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Case history of one of the few successful Superfund remediation sites: A site at Salinas, California, USA
Engineering Geology, 34 (1993) 189-203
189
Elsevier Science Publishers B.V., Amsterdam
Case history of one of the few successful Superfund remediation sites: A site at Salinas, California, USA H a r r y W . S m e d e s a, N i c o l a s S p y c h e r b a n d R. L e o n a r d A l l e n b
aHarry W. Smedes Associates, Carson City, NF 89703, USA biT Corporation, Irvine, CA 92714, USA (Revised manuscript accepted December l, 1992)
ABSTRACT Smedes, H.W., Spycher, N. and Allen, R.L., 1993. Case history of one of the few successful Superfund remediation sites: A site at Salinas, California, USA. In: M. Arnould, T. Furuichi and H. Koide (Editors), Management of Hazardous and Radioactive Waste Disposal Sites. Eng. GeoL, 34: 189-203. In 1985, a former tire manufacturing plant surrounded by agricultural fields in the Salinas Valleywas designated a Superfund Site by the US Environmental Protection Agency. The plant had been operating for seventeen years, from 1963 until 1980. When dismantling of the plant was started, it was determined that toxic hydrocarbon solvents and oils from the plant had contaminated soil and groundwater in alluvial deposits alongside the plant. It was determined later that the groundwater contamination also lay beneath the agricultural fields in a narrow groundwater plume that extends about 4.3 km downgradient from the plant. Because of the complex architecture of the aquifer system, the gradient, and extensive pumping of agricultural wells, the contaminants migrated northwestward and downward to deeper levels away from the plant. The agricultural fields are underlain by an unconfined shallow aquifer and by a system of confined aquifers that extend to more than 180 m below surface. Aquifers are discontinuous beds of channel sand and gravel; confining beds are overbank clay and silt, and estuarine clay. Geophysical data, logs of existing agricultural and other wells, and careful consideration of the stratigraphic architecture of the depositional environment provided the basis for a conceptual hydrogeologic model and for locating characterization wells for detailed visual and geophysical logging and hydrologic testing. Successive refinements of the characterization by sequential installation of wells indicated optimal locations for installation of extraction and monitoring wells. Validity of the concept of the hydrogeologic regime was verified by close match of predictions made by modeling with the later results of pumping from the extraction wells in a pump-and-treat system. Successful remediation was accomplished by analyzing data from 110 agricultural wells, the few domestic water wells, nearly 200 sequentially installed stratigraphic-characterization and monitoring wells, 25 extraction wells, and by close cooperation among federal, state and local agencies, and the ranchers and growers. Total contaminants recovered from activated-charcoal strippers of the treatment system totalled < 230 kg. Large quantities were harmlessly volatilized and dispersed into the atmosphere by air strippers and by agricultural sprinkling systems spraying water onto the fields. Crop testing showed no contamination of food crops. The activity has taken seven years and has cost more than US$22 million.
Location and history of the site Location and land use A S u p e r f u n d Site is designated by the U n i t e d States E n v i r o n m e n t a l P r o t e c t i o n A d m i n i s t r a t i o n as one that potentially c o n t a i n s s u b s t a n t i a l a m o u n t s o f h a z a r d o u s c o n t a m i n a t i o n a n d is potentially o f i m m i n e n t threat to public health a n d 0013-7952/93/$06.00
safety. O n e of the very few successfully remediated S u p e r f u n d Sites is at a n d d o w n g r a d i e n t from a former tire m a n u f a c t u r i n g facility located in the Salinas River Valley (Fig. 1). It lies a b o u t 10 k m SE o f a b u i l t - u p industrial area of the city of Salinas, C a l i f o r n i a a n d comprises a b o u t 1 k m 2. The principal feature o f the facility is the former tire m a n u f a c t u r i n g plant, which occupies a b o u t 0.18 k m 2.
© 1993 - - Elsevier Science Publishers B.V. All rights reserved.
190
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a variety of chlorinated and aromatic solvents. The aromatic solvents used at the plant were principally a mixture of toluene and xylenes, with some benzene and ethylbenzene as well. The principal chlorinated solvent used was 1,1,l-trichloroethane (TCA). Some trichloroethene (TCE) and perchloroethane (PCE, also called tetrachloroethene) apparently also were used. TCE and PCE were found in amounts typically one-tenth to oneone hundredth of the levels of TCA. Other chlorinated ethanes and ethenes were found; they are believed to be contaminants or degradation products of the solvents used.
Investigations
Fig. 1. Index map. (State Highway is labelled by its number,
ioi.) Most of the land in the immediate vicinity of the plant is prime agricultural land used for growing field crops. In fact, much of the premium lettuce produced in the western U.S. is grown in the Salinas area.
Operation and shutdown From 1963 to 1980, Firestone Tire and Rubber Company operated the plant. The company is now called the Bridgestone/Firestone Company; in this paper it will be referred to simply as Firestone. After ceasing operations in 1980, the company began closure of the areas that were used to handle hazardous materials. These included railcar unloading and storage areas, fuel storage and waste-tank storage areas, sludge beds, wastewater holding ponds, evaporation ponds, and oxidation and seepage ponds near the plant. Hazardous materials were removed from the plant, all equipment and all areas where material had spilled were decontaminated or removed. Removal of contaminated soil and capping of those areas with uncontaminated soil cleaned up most of the near-plant contamination; however, sampling of the groundwater and soil in the unsaturated zone near the plant showed the presence of
Firestone contracted to have site investigations conducted, in coordination with the California Department of Health Services (DHS, reorganized in 1992 to the Department of Toxic Substances Control) and other agencies. The purpose of those investigations was to determine the nature and extent of soil and groundwater contamination by substances released from the plant, the potential pathways and rates of migration of those substances, and to take appropriate corrective action. Investigations were conducted during the first half of the project by personnel of Woodward-Clyde Consultants and during the last half by International Technology Corporation (IT).
Contaminants From the results of analytical data of those investigations, the DHS concluded that the principal contaminants of concern at the site were dichloroethene (1,1-DCE), dichloroethane (1,2-DCA and some 1,1-DCA), tetrachloroethene (perchloroethene, PCE), trichloroethane (1,1,1-TCA), trichloroethene (TCE), toluene, xylene, ethylbenzene, and benzene.
Cleanup Cleanup at the site removed the primary source of contaminants. More than 34,000 m 3 of hazardous liquids were removed and about 50,000 m 3 of contaminated soil was excavated. A secondary
A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
source was the contamination in the shallow and the intermediate aquifers; it became the focus of the investigations. In October, 1985, Firestone entered into an agreement (Remedial Action Order) with DHS to investigate potential environmental impact caused by migration of contaminants into aquifers downgradient from the plant. During that investigation, contaminants above state Action Levels for drinking water were found in groundwater in the shallow aquifer. Further investigation of the extent of groundwater contamination revealed a very complex geologic and hydrogeologic setting of the site, described below.
Treatment System From the results of those early investigations and in accordance with its agreement with the DHS, Firestone installed an on-site groundwaterextraction and water-treatment system for the shallow aquifer and, subsequently, for the intermediate aquifer. Interim remedial action began in 1986. A vapor-extraction system of six wells was installed to depths of 10 m in the unsaturated zone. Fifteen on-site and five off-site extraction wells were installed in the shallow aquifer and five extraction wells were installed in the intermediate aquifer. The treatment plant uses an air stripper and a parallel activated-charcoal absorption unit. The treated effluent from the treatment plant is discharged to the Salinas River bed, which normally is dry during the summer. The effluent recharges the aquifers below the river bed; this constitutes a significant recharge inasmuch as the groundwaterextraction system can handle as much as 650 gpm (0.04 m3/s) and commonly handles at least 400 gpm (0.025 m3/s).
Status In June 1992, all extraction wells had reached cleanup level. It is expected that the responsible state agencies will permit the treatment plant to be shut down before the end of 1992.
191
Regional hydrogeologicsetting The Salinas Valley is located in the California Coastal Range Province. It is a 150-km long northwest-trending asymmetrical graben filled with semiconsolidated and unconsolidated sedimentary deposits of Pliocene and Quaternary age. The deepest part of the graben is along the southwestern side, where basement rocks lie as deep as 2400 m below the surface (Ross and Brabb, 1973). Exposed rocks adjacent to the graben are steeply dipping Upper Tertiary sedimentary rocks to the southwest and granitic rocks to the northeast. The upper 300-400 m of graben deposits constitute the principal aquifers of the Salinas Valley groundwater basin. The deposits include the Paso Robles Formation of Pliocene and Pleistocene age and alluvium of Pleistocene and Holocene age. The Paso Robles Formation consists of alluvial and lacustrine deposits (Greene, 1970); they lie well below the depth of concern at the site, and are not discussed further. The younger deposits are those encountered in the investigations of the site; they consist principally of coarse alluvial fan deposits, sand and gravel channel deposits, silt and clay overbank (floodplain) deposits, estuarine clays, and some windblown sand. Alluvial fans lie along the graben edges and form coalesced aprons which interfinger with the floodplain deposits. The deposits were laid down by the ancestral Salinas River and its tributaries. The valley was repeatedly eroded and then aggraded with alluvium in response to cyclic glacio-eustatic and tectonic changes of sea level (Tinsley, 1975). At times, the rate of rise of sea level was greater than the rate of sedimentation; as a result, the lower part of the Salinas Valley was submerged and received estuarine deposits of massive clay - in places as much as 45 m thick. These alluvial deposits are dominated by floodplain silt and clay of low permeability in which are encased the subordinate sand and gravel channel deposits of high permeability. These channel deposits are mostly of meandering streams and are serpentine in plan and lensoid in cross-section. They represent only a small proportion of the total width of the valley at any given time. Because the meanders migrate laterally with time, the channel
192 deposits tend to have only moderate hydraulic connectivity in a vertical cross-section. At several deep horizons, the highly permeable sand and gravel deposits appear to form a continuous blanket across the paleo-valley floor; those may represent a braided stream valley, the entire width of which was traversed by interlaced distributary channels. The Salinas groundwater basin is comprised of an upper valley area, a pressure area, an east-side area, and a forebay area. The site lies in the pressure area, so named because it comprises a system of confined and semiconfined aquifers that are separated by quasi-continuous layers of clay that act as regional aquitards. About 50% of the basin recharge is by infiltration from the Salinas River (Durham, 1974). Groundwater movement is generally down the valley (northwestward), toward Monterey Bay. The average hydraulic gradient is about 0.0012 and closely follows the gradient of the Salinas River. In many areas of the Salinas Valley groundwater basin, pumping of agricultural wells imparts a cross-valley component to the potentiometric surface. Groundwater moves from the river toward areas of pumping. That pumping has been increasing since 1900, when groundwater usage began in the valley. Groundwater levels have been declining in response to this increased pumping but have partially stabilized in response to increased recharge resulting from the regulation of surface-water inflow to the valley by construction of dams and reservoirs. High-volume agricultural pumping from deep aquifers imposes a downward component to the NNW gradient.
