Twin Lake Tracer Tests: Setting, methodology, and hydraulic conductivity distribution

Abstract

Following evaluation of three potential sites, a fluvial sand aquifer with high (0.7–1.6 m d−1) linear average groundwater flow velocities was instrumented with a fully penetrating injection well and 82 monitoring installations over a 40‐m flow path length. Two natural gradient tracer migration experiments have employed pulse injections of a nonreactive radioisotope (131I) in a study of hydraulic conductivity structure and dispersive characteristics of the flow system. The use of a gamma‐emitting radionuclide in these experiments allowed measurement of tracer distribution by scanning fully penetrating boreholes with a scintillation detector. This technique provided many advantages in data collection over conventional tracers, including unprecedented resolution of the vertical distribution of tracer, measurements without pumping and the attendant concerns for flow system perturbation, and minimal manpower for sample analysis. Tracer breakthroughs at monitors along the axis of flow revealed the presence of heterogeneities in aquifer hydraulic conductivity at two scales. Over the 40‐m path length, six regions with hydraulic conductivities substantially different from adjacent strata were identified. Thicknesses of these regions varied from 0.2 to more than 3 m; their lateral extents (in the direction of groundwater flow) ranged from 15 m to more than 40 m. Smaller strata, with an average thickness of 0.34 m and lengths of less than 10 m, were detected within all six of the hydraulic conductivity regions, but there was no evidence of increases in local dispersivity with increased transport distance as a result of these variations. The six hydraulic conductivity regions do, however, have measurable effects on whole‐aquifer dispersion. Stratification characteristics of the tested aquifer, believed to have been deposited in a sandy braided river, are consistent with those observed in similar modern environments. Estimation of the magnitude and spatial structure of hydraulic conductivity variations in aquifers of concern would be greatly aided by studies of the stratification characteristics in well‐exposed modern or lithified sedimentary analogues.