POROUS-MEDIA MODELING OF BEDROCK WELLS FOR MAINE'S WELLHEAD PROTECTION PROGRAM

We used porous-media numerical models to estimate the recharge area to 26 public supply wells drilled into fractured bedrock. This work was done in support of the Source Water Assessment Program, in an effort to improve understanding of the contributing areas for assessment.

We attempted to characterize the fracture system of each wellfield using outcrop fracture mapping and lineament studies. Because no system has sufficient data to model the ground-water flow using discrete-fracture models, we modeled the ground-water systems using an equivalent porous-media model. Justification for the use of a porous-media model came from the analysis of well tests using techniques outlined in Gringarten (1982) and from characterization of the fracture density measured at the outcrops.

On a log-log plot of drawdown versus time, the slope of the early data will follow a straight line with a slope of 0.5 if the flow to the well is linear to a single fracture (Gringarten, 1982). Pump-test data from many of the sites show less of a linear response with increasing distance from the pumping well. Linear flow was not detected in any wells farther than 92 meters from the pumping well. It is reasonable to assume a porous-media response at the scale of the capture zone.

Uncertainties in hydraulic properties were taken into account by varying them across a range of values in a series of simulations. The resulting contributing-zone delineations were used to display zones of low, medium and high confidence based on the number of overlapping polygons.

The primary lesson learned from this study is that at the scale of the well capture zone, fractured bedrock behaves as a porous equivalent medium and may be modeled as such.

Gringarten, A. C., 1982. Flow-test evaluation of fractured reservoirs. In Recent Trends in Hydrogeology, T. N. Narasimhan (Ed.), Geological Society of America Special Paper. p. 237-264.