Using heat as an active tracer for aquifer characterization is a topic of increasing interest for providing complementary insights on groundwater flow and transport pathways. Groundwater temperature profiles are being extensively used in fractured and karst aquifers to provide a better understanding of the main flow and transport structures that dominate a system. Alluvial gravel aquifers are inherently heterogeneous as a consequence of their complex sedimentary architecture, with significant implications for reliable predictive contaminant transport modelling required in water resource management decision making. This heterogeneity is further accentuated by the existence of open framework gravel (OFG) facies, set within a sandy gravel matrix, that exhibit hydraulic conductivity up to three orders of magnitude higher than that of the matrix.
An extended heat and solute tracing experiment was undertaken at ESR’s experimental site at Burnham, 25 km southwest of Christchurch, NZ. During the experiment warm (37 degrees) Rhodamine WT solution was injected for 16 days and temperatures and RWT concentrations were periodically monitored in an array of 22 observation wells over an 80 days period. A three dimensional, dual species, transient, contaminant transport model has been developed to provide an understanding of how the two tracers moved through this highly heterogeneous aquifer. The concept of a dual domain system has been tested numerically against field observations, while inversion modelling of the underlying hydraulic conductivity field provides interesting results regarding the delineation of preferential flow paths and the characterization of the aquifer heterogeneity.