Characterization of preferential flow paths is important for assessing the delivery of treatment amendments to contaminated portions of an aquifer. Field investigation methods with high spatial resolution are required to capture the flow variability in unconsolidated or discretely fractured aquifers, and to identify the nature of flow pathways (i.e. fracture vs matrix flow) relative to the contaminant distribution. The active distributed temperature sensing (A-DTS) method, originally developed for fractured rock boreholes, was adapted for application in a poorly cemented sandstone aquifer contaminated with chlorinated solvents in southern France.
The composite fiber optic cable was attached to a PVC pipe and grouted in the borehole to avoid cross-connected flow and to recreate natural-gradient flow conditions in the aquifer representative of flow conditions over the past decades of plume transport at this aged contaminated site. The A-DTS tests consist of heating the cable for up to 24 hours with constant heat input and recording temperature along the cable continuously using a DTS unit. Active groundwater flow in preferential flow paths causes an enhancement of heat transfer from the cable creating a cooler thermal response than zones with lower or no flow. The geometry of the test was recreated in a numerical heat transport model and a relationship was developed between the thermal responses measured with the A-DTS, and the volume of water flowing through the preferential flow paths.
The results show variable flow rates along the borehole indicating the presence of preferential flow zones. A fluorescein tracer injection experiment followed by detailed core logging and sampling, with visual inspection of fluorescein tracer distributions under UV light to guide high frequency, depth-discrete rock core sampling, provided additional evidence for the presence and distribution of preferential flow paths, also indicating flow anisotropy and tracer transport retardation. The results from these two methods, combined with detailed profiles of contaminant concentration distribution from rock core and groundwater sampling, inform and optimize the design for in-situ remediation.