Survey of chromium and selected element concentrations in rock, alluvium, and core material

Abstract

Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from the Pacific Gas and Electric Company (PG&E) compressor station in Hinkley, California, in the western Mojave Desert 80 miles northeast of Los Angeles, California. In 2015, the extent of anthropogenic Cr(VI) in groundwater in Hinkley and Water Valleys was uncertain, and some Cr(VI) in groundwater may be naturally occurring from rock and aquifer material. On the basis of more than 1,500 portable (handheld) X-ray fluorescence (pXRF) measurements on more than 250 samples of rock, surficial alluvium, and core material from selected wells in Hinkley and Water Valleys, chromium concentrations are commonly low compared to the average bulk continental abundance of 185 milligrams per kilogram (mg/kg). However, chromium concentrations are as high as 530 mg/kg in mafic hornblende diorite that crops out along the western margin of Hinkley Valley in Iron Mountain. Other chromium-containing rocks in the area are either (1) not consistently high in chromium, (2) have limited areal extent, or (3) in the case of basalt, are present only in Water Valley. Chromium concentrations in core material adjacent to the screened intervals of wells sampled for water chemistry and isotopic composition as part of the U.S. Geological Survey Cr(VI) background study ranged from less than the study reporting level (SRL) of 5 mg/kg to 410 mg/kg, with a median concentration of 23 mg/kg. Chromium concentrations in core material were lower in the eastern subarea and higher in the western and the northern subareas of Hinkley Valley and in Water Valley. The highest chromium concentration in core material was in weathered hornblende diorite bedrock. Chromium concentrations in core material adjacent to the screened interval of sampled wells were log-normally distributed below a threshold of 85 mg/kg, and 3 percent of chromium concentrations were greater than 85 mg/kg. Manganese can oxidize trivalent chromium, Cr(III), to Cr(VI). Similar to chromium, manganese concentrations in core material also were log-normally distributed below a threshold of 970 mg/kg, and 5 percent of manganese concentrations were greater than 970 mg/kg. Both chromium and manganese concentrations were higher in fine-textured core material and in visually abundant iron- and manganese-oxide coatings on the surfaces of mineral grains. High concentrations of chromium and manganese in core material commonly co-occurred. Fine-textured core material, chromium concentrations greater than 85 mg/kg, and manganese concentrations greater than 970 mg/kg in core material adjacent to the screened interval of sampled wells were selected for use as metrics (threshold values) within a summative-scale analysis (SSA) developed to identify natural and anthropogenic Cr(VI) in water from wells later within this professional paper (chapter G). Principal component analysis (PCA) of 18 elements within surficial alluvium, rock, and core material measured using pXRF shows distinct elemental assemblages associated with (1) older and more recent “Mojave-type” deposits, including alluvium and lake-margin (beach) deposits sourced from the Mojave River, (2) alluvium eroded from mafic rock, including hornblende diorite that crops out on Iron Mountain, (3) alluvium eroded from felsic volcanic and hydrothermal rock that crops out on Mount General along the eastern margin of Hinkley Valley, (4) playa/mudflat and other fine-textured deposits, and (5) material with visually abundant iron- and manganese-oxide coatings. Most wells sampled as part of this study were completed in Mojave-type deposits. Portable (handheld) X-ray fluorescence data measured on core material from those wells do not appear to be different or unusual compared to the magnitude and range of data from the larger Mojave River groundwater basin, and the core material has a low-chromium, felsic composition consistent with a Mojave River origin. In general, the elemental composition of core material from wells was not measurably altered by admixtures with local mafic, felsic volcanic, or hydrothermal source materials; although, where present, admixtures with basalt may contribute chromium to core material.