Simulation of Sampling and Hydraulic Tests to Assess a Hybrid Monitoring Well Design

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Abstract
Determination of the nature, extent, and rate of off‐site chemical migration are common objectives of hazardous waste site investigations. Chemical analyses of water samples from monitoring wells and measurements of hydraulic head and hydraulic conductivity provide the basis for making these determinations. Accurate site assessment, therefore, depends upon the appropriate monitoring well design and sampling and testing procedures.During the course of remedial investigations in Niagara Falls, New York, it has been necessary to evaluate the ground water quality and hydraulic characteristics of 5‐ to 30‐feet thick overburden formations. Many of the monitoring wells completed to these formations consist of a partially penetrating screen (5 feet at the base of the formation) with a fully penetrating sandpack. Questions regarding how this well design influences the source of sampled ground water and hydraulic tests were examined using an extremely fine axisymmetric grid with SATURN, a two‐dimensional, finite‐element ground water model, and a particle tracking post‐processor.A discrete sensitivity analysis was made to determine how flow patterns induced by pumping at 1 gpm are affected by: different screen and sandpack configurations, the ratio of sandpack to formation hydraulic conductivities, heterogeneity, anisotropy, and sandpack thickness. The simulations show that the source (and chemistry given a non‐uniform chemical distribution) of ground water sampled will vary considerably depending on a number of factors. Analysis of simulated drawdowns in the monitoring well during purging shows that calculated transmissivities for the range of well designs and conditions modeled will be accurate to within one‐half order of magnitude.

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