PIEDMONT GROUNDWATER SYSTEM, PART 1—THE TRANSITION ZONE BETWEEN REGOLITH AND BEDROCK: EXISTENCE

The groundwater system in the Piedmont Province is comprised of two interconnected layers: 1) residuum/saprolite and weathered rock (regolith) overlying 2) fractured crystalline bedrock. A transition zone (TZ) at the base of the regolith has been interpreted to be present in many areas of the Piedmont and to be the most permeable part of the system. Most of the information supporting the existence of the TZ is qualitative based on observations made during the drilling of boreholes and water-wells, although some quantitative data is available for the Piedmont region. A data base of 669 horizontal conductivity measurements in boreholes at six locations in the Carolina Piedmont is utilized for a quantitative assessment of the existence of the TZ. The horizontal hydraulic conductivity measurements at each location are grouped into four hydrostratigraphic layers based on Standard Penetration Testing (SPT; N-Values) and Rock Core Recovery (REC)/Rock Quality Designation (RQD): 1) M1 – Soil/Saprolite; SPT, N < 100, 2) M2 – Saprolite/Weathered Rock; SPT, N > 100 or REC < 50%, 3) WF – Partially Weathered/Fractured Rock; REC > 50% and RQD < 50%, and 4) D – Sound Rock; REC > 85% and RQD > 50%. The definitions were strictly adhered to when assigning the measurements to the hydrostratigraphic units. The WF layer corresponds to the TZ of the groundwater system. The six locations are then grouped into two conceptual models for Piedmont bedrock, 1) a layered/foliated bedrock model (N = 291) and 2) a massive/plutonic bedrock model ( N = 311) based on the bedrock at the locations. To test for the existence of the TZ, the following hypothesis was formulated corresponding to the definition of the zone in the literature: the hydraulic conductivity of the TZ is greater than the hydraulic conductivity of the overlying regolith and underlying bedrock. The hypothesis was statistically tested on the two conceptual models utilizing a 2-Sample T-Test on the log values of hydraulic conductivity (k; has a log normal distribution). The tests on the two models indicate that a transition zone of higher hydraulic conductivity exists in the Piedmont groundwater system when considered within this conceptual framework.