T22 Natural Arsenic in Groundwater: Science, Regulation, and Health Implications (Posters)

Monday, 5 November 2001
8:00 AM - 12:00 PM

GROUND WATER DISCHARGE FROM MID-TERTIARY RHYOLITIC ASH-RICH SEDIMENTS AS THE SOURCE OF ELEVATED ARSENIC IN SOUTH TEXAS SURFACE WATERS

Paper number: T22-2
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Lake Corpus Christi, the drinking water source for 500,000 people, exhibits seasonally mediated changes in arsenic concentration ranging from 2 to 15 ug/L. Arsenic also exhibits a depth-concentration gradient linked to seasonal thermal and redox stratification with attendant flux in iron and manganese.

The source of the arsenic in Lake Corpus Christi is not currently known. We used a Geographic Information System (GIS) to display, analyze and interpret existing data to evaluate potential As sources within the Nueces River catchment. Data include concentrations of As in ground and surface waters, stream sediment, soil and bedrock; the distribution of industrial and mining sources and potential additions from agricultural and rangeland activities. We evaluated National Uranium Resource Evaluation (NURE), Texas Water Development Board (TWDB) and National Water Information System (NWIS) datasets.

The Nueces experiences a steep precipitation gradient (48 cm/yr -west: 79 cm/yr -east) and an increasing component of baseflow discharge from west to east. The hydrology of the Nueces is strongly influenced by rare, short duration precipitation events followed by longer periods of baseflow recession and evapo-transpiration driven solute concentration. Arsenic concentrations in Lake Corpus Christi reflect this; increasing with high summer evaporation rates and decreasing after storm dilution.

GIS display of the data permit unambiguous interpretation of the dominant source of As within the Nueces watershed. A pronounced increase in ground water As concentrations is observed in all of the datasets and is clearly tied to bedrock geology. Arsenic concentrations dramatically increase in in association with upper Eocene and Oligo-Miocene coastal margin sediments containing a large component of rhyolitic volcanic sediments. These sediments also generated roll-front uranium deposits enriched in molybdenum and selenium along with arsenic. Arsenic concentrations measured from seeps near former open pit uranium mines exemplify the geological control of As, perhaps augmented by human agency.