Local hydrogeologic setting Hydrogeologic conditions in the vicinity of the site were studied through extensive subsurface investigations which resulted in the delineation of the subsurface hydrostratigraphic framework and the definition of principal contaminant flowpaths. The environment of deposition was confirmed to be mainly of a meandering stream system, locally changing to a braided stream system in which a blanket sand and gravel was deposited, or to an estuary in which thick clay was deposited.
H.w SMV:L)ESHAL In places, deposits of windblown sand occur. Coalesced alluvial fans occur along the valley margins and extend into the study area at some stratigraphic levels. The thickness of alluvial deposits at the site is more than 300 m. Three aquifers, with varying degrees of interconnection, were recognized below the unsaturated zone in the region of the site as a result of the detailed investigations described below. They are designated the shallow aquifer, intermediate aquifer, and deep aquifer system. The deep aquifer system can be subdivided into the 60-,90-,120-, and 150-m aquifers. The shallow aquifer is unconfined; the others are confined or partly confined. Each is a complex hydrostratigraphic system dominantly of low-permeability overbank deposits within which there are numerous high-permeability sand-gravel channel deposits that may overlap vertically, are subordinate to the low-permeability highly plastic clay and silt overbank deposits, and have only moderate connectivity. Each of these complex aquifers is separated from the one above and below by more-widespread units of lowpermeability materials that lack connected channel deposits and thereby constitute aquitards. That is, within a given aquifer, there are large volumes of material that do not contribute significantly to contaminant transport and that, by themselves, would not be considered an aquifer. Unsaturated zone
The unsaturated zone ranges from about 7 to 12 m thick. A soil-gas survey of the unsaturated zone established a plume which was used to focus the investigation of areas offsite where chlorinated hydrocarbons were likely to be found in the underlying shallow aquifer. Shallow and intermediate aquifers
The shallow aquifer extends from near the surface in places to a depth of about 27 m. It merges downgradient into a thick clay sequence where it becomes discontinuous and separated into isolated zones interbedded in thick clay. In the area of the former tire plant, the shallow aquifer is underlain by a generally continuous estuarine blue clay hori-
193
A SUPERFUND REMEDIATIONSITE AT SALINAS,CALIFORNIA, USA
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Fig. 2. Potentiometric m a p of the shallow aquifer in the vicinity of the plant. (Contours are in feet above mean sea level.)
zon that constitutes an effective aquitard between the shallow and underlying aquifers. Downgradient, about where the aquifer disappears by merging into a clay sequence, this aquitard becomes thin and discontinuous, thus providing a hydraulic connection between the shallow and the underlying intermediate aquifers. A potentiometric map of the shallow aquifer near the plant is shown in Fig. 2. These data were obtained during a period when no extraction wells were pumping. The figure shows a steep gradient along the SW side which indicates resistance to groundwater flow caused by materials of low conductivity. The intermediate aquifer consists of strata within the depth interval of about 33 to 45 m. Farther downgradient, the clay aquitard at the base of the intermediate aquifer thins and becomes discontinuous; this provides a hydraulic connection between the intermediate and the deep aquifers.
Deep aquifer system Four zones were identified in the deep-aquifer system; they are referred to as the 60-, 90-, 120-, and 150-m aquifers (referred to as 200-, 300-, 400-, and 500-ft aquifers in the reports of the investigations). These hydrostratigraphic zones are separated by generally discontinuous fluvial and estuarine clays. The stratigraphic and potentiometric data indicate that these aquifers are hydrau-
lically connected to varying degrees at various places. The 60-m aquifer generally is encountered at about 57 m and is about 21 to 27 m thick. It is a regional aquifer. In the area of the former tire plant, an estuarine blue clay aquitard forms the top of the 60-m aquifer and separates it from the intermediate aquifer. Downgradient, this aquitard is missing in several places, providing a hydraulic connection between the intermediate and 60-m aquifer. The 90-m aquifer is encountered at a depth of about 90 m and is from about 8 to 15 m thick.It is capped by a 6-12-m-thick estuarine blue clay which appears to be continuous beneath the plant and for some distance downgradient, and then it grades into or is taken over by a tan clay of probable fluvial origin which is discontinuous and allows for hydraulic connection with the overlying 60-m aquifer. Another bed of blue clay, 1.5-8 m thick, underlies the 90-m aquifer. This clay aquitard thins and becomes discontinuous downgradient from the plant as it too is taken over by a brown clay of probable alluvial origin. The 120-m aquifer is generally encountered at about 105 m below the surface and is from about 18 to 30 m thick. The underlying aquitard is similar to that above it and is also discontinuous. The 150-m aquifer is encountered at a depth of about 135 m below surface and is thicker than the overlying aquifers. Little information is known of
194
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Fig. 3. Hydrographsof a cluster of wells, each screened to sample a differentaquifer. Aquifers listed are referred to herein as: shallow, intermediateand deep (60 m, 90 m, 120 m and 150 m). this deep aquifer; neither it nor the 120-m aquifer are involved in any contamination. Groundwater flow in the deep aquifer system is generally north-northwestward. Potentiometric levels measured in the deep aquifer monitoring wells decrease with depth, as shown in the hydrographs in Fig. 3. This is especially noticeable during periods of high-volume agricultural pumping seasons (summer and fall). The decreases cause a downward component to the gradient and promote downward flow and chemical transport into deeper aquifers, especially near the pumping wells.
Aquifer characteristics Hydrologic data sources consist of existing onsite and agricultural pumping data, aquifer pumping tests, and geotechnical data for the subsurface materials. Transmissivity values were calculated for the shallow and the intermediate aquifers from field data and from performance tests of several agricultural wells in the area. These data indicate that the transmissivity of the intermediate aquifer generally ranges from about 10,000 ft2/day (30 m2/day) to about 50,000 ft:/day (65 m2/day). The average hydraulic gradient is about 0.0012. Hydraulic conductivity was calculated to be about 100 ft/day (30 m/day) for the shallow aquifer and to range from about 200 to 1200 ft/day (60-360
m/day) for the intermediate and deep aquifers. Analyses of time for a pulse of contamination first detected at one well to travel to another give a useful conservative measure of the net transport time of about 0.4 to 0.6 m/day (if the transport were in a straight line from well to well, an unlikely but conservative scenario). Hydraulic conductivity of the various clay aquitards ranged from about 3 x 10 - 4 t o 8 X 10 - 6 ft/day, and is controlled by the finest 10% of the particle sizes. The low hydraulic conductivity and the high plasticity of the aquitard materials suggest that vertical contaminant migration occurs principally through discontinuities in the aquitard rather than by leakage through the clay itself.
Investigative approach From the local geologic setting, the probable depositional environment was considered to be that of a meandering, aggrading stream system, at times a braided stream system, much as is shown in the schematic drawings of Fig. 4, and sporadically an estuarine environment. The regional hydrogeologic base indicated that the gradient most likely was northwestward. These assumptions formed the basis for development of a plan for a sequential approach to the characterization of the site, including the determination of the nature and
195
A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
Fig. 4. Schematic block diagrams of meandering stream system (A) and braided stream system (B). (Dot pattern is for sand and gravel channel deposits, gray is silt and sand, black is clay.)
extent of contamination and of contaminant transport. Although sequential overall, many activities were conducted synchronously or in overlapping fashion.
Removal of sources The first concern was to remove the source of contamination. Since the primary source of contaminants had been removed during the cleanup, and geophysical surveys showed no likely buried additional sources (drums, tanks, etc.), the following actions were taken: (1) A soil-gas survey was made to determine locations of chlorinated hydrocarbons in the unsaturated zone that could serve as secondary sources of contamination of groundwater. (2) Six soil-vapor-extraction wells were installed to remove contaminants from the unsaturated zone. (3) Fifty-seven monitoring wells were installed to more-closely define the contaminant plume in the shallow aquifer; of those, 38 were onsite and
19 offsite as far as 480 m away from the plant. Evaluation of analyzed concentrations of chemicals of concern established the optimum locations for installation of shallow extraction wells. Fifteen extraction wells were installed in a row across the hydraulic gradient, near the edge of the plant, in order to aggressively start to remove contaminants and prevent them from serving as secondary sources and spreading farther. Their spacing was designed to ensure that the capture zone of each intercepted that of its neighbors so that no contamination would pass through the row of guard wells.
