1

Prepared in cooperation with the Lahontan Regional Water Quality Control Board

A Plan for Study of Natural and Man-Made Hexavalent 

Chromium, Cr(VI), in Groundwater near a Mapped Plume, 

Hinkley, California

By John A. Izbicki and Krishangi Groover

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2016–1004

January 2016

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/ofr20161004

Figure 1. Pacific Gas and Electric (PG&E) compressor station, Hinkley, California, March 2009. (Photo by Steven 

Perry, Arcadis, Inc., courtesy of PG&E).

The Pacific Gas 

and Electric Company 

(PG&E) Hinkley 

compressor station 

(fig. 1), in the Mojave 

Desert 80 miles northeast of Los Angeles, 

is used to compress 

natural gas as it is 

transported through a 

pipeline from Texas to 

California. Between 

1952 and 1964, cooling 

water used at the 

compressor station 

was treated with a 

compound containing 

chromium to prevent 

corrosion. After 

cooling, the wastewater 

was discharged to 

unlined ponds, resulting 

in contamination of 

soil and groundwater in 

the underlying alluvial 

aquifer (Lahontan 

Regional Water Quality Control Board, 

2013). Since 1964, cooling-water 

management practices have been used 

that do not contribute chromium to 

groundwater.

In 2007, a PG&E study of the 

natural background concentrations 

of hexavalent chromium, Cr(VI), 

in groundwater estimated average 

concentrations in the Hinkley area to be 

1.2 micrograms per liter (μg/L), with 

a 95-percent upper-confidence limit 

of 3.1 μg/L (CH2M-Hill, 2007). The 

3.1 μg/L upper-confidence limit was 

adopted by the Lahontan Regional Water 

Quality Control Board (RWQCB) as the 

maximum background concentration used 

to map the plume extent. In response to 

criticism of the study’s methodology, 

and an increase in the mapped extent of 

the plume between 2008 and 2011, the 

Lahontan RWQCB (Lahontan Regional 

Water Quality Control Board, 2012) 

agreed that the 2007 PG&E backgroundconcentration study be updated. 

The purpose of the updated 

background study is to evaluate the 

presence of natural and man-made 

Cr(VI) near Hinkley, Calif. The study 

also is to estimate natural background 

Cr(VI) concentrations in the aquifer 

upgradient and downgradient from the 

mapped Cr(VI) contamination plume, 

as well as in the plume and near its 

margins. The study was developed by 

the U.S. Geological Survey (USGS) in 

collaboration with a technical working 

group (TWG) composed of community 

members, the Independent Review Panel 

(IRP) Manager (Project Navigator, Ltd.), 

the Lahontan RWQCB, PG&E, and 

consultants for PG&E.

The scope of the study includes 

eight tasks and publication of four 

reports (table 1, shown on page 12). 

The proposal is available at http://

ca.water.usgs.gov/projects/hinkley/; a 

video describing the study is available at 

http://ca.water.usgs.gov/media/hinkleygroundwater-chromium.html. The study 

agreement was approved by the State 

Water Resources Control Board in 

January 2015; field-data collection began 

in March 2015; the study is scheduled to 

be completed in December 2019.
2 

Figure 2. Location of Pacific Gas and Electric monitoring-well sites, Hinkley, California; the maximum 

extent of hexavalent chromium, Cr(VI), greater than the 3.1 microgram per liter clean-up level for the 

site; and the extent of the mapped Cr(VI) plume, April 2015.

Task 1: Evaluation of 

Existing Data 

The purpose of this 

task is to identify areas 

near the mapped Cr(VI) 

plume where water-quality 

is a concern. Results of 

this task are to provide an 

assessment of the changes 

in Cr(VI) concentrations in 

groundwater in the Hinkley 

area through time.

PG&E has monitored 

Cr(VI) contamination 

near Hinkley since the late 

1980s. By December 2014, 

more than 630 monitoring 

wells were installed in 

the area (fig. 2), and more 

than 15,900 water samples 

had been collected from 

those wells and analyzed 

for Cr(VI). Many of the 

monitoring locations include 

multiple wells completed 

at different depths in the 

aquifer to provide depthspecific water-level and 

water-quality data. In 

addition, more than 5,300 

samples were collected 

from 540 domestic and 

agricultural wells in the area 

and analyzed for Cr(VI). 

The more recent data were 

not available to the 2007 

background study, but have 

been used by PG&E and the 

Lahontan RWQCB to define 

the extent of the Cr(VI) 

plume.

As part of task 1, 

existing data are to be 

examined to determine 

if there are trends of increasing or 

decreasing Cr(VI) concentrations 

with time. Increasing Cr(VI) 

concentrations with time could indicate 

areas where the Cr(VI) plume is 

expanding. Decreasing concentrations 

could indicate areas where plume 

management has been successful at 

controlling the movement of the plume. 

