CHROMIUM ALONG GROUNDWATER FLOW PATHS: THE CARRIZO SAND AND FLORIDAN AQUIFERS, TEXAS AND FLORIDA, USA

Groundwater samples were collected along the flow path in the Carrizo Sand aquifer in south Texas (October 2002 and June 2003) and the Upper Floridan (i.e., carbonate) aquifer, west-central Florida (June 2003). Samples were collected for measurement of both total dissolved Cr (filtered through 0.45 micron in-line filter-capsules, acidified to pH 2 with ultra-pure nitric acid) and Cr(III). The Cr(III) samples were collected following the Fe(III) hydroxide coprecipitation technique. As of 15 July 2003, we are still measuring Cr(III) in groundwaters from both aquifers. However, total dissolved Cr concentrations (i.e., October 2002 samples) were measured for the Carrizo aquifer and the values range from 1.8 nmol/kg to 7.3 nmol/kg. In addition, Cr concentrations exhibit systematic changes along the general groundwater flow path. Initially, Cr values decrease along the flow path from 4.5 nmol/kg in the recharge zone to 1.8 nmol/kg at a distance of 9.4 km down-flow. With further flow down-gradient in the Carrizo, Cr concentrations increase attaining a value of 7.3 nmol/kg at a distance of 66 km from the recharge zone. Groundwater samples were not collected further down-gradient in this study because of salinity increases. Total dissolved U and Re concentrations, which are relatively high in groundwaters from the recharge zone (i.e., 670 and 180 pmol/kg, respectively), indicate the presence of a redox boundary in the Carrizo aquifer at roughly 10 km down-gradient from the recharge area. For example, with flow beyond this "10 km redox-boundary", U and Re concentrations fall precipitously to values below 10 pmol/kg. The Cr data also appear to reflect this redox boundary as Cr concentrations decrease with flow towards the boundary, and subsequently increase again down-gradient of the "10 km redox-boundary".