Occurrence of natural and anthropogenic hexavalent chromium (Cr VI) in groundwater near a mapped plume, Hinkley, CA

Hydrogeologic Setting

Figure 1.: Mojave River Groundwater Basin

Figure 1.: Mojave River Groundwater Basin

Figure 2.: Study area location

Figure 2.: Study area location

Hydrogeologic setting: Hinkley Valley, within the Harper Valley Groundwater Basin (Department of Water Resources, 2004), is part of the Mojave River groundwater basin (Stamos and others, 2001) (fig. 1). The geologic development of the Mojave River within the Pleistocene Epoch is the result of movement along the San Andreas Fault and the subsequent opening of Cajon Pass between the San Bernardino and San Gabriel Mountains (Meisling and Weldon, 1989). As the pass opened, increased precipitation within the Mojave Desert near the pass gave rise to the Mojave River. Transport of alluvium as the river extended farther into the Mojave Desert created interconnected alluvial aquifers, including Hinkley Valley, that extend from near Cajon Pass to Soda (dry) Lake more than 100 miles from the mountain front (Tchakerian and Lancaster, 2002; Enzel and others, 2003).

Hinkley Valley is bounded to the west by Iron Mountain composed of quartzite and marble, with smaller hills to the north composed of quartz monzonite. The valley is bounded to the east by Mount General composed of quartzite, marble, and Tertiary-age dacitic volcanics, with smaller hills to the north composed of quartz monzonite (Dibblee, 2008). The northwest trending Lockhart and Mount General Faults are present along the southwest and northeast parts of the valley, respectively. To the north, there is a narrow gap separating Hinkley and Water valleys. The Mount General Fault passes through this gap and volcanic rocks are exposed within the gap (fig. 2).

Alluvial deposits within the valley consist of alluvial-fan deposits eroded from highlands along the valley margins, and alluvium from the Mojave River eroded largely from granitic rock in the San Bernardino Mountains 40 miles to the south. Alluvium within the valley is divided into an upper and lower aquifer by the "blue clay." The upper aquifer is further divided by the "brown clay". Alluvium is underlain by bedrock or weathered bedrock. Where the blue clay is not present the upper aquifer is in direct hydraulic communication with the surrounding bedrock. Detailed descriptions of these alluvial aquifers and confining clay units are available in (CH2M-Hill, 2013a). Alluvium from the Mojave River composes the floodplain aquifer (Stamos and others, 2001) within the upper aquifer. The floodplain aquifer is present through the center of the valley and through the gap at the north end of Hinkley Valley connecting into Water Valley.

The climate is arid, with hot summers and cool winters. Average annual precipitation is less than 110 millimeters per year (Barstow, CA, station 040519, 1903-1980 http://www.wrcc.dri.edu/cgi-bin/cliMONtpre.pl?ca0519, accessed July 16, 2003). In the western Mojave Desert, little or no groundwater recharge occurs from infiltration of precipitation or from infiltration of intermittent runoff in small streams (Izbicki and others, 2007). Most groundwater recharge to the Hinkley Valley occurs as infiltration of streamflow from the Mojave River along the southern edge of the valley (Thompson, 1929; Stamos and others, 2001; Izbicki, 2004). Streamflow in the Mojave River originates primarily as precipitation and runoff from near Cajon Pass and the San Bernardino Mountains (Izbicki, 2004). Large streamflows in the Mojave River that recharge the alluvial aquifer within Hinkley Valley occur infrequently, and many years may pass without significant flow along this reach of the river, and without groundwater recharge (Stamos and others, 2001).

Under predevelopment conditions, groundwater flow through Hinkley Valley was from intermittent recharge areas along the Mojave River, to the north through a gap at the northern end of the valley into Water Valley towards discharge areas near Harper (dry) Lake (Thompson, 1929; Stamos and others, 2001; Izbicki, 2004). Groundwater levels in parts of Hinkley Valley were within 15 feet of land surface and flowing wells were present to the north in Water Valley (Thompson, 1929). On the basis of water level differences, the Lockhart Fault is an impediment to groundwater flow in the western part of Hinkley Valley (California Department of Water Resources, 1967). The Lockhart Fault does not impede groundwater flow in recent alluvial deposits along the Mojave River (Stamos and others, 2001). The extent to which groundwater flow is impeded by the Mount General Fault between Hinkley and Water Valleys is not known.

