Twin Lake Tracer Tests: Transverse dispersion

Two natural-gradient dispersion tests were performed at the Chalk River Nuclear Laboratories in Ontario in 1982 and 1983. During the field tests 131I was used to label the groundwater, and its activity was measured in situ, collecting continuous records of activity versus depth at selected times and of activity versus time at selected locations. Curves generated by a three-dimensional analytical solution to the advection dispersion equation were fit to activity versus depth scans recorded at various locations in the test aquifer. Within the individual velocity regions the vertical dispersivities were 0.0006 and 0.0008 m, with the arithmetic average 0.0007 m as calculated from the 1982 data. The vertical dispersivity values calculated from 1983 data were 0.0014 and 0.0010 m for the high- and intermediate-velocity regions, respectively, with an arithmetic average 0.0012 m. The increase in dispersivity was attributed to the error associated with the sampling and fitting procedures. The average vertical dispersivity of the tracer test aquifer was an order of magnitude less than the average longitudinal dispersivity (0.0010 m versus 0.010 m for the intermediate-velocity region). Within individual velocity regions the vertical dispersivity of the porous medium was independent of the travel distance of the tracer and was proportional to the velocity.