Inventory of wells At the same time, available data were collected for all wells located within 4 miles (about 6.4 km) in a large sector downgradient from the plant. Logs were available for 350 of the 370 wells inventoried. Although the quality of the logs ranged widely, analysis of those data provided a first approximation of the subsurface hydrostratigraphy, and served as the basis for planning the
196
locations for installation of four deep stratigraphic wells to establish detailed stratigraphic relations at key points. Of those existing wells, 208 were sampled to determine the lateral and vertical extent of contamination. Of 110 agricultural wells, all of which were sampled, only seven showed contamination. Even though the logs of most agriculture and industrial wells were of variable quality, they were available for the area immediately downgradient (agriculture wells) and, as it turned out, around the nose of the plume (industrial wells). The plume established in a reconnaissance way by this testing provided the basis for installation of additional monitoring wells whose logs also provided data for refining the understanding of the hydrostratigraphy. Downhole geophysical surveys conducted in these monitoring wells further refined the details of the three-dimensional architecture of the hydrostratigraphy. Eventually, a total of 189 monitoring wells, four stratigraphic-characterization wells, and 25 extraction wells were installed in the three groundwater zones. An ongoing groundwater monitoring program has provided a detailed understanding of the subsurface hydrostratigraphic architecture and a substantial geochemical data base from the monitoring and extraction wells and from agricultural, industrial, municipal, and domestic wells in the area.
Soil-gas survey A soil-gas survey was made in the unsaturated zone in a 3 km 2 area adjacent to and extending about 1.6 km down gradient from the plant. The unsaturated zone in the area ranges from about 7 to 12 m thick. The purpose of that survey was to obtain information on the distribution of chlorinated hydrocarbon vapors in the unsaturated zone in order to plan exploration of the underlying shallow aquifer. Soil-gas samples were collected from six wells and analyzed from depths of about 3, 6, and 9 m at 44 locations. Although the samples were analyzed for 1,!-DCA, 1,I-DCE, 1,1,I-TCA, and PCE, only 1,1,1-TCA and 1,1-DCE were detected frequently enough to establish a soil-gas plume.
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TCE was detected only rarely and I,I-DCA and PCE were not detected at all. The resulting soil-gas plume was used to focus the investigation of the shallow aquifer off site. The results were a first approximation of the plume and flowpath subsequently established from sampling of groundwater in the shallow aquifer.
Geophysical surveys While the activities stated above were underway, geophysical surveys were made. A terrain conductivity survey was made of the approximate area of the plume to search "depths" of about 15, 30, and 60 m in what later was determined to be the shallow and intermediate aquifers. This survey aided in the recognition of the major clay-rich aquitards, which were electrically moreconductive. A generalized map of the results of that survey is shown in Fig. 5. The overall configuration shown was corroborated by subsequent extensive exploration, including such things as maps of net sand and gravel, analytical testing of groundwater samples, and potentiometric maps. Resistivity soundings were made to better-define particular features shown in the conductivity survey. Ten resistivity soundings were taken at selected locations to help define the continuity, depth, and thickness of hydrostratigraphic units. A vertical-gradient magnetometer survey was made in selected places just off site in order to detect possible metal well casings at abandoned and buried wells. Candidate survey locations were selected from conversations with residents and from anomalies detected by the terrain conductivity survey. Identification of such wells is important because they may provide conduits for contaminants to flow from the unconfined (shallow) aquifer to the lower aquifers. Two wells were found by this survey and were tested to be certain that they did not provide such a conduit. Downhole geophysical surveys were made. They consisted of natural-gamma-ray logs and electric geophysical logs made at 26 selected deep borings across the study area to provide detailed stratigraphic data relevant to geohydrologic characteristics and to select screen intervals for monitoring.
197
A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
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Fig. 5. Generalized geohydrologic interpretation of composite terrain conductivity data to a search "depth" of 60 m,
Hydrogeochemicalevaluation For some wells in the intermediate and deep aquifers, the chemical concentrations show patterns that reflect seasonal variation due to agricultural pumping. Agricultural-well pumping from the deep aquifers induces flow from overlying aquifers, which results in downward transport of chemicals present at locations where aquitards are absent. Hydrographs of water pressures in wells show that during the summer, when pumping is greatest, a downward hydraulic gradient exists
,0t
between shallow and deep aquifers. In winter, when pumping is least, the vertical gradient declines to virtually zero, as shown in Fig. 3. A representative example of the time variability of concentrations of contaminants for a well in the shallow aquifer is shown in Fig. 6. The graph shows the geometric mean of the amount detected, in pg/l, Although some seasonal fluctuations in concentrations occur for various wells and chemicals, the general trend is a decrease of all concentrations in all wells due to the groundwater pumping and treatment system that has been oper-
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Fig. 6. Representative concentrations of I,I-DCE in the shallow aquifer. (Each triangle represents the geometric mean of the amounts detected (in pg/l) from five wells.)
198
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ating for more than six years. By June of 1992, all wells had concentrations reduced to below the Maximum Contaminant Levels (MCL). Because it is the last constituent to be cleaned up (all others fall below M C L before it does), 1,1D C E serves as an indicator of contamination and of progress of cleanup. Data for I, 1-DCE concentrations at the M C L for different time periods are shown in the m a p of Fig. 7. The reduction in area (which is proportional to volume) of the M C L concentration is shown for a five-year period
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during which time the area within the M C L contour (6 ~tg/l for I , I - D C E ) decreased 98%. The area has subsequently been reduced to zero, that is, cleanup has been achieved. The pattern shown in Fig. 7 is compatible with the interpretation of a buried stream channel as controlling the transport. The area of the contaminant plume lies along the trough shown in the potentiometric map of Fig. 2. Concentrations of 1,1-DCE in the intermediate aquifer show a pattern (Fig. 8) similar to that for the shallow aquifer (Fig. 6) wherein concentrations
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207 15
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Fig. 8. Representativeconcentrations of I,I-DCE in the intermediate aquifer. (Each datum point represents the geometric mean of the amounts detected (in/ag/l) from five wells. Time scale is labelled with the initial letter of the month and the last two digits of the year.)
199
A SUPERFUNDREMEDIATIONSITE AT SALINAS,CALIFORNIA,USA
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Fig. 9. Representative concentrations of 1,I-DCE in four agricultural wells in the deep aquifer. (Time scale as in Fig. 8. Each symbol
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are affected by seasonal fluctuations and have been decreasing with time. The intermediate aquifer has also been remediated to concentrations below MCL. Concentrations of I,I-DCE in the deep aquifer system (deeper than 60 m) are shown in Fig. 9. Seasonal fluctuations are seen; however, the concentrations generally are less than 10 p.g/l (usually about 2 pg/1) and, at those low levels, some of the apparent fluctuations may simply be due to variations in sampling and laboratory precision. It was noted that some other chemicals such as 1, I-DCA, TCE, PCE, benzene, and xylene are detected only during times of heavy agricultural pumping, suggesting that the heavy withdrawal rate for irrigation influences transport of those components. The general composite position of the contaminant plume at its maximum known extent is shown schematically in Fig. 10. The figure represents a map projection of the maximum lateral extent of the irregularly-stacked channel deposits that serve as the conduits for contaminant transport. If viewed in cross-section, the plume would appear like a complex stack of lenses or pods whose degree of connectedness is irregular, and would shift from side to side depending upon the nature of stacked stream channels at the position of the section (see Fig. 4). A schematic cross-section of the composite plume showing the general architecture of the major hydrostratigraphic units is presented in Fig. 11, in which the vertical exaggeration is more
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Fig. 10. Schematiccomposite map of contaminant plumes at their probablemaximumextentin the shallow(S), intermediate (1) and deep (D) aquifers. (State Highwayis labelled by its number, 101.)
than five times. That figure shows schematically how the contaminant transport is accomplished by discontinuities in aquitards and by high-volume pumping of agricultural wells. Risk assessment
Detailed site characterization provided the basis for risk assessment. It is interesting to note that the risk assessment showed that there was no present or future toxicologically significant risk associated with exposure to groundwater downgradient from the plant. That conclusion held even for the unlikely worst-case scenario for future lifetime exposure, with no decrease in chemical concentrations over 70 years. The risk was also
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Fig. 11. Schematic cross-section of the contaminant plume, showing interconnections among the aquifers. (Scale is only approximate. Vertical exaggeration is about 25 x .) Only the major aquitards are shown, in black. Clear areas represent zones of moderately interconnected stream channels of high permeability and abundant silt and clay overbank material of low permeability, not shown separately. (Dotted line portrays the water table; dashed lines portray the contaminant plume; arrows indicate direction of contaminant transport.)
significantly below other risks associated with exposures commonly present in everyday life, such as drinking diet sodas or eating charcoal-broiled steaks or peanut butter. The only scenario that showed risks above the acceptable levels was an unrealistic one: a hypothetical new domestic well placed in the maximum contamination point of the shallow aquifer, in the middle of agricultural cropland, with no provision for any decrease in concentrations over the next 70 years - - not even by the natural dispersion, dilution, and chemical degradation that would decrease the concentrations. That worst-case scenario also required that, in order to be at risk, an individual would have to use that well as sole source of drinking water, and would have to drink that water for 70 years! To determine whether crops were contaminated from irrigation by the sprinkler systems from the agricultural wells, crops were sampled and analyzed at various intervals and seasons. No contamination was ever found. It turns out that the pumping and spraying of crops is a highly effective means of remediation of the deep aquifer even though the concentrations were already well below the cleanup levels. It was both fortunate and commendable that the regulators allowed the
growers to continue to irrigate from the deep aquifer in that manner.