Changes in concentrations of Cr(VI) 

and other constituents also could be 

related to other factors, such as water 

levels changing as a result of changes 

in plume-management activities, 

groundwater pumping, or intermittent 

groundwater recharge from the Mojave 

River. The existing data also are to 

be used to select wells for sample 

collection as part of this study (task 3).

Although existing data are to 

be examined primarily with respect HINKLEY ROAD

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PG&E monitoring-well sites

 by year installed:

2010 or earlier

2011

2012

2013

2014

EXPLANATION

Extent of mapped Cr(VI) plume,

 April 2015

Maximum mapped extent of 

 3.1 microgram per liter 

 hexavalent chromium, Cr(VI), 

 2008–15

Pacific Gas and Electric (PG&E)

 compressor station

0 2 4 KILOMETERS

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Faults

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area

Los Angeles

San Diego

CALIFORNIA

Pacific Ocean

Barstow

to total chromium, Cr(t), and 

Cr(VI) concentrations, water from 

monitoring wells and other wells in 

the area also have been monitored 

for other constituents of concern 

to the community, such as arsenic, 

manganese, uranium, and nitrate. 

Concentrations and trends of these 

constituents also are to be examined as 

part of this task.
 3

Task 2: Analyses of Rock and 

Alluvium

The purpose of this task is to 

determine if there are natural geologic 

sources of chromium in the area and 

if any such sources are contributing 

Cr(VI) to groundwater. The natural 

presence of Cr(VI) in groundwater 

is influenced by a number of factors, 

including (1) the concentration 

of chromium in rock and aquifer 

materials; (2) the minerals in those 

materials and their weathering rates; 

and (3) the abundance and reactivity 

of manganese oxides on the surfaces 

of mineral grains that can convert 

trivalent chromium, Cr(III), weathered from minerals, to Cr(VI). The pH 

(a measure of the acidity or alkalinity 

of water) and reduction-oxidation 

(redox) conditions of groundwater, 

particularly with respect to dissolved 

oxygen that allows Cr(VI) to enter and 

remain in solution, are to be assessed 

on the basis of existing water-quality 

data (task 1) and data collected as part 

of this study (task 3).

Alluvium from the 

Mojave River and eroded 

from rock in the hills 

surrounding the study 

area is to be examined in 

the field by using a handheld X-ray fluorescence 

(XRF) instrument 

(fig. 3). Results are to 

be compared with XRF 

measurements of core 

material from wells 

installed in the area by 

PG&E (1) to identify 

areas where alluvium 

is derived from local 

sources in the nearby hills surrounding 

Hinkley Valley or from more distant 

sources, such as rocks in the San 

Gabriel or San Bernardino Mountains 

that were transported to and deposited 

in the area by the Mojave River, and 

(2) to determine whether chromium 

concentrations in the alluvium that 

composes the aquifers are high or 

low relative to average continental 

abundances. XRF measurements are to 

include core material from the screened 

intervals of wells selected for sample 

collection in task 3 and to be compared 

with water-quality data from those 

wells (task 6).

The XRF data also are to be used 

as a screening tool to select materials 

for additional analyses. Those analyses 

include (1) chemical extractions 

using increasingly aggressive 

(reactive) solutions to determine how 

tightly bound chromium and other 

selected elements are to the surfaces 

of mineral grains (fig. 4); 

(2) thin-sections to identify 

minerals associated with 

high chromium abundance; 

(3) separation of heavier, 

denser minerals from 

lighter, less-dense minerals, 

coupled with optical and 

X-ray diffraction (XRD) 

identification of the denser 

minerals possibly associated 

with high chromium 

abundance; and (4) digestion 

and chemical analyses of the 

denser minerals to determine 

their elemental composition.

The data are to be used 

to describe the abundance 

and potential for weathering 

of chromium-containing 

minerals. Results from task 

2 are to be considered during 

selection of wells for sample 

collection (task 3) and for 

evaluation of the presence 

of natural and anthropogenic 

chromium (task 6).

Figure 3. Hand-held X-ray fluorescence (XRF) instrument used to measure 

concentrations of up to 28 elements (including chromium) in rock, alluvium, and core 

material from Pacific Gas and Electric (PG&E) monitoring wells, Hinkley, California, 

March 2015 (core material archived by PG&E in background).

Figure 4. Preparation of sequential chemical 

extractions from alluvium and core material, 

Hinkley, California, June 2015.
4 

Figure 5. U.S. Geological Survey personnel and consultants for Pacific Gas and Electric sampling a 

monitoring well, Hinkley, California, March 2015.

Task 3: Analyses of Chemical and 

Environmental Tracers in Water 

from Wells

The purpose of this task is to 

determine the chemical and isotopic 

(including other environmental tracers) 

composition of water from selected 

wells throughout the study area with 

respect to (1) the sources and chemical processes controlling Cr(VI) and 

(2) the source, movement, and “age” of 

the groundwater relative to the timing of 

Cr(VI) releases and migration from the 

PG&E compressor station.