Shallow depths to water enabled agricultural development by early settlers. Agricultural pumping peaked in this part of the Mojave River groundwater basin in the mid-1950's, and gradually declined in the following decades (Stamos and others, 2001). Water-level declines in some areas as a result of agricultural pumping were between 70 and 90 feet (California Department of Water Resources, 1967; LRWQCB, 2013a). In parts of the valley, formerly saturated alluvium was dry, and limited pumping by domestic wells was sustained by withdrawals from the underlying bedrock aquifer. The bedrock aquifer is hydraulically connected to the alluvial deposits. Regionally, water-level declines led to a series of lawsuits culminating in adjudication of the Mojave River groundwater basin in 1996. Subsequent reduction in agricultural pumping, natural recharge from the Mojave River, and artificial recharge of imported water along the river contributed to partial recovery of water levels in the area.

Under present-day conditions, groundwater flow is from recharge areas along the Mojave River toward a pumping depression underlying land treatment units operated by PG&E near the northern extent of the contaminant plume to remove Cr VI through reduction to Cr III by application to agricultural fields (CH2M-Hill, 2013a). Historically saturated alluvium below the predevelopment water table and above the present-day water table is unsaturated.

Discharges of wastewater containing chromium from the compressor station began in 1952 and continued until 1964 (LRWQCB, 2012a). Although seasonal flows in the Mojave River occurred annually between 1940 and 1945, only a few small flows and consequently only small quantities of groundwater recharge occurred along this reach of the Mojave River during the time of chromium releases from the compressor station. Presumably during this time, chromium from the compressor station that reached the water table moved with groundwater towards pumping wells within the valley. In 1969 large flows in the Mojave River and subsequent large quantities of groundwater recharge increased water levels and changed groundwater flow within the system. The water table within Hinkley Valley, although mapped as part of regional investigations of groundwater conditions within the Mojave River basin (California Department of Water Resources, 1967), was not closely monitored during the period of chromium releases or during recharge associated with the 1969 streamflows. As a consequence, the movement of Cr VI, the dimensions of the plume, and the potential for mixing of native (uncontaminated) groundwater near the plume margin with small amounts of wastewater containing Cr VI from the compressor station are not precisely known. Uncertainty concerning plume movement is increased as a result of water level changes occurring initially as a result of declining agricultural pumping and later as a result of management activities intended to control the plume.

The total mass of chromium released from the compressor station has been estimated to be about 10,000 pounds (LRWQCB, 2012a). More than 350 wells at more than 100 sites within the study area have been installed to monitor the plume. Historical Cr VI concentrations within the plume exceed 9,000 µg/L (LRWQCB, 2012a). The mass of chromium identifiable in groundwater within the mapped plume in 2011 was about 4,200 pounds (ARCADIS US, Inc., written commun., 2013). Most of this mass was present within the core of the plume in areas having higher Cr VI concentrations. Some removal of chromium from groundwater occurred as a result of a combination of natural processes, management activities, and past agricultural use of contaminated water. However, Cr VI is highly soluble and mobile in alkaline, oxic groundwater; and Cr VI contamination in groundwater can migrate great distances with limited attenuation (Perlmutter and others, 1963; Blowes, 2002). In some areas, identifying the extent of Cr VI contamination near plume margins can be complicated by the presence of naturally-occurring Cr VI from weathering of rocks and minerals (Izbicki and others, 2008a), by potential mobilization of Cr VI within the unsaturated zone by agricultural activities (Izbicki, 2008b and 2008c; Mills and others, 2011), and by reduction of Cr VI to Cr III with subsequent mixing of native and contaminated groundwater near the plume margin (Izbicki and others, 2012).

In addition to Cr VI, other trace elements (including manganese, arsenic, and uranium) are present at concentrations of public health concern in parts of the valley (LRWQCB, 2012a). Concern has been expressed by local residents that management activities intended to control the Cr VI plume may contribute to high-concentrations of these elements. Specific concerns have been raised about: 1) manganese and arsenic by-products resulting from the use of ethanol to reduce Cr VI to Cr III within the In-situ Reactive Zone (IRZ), and 2) the fate of chromium on aquifer solids during decadal, or longer, time-scales as groundwater within the IRZ reoxygenates through natural processes.

To facilitate understanding of geology, hydrology, and the occurrence of natural and anthropogenic Cr VI, for the purposes of this study the site has been divided into the western, northern, and eastern subareas (CH2M-Hill, 2013b). In addition to areas east of the mapped plume, the eastern subarea also includes the mapped plume, and areas upgradient from the plume along the Mojave River. Each subarea has different geologic, hydrologic, geochemical, and land-use histories that may affect naturally occurring Cr VI concentrations and the potential for occurrence of Cr VI associated with the compressor station.

