Natural Occurrence of Hexavalent Chromium in the Aromas Red Sands Aquifer, California

• Published in: Environmental science & technology Vol. 39; no. 15; pp. 5505 - 5511
• Main Authors: Gonzalez, A. R, Ndung'u, K, Flegal, A. R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.08.2005
• Subjects: Applied sciences; Aquifers; California; Chromium; Chromium - analysis; Drinking water; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental Monitoring - methods; Exact sciences and technology; Fresh Water - chemistry; Geologic Sediments - chemistry; Groundwater; Groundwaters; Natural water pollution; Pollution; Pollution, environment geology; Scientific imaging; Sediments; Water Pollutants, Chemical - analysis; Water pollution; Water treatment and pollution; California; United States; North America; America; United States > US; Santa Cruz California; USA, California, Santa Cruz; USA, California, Santa Cruz Cty; Natural origin pollution; Drinking water; Geochemistry; Identification; Sediments; Heavy metal; Aquifers; Concentration measurement; Pollution source; Geogenic factor; Water pollution; Chromium VI; Concentration distribution; Ground water
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es048835n

To address increasing concerns of chromium contamination in the drinking water of Santa Cruz County, we designed a study to investigate the source(s) and spatial gradients of the chromium concentration and speciation in local aquifers. This study was catalyzed by a report (January 2001) by the Soquel Creek Water District of elevated hexavalent chromium concentrations ranging from 6 to 36 μg L-1, approaching the state's maximum concentration limit of 50 μg L-1, in the Aromas Red Sands aquifer. To test the accuracy of those preliminary measurements, we collected groundwater using trace metal clean techniques from 11 sites in Santa Cruz County, including 10 from the aquifer with reportedly elevated chromium concentrations and 1 from an adjacent aquifer, the Purisima, and analyzed them for total chromium using inductively couple plasma mass spectrometry. Nine of the reportedly 10 contaminated sites had total chromium concentrations ranging from 5 to 39 μg L-1, while one from the control site was below the limit of detection (0.01 μg L-1). We also measured the speciation of chromium at all sites using a solid supported membrane extraction coupled with graphite furnace atomic absorption spectrometry and determined that on average 84% of total chromium was Cr(VI). In addition to the groundwater analyses, a series of extractions were performed on sediment samples from both the Aromas Red Sands and Purisima aquifers. These tests were used to empirically characterize sediment trace metal (Cr, Fe, Mn) distributions in five phases providing information about the origin, availability, reactivity, and mobilization of these trace metals. Results from groundwater and sediment samples indicate that the chromium is naturally occurring in the Aromas Red Sands aquifer, possibly by Cr(III) mineral deposits being oxidized to Cr(VI) by manganese oxides in the aquifer.