INVESTIGATION OF STRUCTURAL CONTROL OF SEEMINGLY ANOMALOUS GROUNDWATER FLOW AT BOILING SPRINGS, PENNSYLVANIA

Boiling Springs is located in the Carlisle Quadrangle, Cumberland County, PA. Here, groundwater comes to the surface by a series of springs at the junction of two diabase dikes. The groundwater flow at the springs is anomalous in two ways. First, the amount of water discharged at this location is roughly 16 million gallons per day. This discharge is four times larger than expected from the 3 mi² recharge area of the Boiling Springs watershed. This implies that a significant amount of water discharged from Boiling Springs is coming from sources outside the local watershed. Second, the water temperature cycle at Boiling Springs lags four to six months behind that of regional air temperature, suggesting a long groundwater flow path.

These observations form the background to the questions examined here: (1) Why is the discharge at Boiling Springs significantly greater than the surface area recharge estimate? (2) Why is there such a long and regular lag time in the water temperature? (3) Do these observations imply that water is moving deeply underground from beyond the local drainage area? Our working hypothesis is that the water discharged at Boiling Springs it being recharged from the northeastern face of South Mountain, just south of the Boiling Springs watershed. There, the bedrock is mainly quartzite of the Weverton Formation, which has high primary and secondary porosity and permeability. The water could then move deep underground before being discharged at Boiling Springs.

Bedrock fractures, analyzed in this study, are thought to control recharge and regional groundwater flow. Since the diabase dikes appear to be impermeable, their extent may define the hydrogeological boundary of the watershed. By mapping, measuring and analyzing bedrock fractures, mapping the locations of the basalt dikes using palemagnetometry, and establishing a regional water budget, the direction and volume of groundwater flow can be estimated and compared to the discharge at Boiling Springs.