Computer modeling When it was felt that the hydrogeology and hydrogeochemistry were reasonably-well known, computer modeling was done to predict the manner in which the contaminant plume would decay in space and time due to operation of the extraction wells. Five alternatives to remediation of the shallow and intermediate aquifers were analyzed in detail. These consisted of a no-action alternative and four alternatives of different combinations of pumping at various rates from different sets of extraction wells. Computer simulations were made of each of the five remedial alternatives in order to assess the relative effectiveness and the time required to achieve cleanup for each alternative. The no-action alternative consisted of stopping the then-current pumping and treatment of groundwater and letting natural dispersion, dilution, and degradation slowly reduce the concentrations of contaminants. An alternative of pumping just the shallow aquifer consisted of continuing the then-current pumping and treatment of groundwater from the
201
A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
shallow aquifer only, as a means of cutting off the source to the deeper aquifers. Chemicals in the intermediate aquifer would be subject to natural dispersion, dilution, and degradation, as in the noaction alternative. The remaining three alternatives consisted of various flow combinations of new extraction wells installed in the intermediate aquifer operating along with the shallow extraction wells. Modelled flow rates ranged from 600 to 1150 gpm (about 2.3 to 4.4 m3/min). The higher flow rates would require expansion of the treatment plant and modifications to existing operating permits. The recommended and adopted remedial alternative consisted of continuing to pump and treat groundwater from the shallow aquifer and, in addition, to install five new extraction wells in a specified configuration in the intermediate aquifer. For this alternative, the combined pumping rates for the shallow and the intermediate aquifers would be maintained at or below 650 gpm (about 2.5 m3/min.) which was the current capacity of the treatment plant and the rate allowed under the permit. Under that alternative, cleanup could be accomplished expeditiously, no new permits were required, no expansion of the treatment plant was needed, and no delays would be encountered. A computer simulation indicated that the pumping rates could be adjusted such that both the shallow and intermediate aquifers could reach cleanup levels in about 60% of the time required for the no-action alternative. Evaluation of the predictions over time versus the actual progress of cleanup provided a basis for modifications in the schedule of pumping rates from each of the extraction wells, and showed that certain wells could be shut down without affecting the cleanup rate. Because the computer modeling was used as an investigative tool that assisted in developing, testing, and modifying our understanding of the system, it is no surprise that cleanup was accomplished close to the time predicted by the modeling. Iteration
At each stage in the sequence of activities, the newly acquired data were incorporated into the
developing conceptual model and that model was modified in realistic ways as appropriate to be compatible with all the data accumulated to that moment. Gradually, the hydrostratigraphic and hydrogeologic model developed. It was tested, modified as appropriate, and used to focus the next stage of activity. Results of each stage provided input to refine the model, test it and modify it again and again in an iterative cycle. From all this activity, the understanding that developed is that of the architecture and hydrologic behavior of the shallow, intermediate, and deep aquifer systems as described above and in the 'Findings' section, below. The successful results of remediation demonstrate that characterization of the site was accomplished very well. Careful evaluation, guided by data from monitoring wells and their response to the agricultural pumping, and from time-analysis of changes in concentrations of chemicals in wells resulted in the understanding of the complex lateral and vertical transport of contaminants and the controlling hydrostratigraphy. That complex hydrostratigraphy and contaminant transport can be best understood by time analysis of three-dimensional diagrams, which cannot be displayed in page size of this paper. Therefore, only a schematic or conceptual sketch is used to show the overall interpretation (Figs. 10, 11).
Findings and conclusions Detailed site characterization through analysis of logs and construction records for existing wells; installation, logging, and monitoring of many monitoring and remediation wells; geophysical surveys; data analysis; and computer modeling, culminated in the subsurface mapping of hydrostratigraphic units and the definition of principal contaminant flowpaths. The hydrogeologic conditions at the site are summarized herein. The deposits beneath the site consist of more than 300 m of alluvial deposits comprised of highpermeability channel sand and gravel encased in low-permeability overbank silt and clay. Alluvial fans lie along the north edge of the site and estuarine clays form blanket deposits at several stratigraphic levels. The depositional environment
202
was that of a meandering system, taken over at times by a braided stream system and at other times by an estuarine environment of deposition of clay. Contaminant transport is controlled by the channel deposits and each of the data sources confirm the interpretation of limited lateral extent of the plume. The unsaturated zone at the site is about 10 m thick. The groundwater aquifer system is comprised of three interconnected zones that are designated the shallow aquifer, intermediate aquifer, and the deep aquifer system. The shallow aquifer is an unconfined aquifer that extends from about 10 to 27 m below surface. The other aquifers are confined to partly confined aquifers. The intermediate aquifer extends from about 30 to 42 m and the deep aquifer system locally consists of four zones, at depths of approximately 60, 90, 120, and 150 m. Analyses of samples from monitoring, agricultural, domestic, industrial, and municipal water wells indicate that the plume of chemical contaminants extends about 4.3 km northwestward from the former Firestone plant (Fig. 11). Starting at the plant, the plume is configured as follows: -A narrow ellipse about 1000 m long and 300 m wide, in the shallow aquifer, flowing nearly due west. - - A second ellipse about 1200 m long and 300 m wide in the intermediate zone, partly overlapping the map projection of the plume of the shallow aquifer and flowing northwestward. - - A third ellipse about 2100 m long and 450 m wide in the deep aquifer system, partly overlapping the west end of the map projection of the plume of the intermediate aquifer and flowing north-northwestward. Although the flow in the deep aquifer system generally is to the NNW, during the agricultural pumping seasons the agricultural wells control the flow directions, and each well generates a local cone of depression. The seasonal and diurnal pumping of the agricultural wells has generated a favorable condition for controlling the extent of the plume both downand across-gradient. No drinking-water well had then or has since exceeded current state action
H . W S M E D E S E'I At.
levels; therefore no imminent or substantial endangerment had ever existed. Operation of extraction wells in the shallow and intermediate aquifers reduced the concentrations in those aquifers to below the MCLs and served to cut off the source of contaminants to the deep aquifers. This indicates that concentrations of organic contaminants in drinking-water wells will always remain below current state action levels. However, as has happened in many places in the Salinas Valley, contamination by nitrates from fertilizing the crops (totally unrelated to operations of the tire plant) may eventually contaminate the water. Status
Since June of 1992, concentrations of the chemicals of concern had been reduced to below the Maximum Contaminant Levels (MCL) in all wells, and have remained below the MCLs. The responsible state regulatory agencies will permit the treatment system to be shut down on November 3, 1992. Cleanup has been achieved. Postlude
This project is unusual in that it is one of the few Superfund Sites that has been successfully remediated. In retrospect, we believe that there are two principal reasons for the success in remediation of this typically complex site, as follows: (1) The positive attitude of the management of Firestone, who agreed that the approach must be conservative and that if it required installation of a great many wells, that is what would be done. In addition, Firestone officials have consistently made a concerted and sincere effort to work cooperatively and to coordinate activities with Federal, state, and local regulatory agencies and other concerned agencies; and to communicate on a regular basis with the local municipalities, industries, growers, farmers, and citizens in the region through public hearings and personal contact. This attitude and response on the part of the owner (Firestone) is especially noteworthy in view of the fact that the entire program was at the request of the state and local regulatory agencies,
ASUPERFUNDREMEDIATIONSITEATSALINAS.CALIFORNIA,USA not by any regulatory order. Although this important activity is not dealt with further in this brief paper, it is nonetheless equally as important as the second reason for success, namely, the investigative approach taken. (2) The selection of and continual adherence to a carefully designed plan. A carefully designed sequential approach for the technical studies was selected and continually adhered to, in which the results of each stage of activity were evaluated and compared with the developing and evolving conceptual hydrogeologic model. This was done in a sequential and iterative process of using the conceptual model to focus studies and indicate types of data needed, acquiring the indicated data, comparing results with model, revising or refining model, and then repeating the cycle again and again, as appropriate. In compliance with regulations, volumes of reports, extensive maps, and analytical data were prepared and filed as the project moved through the numerous phases o f risk assessment and consideration of alternative remedial strategies, remedial investigations, feasibility studies,.., ad
infinitum. The project took seven years and cost more than US$22 million. The total amount of contaminant material retrieved from the treatment system was about 225 kg. The cost was borne entirely by Firestone. The project serves as a stern reminder of the old adage: "an ounce of prevention is worth a pound of cure".
203
Acknowledgements The first half of the investigations was carried out by the staff of Woodward-Clyde Consultants and the latter half by staff of the IT Corporation. Our thanks to the countless scientists and engineers of those companies, whose data we have used extensively. We also extend our thanks to the many people who were involved with the project as members of regulatory agencies, growers, industries, and the local citizenry. For information and insights into the problem, we wish especially to express our appreciation to George Markert and Leonard Smith of Bridgestone/Firestone Company, who were involved in the program since its beginning; and to dr. Larry Froebe, dr. R.Nichols Hazelwood, Chandler Weisel, and Ed Wing, all of the IT Corporation.
