Carolina Piedmont Groundwater System—Existence of the Transition Zone Between Regolith and Bedrock

The groundwater system in the Carolina Piedmont Province is comprised of two interconnected layers: regolith (residuum/saprolite and weathered rock) overlying fractured crystalline bedrock. The presence of a transition zone (TZ) at the base of the regolith has been interpreted in many areas of the Piedmont and is theorized to be the most permeable part of the system. Most information supporting the existence of the TZ is qualitative and based on observations made during drilling operations. A database of 1410 horizontal conductivity measurements in boreholes at 12 locations in the Piedmont is utilized to quantitatively assess the existence of the TZ. Hydraulic conductivity measurements are grouped into four hydrostratigraphic units based on Standard Penetration Testing (N-Values) and Rock Core Recovery (REC)/Rock Quality Designation (RQD): (1) M1—Soil/Saprolite; N < 50, (2) M2—Saprolite/Weathered Rock; N ≥ 50 or REC < 50%, (3) WF—Partially Weathered/Fractured Rock—TZ; REC ≥ 50% and RQD < 50%, and (4) D/BR—Sound Rock; REC ≥ 85% and RQD ≥ 50%. The 12 locations are grouped into two conceptual models for Piedmont bedrock: layered/foliated bedrock and massive/plutonic bedrock. The following hypothesis was formulated corresponding to the definition of the TZ in the literature: the hydraulic conductivity of the TZ is greater than the hydraulic conductivity of both 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 the hydraulic conductivity measurements. Results indicate a TZ of higher hydraulic conductivity exists between regolith and bedrock in both conceptual models.