Results of Hexavalent Chromium Background Study in Hinkley, California

Context: 

Chromium, identified in the periodic table using the symbol “Cr,” is a metallic element. As a solid metal, chromium is hard, lustrous, and steel-gray. Chromium metal is durable, highly resistant to corrosion, and used in various industrial applications. The oxidized form of chromium, hexavalent chromium, can form various chemical compounds that can exist in different phases, including liquids and gases.

Hexavalent chromium is a toxic form of chromium commonly found in industrial processes such as electroplating, welding, and pigment production. In Hinkley Valley, hexavalent chromium was used to prevent corrosion in industrial machinery during 1952-64. Hexavalent chromium can also occur naturally in groundwater as a result of weathering of chromium-containing minerals, and the conversion of generally insoluble trivalent chromium within minerals to the more soluble form hexavalent chromium. Hexavalent chromium is soluble in alkaline (high pH water), oxic (contains dissolved oxygen) water.

The World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) have established maximum contaminant levels (MCLs) for hexavalent chromium in drinking water to protect public health. The EPA has set an MCL of 100 micrograms per liter (µg/L, equivalent to parts per billion, ppb) for total chromium, which includes both trivalent chromium, Cr(III), and hexavalent chromium, Cr(VI). The California Water Boards (the State Water Resources Control Board plus nine Regional Water Quality Control Boards) had established a more stringent MCL of 10 µg/L specifically for hexavalent chromium. This MCL was rescinded in 2017 but was reproposed for public comment by the California Water Boards in March 2022.

PG&E currently operates natural gas compressor stations along a pipeline that transports natural gas from Texas to California. A natural gas compressor station is a facility that compresses natural gas to a higher pressure for more efficient transport through pipelines. The Hinkley compressor station is located two miles southeast of the town of Hinkley and a dozen miles west of Barstow in the Mojave Desert of San Bernardino County, California.

Issue: 

To prevent corrosion in the cooling water towers, a common practice during the mid-20^th century was to add the hexavalent chromium chemical into water used to cool machinery used within the compressor station. From 1952 to 1964, the Hinkley compressor station discharged wastewater containing hexavalent chromium from the cooling towers to unlined ponds at the site. Some of this wastewater infiltrated to the groundwater, resulting in hexavalent chromium pollution. In 2012, the extent of hexavalent chromium released from the Hinkley compressor station was uncertain; however, hexavalent chromium was present at concentrations greater than the interim regulatory background concentration of 3.1 µg/L in an area at least eight miles long and two miles wide.

The Lahontan Regional Water Quality Control Board (RWQCB) oversees the Hinkley area. PG&E is under regulatory orders from the Lahontan RWQCB to stop plume expansion and clean up the chromium plume.  

Study:

As part of the regulatory clean-up requirements, PG&E has funded several studies through the years, including a 2007 study to establish the background concentrations of hexavalent chromium in Hinkley Valley. Background refers to the amount of a chemical that would be present in the environment if hexavalent chromium had not been released from the Hinkley compressor station. However, questions about limitations of the 2007 PG&E-funded study and disagreement about the background concentrations of hexavalent chromium led to uncertainty about the mapped extent of the hexavalent chromium regulatory plume. In addition, the relative proportions of hexavalent chromium in groundwater attributable to PG&E’s actions versus other sources could not be determined. Therefore, a more comprehensive study was needed to distinguish natural and anthropogenic (human made) hexavalent chromium and provide an updated estimate of hexavalent chromium background concentrations. In 2015, the USGS was approached by the Lahontan RWQCB to do this study, which was led by Dr. Izbicki. 

Scientists from the USGS CAWSC collected and analyzed a wide variety of geologic, hydrologic, and geochemical data. The data were interpreted in a transparent, unbiased process in which all interpretations were traceable to measured data. Eight “yes or no” questions were applied to water samples from 100 different wells within the Hinkley Valley, and results were assembled into easy to visualize maps that the Lahontan Water Board, Hinkley community, and PG&E could easily follow. The advantage of this approach (referred to as a “summative-scale approach”) was that stakeholders use the same questions and metrics for those questions to reach the same conclusion about the extent of hexavalent chromium released from the Hinkley compressor station. 

The questions covered the kinds of geologic material at well sites, chromium and manganese concentrations in those geologic materials, whether chromium concentrations in water from wells were trended (went up or down) with time, how chromium concentrations varied compared to the pH of the water and with other trace element concentrations in the water, whether water was recharged from the Mojave River, and if the age of the water was consistent with recharge during the period of hexavalent chromium releases from the Hinkley compressor station after 1952. Answers to these questions were used to draw an updated map showing the extent of human-introduced hexavalent chromium downgradient from the PG&E discharge. Background concentrations for hexavalent chromium were then calculated using data from wells outside the area impacted by the PG&E discharge. An overall background concentration of hexavalent chromium was estimated for Hinkley Valley, and background concentrations of hexavalent chromium were also estimated for different areas within Hinkley Valley and for Water Valley to the north.

Results:

The study concluded that the extent of the anthropogenic hexavalent chromium plume estimated from the study was larger than the previously established regulatory plume extent but less than the maximum extent of groundwater containing greater than the previous interim regulatory background value of 3.1 µg/L. 

The overall 3.8 microgram per liter value and the different background concentrations for different areas of the valley are scientifically defensible estimates of the background hexavalent chromium concentrations in groundwater within deposits sourced from the Mojave River within Hinkley Valley, said John Izbicki, PhD, the USGS Research Hydrologist who led the study. 

Although the Hinkley area geology is naturally low in chromium, the study found different background concentrations within the study area. For example, the northern part of Hinkley Valley had a background concentration of 4.8 µg/L, but the eastern part of the valley containing the Hinkley compressor station and most of the plume had a lower background concentration of 2.8 µg/L. 

PG&E injects ethanol into groundwater to reduce soluble hexavalent chromium to insoluble trivalent chromium and remove chromium from groundwater. The USGS was asked by the Lahontan RWQCB to determine if chromium is permanently removed from groundwater. Laboratory experiments showed that if dissolved oxygen re-enters groundwater, the reduced trivalent may re-oxidize to hexavalent chromium in groundwater. 

Uses:

The estimated background concentrations of hexavalent chromium can be used by the Lahontan RWQCB to update regulatory background concentrations and cleanup goals. The Lahontan RWQCB has the sole authority to establish regulatory background concentrations for hexavalent chromium.

The background concentrations in different areas of Hinkley Valley can be used to develop a more refined understanding of the extent of anthropogenic (human-caused) hexavalent chromium released from the Hinkley compressor station and help the Lahontan Regional Water Quality Control Board more closely track the movement of the hexavalent chromium plume in the future and establish cleanup goals, Izbicki said.