References Durham, D.L., 1974. Geology of the southern Salinas Valley area, California. U.S. Geol. Surv., Prof. Pap. 819, map scale 1:25,000. Greene, H.G., 1970. Geology of southern Monterey Bay and its relationship to the groundwater basin and seawater intrusion. U.S. Geol. Surv., Open-File Rep. Ross, D.C. and Brabb, E.E., 1973. Petrography and structural relations of granitic basement rocks in the Monterey Bay area, California. J. Res. U.S. Geol. Surv., 1(3): 273-282. Tinsley, J.C., 1975. Quaternary Geology of northern Salinas Valley, Monterey County, California. Unpubl. Diss., Stanford Univ., Stanford, Calif.
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Elsevier Science Publishers B.V., Amsterdam
Case history of one of the few successful Superfund remediation sites: A site at Salinas, California, USA H a r r y W . S m e d e s a, N i c o l a s S p y c h e r b a n d R. L e o n a r d A l l e n b
aHarry W. Smedes Associates, Carson City, NF 89703, USA biT Corporation, Irvine, CA 92714, USA (Revised manuscript accepted December l, 1992)
ABSTRACT Smedes, H.W., Spycher, N. and Allen, R.L., 1993. Case history of one of the few successful Superfund remediation sites: A site at Salinas, California, USA. In: M. Arnould, T. Furuichi and H. Koide (Editors), Management of Hazardous and Radioactive Waste Disposal Sites. Eng. GeoL, 34: 189-203. In 1985, a former tire manufacturing plant surrounded by agricultural fields in the Salinas Valleywas designated a Superfund Site by the US Environmental Protection Agency. The plant had been operating for seventeen years, from 1963 until 1980. When dismantling of the plant was started, it was determined that toxic hydrocarbon solvents and oils from the plant had contaminated soil and groundwater in alluvial deposits alongside the plant. It was determined later that the groundwater contamination also lay beneath the agricultural fields in a narrow groundwater plume that extends about 4.3 km downgradient from the plant. Because of the complex architecture of the aquifer system, the gradient, and extensive pumping of agricultural wells, the contaminants migrated northwestward and downward to deeper levels away from the plant. The agricultural fields are underlain by an unconfined shallow aquifer and by a system of confined aquifers that extend to more than 180 m below surface. Aquifers are discontinuous beds of channel sand and gravel; confining beds are overbank clay and silt, and estuarine clay. Geophysical data, logs of existing agricultural and other wells, and careful consideration of the stratigraphic architecture of the depositional environment provided the basis for a conceptual hydrogeologic model and for locating characterization wells for detailed visual and geophysical logging and hydrologic testing. Successive refinements of the characterization by sequential installation of wells indicated optimal locations for installation of extraction and monitoring wells. Validity of the concept of the hydrogeologic regime was verified by close match of predictions made by modeling with the later results of pumping from the extraction wells in a pump-and-treat system. Successful remediation was accomplished by analyzing data from 110 agricultural wells, the few domestic water wells, nearly 200 sequentially installed stratigraphic-characterization and monitoring wells, 25 extraction wells, and by close cooperation among federal, state and local agencies, and the ranchers and growers. Total contaminants recovered from activated-charcoal strippers of the treatment system totalled < 230 kg. Large quantities were harmlessly volatilized and dispersed into the atmosphere by air strippers and by agricultural sprinkling systems spraying water onto the fields. Crop testing showed no contamination of food crops. The activity has taken seven years and has cost more than US$22 million.
Location and history of the site Location and land use A S u p e r f u n d Site is designated by the U n i t e d States E n v i r o n m e n t a l P r o t e c t i o n A d m i n i s t r a t i o n as one that potentially c o n t a i n s s u b s t a n t i a l a m o u n t s o f h a z a r d o u s c o n t a m i n a t i o n a n d is potentially o f i m m i n e n t threat to public health a n d 0013-7952/93/$06.00
safety. O n e of the very few successfully remediated S u p e r f u n d Sites is at a n d d o w n g r a d i e n t from a former tire m a n u f a c t u r i n g facility located in the Salinas River Valley (Fig. 1). It lies a b o u t 10 k m SE o f a b u i l t - u p industrial area of the city of Salinas, C a l i f o r n i a a n d comprises a b o u t 1 k m 2. The principal feature o f the facility is the former tire m a n u f a c t u r i n g plant, which occupies a b o u t 0.18 k m 2.
© 1993 - - Elsevier Science Publishers B.V. All rights reserved.
190
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a variety of chlorinated and aromatic solvents. The aromatic solvents used at the plant were principally a mixture of toluene and xylenes, with some benzene and ethylbenzene as well. The principal chlorinated solvent used was 1,1,l-trichloroethane (TCA). Some trichloroethene (TCE) and perchloroethane (PCE, also called tetrachloroethene) apparently also were used. TCE and PCE were found in amounts typically one-tenth to oneone hundredth of the levels of TCA. Other chlorinated ethanes and ethenes were found; they are believed to be contaminants or degradation products of the solvents used.
Investigations
Fig. 1. Index map. (State Highway is labelled by its number,
ioi.) Most of the land in the immediate vicinity of the plant is prime agricultural land used for growing field crops. In fact, much of the premium lettuce produced in the western U.S. is grown in the Salinas area.
Operation and shutdown From 1963 to 1980, Firestone Tire and Rubber Company operated the plant. The company is now called the Bridgestone/Firestone Company; in this paper it will be referred to simply as Firestone. After ceasing operations in 1980, the company began closure of the areas that were used to handle hazardous materials. These included railcar unloading and storage areas, fuel storage and waste-tank storage areas, sludge beds, wastewater holding ponds, evaporation ponds, and oxidation and seepage ponds near the plant. Hazardous materials were removed from the plant, all equipment and all areas where material had spilled were decontaminated or removed. Removal of contaminated soil and capping of those areas with uncontaminated soil cleaned up most of the near-plant contamination; however, sampling of the groundwater and soil in the unsaturated zone near the plant showed the presence of
Firestone contracted to have site investigations conducted, in coordination with the California Department of Health Services (DHS, reorganized in 1992 to the Department of Toxic Substances Control) and other agencies. The purpose of those investigations was to determine the nature and extent of soil and groundwater contamination by substances released from the plant, the potential pathways and rates of migration of those substances, and to take appropriate corrective action. Investigations were conducted during the first half of the project by personnel of Woodward-Clyde Consultants and during the last half by International Technology Corporation (IT).
Contaminants From the results of analytical data of those investigations, the DHS concluded that the principal contaminants of concern at the site were dichloroethene (1,1-DCE), dichloroethane (1,2-DCA and some 1,1-DCA), tetrachloroethene (perchloroethene, PCE), trichloroethane (1,1,1-TCA), trichloroethene (TCE), toluene, xylene, ethylbenzene, and benzene.
Cleanup Cleanup at the site removed the primary source of contaminants. More than 34,000 m 3 of hazardous liquids were removed and about 50,000 m 3 of contaminated soil was excavated. A secondary
A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
source was the contamination in the shallow and the intermediate aquifers; it became the focus of the investigations. In October, 1985, Firestone entered into an agreement (Remedial Action Order) with DHS to investigate potential environmental impact caused by migration of contaminants into aquifers downgradient from the plant. During that investigation, contaminants above state Action Levels for drinking water were found in groundwater in the shallow aquifer. Further investigation of the extent of groundwater contamination revealed a very complex geologic and hydrogeologic setting of the site, described below.
Treatment System From the results of those early investigations and in accordance with its agreement with the DHS, Firestone installed an on-site groundwaterextraction and water-treatment system for the shallow aquifer and, subsequently, for the intermediate aquifer. Interim remedial action began in 1986. A vapor-extraction system of six wells was installed to depths of 10 m in the unsaturated zone. Fifteen on-site and five off-site extraction wells were installed in the shallow aquifer and five extraction wells were installed in the intermediate aquifer. The treatment plant uses an air stripper and a parallel activated-charcoal absorption unit. The treated effluent from the treatment plant is discharged to the Salinas River bed, which normally is dry during the summer. The effluent recharges the aquifers below the river bed; this constitutes a significant recharge inasmuch as the groundwaterextraction system can handle as much as 650 gpm (0.04 m3/s) and commonly handles at least 400 gpm (0.025 m3/s).
Status In June 1992, all extraction wells had reached cleanup level. It is expected that the responsible state agencies will permit the treatment plant to be shut down before the end of 1992.
