MODELING CONCENTRATION VARIATIONS IN HIGH-CAPACITY WELLS - IMPLICATIONS FOR GROUNDWATER SAMPLING

High-capacity wells are used as a convenient and economical means of s ampling groundwater quality. Although the inherent limitations of usin g these wells are generally recognized, little has been done to invest igate how these wells actually sample groundwater. A semi-analytical p article tracking model is used to illustrate the influence of variable vertical contaminant distributions and aquifer heterogeneity on the c omposition of water samples from these wells during short pumping peri ods. The hypothetical pumping well used in the simulations is located in an unconfined, alluvial aquifer with a shallow water table and conc entration gradients of nitrate-nitrogen contamination. This is a typic al setting for many irrigated areas in the United States. The main con clusions are: (1) high-capacity wells underestimate the average amount of contamination within an aquifer; (2) shapes of concentration-time curves for high-capacity wells appear to be governed by the distributi on of the contaminant and travel times to the well; (3) variables such as well construction, pumping rate, and hydrogeologic properties cont ribute to the magnitude of the concentration-time curves at individual high-capacity wells; and (4) a sampling strategy using concentration- time curves based on the behavioral characteristics of the well rather than individual samples will provide a much better framework for inte rpreting spatial contaminant distributions.