Groundwater Quality in Pumping Wells Located Near Surface Water Bodies
Well-water quality depends on the relative amounts of water drawn from different hydrologic units (aquifers, streams, etc.), and the particular capture zones within those units. A contaminant spilled within an aquifer capture zone will eventually enter the well. A spill outside the capture zone may eventually discharge elsewhere, perhaps to a nearby stream. If under normal circumstances the stream is gaining, pumping can locally reverse gradients, causing it to become losing. Stream water can then enter the well by induced infiltration. For wells located near surface water bodies, a variety of two and three-dimensional analytical and numerical models of induced infiltration and capture zones are presented. The capture zone models use particle tracking. The propensity for and rate of induced infiltration is enhanced by higher pumping rates, closer streams, less well penetration, increased stream penetration, less ambient aquifer discharge to the stream, nearby aquifer pinch outs, and a variety of other factors. Induced infiltration does not seem to be very sensitive to the ambient flow’s source, whether it is local vertical recharge or lateral inflow. Two types of capture zones are defined: ultimate capture zones that include that portion of the aquifer that will eventually discharge to the well, and time-dependent capture zones that include only that portion of the aquifer that discharges to the well within a prescribed time. Capture zones are strongly influenced by the source and direction of ambient aquifer flow, nearby aquifer boundaries, and three-dimensional flows caused by partial penetration of the well or the stream. For example, when local vertical recharge is the principle source of ambient flow,the capture zone tends to ‘cusp’ (elongate) along groundwater divides and barrier (pinchout) boundaries. This allows the well to capture contamination from long distances away, albeit at great travel times. Partial penetration of the stream allows the well to capture by underflow ambient groundwater from the opposite stream bank, even though the well may not induce infiltration from the stream itself. Capture zone models involve uncertain parameters that lead to ‘fuzzy’ capture zone boundaries. These boundaries are represented by contour lines of equal capture probability.The concept of ‘fuzzy’ capture zones is illustrated for a simple model. Hydrodynamic and macro dispersion are other sources of uncertainty. They allow a particle to wander across the capture zone boundary. Thus a source of contamination outside of the capture zone has a finite probability of being incor porated into the well water. Another interpretation of these probabilities, at least for long-term continuous sources of contamination, is that they define the proportion of the contaminant that will eventually find its way to the well.
keywords: Groundwater quality, contaminant, numerical models, aquifer discharge