PARTICLE-TRACKING TECHNIQUES TO EVALUATE MIGRATION OF INFILTRATED RIVER WATER INTO A NEARBY AQUIFER DUE TO INDUCED GRADIENT BY GROUNDWATER WITHDRAWAL

Less attention has been paid to the analysis of movement of infiltrated surface water from an adjacent stream to a nearby aquifer due to induced gradient by groundwater withdrawal. This analysis is particularly important in agricultural areas, where pumping from streams and irrigation ditches is widespread. This paper presents a method of using particle-tracking techniques provided by MODPATH (GMS) to evaluate the transport of stream water leaked into the nearby aquifer. Simple stream-aquifer conditions are used to demonstrate the usefulness of the analysis. Temperature analyses indicated that after several days of pumping, almost the entire pumpage was of infiltrated stream water. The mean travel time along the most direct pathline was 9 days, whereas in pathlines offset only 17 meters upstream the mean travel time was 17 days, This model indicates a very restricted area of influence between a stream and a nearby pumping well for a seasonal ground water extraction. Analyses were conducted for transient conditions, and both pumping and nonpumping periods were considered. Infiltrated stream water may remain in the aquifer for several days before arriving at the pumping well. The percentage of stream water extracted from the well was determined to be 39 % of the total water flowing into the well. Simulations of different scenarios indicate that the pumping rate has more effect on mean travel time than the location of the pumping well. Changing the distance between the well and stream did not influence travel times as much as doubling the pumping rate.