Water samples are to be collected 

from up to 90 domestic, agricultural, 

and monitoring wells. Sample collection 

will be done in 3 phases over a 3-year 

period with 40, 30, and 20 wells 

sampled in each phase. Because of their 

known construction characteristics, 

relatively short screen-intervals, and the 

availability of supporting geologic data, 

sample collection from monitoring wells, 

where available, is preferred to sample 

collection from domestic or agricultural 

wells. Use of monitoring wells addresses 

one of the criticisms of the 2007 PG&E 

background study, which relied on 

samples collected from domestic wells 

that had limited construction information. 

Data collection for each phase is planned 

to be done approximately 1-year apart 

in March, at the end of the rainy season, 

beginning in 2015. This allows for 

completion of laboratory analyses, such 

as tritium—which requires more than 

9 months to analyze—and for review and 

preliminary interpretation of results from 

the first phase to support selection of 

wells for the second phase. Well selection 

for the third phase is planned to fill any 

remaining data gaps and to collect data 

from previously sampled wells that show 

trends in Cr(VI) concentrations over 

time (task 1). Samples from monitoring 

wells are to be collected by USGS field 

personnel in collaboration with PG&E 

consultants (fig. 5) to ensure similar 

well-purging and other data-collection 

procedures are used.

Chemical data to be collected 

include field parameters, major ions, 

nutrients, selected minor ions and trace 

elements, including Cr(t), Cr(VI), 

manganese, arsenic, and uranium 

(table 2). Chemical data are to be 

interpreted in terms of (1) health-based 

standards, (2) their distribution with 

respect to the mapped plume, (3) the 

solubility of chromium-containing 

minerals identified in task 2, and (4) the 

solubility of Cr(VI) with respect to pH 

and redox conditions.

Environmental tracers are 

constituents that can be used to 

understand the hydrologic history 

of water or the processes that affect 

constituents in the water. Environmental 

tracers can be measured very precisely 

at low concentrations and can be used 

to track the movement of contaminants of interest in areas where natural 

background concentrations could be 

present. Isotopes, a type of environmental 

tracer, are atoms of the same element 

(and, therefore, have the same number 

of protons in the atomic nucleus) that 

differ in the number of neutrons in the 

nucleus. Isotopes can be either stable or 

radioactive. Stable isotopes have slight, 

but measurable, differences in their 

physical properties or chemical reactivity. 

Radioactive isotopes 

decay at known and 

constant rates over time; 

if their initial abundance 

is known, their measured 

abundance can be used to 

determine the time since 

recharge, or “age,” of a 

water sample.

Environmental 

tracers used in this 

study can be divided 

into three categories: 

(1) tracers of the source 

and movement of water, 

(2) tracers of the “age” of 

water, and (3) tracers of 

chemical reactions and 

environmental processes. 

Tracers of the 

source and movement of 

water used in this study 

include (1) the stable 

isotopes of oxygen and 

hydrogen in the water 

molecule (oxygen-18 and 
 5

Table 2. Chemical constituents to be analyzed in water from wells as part of task 3 in the 

U.S. Geological Survey hexavalent chromium, Cr(VI), updated background study, Hinkley, 

California.

[Field parameters to be measured include water temperature, pH, dissolved oxygen, specific conductance, 

and alkalinity]

deuterium), which provide information 

on the source of recharge (for example, 

Mojave River or runoff in local streams) 

and the evaporative history of water, 

and (2) dissolved atmospheric gasses 

(nitrogen and argon), which record 

information on recharge processes.

Tracers of the “age” of water 

include (1) dissolved industrial gasses, 

including chlorofluorocarbons and 

sulfur hexafluoride; (2) tritium, a 

radioactive isotope of hydrogen that 

analyses of water samples and a variety 

of interpretive tools developed for this 

purpose (fig. 6).

Tracers of chemical reactions 

and environmental processes include 

(1) chromium-53, a stable isotope of 

chromium that can be used to distinguish 

natural and man-made sources of 

chromium and processes that control 

the reduction of Cr(VI) to Cr(III), and 

(2) strontium-87/86, a ratio of the stable 

isotopes of strontium that can be used to 

evaluate the geologic material with which 

the water has been in contact over time.

Collectively, chemical and 

environmental tracer data are to be 

used to evaluate the source, movement, 

and “age” of water in aquifers 

underlying the study area to determine 

(1) the geochemical “footprint” (areal 

extent) of water recharged during the 

period of Cr(VI) releases from the 

PG&E compressor station; (2) older 

groundwater recharged prior to the 

releases; and (3) the effect of mixing of 

water from different recharge sources 

that have different hydrologic histories, 

different “ages,” and have been in 

contact with different geologic materials. 

