Direct-push EC profiling to define brine-impacted groundwaters
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Delineating the extent of brine contamination in shallow groundwater systems using piezometers is costly and does not provide adequate data resolution. Direct-push (D-P) electrical conductivity (EC) profiling enables rapid in situ measurements of bulk soil EC (ECa) at the cm scale. Previous studies using D-P EC profiling to detect contaminant plumes have solely relied on ECa measurements, and where attempts were made to isolate pore-water salinity variations from changes in ECa they were accomplished using simple linear methods. In this study D-P EC profiling was used to define groundwater salinity distributions using an established soil conductance model and estimate the timing of groundwater contamination at a long-term potash mine in south-central Saskatchewan, Canada. The site was dominated by fine-grained postglacial and glacial sediments with known Na-K-Cl brine impacts resulting from mining activities. Coreholes (n=22) were drilled to 7.6-12.2 m below ground (mbg) to obtain continuous cores for detailed geologic descriptions and measurements of index parameters (n=522) below the water table. Pore-water EC (ECw) and Cl- results from squeezed core samples (n=142) at 12 locations were compared to ECa measurements collected using a D-P probe adjacent to each corehole. Measured ECw and pore-water Cl- results ranged from 1.94-55.1 mS/cm and 87-20,700 mg/L, respectively. In situ D-P EC values from logs collected adjacent to all 22 coreholes ranged from 2-8 mS/cm within the oxidized zone (5-6 mbg) and decreased to background values of 0.3-2 mS/cm within the underlying unoxidized zone. Significant linear ECa–ECw regressions established for four lithological groups (r2=0.78-0.95) were used with porosity and dry density measurements to generate high-resolution depth profiles of ECw from D-P EC measurements. A significant linear ECw–Cl regression (r2=0.92) further enabled the generation of pore-water Cldepth profiles from ECw predictions. Observed 1D vertical profiles of Cl-, ECw, and ECa at three locations were modeled. Results suggested solute transport can be described as diffusion-dominated below depths of 3-5 mbg and that groundwater contamination began shortly after the onset of mining. Based on the results attained, this method can generate high-resolution depth profiles of pore-water salinity that can be used to define the lateral and vertical extent of brine contamination, dominant solute transport mechanisms, and timing of groundwater contamination.
DegreeMaster of Science (M.Sc.)
CommitteeHendry, M J.; Barbour, S L.; Lindsay, M B.; Merriam, J B.
Copyright DateJune 2015