HYDRAULIC EVIDENCE OF FRACTURE FLOW IN A CARBONATE AQUIFER, SOUTH MIDDLETON TOWNSHIP, PENNSYLVANIA

Hydraulic tests were conducted to characterize the flow regime of a fractured carbonate aquifer at the Dickinson College research well field located in South Middleton Township, Pennsylvania. The well field includes one pumping well and four observation wells installed within the Elbrook Formation, which is composed of massive limestone beds interbedded with thin shales and porous dolomite. Well yield tests conducted during drilling indicate that the most productive zones are associated with fractures, bedding planes, and solution openings. Borehole geophysics logs and an analysis of drill core revealed that the mean fracture/secondary porosity spacing is 2.3 m.

To evaluate the hydraulic properties of the aquifer, a constant-discharge pumping test was conducted with a flow rate of 34.3 L/min for 10 hours. Drawdown data were recorded at the four observation wells using pressure transducers and a datalogger, and were analyzed using the Bourdet-Gringarten method to determine fracture transmissivity (Tf), fracture storativity (Sf), and storativity of the aquifer matrix (Sm). Results from slug tests were evaluated using the Bouwer and Rice method to determine the bulk hydraulic conductivity (K) of each observation well. Transmissivity values and fracture porosity (nf, determined by the cubic law) were used to estimate the fracture flow velocity.

The drawdown data developed an elongate, pseudo-steady-state segment during the central portion of the test, which suggests that the aquifer behaves as a double-porosity flow system. The geometric mean Tf was 3.2 x 10-03 m2/s with a range of 1.8 x 10-03 to 1.2 x 10-02 m2/s, Sf ranged from 0.001 to 0.065, and Sm ranged from 0.012 to 0.014. Hydraulic conductivity from slug tests compared well with the pumping test results, with a slug-test geometric mean K of 2.9 x 10-05 m/s vs. a geometric mean Kf of 7.4 x 10-05 m/s calculated from Tf. The mean estimated nf was 2.9 x 10-04, which could produce a flow velocities up to 400 m/d under the natural gradient of 0.018. Results from this study demonstrate that groundwater movement is controlled by fractures and/or other zones of secondary porosity, and that future solute transport studies planned at the site should quantify the effect of these preferential flow paths.