ABSTRACTA plume of contaminated water may extend downstream for several miles in an alluvial aquifer. Hydrologic forces acting on the aquifer generally tend to dilute the contaminant downstream from the source. After the pollution has been stopped, how rapidly will these hydrologic forces flush out the contaminant? The answer to this question may be calculated by using the mass‐balance equation herein developed.This mass‐balance mathematical model incorporates all the hydrologic forces acting on the alluvial aquifer that affect the concentration of the contaminant. Methods are presented to define and quantify each of the hydrologic forces. These forces include (1) present quantity and quality (with respect to contaminant) of water in the alluvium and the change in the quality downstream, (2) quantity and quality of ground‐water inflow, (3) quantity and quality of flood inflow, (4) quantity and quality of base flow, (5) quantity of recharge from precipitation, and (6) quantity of loss from evapotranspiration.The equation is first balanced to agree with past conditions and observed field data and is then used to predict future quality changes after the pollution is stopped.The method is applicable primarily to situations in which the pollution has been taking place over a long period of time and the water quality in the alluvium has reached equilibrium at any given point. Modifications may be possible to permit use of the general approach to short‐term or slug‐type polluti
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