VULNERABILITY OF THE TERTIARY AQUIFER SYSTEM IN SOUTH CAROLINA: A CONTINUING SURVEY USING GROUND-WATER RADIOCARBON AGES

STONE, Peter A. (stonepa@columb32.dhec.state.sc.us); LOGAN, W. Robert; ADAMS, R. Todd; and DEVLIN, Robert J., Ground Water, SC Dept. of Health and Environmental Control, Columbia, SC 29201

Ground-water ages can index aquifer vulnerability to contamination in an efficient and relatively inexpensive manner. Highest vulnerability occurs where water is recharged, and vulnerability thus is indicated by young (or relatively young) ground-water ages. Isolation and high natural protection is instead suggested by very old ages. Middle values sometimes must be evaluated by trends. Examined here (in a broad regional-survey mode) is an interconnected system of sand aquifers (lying mainly updip and upflow) and a limestone aquifer (downdip, downflow). Upper zones of the limestone aquifer and the area of the middle and lower coastal plain are emphasized. Sampling of heavily pumped established wells allows testing of induced as well as natural recharging.

Computed ground-water ages range from "modern" to 30,000+ 14C years. Sand aquifers in the inner coastal plain that lie far updip of the limestone show young ages and high vulnerability. Close to the Orangeburg scarp substantial age and apparent low vulnerability can exist. Hydrologic isolation and low vulnerability persist over wide areas farther downflow on the middle and outer coastal plain, but in several major areas the limestone aquifer shows resumed recharging. These latter vulnerable areas lie distant from the updip outcrop area of the Tertiary aquifer system that is normally associated with recharging and vulnerability. River incisement and aquifer drainage seem to promote recharging close to the Santee River valley. Outlying downdip recharge areas also exist in Allendale and Beaufort counties.

Recharging into old ground water causes high vulnerability but does not necessarily result in a young age in the mixed ground water. Comparison with results at nearby or upflow sites is necessary to reveal the addition of the young component.