Interpretation of these data is to be 

used to help answer “What Cr(VI) is 

associated with PG&E releases from the 

compressor station, and what is not?” 

(task 6).

Figure 6. U.S. Geological Survey field 

personnel organizing bottle sets used to collect 

water samples, Hinkley, California, March 2015.

has a half-life of 12.3 years, and 

its decay product helium-3; and 

(3) carbon-14, a radioactive isotope 

of carbon that has a half-life of 

5,730 years, and carbon-13, a stable 

isotope of carbon. Many tracers 

of the “age” of water are affected 

by processes during groundwater 

recharge, chemical reactions in the 

aquifer, and mixing of water from 

different sources. These processes are 

to be evaluated by using results from 

Nutrients,

in milligrams per liter

Reporting limit

Ammonia, as nitrogen 0.01

Ammonia plus organic 

nitrogen as nitrogen 0.07

Nitrite, as nitrogen 0.001

1

Nitrite plus nitrate, as 

nitrogen 0.04

Phosphorous 0.02

Orthophosphorous, as 

phosphorous 0.004

1

Nitrate calculated by difference from nitrite 

and nitrite plus nitrate.

Trace elements,

in micrograms per liter

Reporting limit

Antimony 0.03

Aluminum 2.2

Arsenic 0.1

Barium 0.3

Boron 2

Cadmium 0.3

Chromium 0.2

Iron 3.2

Lithium 0.1

Manganese 0.1

Uranium 0.1

Vanadium 0.6

Major ions,

in milligrams per liter

Reporting limit

Alkalinity (bicarbonate) 4.6

Calcium 0.02

Chloride 0.02

Fluoride 0.04

Mangnesium 0.01

Potassium 0.03

Silica 0.02

Sodium 0.06

Sulfate 0.02

Residue on evaporation

(dissolved solids) 20

Minor ions,

in micrograms per liter

Reporting limit

Bromide 30

Iodide 1

Strontium 0.2

Reduction-oxidation couples,

in micrograms per liter

Reporting limit

2

Iron/iron II 2 / 2

2

Arsenic/arsenic III 0.2 / 0.5

2

Chromium/chromium VI 0.2 / 0.06

2

Iron III, arsenic V, and chromium III 

calculated by difference from their respective 

reduction-oxidation couples.
6 

geology and hydrology in the western, 

northern (including Water Valley), 

and eastern (including the plume and 

upgradient areas) subareas (fig. 7) 

influence concentrations of natural 

Cr(VI) in groundwater and the 

movement of Cr(VI) released from the 

compressor station. The scope of work 

Figure 7. Map showing features of hexavalent chromium study, Hinkley, California: western, northern (including 

Water Valley), and eastern (including the mapped plume and upgradient) subareas, Pacific Gas and Electric 

monitoring wells, monitoring wells installed for flow-path studies, U.S. Geological Survey multiple-well monitoring 

sites, and domestic wells.

Task 4: Evaluation of Local 

Conditions

The purpose of this task is to 

determine how differences in local 

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Iron

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EXPLANATION

Pacific Gas and Electric (PG&E)

 compressor station

Subareas

Eastern and upgradient

Faults

Extent of mapped Cr(VI) plume 

April 2015; dashed where uncertain.

U.S. Geological Survey multiple-well 

monitoring sites

Domestic wells sampled by PG&E

PG&E monitoring-well sites

PG&E monitoring-well sites for flowpath 

 studies

Maximum mapped extent of 3.1 microgram 

per liter hexavalent chromium VI, Cr(VI), 

 2008–15

Northern

Western

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Figure 9. Test-drilling by consultants for 

Pacific Gas and Electric to install monitoring 

wells for flow-path studies, Hinkley, California, 

August 2015.

Figure 8. U.S. Geological Survey field crew preparing to set a temporary 

pump for borehole-geophysical data collection in a well, Hinkley, California, 

June 2015 (Roger Killian, Hinkley Technical Working Group, in foreground).

for this task is intended to be flexible, so 

that issues important to the background 

study that were not apparent during the 

initial development of the study can be 

addressed as the study progresses. This 

task includes a combination of (1) test 

drilling and monitoring-well installation 

for groundwater flow-path studies, 

(2) collection of surface-geophysical 

data (primarily gravity measurements) to 

estimate aquifer thickness, (3) collection 

of borehole-geophysical data to assess 

aquifer properties and lithology (fig. 8), 

and (4) collection of hydraulic data for 

estimation of aquifer properties that 

control the movement of water and 

Cr(VI) in the plume and in downgradient 

areas.

In the western subarea, data are 

to be collected (1) to estimate the 

thickness of alluvial deposits; (2) to 

assess the hydraulic connection between 

weathered bedrock and overlying 

alluvial deposits in the aquifer; and 

(3) to evaluate the effect of the Lockhart 

Fault, which crosses the western 

subarea, on groundwater movement. 

