ARSENIC CONCENTRATIONS AND SPECIATION 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 October 2002 and June 2003. Groundwater samples were also collected along a flow path in the Upper Floridan (i.e., carbonate) aquifer in June 2003. The Carrizo aquifer samples collected in October 2002 were filtered in the field through 0.45 micron in-line filter-capsules, acidified to pH 2 with ultra-pure nitric acid, and analyzed by ICP-MS. Groundwater samples collected from both aquifers in June 2003 were filtered as above, and subsequently frozen (Carrizo samples could not be frozen, and thus As speciation could not be determined for these samples), and then shipped back to the laboratory for analysis by high-performance liquid chromatography/ICP-MS. The HPLC is used to separate arsenite and arsenate species, whereas detection of both species is accomplished by ICP-MS (Perkin Elmer Elan 6200 equipped with a reaction cell). Arsenic concentrations (i.e., October 2002) range from 2.5 to 7.9 nmol/kg in groundwaters from the Carrizo Sand aquifer, and from 1.6 to 15 nmol/kg in the Floridan aquifer (June 2003). Arsenic concentrations initially decrease by a factor of 2 along the flow path in the Carrizo aquifer (7.9 nmol/kg in recharge zone to 4 nmol/kg 25 km down-flow). Arsenic concentrations subsequently rise with continued flow down-gradient, reaching 6.6 nmol/kg approximately 66 km from the recharge zone. Floridan groundwaters exhibit a similar pattern in total As concentrations along the flow path, with an initial drop in As concentration from 6 nmol/kg in the recharge zone to 1.6 nmol/kg roughly 40 km down-flow. With further flow beyond 40 km, As concentrations rise and reach a maximum value of 15 nmol/kg approximately 138 km from the recharge zone. The Floridan groundwater samples indicate that arsenate dominates in this aquifer for all but one groundwater sample. This one sample, located 46 km down-flow from the recharge area has roughly equal concentrations of arsenate and arsenite (0.6 and 0.95 nmol/kg, respectively). With further flow down-gradient in the Floridan aquifer, arsenite decreases, whereas arsenate concentrations continue to rise.