BAROMETRIC EFFECTS ON GROUNDWATER DISCHARGE AT A HUMAN-INDUCED SUBTERRANEAN SPRING

The drilling of a water well in rural Monroe County, West Virginia resulted in breaching of a confining unit, and subsequent upward leakage of water from a confined aquifer in to an overlying/adjacent air-filled cave. The subterranean spring so formed has been flowing in this cave about 10 years, at a current rate of 6 cubic meters per hour. Long-term monitoring of flow, at 20-minute intervals, has revealed that flow decreases in response to rising barometric pressure, and vice-versa. For example, a atmospheric pressure increase of 440 Pa (0.13 inches Hg) results in a 0.03 cubic meter/hr decrease in discharge. This is a 0.24% reduction of flow. There are 3 potential mechanisms that could cause this. First, the column of water in the well bore can be depressed, limiting the “spillway” for the leakage. Second, the increase in total head in the vicinity of the borehole can cause a localized decrease, or even reversal, of gradient towards the spring. Third, regional increase of stress on the fractured shale aquifer caused by atmospheric loading can decrease permeability of the aquifer. All 3 of these effects seem to play a role in the response. Difficulties in apportioning causality come from: lack of regional groundwater level/aquifer data, uncertainties in borehole/surficial geologic delineation, and the absence of a definitive topology for the borehole-spring system.