191
Regional hydrogeologicsetting The Salinas Valley is located in the California Coastal Range Province. It is a 150-km long northwest-trending asymmetrical graben filled with semiconsolidated and unconsolidated sedimentary deposits of Pliocene and Quaternary age. The deepest part of the graben is along the southwestern side, where basement rocks lie as deep as 2400 m below the surface (Ross and Brabb, 1973). Exposed rocks adjacent to the graben are steeply dipping Upper Tertiary sedimentary rocks to the southwest and granitic rocks to the northeast. The upper 300-400 m of graben deposits constitute the principal aquifers of the Salinas Valley groundwater basin. The deposits include the Paso Robles Formation of Pliocene and Pleistocene age and alluvium of Pleistocene and Holocene age. The Paso Robles Formation consists of alluvial and lacustrine deposits (Greene, 1970); they lie well below the depth of concern at the site, and are not discussed further. The younger deposits are those encountered in the investigations of the site; they consist principally of coarse alluvial fan deposits, sand and gravel channel deposits, silt and clay overbank (floodplain) deposits, estuarine clays, and some windblown sand. Alluvial fans lie along the graben edges and form coalesced aprons which interfinger with the floodplain deposits. The deposits were laid down by the ancestral Salinas River and its tributaries. The valley was repeatedly eroded and then aggraded with alluvium in response to cyclic glacio-eustatic and tectonic changes of sea level (Tinsley, 1975). At times, the rate of rise of sea level was greater than the rate of sedimentation; as a result, the lower part of the Salinas Valley was submerged and received estuarine deposits of massive clay - in places as much as 45 m thick. These alluvial deposits are dominated by floodplain silt and clay of low permeability in which are encased the subordinate sand and gravel channel deposits of high permeability. These channel deposits are mostly of meandering streams and are serpentine in plan and lensoid in cross-section. They represent only a small proportion of the total width of the valley at any given time. Because the meanders migrate laterally with time, the channel
192 deposits tend to have only moderate hydraulic connectivity in a vertical cross-section. At several deep horizons, the highly permeable sand and gravel deposits appear to form a continuous blanket across the paleo-valley floor; those may represent a braided stream valley, the entire width of which was traversed by interlaced distributary channels. The Salinas groundwater basin is comprised of an upper valley area, a pressure area, an east-side area, and a forebay area. The site lies in the pressure area, so named because it comprises a system of confined and semiconfined aquifers that are separated by quasi-continuous layers of clay that act as regional aquitards. About 50% of the basin recharge is by infiltration from the Salinas River (Durham, 1974). Groundwater movement is generally down the valley (northwestward), toward Monterey Bay. The average hydraulic gradient is about 0.0012 and closely follows the gradient of the Salinas River. In many areas of the Salinas Valley groundwater basin, pumping of agricultural wells imparts a cross-valley component to the potentiometric surface. Groundwater moves from the river toward areas of pumping. That pumping has been increasing since 1900, when groundwater usage began in the valley. Groundwater levels have been declining in response to this increased pumping but have partially stabilized in response to increased recharge resulting from the regulation of surface-water inflow to the valley by construction of dams and reservoirs. High-volume agricultural pumping from deep aquifers imposes a downward component to the NNW gradient.
Local hydrogeologic setting Hydrogeologic conditions in the vicinity of the site were studied through extensive subsurface investigations which resulted in the delineation of the subsurface hydrostratigraphic framework and the definition of principal contaminant flowpaths. The environment of deposition was confirmed to be mainly of a meandering stream system, locally changing to a braided stream system in which a blanket sand and gravel was deposited, or to an estuary in which thick clay was deposited.
H.w SMV:L)ESHAL In places, deposits of windblown sand occur. Coalesced alluvial fans occur along the valley margins and extend into the study area at some stratigraphic levels. The thickness of alluvial deposits at the site is more than 300 m. Three aquifers, with varying degrees of interconnection, were recognized below the unsaturated zone in the region of the site as a result of the detailed investigations described below. They are designated the shallow aquifer, intermediate aquifer, and deep aquifer system. The deep aquifer system can be subdivided into the 60-,90-,120-, and 150-m aquifers. The shallow aquifer is unconfined; the others are confined or partly confined. Each is a complex hydrostratigraphic system dominantly of low-permeability overbank deposits within which there are numerous high-permeability sand-gravel channel deposits that may overlap vertically, are subordinate to the low-permeability highly plastic clay and silt overbank deposits, and have only moderate connectivity. Each of these complex aquifers is separated from the one above and below by more-widespread units of lowpermeability materials that lack connected channel deposits and thereby constitute aquitards. That is, within a given aquifer, there are large volumes of material that do not contribute significantly to contaminant transport and that, by themselves, would not be considered an aquifer. Unsaturated zone
The unsaturated zone ranges from about 7 to 12 m thick. A soil-gas survey of the unsaturated zone established a plume which was used to focus the investigation of areas offsite where chlorinated hydrocarbons were likely to be found in the underlying shallow aquifer. Shallow and intermediate aquifers
The shallow aquifer extends from near the surface in places to a depth of about 27 m. It merges downgradient into a thick clay sequence where it becomes discontinuous and separated into isolated zones interbedded in thick clay. In the area of the former tire plant, the shallow aquifer is underlain by a generally continuous estuarine blue clay hori-
193
A SUPERFUND REMEDIATIONSITE AT SALINAS,CALIFORNIA, USA
i ~ORMER
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zon that constitutes an effective aquitard between the shallow and underlying aquifers. Downgradient, about where the aquifer disappears by merging into a clay sequence, this aquitard becomes thin and discontinuous, thus providing a hydraulic connection between the shallow and the underlying intermediate aquifers. A potentiometric map of the shallow aquifer near the plant is shown in Fig. 2. These data were obtained during a period when no extraction wells were pumping. The figure shows a steep gradient along the SW side which indicates resistance to groundwater flow caused by materials of low conductivity. The intermediate aquifer consists of strata within the depth interval of about 33 to 45 m. Farther downgradient, the clay aquitard at the base of the intermediate aquifer thins and becomes discontinuous; this provides a hydraulic connection between the intermediate and the deep aquifers.
Deep aquifer system Four zones were identified in the deep-aquifer system; they are referred to as the 60-, 90-, 120-, and 150-m aquifers (referred to as 200-, 300-, 400-, and 500-ft aquifers in the reports of the investigations). These hydrostratigraphic zones are separated by generally discontinuous fluvial and estuarine clays. The stratigraphic and potentiometric data indicate that these aquifers are hydrau-
lically connected to varying degrees at various places. The 60-m aquifer generally is encountered at about 57 m and is about 21 to 27 m thick. It is a regional aquifer. In the area of the former tire plant, an estuarine blue clay aquitard forms the top of the 60-m aquifer and separates it from the intermediate aquifer. Downgradient, this aquitard is missing in several places, providing a hydraulic connection between the intermediate and 60-m aquifer. The 90-m aquifer is encountered at a depth of about 90 m and is from about 8 to 15 m thick.It is capped by a 6-12-m-thick estuarine blue clay which appears to be continuous beneath the plant and for some distance downgradient, and then it grades into or is taken over by a tan clay of probable fluvial origin which is discontinuous and allows for hydraulic connection with the overlying 60-m aquifer. Another bed of blue clay, 1.5-8 m thick, underlies the 90-m aquifer. This clay aquitard thins and becomes discontinuous downgradient from the plant as it too is taken over by a brown clay of probable alluvial origin. The 120-m aquifer is generally encountered at about 105 m below the surface and is from about 18 to 30 m thick. The underlying aquitard is similar to that above it and is also discontinuous. The 150-m aquifer is encountered at a depth of about 135 m below surface and is thicker than the overlying aquifers. Little information is known of
194
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Fig. 3. Hydrographsof a cluster of wells, each screened to sample a differentaquifer. Aquifers listed are referred to herein as: shallow, intermediateand deep (60 m, 90 m, 120 m and 150 m). this deep aquifer; neither it nor the 120-m aquifer are involved in any contamination. Groundwater flow in the deep aquifer system is generally north-northwestward. Potentiometric levels measured in the deep aquifer monitoring wells decrease with depth, as shown in the hydrographs in Fig. 3. This is especially noticeable during periods of high-volume agricultural pumping seasons (summer and fall). The decreases cause a downward component to the gradient and promote downward flow and chemical transport into deeper aquifers, especially near the pumping wells.
Aquifer characteristics Hydrologic data sources consist of existing onsite and agricultural pumping data, aquifer pumping tests, and geotechnical data for the subsurface materials. Transmissivity values were calculated for the shallow and the intermediate aquifers from field data and from performance tests of several agricultural wells in the area. These data indicate that the transmissivity of the intermediate aquifer generally ranges from about 10,000 ft2/day (30 m2/day) to about 50,000 ft:/day (65 m2/day). The average hydraulic gradient is about 0.0012. Hydraulic conductivity was calculated to be about 100 ft/day (30 m/day) for the shallow aquifer and to range from about 200 to 1200 ft/day (60-360
m/day) for the intermediate and deep aquifers. Analyses of time for a pulse of contamination first detected at one well to travel to another give a useful conservative measure of the net transport time of about 0.4 to 0.6 m/day (if the transport were in a straight line from well to well, an unlikely but conservative scenario). Hydraulic conductivity of the various clay aquitards ranged from about 3 x 10 - 4 t o 8 X 10 - 6 ft/day, and is controlled by the finest 10% of the particle sizes. The low hydraulic conductivity and the high plasticity of the aquitard materials suggest that vertical contaminant migration occurs principally through discontinuities in the aquitard rather than by leakage through the clay itself.
Investigative approach From the local geologic setting, the probable depositional environment was considered to be that of a meandering, aggrading stream system, at times a braided stream system, much as is shown in the schematic drawings of Fig. 4, and sporadically an estuarine environment. The regional hydrogeologic base indicated that the gradient most likely was northwestward. These assumptions formed the basis for development of a plan for a sequential approach to the characterization of the site, including the determination of the nature and
195
A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
Fig. 4. Schematic block diagrams of meandering stream system (A) and braided stream system (B). (Dot pattern is for sand and gravel channel deposits, gray is silt and sand, black is clay.)
extent of contamination and of contaminant transport. Although sequential overall, many activities were conducted synchronously or in overlapping fashion.
