9 1984 Springer-Verlag New York Inc </p></li><li><p>2 Donald J Stierman </p><p>k'4 s / CONTOUR 'NTERV2AL : 200 FKEMET ~ / </p><p>Figure 1. Index map showing the location of the Stringfetlow site in the Jurupa Mountains of southern California. </p><p>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). </p><p>Instrumentation </p><p>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. </p><p>Electrical Measurements </p><p>Spontaneous Potential (SP) </p><p>When fluids of different electrochemical characteristics occur together in the earth, a voltage difference develops that </p><p>Figure 2. Map showing the locations of toxic waste lagoons, elec- trical survey lines, and concrete barrier in Pyrite Canyon. </p><p>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. </p><p>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 </p></li><li><p>Detecting Contaminated Groundwater 13 </p><p>TO , REFERENCE </p><p>(ROVING PROBE) </p><p>SPONTANEOUS POTENTIAL (SP) </p><p>| </p><p>A M N [] </p><p>WENNER ARRAY </p><p>A M N [] </p><p>SCHLUMBERGER ARRAY </p><p>Na ~ DIPOLE-DIPOLE </p><p>(N = I, 2, 3 ,4 , - - ) </p><p>TO REFERENCE </p><p>O0 CO r ,.s {~) , MISSE-A-LA-MASSE </p><p>FIXED ROVING (IN CONOUCTIVE BODY ) </p><p>Figure 3. Schematics of electrode configurations for arrays employed in this study. I denotes current source, V denotes voltage measurement. </p><p>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. </p><p>Direct Current Resistivity </p><p>The electrical resistivity of a fluid-saturated rock is gener- ally 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><p>p (rock) = p (solution) aP -m </p><p>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 elec- trical resistivity of water generally decreases as the concentra- tion of dissolved ions in the watei" increases, increases in the </p><p>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. </p><p>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 resistiv- ity 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. </p><p>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 inter- preted 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. </p><p>Electrical soundings were conducted using both the Wenner and Schlumberger array geometries (Fig. 3). In electrical sounding, increasing the distance between the current elec- trodes (A and B) increases the depth of geophysical interroga- tion. The Schlumberger array requires that potential electrode separation MN 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). </p><p>Electrical soundings were interpreted by comparing the data with hypothetical curves generated using computer pro- grams published by Campbell and Watts (1978). Implicit in calculating both apparent resistivity and hypothetical curves is the presumption that the earth consists of homogeneous, </p></li><li><p>14 Donald J Stierman </p><p>A A' </p><p>SW </p><p>i ioo </p><p>z 9 </p><p>, ooo J </p><p>~jv _J L~J </p><p>900 </p><p>Figure 4. </p><p>400 300 200 </p><p>700 600 ~ " ~ 19 </p><p>19 ",-~v 15 ~ / /20 L,~ x~ - - 25/ / </p><p>DIPOLE SPACING A=IO0' I 50 meters I </p><p>Pseudosection showing apparent resistivities for dipole-dipole line AA', adjusted to reflect effective depth </p><p>NE </p><p>0 100 ._----J </p><p>60 </p><p>'Edwards 1977). </p><p>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 </p><p>Mise-a-la-masse, which can be translated as "excitation of 100! the mass, is used by mining geophysicists to map the sub- </p><p>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 ~&lt; ! 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. </p><p>:/ </p><p>3 I ; . . . . [0 2:0 3=O 1 5i 0 . . . . i ; 0 2;0 300 i500 I i i'0; 0 AB/2 meters </p><p>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. </p><p>Leak Detect ion </p><p>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 (appar- ent 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). </p><p>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 </p><p>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 th...</p></li></ul>