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

Abstract

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) bythe Soquel Creek Water District of elevated hexavalent chromium concentrations ranging from 6 to 36 microg L(-1), approaching the state's maximum concentration limit of 50 microg 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 fortotal chromium using inductively couple plasma mass spectrometry. Nine of the reportedly 10 contaminated sites had total chromium concentrations ranging from 5 to 39 microg L(-1), while one from the control site was below the limit of detection (0.01 microg 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.

Genesis of hexavalent chromium from natural sources in soil and groundwater

“…Mexico, having Cr(VI) concentrations of 12 g liter Ϫ1 attributed to ultramafic rocks of the Sierra de Guanajuato (4); (v) groundwater from mafic alluvial deposits in the Mojave Desert having Cr(VI) concentrations of 60 g liter Ϫ1 (6); (vi) groundwater from the Aromas Red Sands aquifer in Santa Cruz County, CA, residing within an ophiolite complex having dissolved Cr(VI) concentrations ranging from 4 to 33 g liter Ϫ1 (12); and (vii) phosphorus-amended soils derived from utramafic rocks in New Caledonia having pore-water concentrations of 700 g liter Ϫ1 (5). The New Caledonia soils have the highest reported Cr(VI) concentration, owing to the presumptive displacement of Cr(VI) from mineral surfaces by phosphate, which has been added to the soil as a nutrient amendment (5).…”

Health risk analysis of the environmental burden

“…The concentrations in soil range between 1 and 3×10 3 mg kg -1 , in sea water 5-800 µg dm -3 , and in rivers and lakes from 26 µg dm -3 to 5.2 mg dm -3 . The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location, but in most cases, the Cr(III) is the dominating species (Kotas and Stasicka [4]), although in some areas the groundwater can contain up to 39 µg of total chromium, of which 30 µg is present as Cr(VI) (Gonzalez et al [3]). Chromium is mined as chromite (FeCr 2 O 4 ) ore.…”

Natural occurrence of hexavalent chromium in a sedimentary aquifer in Urânia, State of São Paulo, Brazil

“…Considering that the residence time of water in aquifers may reach thousands of years and pH values are quite elevated in the deep aquifer, the possibility cannot be neglected that this reaction may occur. However, manganese oxides, generally present as grain coatings, crack deposits or finely disseminated grains, are known to be the main agent responsible for the oxidation of Cr 3+ to Cr 6+ in aquifers (Apte et al 2006, Sedlak and Chan 1997, Richard and Bourg 1991, Eary and Rai 1987, Fendorf and Zazoski 1992, Fendorf 1995, Bartlett and James 1979 and Mn 2+ ) has also been observed by Gonzalez et al (2005) in the Aroma Red Sands Aquifer in California.…”

“…Nevertheless, mafic and ultramafic rocks generally show higher concentrations of chromium than other rock types. centrations of naturally occurring hexavalent chromium have been reported in groundwater in the Aroma Red Sands Aquifer, California (Gonzalez et al 2005), at Léon Valley, Mexico (Robles-Camacho and Armienta 2000), in the western Mojave Desert, California (Ball and Izbicki 2004) and Arizona (Robertson 1995).…”