NUMERICAL EVALUATION OF FLOWMETER TEST INTERPRETATION METHODOLOGIES

We evaluate systematic errors inherent in two flowmeter test interpret ation methodologies. A physically consistent two-dimensional groundwat er flow model is used to numerically simulate the single flowmeter tes t and the double flowmeter test in 25 different two-layer confined aqu ifers of a priori known horizontal hydraulic conductivities, K-i; spec ific storativities, S-si; and hydraulic diffusivities, v(i) = K-i/S-si , in each layer (i = 1, 2). Values selected for the layer hydraulic pa rameters span those encountered in natural sandy formations with the r atios of the corresponding parameters in the two layers falling in the following ranges: 1 less than or equal to K-1/K-2 less than or equal to 100, 0.0001 less than or equal to S-s1/S-s2 less than or equal to 1 00, and 1 less than or equal to v(1)/v(2) less than or equal to 10,000 . We find that the size of the hydraulic diffusivity contrast rather t han the hydraulic conductivity ratio is the dominating factor for the parameter estimation accuracy in the two flowmeter methodologies. For the single flowmeter test methodology the ratio of the estimate over t he corresponding true value falls within the range 0.97 less than or e qual to (K) over cap(i)/K-i less than or equal to 1.55, whereas for th e double flowmeter test it falls within 0.99 less than or equal to (K) over cap(i)/K-i less than or equal to 3.40. The double flowmeter test also provides estimates of layer specific storativity, S-si. These ar e accurate only for the special case of equal layer hydraulic diffusiv ities. The test yields order-of-magnitude estimates of S-si for layer hydraulic diffusivities differing from each other by up to an order of magnitude. For larger differences the estimation errors are much larg er. Hydraulic parameters of a given layer are better estimated when th e flowmeter is placed either exactly at the interlayer boundaries or, ideally, at two different points within each layer away from the bound aries. The results from simulated flowmeter tests in a five-layer syst em are consistent with those for the two-layer aquifers. This implies that the presented results most likely apply to flowmeter tests in arb itrary multilayer aquifers. Existence of significant errors in aquifer parameters estimated from synthetic flowmeter data demonstrate that t he Theis [1935] model, which is assumed to be valid in each layer by t he two considered interpretation methodologies, does not fully capture the flow dynamics in layered aquifers. This is illustrated by numeric ally calculated examples of simultaneously nonuniform and transient we ll face flux distributions in a layered aquifer. A model more sophisti cated than that of Theis [1935] needs to be found.