INTERPRETATION OF TRACER DISPLACEMENT DURING INFILTRATION OF RIVER WATER TO GROUNDWATER

The accidental release of a tritium pulse of about 500 Ci into a river in northern Switzerland allowed the measurement of travel times at a site where the river recharges a granular aquifer almost perpendicularly to the groundwater flow. A series of sampling wells had been previously installed in the aquifer. Most wells give access to shallow freshly infiltrated water. The infiltrated water flows along in the top layer of the aquifer, thus allowing for one-dimensional analysis of the spreading process from the river to the groundwater in the wells. The linear groundwater flow velocities calculated from the tritium responses at distances from the river of up to 500 m confirm the velocities determined from an earlier local-scale uranine experiment at the same site. A linearly scale-dependent dispersivity is evaluated from the mode of increase of the temporal variance of the tracer concentration distribution.