These data were collected using field portable (handheld) X-ray fluorescence (pXRF) equipped with a 4-watt Ta/Au X-ray tube. Samples of surficial alluvium, rock, and archived core material from existing auger- or sonic-drilled monitoring wells in Hinkley Valley and the adjoining Water Valley, 140 kilometers (km) northeast of Los Angeles, California, were measured as part of an investigation of naturally-occurring and anthropogenic hexavalent chromium, Cr(VI), concentrations in local groundwater. Surficial alluvium samples were collected from small stream channels draining distinct geologic units, or from previously mapped river deposits, and generally consisted of silt, sand, and granules to small pebbles. Twigs and other detritus were removed prior to measurement. Rocks were collected from outcrops or from colluvium eroded from nearby outcrops and were broken to expose fresh surfaces whenever possible. Core material was measured within the screened interval of wells sampled for water-quality as part of the study, along with additional core material from other intervals of geologic or lithologic interest to the study. Some measurements of core material were made on materials from selected geologic settings including oxide-rich zones formed near lithologic and redox contacts, groundwater discharge deposits, and weathered bedrock. Measurements were made on 155 samples of alluvium and rock, and archived core material from 69 monitoring well sites, located over an approximately 200 square km area between March 2015 and May 2018.
Measurements on National Institute of Standards and Technology (NIST) and U.S. Geological Survey standard reference materials (available in a separate child page associated with https://doi.org/10.5066/P9CU0EH3) indicated the pXRF was sufficiently accurate for chromium and selected trace elements for the intended purpose of the dataset. Standard reference material indicated a need to adjust instrument beam times to optimize measurements of chromium. Measurements on a silica dioxide blank showed consistent clean (few to no measurable elements) data.