The western subarea contains alluvial fan deposits eroded from Iron Mountain and the surrounding hills, interfingered with Mojave River alluvium. The Lockhart Fault to the southwest has been recognized as an impediment to groundwater flow (California Department of Water Resources, 1967; Stamos and others, 2001). Alluvium north of the fault thins to the west as bedrock slopes upward to the surrounding hills, and to the north over a bedrock high. Much of the formerly saturated alluvium in the western subarea is unsaturated as a result of past pumping. Under present-day conditions the water table slopes to the east, and water-level gradients steepen near the Lockhart Faultóconsistent with an impediment to flow in that area (CH2M-Hill, 2013a). Present-day pumping for domestic and remaining agricultural use is sustained by wells often completed partly, or entirely, into underlying bedrock. Increasing Cr VI concentrations in part of the western subarea have called into question the effectiveness of injection wells installed near the mapped plume boundary to limit westward movement of Cr VI (LRWQCB, 2013). Some other issues of concern in the western subarea include: 1) Has Cr VI associated with the plume entered the area in the past, and is this Cr VI still present to the west of injection wells installed to control plume movement?; 2) Does bedrock and alluvium eroded from local sources contain chromium that may weather and contribute Cr VI to groundwater under certain geochemical conditions?; 3) Does pumping from bedrock wells hydraulically connected to the overlying alluvial aquifer cause unforeseen movement of Cr VI associated with the plume?; and 4) Does oxidation of chromium-containing minerals in historically saturated alluvial deposits above the present-day water table (Izbicki and others 2008), and mobilization of soluble salts (including Cr VI) from the unsaturated zone by past agricultural activity (Izbicki and others, 2008; Mills and others, 2011), contribute Cr VI to the underlying groundwater?

The northern subarea includes parts of Hinkley and Water Valleys. The subarea contains Mojave River alluvium that composes the highly-permeable floodplain aquifer, surrounded by bedrock covered by alluvium and alluvial-fan deposits eroded from the surrounding hills. Saturated alluvium is within a bedrock channel extending from Hinkley Valley to the north through the gap in the surrounding hills into Water Valley. The thickness of alluvium within the gap and the influence of the Mount General Fault on groundwater flow through the gap are not known. Past agricultural pumping that lowered the water table reduced or eliminated groundwater flow through the gap, ultimately eliminating groundwater discharge from springs and flowing wells in Water Valley. Under present-day conditions, pumping as part of remediation and land-treatment of Cr VI maintains a depression in the water table, limiting groundwater flow to the north. An area of groundwater having Cr VI concentrations greater than the 3.1 µg/L background concentration is present in the northern subarea, and there is concern that the northern extent of the plume has not been adequately defined (Lahontan Regional Water Quality Control Board, 2013). Some other issues of concern in the northern subarea include: 1) Has Cr VI associated with the plume entered part of the subarea in the past?; 2) Does bedrock and alluvium eroded from local sources, especially volcanic rocks, contain Cr VI that could weather and contribute Cr VI to groundwater under certain geochemical conditions?; 3) What is the depth of alluvium within the gap between Hinkley and Water Valleys and does the Mount General Fault impede groundwater flow through the gap?; and 4) What is the hydraulic connection between Hinkley Valley and Water Valley potentially affecting plume migration?

The eastern subarea contains alluvial deposits from the Mojave River and alluvial fan deposits eroded from Mount General. The "blue clay" that separates the upper aquifer from the lower aquifer and the "brown clay" within the upper aquifer are present throughout much of the eastern subarea (CH2M-Hill, 2013a). In some areas near the margins of the eastern subarea (and adjacent parts of the western and northern subareas), the blue clay was deposited upon bedrock and the lower aquifer is absent. Although some Cr VI contamination has been reported in the lower aquifer (LRWQCB, 2012a); at this time, the small amount of Cr VI contamination within the lower aquifer is a lesser concern to the TWG than Cr VI contamination in the upper aquifer and bedrock aquifer near the mapped plume margins. Water-level declines as a result of agricultural pumping were as great as 90 ft within the eastern subarea (California Department of Water Resources, 1967; LRWQCB, 2013a). Under present-day conditions, pumping for agriculture continues in the eastern area, with additional pumping for land-treatment of Cr VI. In-situ reduction of Cr VI to Cr III through injection of ethanol occurs within the In-situ Reactive Zone (IRZ) within the eastern subarea. Manganese and other by-products of in-situ reduction of Cr VI to Cr III, and the long-term fate of chromium within the in-situ treatment area as groundwater reoxygenates are a concern to residents and regulators. Recharge from irrigation return and dairy waste disposal has contributed to increased dissolved solids and nitrates in some areas. Some issues of concern in the eastern subarea include: 1) Does large-scale agricultural pumping of groundwater allow unexpected movement of Cr VI near production wells?, and 2) What are the chemical composition and Cr VI concentration of recently recharged water along the Mojave River upgradient from the compressor station?

Cooperating Agency: Lahontan Regional Water Quality Control Board
Project Chief: John A. Izbicki
Phone: 619-225-6131
Email: jaizbick@usgs.gov

California Water Conditions

Real-Time California Streamflow Conditions