In the northern subarea, data are to 

be collected on aquifer thickness and 

hydraulic properties. Spatial 

variation in aquifer properties 

to the north are to be related 

to differences in the sources 

and depositional environments 

of alluvium identified on the 

basis of XRF data (task 2) and 

chemical and isotopic data 

(task 3). In the eastern subarea, 

borehole geophysical data are to 

be collected to assess changes in 

the hydraulic properties of the 

aquifer with depth. Additionally, 

test drilling and monitoring-well 

installation are to be done by 

PG&E consultants in upgradient 

areas to facilitate flow-path 

studies (fig. 9). The flow-path 

studies are designed to provide 

data on Cr(VI) concentrations, aquifer properties, rates 

of groundwater movement, 

and geochemical reactions in 

the aquifer upgradient of the 

compressor station. Data collection in each subarea supplements 

data collected by PG&E during 

recent years.

As part of task 4, a conceptual 

geologic framework is to be developed 

to explain the origin of alluvial and 

lacustrine deposits in the Hinkley Valley. 

The conceptual geologic framework is 

to be based on interpretations made from 

examinations of core material (task 2) 

that are supported by water chemistry, 

environmental-tracer (task 3), geophysical, 

and hydraulic-property data (collected as 

part of this task). The conceptual geologic 

framework is intended to explain some 

of the differences in geohydrologic units, 

hydrology, and geochemistry among the 

western, northern, and eastern subareas. 

The conceptual geologic framework 

can provide information to improve the 

existing conceptual hydrologic model and 

simulations of groundwater flow in the 

study area (task 5).
8 

Figure 10. Model extent for existing Pacific Gas and Electric (PG&E) groundwater-flow 

model and for updated groundwater-flow model, Hinkley, California.

and infiltration of streamflow from the 

Mojave River, are to be derived from 

the existing PG&E or USGS models 

and data from other recent studies. Areal 

and local recharge data are to be derived 

from basin-scale climate and hydrologic 

data (http://ca.water.usgs.gov/projects/

reg_hydro/projects/dataset.html). Model 

calibration is to include comparison of 

field-measured and model-simulated 

hydrographs and water-level contours. 

A comparison of water-budget estimates 

from the updated and existing models is 

planned.

The updated model is to 

be constrained by chemical and 

environmental-tracer data (task 3) by 

using a technique known as “particle 

tracking.” The simulated movement 

of model particles can be used 

(1) to evaluate the “footprint” of water 

recharged during the period of Cr(VI) 

releases and (2) to estimate the extent of 

the PG&E Cr(VI) contaminant plume 

through time. The model simulation 

results can provide a better understanding 

of groundwater flow than can be 

determined from analyses of field data 

alone. The updated model is not intended 

to simulate chemical processes affecting 

the movement of Cr(VI).

Task 5: Evaluation of Groundwater 

Movement

The purpose of this task is to 

evaluate how changes in hydrologic 

conditions over time have influenced the 

movement of water and Cr(VI) through 

the aquifer underlying Hinkley Valley.

It is not possible to go back in time 

and measure the movement of Cr(VI) 

from the PG&E compressor station 

following the releases. It is possible, 

however, to assemble available geologic 

and hydrologic data in a computer 

simulation, or model, of groundwater 

flow in Hinkley Valley and to use that 

model to estimate what could have 

happened after the Cr(VI) releases. 

Two models of groundwater flow 

have been developed for the study area. 

Both models use the USGS computer 

code MODFLOW. The PG&E model 

simulates groundwater flow in Hinkley 

Valley near and downgradient from the 

compressor station (fig. 10; ARCADIS/

CH2M-Hill, 2011) from the period 

1990 to 2010 and is used to evaluate 

various plume-management alternatives. 

The USGS model simulates regional 

groundwater flow in the larger Mojave 

River basin (Stamos and others, 2001) 

and is used to evaluate regional watermanagement alternatives. The USGS 

model has limited hydrogeologic detail 

in the Hinkley area, but simulates 

groundwater flow from 1931 to 1999, 

which includes the period of Cr(VI) 

releases. Although each model is suitable 

for its intended use, neither model is 

suitable, without modification, for the 

purposes of this background study.

As part of this task, the PG&E flow 

model is to be updated and expanded 

to include a larger area so that model 

boundaries coincide with hydrologic 

boundaries (fig. 10). The updated model 

is to simulate groundwater flow from 

1931 to 2010, which includes the period 

of the releases of Cr(VI) from the 

compressor station. 

