PLUME DISTORTION AND APPARENT ATTENUATION DUE TO CONCENTRATION AVERAGING IN MONITORING WELLS

Mathematical models that simulate common monitoring well sampling demo nstrate the distortions that vertical concentration averaging causes d uring the mapping and modeling of an idealized, three-dimensional cont aminant plume emanating from a simple source of constant solute concen tration, The apparent extent of the plume, mapped using simulations of a regular grid of screened monitoring wells, ranged from a worst case of 0% of the original plume area for long screens (4 m) in a low-perm eability formation to 90% for short screens (1 m) in a high-permeabili ty formation. When well design and purging procedure were inconsistent among wells, the mapped plume exhibited spurious directional skewing, bifurcation, zones of low concentration, intermittent sources, or mul tiple sources. Although the study plume was not retarded, calibrating a transport model to the monitoring well data resulted in retardation factors of up to 23. If first-order decay was assumed, the apparent de cay constant was found to be as much as 1.8 x 10(-7) sec(-1) (T-1/2 = 45 days), Apparent retardation or decay was inconsistent from well to well, depending on the saturated screen length, the degree of screen d esaturation during purging, and the distance from the source. The stud y indicates that the quantitative assessment of contaminant distributi ons and transport processes requires discrete vertical sampling in the common situation where concentrations vary sharply with depth, even i n the most ideal hydrogeologic environment. If screened monitoring wel ls are used, screen lengths and placements should be appropriate to th e contamination situation being assessed and inherent biases must be c onsidered. Even so, vertical concentration averaging biases and the re sulting inconsistencies can result in highly misleading evaluations of ground-water contamination problems.