PETROLOGIC AND HYDROGEOLOGIC CHARACTERISTICS OF A FELSIC METAVOLCANIC TUFF HYDROGEOLOGIC UNIT IN THE CAROLINA TERRANE: PRELIMINARY FINDINGS FROM THE NC ZOO GROUNDWATER RESEARCH STATION, ASHEBORO, RANDOLPH COUNTY, NC
The NC Zoo groundwater research station is located at the southernmost part of the North Carolina Zoo in Asheboro, North Carolina. The station includes 3 monitoring well clusters installed in a nearly linear well transect parallel to an assumed flow path, from a topographic high (recharge area) to low (discharge) setting. The station is geologically located within the Carolina terrane of the Carolina Zone and is underlain primarily by felsic metavolcanic tuffs with massive fabrics to steeply dipping foliations. One core contained a mafic lithology.
Whole-Rock geochemistry analyses performed on eight bedrock core samples indicate SiO2 ranging from 69.8 to 77.6 percent, with one exception of 40.3 percent. Analyses from felsic lithologies generally plot within the rhyodacite and rhyolite fields of a TAS diagram. Rock core and petrographic thin section observations indicate that the felsic rocks are likely lapilli tuffs with fiamme-shaped pumice clasts common.
Groundwater level ranged from two to 36 feet below the surface, with up to 10 feet of drop in the regolith aquifer and up to 7 feet of drop in bedrock aquifer from April to September 2007 due to the unusual drought. Aquifer testing results suggest an average horizontal hydraulic conductivity of 0.39 ft/day and an average transmissivity of 29.25 ft2 /day at the low-cluster area, and 0.78 ft/day and 139.50 ft2 /day respectively at the mid-cluster area.
Groundwater sampling results indicate a bicarbonate type of ground water throughout the station. However, cation types varied from Na+ - K+ dominant type, Na+ - K+ - Ca+2 mixed type, Ca+2 dominant-mixed type, to Ca+2 dominant type. Groundwater pH ranged from 5.43 to 7.97, and was lower in the regolith aquifer than in the bedrock aquifer. Total dissolved solids, specific conductance, calcium, magnesium, and potassium showed the same trend. The ground water at the low-cluster area had the highest pH and the highest mineralization.
Manganese was detected above the North Carolina state groundwater quality standard of 50 parts per billion (ppb) in most monitoring wells. Zinc was detected in some bedrock wells at concentrations exceeding the state standard of 1050 ppb. Total iron was detected at concentrations exceeding the state standard of 300 ppb in shallow monitoring wells. In addition, heavy metals were detected in some samples.