The model updates are to be done by 

PG&E consultants in collaboration with 

the USGS and include additional data on 

the distribution and layering of hydraulic 

properties that conform to the conceptual 

geologic framework developed as part of 

task 4. Model inputs, including pumping 

58

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Existing groundwater-flow 

model boundary

Updated groundwater-flow 

model boundary

Fault

EXPLANATION

0 2 4 MILES

0 2 4 KILOMETERS

PG&E compressor station

117°15' 117°10' 117°05'

35°

05'

35°

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55'

34°

50'

San Francisco Study

area

Los Angeles

San Diego

CALIFORNIA

Pacific Ocean
 9

Figure 12. U.S. Geological Survey mobile laboratory and 

equipment to analyze chromium (VI).

Figure 11. Chromium-containing rocks and soils in California: A, chromium-containing rocks, and, 

B, chromium-containing soils (modified from Izbicki and others, 2015).

of the study area affected by discharges 

from the PG&E compressor station. 

The estimated Cr(VI) background 

concentrations could differ among 

the western, northern, and eastern 

subareas as a result of local differences 

in geology, hydrology, land use, and 

other factors. As a consequence, there 

could be different values used for plume 

management and clean-up goals in 

different parts of the study area.

The exact nature and statistical rigor 

associated with estimates of background 

Cr(VI) concentrations have not yet been 

determined. For example, is it necessary 

to precisely estimate background Cr(VI) 

concentrations in areas 

that have distinctive 

geology or hydrology 

and where results of 

task 6 show Cr(VI) is 

natural and not related 

to releases from the 

compressor station? 

How are background 

Cr(VI) concentrations 

to be estimated in areas 

where contamination is 

present at concentrations 

greater than what would 

be natural, but existing 

wells representative of 

pre-release conditions do 

not exist? These issues 

are to be discussed in the 

second report from the 

study, to be prepared in 

2016.

from domestic wells in 

the study area to provide 

information on Cr(VI) 

concentrations in areas not 

sampled by monitoring 

wells. These data are 

to be analyzed onsite 

in a mobile laboratory 

(fig. 12). Data from task 6 

are to be used to supplement data collected as 

part of task 3 and, if 

appropriate, to guide 

selection of additional 

wells for analyses as part 

of task 3.

The USGS plans to assemble data 

from the previous five tasks, ranking 

relevant data to simplify complex 

data sets and produce understandable 

graphics to facilitate input and discussion 

from TWG members during data 

interpretation. Final interpretation of the 

data is to be process oriented, and data 

interpretation is the responsibility of the 

USGS.

Task 7: Estimation of Background 

Chromium (VI) Concentrations 

The purpose of this task is to 

estimate background Cr(VI) in the parts 

Hydrogeologic

 province boundary

Soil chromium

(depth = 0–5 centimeters),

in milligrams per kilogram

56–2,850

46–55

40–45

36–39

31–35

27–30

22–26

17–21

11–16

<1–10

EXPLANATION

116°

118°

124° 122° 120° 42°

40°

38°

36°

34°

San Francisco

Los Angeles

Hinkley Hinkley

B

EXPLANATION

Chromium-

 containing

 ultramafic 

 rock

Hydrogeologic 

 province

 boundary

0 50 100 MILES

0 100 KILOMETERS 50

116°

118°

124° 122° 120° 42°

40°

38°

36°

34°

PACIFIC OCEAN

San Francisco

Los Angeles

A

Hydrogeologic

province

1 Central

 Valley

2 Desert

3 Basin and Range

4 Klamath Mountains

5 Modoc Plateau and 

 Cascades

6 Northern Coast Ranges

7 Southern Coast Ranges

8 San Diego

9 Sierra Nevada

10 Transverse and selected 

 Peninsular Ranges

1

2

3

4 5

6

7

8

9

10

PACIFIC OCEAN

Task 6: Evaluation of Natural and 

Man-Made Chromium (VI)

The purpose of this task is to 

identify areas in the aquifer containing 

man-made Cr(VI) from releases at the 

PG&E compressor station and areas 

that contain Cr(VI) from other sources. 

Although rocks and soils in the study 

area have comparative low chromium 

concentrations (fig. 11), some natural 

chromium may be present.

Data and results from the previous 

five tasks can be interpreted to answer 

the question “What Cr(VI) is associated 

with PG&E releases from the compressor 

station, and what is not?” This question 

was not addressed as part of the 2007 

background study. 

Mineralogical (task 2), chemical, 

environmental-tracer, and groundwater 

“age” information (task 3); increased 

understanding of local geology and 

hydrology (task 4); and results from the 

groundwater-flow model (task 5) are 

expected to provide information on areas 

that likely were affected by releases 

from the PG&E compressor station and 

to identify areas that likely were not 

affected by releases, but could contain 

natural Cr(VI). As part of this task, 

additional Cr(VI) data are to be collected 
10 

Figure 13. Experimental microcosms used to evaluate the 

potential reoxidation of trace elements over time.

