A fluoride borehole dilution apparatus for groundwater velocity measurements

Abstract

A borehole dilution method using fluoride in low concentrations as the tracer and an adapted, commercially available fluoride ion electrode to measure the tracer dilution has been developed and employed at two sites. The sites are shallow groundwater systems consisting of alluvial surficial gravels near Fort Macleod, Alberta, and glaciodeltaic sand deposits at the Chalk River Nuclear Laboratories, Ontario. Zones of relatively high groundwater velocities determined at Fort Macleod with the fluoride apparatus are also evident in the results of a large-scale tracer test that documented chloride profiles over the saturated gravel thickness from point samples at 0.3 m depth intervals. The performance of the fluoride electrode compared favorably with a radio tracer (131I) technique in a simultaneous dilution experiment at the Chalk River site. Other ion-selective electrodes such as the chloride electrode may provide an alternative sensor in areas where hydrochemical conditions may restrict the use of the fluoride electrode.