COMPARISON OF GROUNDWATER FLOWS INTO THREE CLOSELY SPACED CRYSTALLINE BEDROCK WELLS

Wells placed in crystalline bedrock pose many challenges for understanding groundwater flow. These challenges include unpredictable well capacities and difficulties in determining the zones of contribution for the wells. In much of central Wisconsin, the crystalline bedrock aquifer is the only available source of groundwater. These rocks have received relatively little study even though many communities and homeowners depend on these rocks for their groundwater needs.

We conducted flowmeter testing, borehole geophysics, and borehole imaging on three test wells in central Wisconsin in the city of Pittsville. These tests identified the rock fabric, fractures, and groundwater flow in the wells. The rocks in which these wells are located are Archean gneisses that were subsequently intruded by Penokean granites. The three wells are located on a 400 ft west-to-east line at 200 ft intervals. The wells are 260 to 300 feet deep and have a diameter of 6-inches. The well located in the center has the greatest specific capacity, 2 gpm/ft drawdown, with the two end wells having lower specific capacities of 0.8 and 0.2 gpm/ft.

Only a few of the fractures seen in the image logs provide flow to the wells. In the case of the western most well, more than ninety percent of the flow originates from a single fracture at a depth of 200 feet. Two fractures, one at 70 feet and the other at 235 feet dominate the flows in the central well and the flows in the easternmost well comes from four fractures located at depths of 56, 96, 140, and 214 feet with each fracture contributing nearly equally to the overall flow. These fractures do not all have the same orientation nor are they not aligned with the rock fabric. It will be necessary to use additional means such as tracer analysis to understand the flow path of groundwater to these wells. There is not a neat picture of a continuous planar fracture zone that moves the groundwater from a recharge zone to the well. Rather it seems likely that a complex interconnected fracture network provides flow to these wells.