Hexavalent chromium, Cr(VI), was released between 1952 and 1964 from the Pacific Gas and Electric Company (PG&E) Hinkley compressor station, in the Mojave Desert about 80 miles northeast of Los Angeles, California. Geologic, geochemical, and hydrologic data from more than 100 wells collected between March 2015 and November 2017 were interpreted using a summative-scale analysis to define the extent of anthropogenic (human-made) Cr(VI) in groundwater. The summative scale consisted of eight questions requiring binary (yes or no) answers for each sampled well. The questions were intended to (1) provide a transparent framework for data interpretation in which all stakeholders participated; (2) provide unbiased interpretation of data traceable to numerical measurements; (3) provide a framework that enabled geologic, geochemical, and hydrologic data to be considered collectively; and (4) consolidate different types of data into a simple, easy-to-understand interpretation. When data from each well are scored using questions and metrics within the summative scale, all stakeholders would score each well the same way and would draw the same summative-scale Cr(VI) plume extent.

The areal extent of the summative-scale Cr(VI) plume was 5.5 square miles (mi²); this is larger than the 2.2-mi² extent of the October–December 2015 (Q4 2015) regulatory Cr(VI) plume but smaller than the 8.3-mi2 maximum mapped extent of Cr(VI) greater than the interim regulatory Cr(VI) background value of 3.1 micrograms per liter (μg/L). The summative-scale Cr(VI) plume is within the area covered by the PG&E monitoring well network and lies within “Mojave-type” deposits composed of low-chromium stream and near-shore lake deposits sourced from the Mojave River. The summative-scale Cr(VI) plume included all shallow wells within the footprint of the Q4 2015 regulatory Cr(VI) plume, but summative-scale scores indicate that anthropogenic Cr(VI) was not present in several wells within the footprint of the regulatory Cr(VI) plume that were screened within the deep zone of the upper aquifer. The summative-scale Cr(VI) plume extent was consistent with mineralogic and geochemical data collected as part of this study that were not used within the summative-scale analysis.

Data from wells outside the summative-scale Cr(VI) plume collected for regulatory purposes from April 2017 through March 2018 were used to estimate Cr(VI) background concentrations as the upper 95-percent tolerance limit (UTL₉₅) in different parts of Hinkley and Water Valleys. The UTL₉₅ values were calculated using the computer program ProUCL 5.1 and are suitable for use by regulatory agencies in support of (1) updating the regulatory Cr(VI) plume extent and management of Cr(VI) near the plume margins, (2) establishing cleanup goals for Cr(VI) within the updated regulatory Cr(VI) plume, and (3) identifying unusual Cr(VI) concentrations outside the regulatory Cr(VI) plume. The nonparametric UTL₉₅ values for wells screened in Mojave-type deposits in the eastern, western, and northern subareas of Hinkley Valley were 3.7, 3.9, and 4.0 μg/L, respectively. The normal UTL₉₅ values for wells screened in undifferentiated, unconsolidated deposits in the eastern and western subareas and the northern subarea upgradient from the Mount General fault were 2.8, 3.8, and 4.8 μg/L, respectively. An overall normal UTL₉₅ value of 3.8 μg/L was calculated for undifferentiated, unconsolidated deposits in these areas. This value is similar to the overall nonparametric UTL₉₅ value of 3.9 μg/L calculated for Mojave-type deposits and similar to the maximum Cr(VI) concentration of older groundwater in contact with Mojave-type deposits of 3.6 μg/L. The provenance of most PG&E monitoring wells is not precisely known, and the UTL₉₅ values for wells screened in undifferentiated, unconsolidated deposits in the different subareas may be more widely applicable for regulatory purposes than the UTL₉₅ values for Mojave-type deposits.

The UTL₉₅ value of 2.8 μg/L for wells screened in undifferentiated, unconsolidated deposits in the eastern subarea is important for plume management because most of the summative-scale Cr(VI) plume is within the eastern subarea. A UTL₉₅ value of 5.8 μg/L was calculated for older (pre-1952) groundwater associated with mudflat/playa deposits in the eastern subarea near Mount General. A UTL₉₅ value of 2.3 μg/L was calculated for Mojave-type deposits within the Cr(VI) plume downgradient from the Hinkley compressor station after regulatory updates. This lower value is consistent with neutral to slightly alkaline, younger (post-1952) groundwater within coarse-textured, low-chromium Mojave-type deposits in this area and may be a suitable metric for Cr(VI) cleanup goals. The UTL₉₅ value of 4.8 μg/L for wells screened in undifferentiated, unconsolidated deposits in the northern subarea upgradient from the Mount General fault provides for possible increases in Cr(VI) concentrations if water levels continue to decline. Downgradient from the Q4 2015 regulatory Cr(VI) plume and the summative-scale Cr(VI) plume, UTL₉₅ values of 9.0 and 6.4 μg/L were calculated for wells screened in undifferentiated, unconsolidated deposits in the northern subarea downgradient from the Mount General fault and for Water Valley, respectively, consistent with different geologic and geochemical conditions in these areas.

The UTL₉₅ values calculated as part of this study provide scientifically defensible estimates of background Cr(VI) concentrations that differ with local geologic, geochemical, and hydrologic conditions in Hinkley and Water Valleys. The regulatory Cr(VI) plume extent can be updated on the basis of these values. The summative-scale Cr(VI) plume extent may contain wells having anthropogenic Cr(VI) concentrations less than the UTL₉₅ values for their respective subareas that may not require regulatory attention, and an updated regulatory Cr(VI) plume extent may be less than the summative-scale Cr(VI) plume extent. The UTL₉₅ values are not background Cr(VI) concentrations for regulatory purposes, and the authority to establish regulatory values resides solely with the Lahontan Regional Water Quality Control Board.