Applicability of Single-Borehole Dilution Tests in Aquifers with Vertical Flow

A set of experimental field single-borehole dilution tests were completed in the Motril–Salobreña detrital aquifer (Spain) in a sector with coarse material in four different moments under variable hydrological conditions. The comparative study of the tracer washing, and the temperature profile patterns for the tests carried out in two wells located hundreds of m from each other, revealed the presence of ascending vertical flows in one of the wells (not detected by other means) that compromises the reliability of the tracer test. The values of both the apparent horizontal velocity and hydraulic conductivity obtained in the affected well were less than half of those estimated in the well not affected by the upward vertical flows. The repetition of the test eight times during different seasons showed that the hydraulic conductivity calculated from the apparent horizontal velocity can vary; therefore, to approximate to a representative hydraulic conductivity value, using this method is recommended to carry out tests under different hydrological conditions and average the results. The difference generated by the changes in conditions for the specific setting of the study area was 25%. Taking this into account, it was considered that an approximation to the more representative value would be an average under variable hydrological conditions, resulting in a horizontal velocity of 6.7 m/d and hydraulic conductivity of 337 m/d. This information is critical for the management of the aquifer as it has strategic resources against droughts that are becoming more frequent in the Mediterranean area.