Task 8: Fate of Chromium During 

and After In Situ Reduction

To create reduced conditions, 

where dissolved oxygen is depleted, 

PG&E is injecting ethanol into wells 

in the Cr(VI) contamination plume in 

an area known as the “in situ reactive 

zone” (IRZ). Reduced conditions in the 

IRZ convert Cr(VI) to Cr(III), which is 

only sparingly soluble and is removed 

from groundwater. The purpose of this 

task is to determine whether chromium 

is permanently removed by the IRZ or 

if changes in hydrology (such as large 

amounts of groundwater recharge from 

the Mojave River) that result in oxic 

conditions (where dissolved oxygen is 

present) in the IRZ can convert Cr(III) 

back to Cr(VI) and allow it to reenter 

groundwater.

There are several factors that 

could control the mobility of chromium 

sorbed to aquifer materials in the IRZ. 

Two factors to be investigated include 

(1) changes in how chromium is sorbed 

and mineralized over time and (2) the 

rate of re-oxidation of sorbed Cr(III) to 

Cr(VI). 

First, chromium sorbed to aquifer 

materials can be weakly sorbed (and 

potentially more reactive), more strongly 

sorbed (and potentially less reactive), or 

incorporated into the crystalline structure 

of iron and manganese oxide coatings 

on the mineral grains that compose the 

aquifer (least reactive). The nature of 

chromium sorption could change with 

time as weakly sorbed chromium is 

incorporated into less reactive materials. 

The less abundant, natural stable 

isotopes of chromium (chromium-50 and 

chromium-54) are to be used to track 

the sorption of chromium on aquifer 

materials in experimental microcosms.

Second, if geochemical conditions 

change, and groundwater once again 

becomes oxic, Cr(III) could oxidize 

to Cr(VI) and reenter groundwater. 

If Cr(III) is oxidized, is the rate of 

oxidation environmentally meaningful 

over decadal time scales? Specialized 

techniques, such as X-ray absorption 

near-edge structure (XANES) 

spectroscopy, can be used to measure 

the presence of small concentrations 

of Cr(VI) oxidized in the experimental 

microcosms.

These issues are to be investigated 

in a series of laboratory microcosm 

studies of aquifer materials under 

controlled conditions for a period 

of 2 years (fig. 13). Numerous 

microcosms are to be set-up at the 

beginning of the experiment and then 

harvested periodically to provide timeseries data. The experimental design 

considers a range of factors, including 

(1) the number of microcosms and 

harvest times needed to provide timeseries data on the composition of the 

water/solid mixture in each microcosm; 

(2) the amount of ethanol to be added 

to each microcosm to create conditions 

similar to those found in the IRZ 

without under-dosing or over-dosing; 

(3) the need for buffers to control 

pH; (4) the use of suitable materials 

to allow exchange with atmospheric 

oxygen (for aerobic microcosms), while 

limiting water loss; and (5) the mass of 

chromium isotopes to be used as tracers.

These experiments cannot perfectly 

replicate field conditions and, because 

of project time constraints, can only be 

run for about 2 years. Results, however, 

are expected to provide a basis for 

management decisions that need to 

consider the longer time frames present 

in the field setting.

Project Timeline and Reports

The study is to be completed in 

December 2019. Important project 

milestones are shown in figure 14. 

Several USGS-authored reports are 

planned: (1) a study overview (this 

report); (2) a mid-term report that refines 

the project scope for remaining work, 

especially tasks 6 and 7, on the basis 

of preliminary results; and (3) a final 

report documenting study results. The 

final report may take the form of a large 

report that has several chapters corresponding to study tasks or of several 

smaller reports. In addition, a fact-sheet 

style report is planned to summarize the 

study results.

Data from the study are to be made 

publically available and, after review 

by the USGS and TWG, posted on the 

USGS online data base, NWIS-web, at 

http://waterdata.usgs.gov/nwis.
 11

Literature Cited

ARCADIS/2M-Hill, 2011, Development 

of a groundwater flow and solute transport model: Feasibility Study, Addendum 3, Appendix G, variously paged, 

http://geotracker.waterboards.ca.gov/

esi/uploads/geo_report/2112934887/

SL0607111288.PDF.

CH2M-Hill, 2007, Groundwater background study report: Hinkley compressor station, Hinkley, Calif., Oakland, 

Calif., variously paged, http://www.

swrcb.ca.gov/lahontan/water_issues/

projects/pge/docs/2007_background_

study_report.pdf.

Izbicki, J.A., Wright, M.T., Seymour 

W.A., McCleskey, R.B., Fram, M.S., 

Belitz, K., and Esser, B.K., 2015, 

Cr(VI) occurrence and geochemistry 

in water from public-supply wells in 

California. Applied Geochemistry, Vol. 

63, pp. 203-217.

Lahontan Regional Water Quality Control 

Board, 2008, Staff report: Background 

chromium study Pacific Gas and 

Electric Company Compressor Station. California Regional Water Quality 

Control Board, Lahontan Region, South 

Lake Tahoe, Calif., variously paged, 

http://www.swrcb.ca.gov/rwqcb6/

water_issues/projects/pge/docs/backgroundstudy_pge8_08.pdf.

