Direct-push EC profiling to define brine-impacted groundwaters
Date
2015-04-15
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Degree Level
Masters
Abstract
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.
Description
Keywords
electrical conductivity, direct-push, aquitard, groundwater, brine, chloride, modelling
Citation
Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geology