Removal of sources The first concern was to remove the source of contamination. Since the primary source of contaminants had been removed during the cleanup, and geophysical surveys showed no likely buried additional sources (drums, tanks, etc.), the following actions were taken: (1) A soil-gas survey was made to determine locations of chlorinated hydrocarbons in the unsaturated zone that could serve as secondary sources of contamination of groundwater. (2) Six soil-vapor-extraction wells were installed to remove contaminants from the unsaturated zone. (3) Fifty-seven monitoring wells were installed to more-closely define the contaminant plume in the shallow aquifer; of those, 38 were onsite and
19 offsite as far as 480 m away from the plant. Evaluation of analyzed concentrations of chemicals of concern established the optimum locations for installation of shallow extraction wells. Fifteen extraction wells were installed in a row across the hydraulic gradient, near the edge of the plant, in order to aggressively start to remove contaminants and prevent them from serving as secondary sources and spreading farther. Their spacing was designed to ensure that the capture zone of each intercepted that of its neighbors so that no contamination would pass through the row of guard wells.
Inventory of wells At the same time, available data were collected for all wells located within 4 miles (about 6.4 km) in a large sector downgradient from the plant. Logs were available for 350 of the 370 wells inventoried. Although the quality of the logs ranged widely, analysis of those data provided a first approximation of the subsurface hydrostratigraphy, and served as the basis for planning the
196
locations for installation of four deep stratigraphic wells to establish detailed stratigraphic relations at key points. Of those existing wells, 208 were sampled to determine the lateral and vertical extent of contamination. Of 110 agricultural wells, all of which were sampled, only seven showed contamination. Even though the logs of most agriculture and industrial wells were of variable quality, they were available for the area immediately downgradient (agriculture wells) and, as it turned out, around the nose of the plume (industrial wells). The plume established in a reconnaissance way by this testing provided the basis for installation of additional monitoring wells whose logs also provided data for refining the understanding of the hydrostratigraphy. Downhole geophysical surveys conducted in these monitoring wells further refined the details of the three-dimensional architecture of the hydrostratigraphy. Eventually, a total of 189 monitoring wells, four stratigraphic-characterization wells, and 25 extraction wells were installed in the three groundwater zones. An ongoing groundwater monitoring program has provided a detailed understanding of the subsurface hydrostratigraphic architecture and a substantial geochemical data base from the monitoring and extraction wells and from agricultural, industrial, municipal, and domestic wells in the area.
Soil-gas survey A soil-gas survey was made in the unsaturated zone in a 3 km 2 area adjacent to and extending about 1.6 km down gradient from the plant. The unsaturated zone in the area ranges from about 7 to 12 m thick. The purpose of that survey was to obtain information on the distribution of chlorinated hydrocarbon vapors in the unsaturated zone in order to plan exploration of the underlying shallow aquifer. Soil-gas samples were collected from six wells and analyzed from depths of about 3, 6, and 9 m at 44 locations. Although the samples were analyzed for 1,!-DCA, 1,I-DCE, 1,1,I-TCA, and PCE, only 1,1,1-TCA and 1,1-DCE were detected frequently enough to establish a soil-gas plume.
H . W S M E D E S [~'! AlL
TCE was detected only rarely and I,I-DCA and PCE were not detected at all. The resulting soil-gas plume was used to focus the investigation of the shallow aquifer off site. The results were a first approximation of the plume and flowpath subsequently established from sampling of groundwater in the shallow aquifer.
Geophysical surveys While the activities stated above were underway, geophysical surveys were made. A terrain conductivity survey was made of the approximate area of the plume to search "depths" of about 15, 30, and 60 m in what later was determined to be the shallow and intermediate aquifers. This survey aided in the recognition of the major clay-rich aquitards, which were electrically moreconductive. A generalized map of the results of that survey is shown in Fig. 5. The overall configuration shown was corroborated by subsequent extensive exploration, including such things as maps of net sand and gravel, analytical testing of groundwater samples, and potentiometric maps. Resistivity soundings were made to better-define particular features shown in the conductivity survey. Ten resistivity soundings were taken at selected locations to help define the continuity, depth, and thickness of hydrostratigraphic units. A vertical-gradient magnetometer survey was made in selected places just off site in order to detect possible metal well casings at abandoned and buried wells. Candidate survey locations were selected from conversations with residents and from anomalies detected by the terrain conductivity survey. Identification of such wells is important because they may provide conduits for contaminants to flow from the unconfined (shallow) aquifer to the lower aquifers. Two wells were found by this survey and were tested to be certain that they did not provide such a conduit. Downhole geophysical surveys were made. They consisted of natural-gamma-ray logs and electric geophysical logs made at 26 selected deep borings across the study area to provide detailed stratigraphic data relevant to geohydrologic characteristics and to select screen intervals for monitoring.
197
A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
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Fig. 5. Generalized geohydrologic interpretation of composite terrain conductivity data to a search "depth" of 60 m,
Hydrogeochemicalevaluation For some wells in the intermediate and deep aquifers, the chemical concentrations show patterns that reflect seasonal variation due to agricultural pumping. Agricultural-well pumping from the deep aquifers induces flow from overlying aquifers, which results in downward transport of chemicals present at locations where aquitards are absent. Hydrographs of water pressures in wells show that during the summer, when pumping is greatest, a downward hydraulic gradient exists
,0t
between shallow and deep aquifers. In winter, when pumping is least, the vertical gradient declines to virtually zero, as shown in Fig. 3. A representative example of the time variability of concentrations of contaminants for a well in the shallow aquifer is shown in Fig. 6. The graph shows the geometric mean of the amount detected, in pg/l, Although some seasonal fluctuations in concentrations occur for various wells and chemicals, the general trend is a decrease of all concentrations in all wells due to the groundwater pumping and treatment system that has been oper-
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198
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ating for more than six years. By June of 1992, all wells had concentrations reduced to below the Maximum Contaminant Levels (MCL). Because it is the last constituent to be cleaned up (all others fall below M C L before it does), 1,1D C E serves as an indicator of contamination and of progress of cleanup. Data for I, 1-DCE concentrations at the M C L for different time periods are shown in the m a p of Fig. 7. The reduction in area (which is proportional to volume) of the M C L concentration is shown for a five-year period
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during which time the area within the M C L contour (6 ~tg/l for I , I - D C E ) decreased 98%. The area has subsequently been reduced to zero, that is, cleanup has been achieved. The pattern shown in Fig. 7 is compatible with the interpretation of a buried stream channel as controlling the transport. The area of the contaminant plume lies along the trough shown in the potentiometric map of Fig. 2. Concentrations of 1,1-DCE in the intermediate aquifer show a pattern (Fig. 8) similar to that for the shallow aquifer (Fig. 6) wherein concentrations
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199
A SUPERFUNDREMEDIATIONSITE AT SALINAS,CALIFORNIA,USA
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are affected by seasonal fluctuations and have been decreasing with time. The intermediate aquifer has also been remediated to concentrations below MCL. Concentrations of I,I-DCE in the deep aquifer system (deeper than 60 m) are shown in Fig. 9. Seasonal fluctuations are seen; however, the concentrations generally are less than 10 p.g/l (usually about 2 pg/1) and, at those low levels, some of the apparent fluctuations may simply be due to variations in sampling and laboratory precision. It was noted that some other chemicals such as 1, I-DCA, TCE, PCE, benzene, and xylene are detected only during times of heavy agricultural pumping, suggesting that the heavy withdrawal rate for irrigation influences transport of those components. The general composite position of the contaminant plume at its maximum known extent is shown schematically in Fig. 10. The figure represents a map projection of the maximum lateral extent of the irregularly-stacked channel deposits that serve as the conduits for contaminant transport. If viewed in cross-section, the plume would appear like a complex stack of lenses or pods whose degree of connectedness is irregular, and would shift from side to side depending upon the nature of stacked stream channels at the position of the section (see Fig. 4). A schematic cross-section of the composite plume showing the general architecture of the major hydrostratigraphic units is presented in Fig. 11, in which the vertical exaggeration is more
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Fig. 10. Schematiccomposite map of contaminant plumes at their probablemaximumextentin the shallow(S), intermediate (1) and deep (D) aquifers. (State Highwayis labelled by its number, 101.)
than five times. That figure shows schematically how the contaminant transport is accomplished by discontinuities in aquitards and by high-volume pumping of agricultural wells. Risk assessment
Detailed site characterization provided the basis for risk assessment. It is interesting to note that the risk assessment showed that there was no present or future toxicologically significant risk associated with exposure to groundwater downgradient from the plant. That conclusion held even for the unlikely worst-case scenario for future lifetime exposure, with no decrease in chemical concentrations over 70 years. The risk was also
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significantly below other risks associated with exposures commonly present in everyday life, such as drinking diet sodas or eating charcoal-broiled steaks or peanut butter. The only scenario that showed risks above the acceptable levels was an unrealistic one: a hypothetical new domestic well placed in the maximum contamination point of the shallow aquifer, in the middle of agricultural cropland, with no provision for any decrease in concentrations over the next 70 years - - not even by the natural dispersion, dilution, and chemical degradation that would decrease the concentrations. That worst-case scenario also required that, in order to be at risk, an individual would have to use that well as sole source of drinking water, and would have to drink that water for 70 years! To determine whether crops were contaminated from irrigation by the sprinkler systems from the agricultural wells, crops were sampled and analyzed at various intervals and seasons. No contamination was ever found. It turns out that the pumping and spraying of crops is a highly effective means of remediation of the deep aquifer even though the concentrations were already well below the cleanup levels. It was both fortunate and commendable that the regulators allowed the
growers to continue to irrigate from the deep aquifer in that manner.
