NUMERICAL EVALUATION OF THE FLOWMETER TEST IN A LAYERED AQUIFER WITH A SKIN ZONE

In this study, we used a two-dimensional groundwater how model to nume rically evaluate the single flowmeter test in a series of two-layer co nfined aquifers for a fully penetrating well with a skin zone (i.e. a disturbed zone, filter-pack envelope, etc.). The skin zone causes erro rs in estimates of the vertical distribution of horizontal hydraulic c onductivity by acting as a channel for cross-how between layers of con trasting hydraulic conductivity. The bias in measured layer discharge rates, Q(i), is dependent on the ratios of the skin zone hydraulic con ductivity to the layer hydraulic conductivities, K-s/K-i, and on the r atios of the skin zone thickness to the thickness of the individual la yers, (r(s)-r(w))/b(i). For any given K-s/K-i, the cross-how increased with increases in the skin zone radius relative to the well radius, r (s)/r(w). The error in estimated K-i due to a skin zone of reduced hyd raulic conductivity (K-s/K-i < 1) is greater than that of a skin zone of a correspondingly enlarged hydraulic conductivity (K-s/K-i > 1). As K-s/K-1 increases, the ratios (K) over cap(1)/K-1 and (K) over cap(2) /K-2 converge approximately to the ranges 0.9-0.95 and 1.0-1.25, respe ctively. As K-s/K-1 decreases from value 1, the ratios of (K) over cap (1)/K-1 and (K) over cap(2)/K-2 diverge from these asymptotic ranges. In all simulated flowmeter tests, where K-1/K-2 = 10, (K) over cap(1) underestimates K-1 and (K) over cap(2) overestimated K-2. (C) 1997 Els evier Science B.V.