Chromium along groundwater flow paths: The Carrizo Sand and Upper Floridan aquifers, Texas and Florida, United States of America

Groundwater samples were collected along flow paths in the Carrizo Sand aquifer in Texas and the Upper Floridan aquifer in Florida for measurement of total dissolved Cr and Cr(III). In the Carrizo Sand aquifer, total dissolved Cr initially decreases, but increases further down-flow. Total dissolved U and Re concentrations indicate the presence of a redox boundary at 10 km down-gradient; the Cr concentrations also reflect this redox boundary. It is possible that Cr(III) and Cr(VI) are responsible for the total dissolved Cr patterns seen in the oxidizing conditions initially found in the aquifer, and the reducing conditions that dominate further down-flow. In the Upper Floridan aquifer, Cr(III) values are constant along flow, and Cr(VI) becomes significant further down the flow path. It is known that manganese oxides can be a source of oxygen for the oxidation of Cr(III) to Cr(VI), so manganese oxides may be driving Cr(III) oxidation.