Computer modeling When it was felt that the hydrogeology and hydrogeochemistry were reasonably-well known, computer modeling was done to predict the manner in which the contaminant plume would decay in space and time due to operation of the extraction wells. Five alternatives to remediation of the shallow and intermediate aquifers were analyzed in detail. These consisted of a no-action alternative and four alternatives of different combinations of pumping at various rates from different sets of extraction wells. Computer simulations were made of each of the five remedial alternatives in order to assess the relative effectiveness and the time required to achieve cleanup for each alternative. The no-action alternative consisted of stopping the then-current pumping and treatment of groundwater and letting natural dispersion, dilution, and degradation slowly reduce the concentrations of contaminants. An alternative of pumping just the shallow aquifer consisted of continuing the then-current pumping and treatment of groundwater from the
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A SUPERFUND REMEDIATION SITE AT SALINAS, CALIFORNIA, USA
shallow aquifer only, as a means of cutting off the source to the deeper aquifers. Chemicals in the intermediate aquifer would be subject to natural dispersion, dilution, and degradation, as in the noaction alternative. The remaining three alternatives consisted of various flow combinations of new extraction wells installed in the intermediate aquifer operating along with the shallow extraction wells. Modelled flow rates ranged from 600 to 1150 gpm (about 2.3 to 4.4 m3/min). The higher flow rates would require expansion of the treatment plant and modifications to existing operating permits. The recommended and adopted remedial alternative consisted of continuing to pump and treat groundwater from the shallow aquifer and, in addition, to install five new extraction wells in a specified configuration in the intermediate aquifer. For this alternative, the combined pumping rates for the shallow and the intermediate aquifers would be maintained at or below 650 gpm (about 2.5 m3/min.) which was the current capacity of the treatment plant and the rate allowed under the permit. Under that alternative, cleanup could be accomplished expeditiously, no new permits were required, no expansion of the treatment plant was needed, and no delays would be encountered. A computer simulation indicated that the pumping rates could be adjusted such that both the shallow and intermediate aquifers could reach cleanup levels in about 60% of the time required for the no-action alternative. Evaluation of the predictions over time versus the actual progress of cleanup provided a basis for modifications in the schedule of pumping rates from each of the extraction wells, and showed that certain wells could be shut down without affecting the cleanup rate. Because the computer modeling was used as an investigative tool that assisted in developing, testing, and modifying our understanding of the system, it is no surprise that cleanup was accomplished close to the time predicted by the modeling. Iteration
At each stage in the sequence of activities, the newly acquired data were incorporated into the
developing conceptual model and that model was modified in realistic ways as appropriate to be compatible with all the data accumulated to that moment. Gradually, the hydrostratigraphic and hydrogeologic model developed. It was tested, modified as appropriate, and used to focus the next stage of activity. Results of each stage provided input to refine the model, test it and modify it again and again in an iterative cycle. From all this activity, the understanding that developed is that of the architecture and hydrologic behavior of the shallow, intermediate, and deep aquifer systems as described above and in the 'Findings' section, below. The successful results of remediation demonstrate that characterization of the site was accomplished very well. Careful evaluation, guided by data from monitoring wells and their response to the agricultural pumping, and from time-analysis of changes in concentrations of chemicals in wells resulted in the understanding of the complex lateral and vertical transport of contaminants and the controlling hydrostratigraphy. That complex hydrostratigraphy and contaminant transport can be best understood by time analysis of three-dimensional diagrams, which cannot be displayed in page size of this paper. Therefore, only a schematic or conceptual sketch is used to show the overall interpretation (Figs. 10, 11).
Findings and conclusions Detailed site characterization through analysis of logs and construction records for existing wells; installation, logging, and monitoring of many monitoring and remediation wells; geophysical surveys; data analysis; and computer modeling, culminated in the subsurface mapping of hydrostratigraphic units and the definition of principal contaminant flowpaths. The hydrogeologic conditions at the site are summarized herein. The deposits beneath the site consist of more than 300 m of alluvial deposits comprised of highpermeability channel sand and gravel encased in low-permeability overbank silt and clay. Alluvial fans lie along the north edge of the site and estuarine clays form blanket deposits at several stratigraphic levels. The depositional environment
202
was that of a meandering system, taken over at times by a braided stream system and at other times by an estuarine environment of deposition of clay. Contaminant transport is controlled by the channel deposits and each of the data sources confirm the interpretation of limited lateral extent of the plume. The unsaturated zone at the site is about 10 m thick. The groundwater aquifer system is comprised of three interconnected zones that are designated the shallow aquifer, intermediate aquifer, and the deep aquifer system. The shallow aquifer is an unconfined aquifer that extends from about 10 to 27 m below surface. The other aquifers are confined to partly confined aquifers. The intermediate aquifer extends from about 30 to 42 m and the deep aquifer system locally consists of four zones, at depths of approximately 60, 90, 120, and 150 m. Analyses of samples from monitoring, agricultural, domestic, industrial, and municipal water wells indicate that the plume of chemical contaminants extends about 4.3 km northwestward from the former Firestone plant (Fig. 11). Starting at the plant, the plume is configured as follows: -A narrow ellipse about 1000 m long and 300 m wide, in the shallow aquifer, flowing nearly due west. - - A second ellipse about 1200 m long and 300 m wide in the intermediate zone, partly overlapping the map projection of the plume of the shallow aquifer and flowing northwestward. - - A third ellipse about 2100 m long and 450 m wide in the deep aquifer system, partly overlapping the west end of the map projection of the plume of the intermediate aquifer and flowing north-northwestward. Although the flow in the deep aquifer system generally is to the NNW, during the agricultural pumping seasons the agricultural wells control the flow directions, and each well generates a local cone of depression. The seasonal and diurnal pumping of the agricultural wells has generated a favorable condition for controlling the extent of the plume both downand across-gradient. No drinking-water well had then or has since exceeded current state action
H . W S M E D E S E'I At.
levels; therefore no imminent or substantial endangerment had ever existed. Operation of extraction wells in the shallow and intermediate aquifers reduced the concentrations in those aquifers to below the MCLs and served to cut off the source of contaminants to the deep aquifers. This indicates that concentrations of organic contaminants in drinking-water wells will always remain below current state action levels. However, as has happened in many places in the Salinas Valley, contamination by nitrates from fertilizing the crops (totally unrelated to operations of the tire plant) may eventually contaminate the water. Status
Since June of 1992, concentrations of the chemicals of concern had been reduced to below the Maximum Contaminant Levels (MCL) in all wells, and have remained below the MCLs. The responsible state regulatory agencies will permit the treatment system to be shut down on November 3, 1992. Cleanup has been achieved. Postlude
This project is unusual in that it is one of the few Superfund Sites that has been successfully remediated. In retrospect, we believe that there are two principal reasons for the success in remediation of this typically complex site, as follows: (1) The positive attitude of the management of Firestone, who agreed that the approach must be conservative and that if it required installation of a great many wells, that is what would be done. In addition, Firestone officials have consistently made a concerted and sincere effort to work cooperatively and to coordinate activities with Federal, state, and local regulatory agencies and other concerned agencies; and to communicate on a regular basis with the local municipalities, industries, growers, farmers, and citizens in the region through public hearings and personal contact. This attitude and response on the part of the owner (Firestone) is especially noteworthy in view of the fact that the entire program was at the request of the state and local regulatory agencies,
ASUPERFUNDREMEDIATIONSITEATSALINAS.CALIFORNIA,USA not by any regulatory order. Although this important activity is not dealt with further in this brief paper, it is nonetheless equally as important as the second reason for success, namely, the investigative approach taken. (2) The selection of and continual adherence to a carefully designed plan. A carefully designed sequential approach for the technical studies was selected and continually adhered to, in which the results of each stage of activity were evaluated and compared with the developing and evolving conceptual hydrogeologic model. This was done in a sequential and iterative process of using the conceptual model to focus studies and indicate types of data needed, acquiring the indicated data, comparing results with model, revising or refining model, and then repeating the cycle again and again, as appropriate. In compliance with regulations, volumes of reports, extensive maps, and analytical data were prepared and filed as the project moved through the numerous phases o f risk assessment and consideration of alternative remedial strategies, remedial investigations, feasibility studies,.., ad
infinitum. The project took seven years and cost more than US$22 million. The total amount of contaminant material retrieved from the treatment system was about 225 kg. The cost was borne entirely by Firestone. The project serves as a stern reminder of the old adage: "an ounce of prevention is worth a pound of cure".
203
Acknowledgements The first half of the investigations was carried out by the staff of Woodward-Clyde Consultants and the latter half by staff of the IT Corporation. Our thanks to the countless scientists and engineers of those companies, whose data we have used extensively. We also extend our thanks to the many people who were involved with the project as members of regulatory agencies, growers, industries, and the local citizenry. For information and insights into the problem, we wish especially to express our appreciation to George Markert and Leonard Smith of Bridgestone/Firestone Company, who were involved in the program since its beginning; and to dr. Larry Froebe, dr. R.Nichols Hazelwood, Chandler Weisel, and Ed Wing, all of the IT Corporation.
References Durham, D.L., 1974. Geology of the southern Salinas Valley area, California. U.S. Geol. Surv., Prof. Pap. 819, map scale 1:25,000. Greene, H.G., 1970. Geology of southern Monterey Bay and its relationship to the groundwater basin and seawater intrusion. U.S. Geol. Surv., Open-File Rep. Ross, D.C. and Brabb, E.E., 1973. Petrography and structural relations of granitic basement rocks in the Monterey Bay area, California. J. Res. U.S. Geol. Surv., 1(3): 273-282. Tinsley, J.C., 1975. Quaternary Geology of northern Salinas Valley, Monterey County, California. Unpubl. Diss., Stanford Univ., Stanford, Calif.