RAPID HYDRAULIC HEAD FLUCTUATIONS IN CRYSTALLINE-ROCK AQUIFERS: EVIDENCE FOR INTIMATELY CONNECTED SURFACE-WATER AND GROUNDWATER IN THE PIEDMONT

Piedmont bedrock-well hydrographs commonly exhibit rapid water-level fluctuations superimposed on gradual, seasonal trends. Rapid fluctuations of up to 3 feet in amplitude spanning time periods of hours to days commonly correlate temporally with major rainfall events and have been interpreted as recharge/discharge phenomena. However, this interpretation is problematic given lack of viable mechanisms to rapidly transmit recharge through the unsaturated zone. This research investigates the hypothesis that rapid water-level fluctuations in many Piedmont bedrock wells may instead be pressure responses to stream stage fluctuations in major streams forming groundwater flow system boundaries. This interpretation is consistent with general observations of rainfall, stream stage and hydraulic head, and with numerical simulations of stream-groundwater interaction. Modeling results indicate that a strong groundwater hydraulic head response to stream stage is dependent upon hydraulic diffusivity and thus may be expected in cases of large hydraulic conductivity or small specific storage. Thus, a direct, high-permeability fracture connection between streambed and well is not required for a strong well response--an equivalent response is produced by small specific storage typical of crystalline bedrock aquifers. Based upon preliminary results stream-driven hydraulic head fluctuations in crystalline-rock wells may be common; therefore, deep groundwater in the Piedmont may be more intimately connected to surface water than previously assumed.