In cooperation with the U.S. Environmental Agency
GOODE, Daniel J., and SENIOR, Lisa A., U.S. Geological Survey, 111 Great Valley Parkway, Malvern PA 19355, djgoode@usgs.gov and lasenior@usgs.gov; AMANTIA, Andrea, Istituto di Geologia e Geofisica, Universita di Catania, Corso Italia 55, 95100 Catania, Italy
ABSTRACT
Transmissivities estimated from aquifer tests in open-hole bedrock wells in Lansdale range from 1 to 500 meters squared per day, with many tests yielding estimates of about 100 meters squared per day. Analyses utilizing different observation wells frequently yield different transmissivity estimates, particularly when they penetrate different strata within the Brunswick Formation, or have different open-interval depths. Horizontal anisotropy oriented with regional strike is suggested by several tests, but these results are not currently considered conclusive because of the lack of directional coverage at the test scale. Many log-log drawdown hydrographs exhibit straight-line response suggesting linear (as opposed to radial) flow or uniform dewatering.
Only a few vertically-isolated zones contribute significant inflow during pumping. When isolated by packers, the vertical hydraulic connection between productive zones is generally weak, suggesting significant vertical anisotropy. Step-drawdown tests indicate that up to 90 percent of the drawdown observed in production wells is due to nonlinear well loss or skin effects, hence analysis of recovery is believed to yield better (and higher) transmissivity estimates than analysis of production-well drawdown during pumping.
A three-dimensional model simulates water levels and ground-water flow directions during 1954 (drought conditions) and 1996. Lansdale is principally a recharge area and is on the regional divide between three drainage systems. Pumping is more than half of recharge, hence baseflow to streams is significantly reduced from pre-pumping levels. Rima's (1955) water table map shows a cone of depression more than 1 kilometer in radius with drawdown of about 50 meters at a well that pumped, on average, about 40 liters per minute. A transient model of drought conditions prior to Rima's measurements exhibits the generally lower levels he observed, but his large cone of depression in the water table is not reproduced.
Citation:
Goode, D.J., Senior, L.A., and Amantia, Andrea, 1997,
Aquifer tests and regional ground-water flow in fractured Triassic rock,
Lansdale, Pennsylvania: (abs.), GSA Abstracts with
Programs, vol. 29, no. 1, p. 49.
Keywords: transmissivity, anisotropy, pumping, Brunswick Formation, fractured rock
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