Influence of Using Purged and Partially Penetrating Monitoring Wells on Contaminant Detection, Mapping, and Modeling

An analytical transport model is presented for evaluating vertically averaged concentration data derived from purged and partially penetrating monitoring wells. The model is applicable to continuous contaminant emission from a finite source with longitudinal advection, and three‐dimensional dispersion. The model is used to investigate how screened length may influence contaminant detection, mapping, and modeling. Sensitivity analyses suggest that contaminant concentrations determined from monitoring wells may underestimate the level of aquifer contamination, depending on screened depth, location of the well relative to the contaminant source, the vertical source dimension, and degree of vertical plume spreading. The magnitude of this effect may be comparable to, and in excess of, concentration biases related to construction materials and sampling procedures. Additional screened interval influences may include: underestimates of contamination extent and migration rate; apparent indications of attenuation processes and steady‐state conditions; over‐ estimates of vertical contaminant spreading; increased time of contaminant detection; and poor transport model predictions, owing to well‐biased concentration distributions.