Lahontan Regional Water Quality Control 

Board, 2012, Peer review of Pacific Gas 

& Electric Company’s 2007 groundwater chromium background study report, 

http://www.swrcb.ca.gov/lahontan/

water_issues/projects/pge/docs/encl_1.

pdf.

Lahontan Regional Water Quality Control 

Board, 2013, Draft Environmental 

Impact Report: Comprehensive groundwater cleanup strategy for historical 

chromium discharges from PG&E’s 

Hinkley Compressor Station, San 

Bernardino County. California Regional 

Water Quality Control Board, Lahontan 

Region, South Lake Tahoe, CA, variously paged, http://www.waterboards.

ca.gov/rwqcb6/water_issues/projects/

pge/feir.shtml.

Stamos, C.L., Martin, P., Nishikawa, 

T., and Cox, B., 2001, Simulation of 

groundwater flow in the Mojave River 

basin, California. U.S. Geological 

Survey Water-Resources Investigations Report 01–4002, 129 p., http://

pubs.usgs.gov/wri/wri014002/pdf/

wrir014002.pdf.

sac15-0586_fig13 2015

20162017

2018

2019

2020

Contract signed, January 2015First sample collection phase, task 3Begin field data collection, task 4Begin field data collection, task 2

Installation of wells for flowpath studies, task 4

Onsite mobile lab chromium VI analyses, task 6

Second sample collection phase, task 3

Third sample collection phase, task 3End field data collection, task 2

Draft mid-term reportEnd of field data collection, task 4

Draft of final report

Project end date, December 2019

Final report preparation

Task 6: Data interpretation

Task 7: Scope to be determined

 by mid-term report

Task 8:

Experimental design

Model development

Microcosm studies

Task 5:

Planning Particle-tracking analysis

Figure 14. Simplified project timeline. (A more detailed project timeline was provided to the Lahontan Regional Water Quality Control Board 

for contractual purposes. Task 1 was completed as part of work for design and development the study. Work done as part of task 1 is planned 

to be updated prior to completion of the study.)
12 

Other Resources

Technical documents and 

regulatory orders related to the Cr(VI) 

contamination at Hinkley are available 

from the Lahontan RWQCB website, 

http://www.waterboards.ca.gov/

lahontan/water_issues/projects/pge/

index.shtml.

Table 1. Tasks and questions addressed by the U.S. Geological Survey background study, January 2015 to December 2019, Hinkley, California.

Task Purpose

Task 1: Evaluation of existing data. Identify areas near the mapped hexavalent chromium, Cr(VI), 

plume having water-quality of concern to the study.

Task 2: Analyses of rock and 

alluvium.

Determine if there are natural geologic sources of chromium in 

the area and if these sources are contributing Cr(VI) to groundwater.

Task 3: Analyses of chemical and 

environmental tracers in 

water from wells.

Determine the chemical and isotopic (including other 

environmental tracers) composition of water from selected 

wells throughout the study area with respect to (1) the sources 

and chemical processes controlling Cr(VI) occurrence and (2) 

the source, movement, and age of the groundwater relative to 

the timing of Cr(VI) releases from the Pacific Gas and Electric 

(PG&E) compressor station.

Task 4: Evaluation of local conditions. Determine how differences in local geohydrology in the western, 

northern (including Water Valley), and eastern (including the 

plume and upgradient area) subareas influence natural Cr(VI) in 

groundwater and the movement of anthropogenic (man-made) 

Cr(VI) from the compressor station.

Task 5: Evaluation of groundwater 

movement.

Evaluate how changing hydrologic conditions in the study area 

over time influence the movement of water and Cr(VI) through 

aquifers underlying Hinkley Valley.

Task 6: Evaluation of the presence of 

natural and anthropogenic 

Cr(VI).

Identify areas in the aquifer containing man-made Cr(VI) from 

releases at the PG&E compressor station and areas that contain 

Cr(VI) from other sources.

Task 7: Estimation of background 

Cr(VI) concentrations.

Estimate background Cr(VI) in parts of the study area affected by 

discharges from the PG&E compressor station.

Task 8: Fate of chromium during and 

after in situ reduction.

Determine if chromium in the in situ reactive zone is permanently 

removed from solution.

Report preparation and project timeline. Four reports are identified in the proposal, and completion of the 

project is scheduled for December 2019.

Other documents intended for the 

public and interested stakeholders are 

available from the IRP Manager website, http://www.hinkleygroundwater.

com/.

Acknowledgments

This work is being done under 

an agreement with the State Water 

Resources Control Board with funding 

provided by Pacific Gas and Electric. 

Funding for the study is held in trust by 

the State in an escrow account to ensure 

unbiased and independent completion 

of the study.