AFFECTS OF GEOLOGIC CONTROLS ON GROUND-WATER FLOW – CHARACTERIZATION USING RESULTS FROM AGE-DATING METHODS ALONG TOPOGRAPHIC TRANSECTS IN REGOLITH-FRACTURED BEDROCK GROUND-WATER SYSTEMS IN NORTH CAROLINA

Age-dating methods, total tritium (3H), chlorofluorocarbons (CFC's), and tritium/helium (3H/3He), were applied at four ground-water research sites in the Piedmont and Blue Ridge Physiographic Provinces in North Carolina to characterize ground-water flow and geologic controls along topographic transects in the regolith-fractured bedrock aquifer system. At all four sites, ground water in the shallow regolith and deeper bedrock is older in discharge areas (near streams) compared to ground water in recharge areas at various points along transects from the topographic high to mid-slope. Older ground water in discharge areas indicates longer flow paths. Shallow ground water in the regolith appears to be a mixture of modern water and older water recharged 10 to 15 years ago. The age of deeper ground water in the bedrock ranges from about 25 to nearly 60 years. The bedrock water has low tritium values, indicating the lack of recharge since the post-1960s hydrogen bomb testing period. Additionally, as expected from the conceptual ground-water-flow model, ground water at depth in the bedrock is older than the shallower ground water in the overlying regolith, likely a result of reduced infiltration of recharge through secondary and commonly discontinuous fracture networks in the bedrock. Ground water in recharge areas is younger at greater depths in the bedrock, whereas ground water in discharge areas is older, even in the shallow regolith.

Geologic controls on ground-water flow were evaluated at one site in the Piedmont. Results from a topographic well transect (high to low elevations) along the dip-slope of foliation in the same lithologic unit indicate the presence of younger ground water throughout except in the the discharge area. Conversely, results from wells drilled along the cut-slope (crossing foliation dip and several lithologic units) indicate the presence of older ground water in the mid-slope and discharge areas.