Time-lapse electrical resistivity monitoring of salt-affected soil and groundwater Kevin Hayley, 1 L. R. Bentley, 1 and M. Gharibi 2,3 Received 26 November 2008; revised 28 April 2009; accepted 8 May 2009; published 28 July 2009. [1] In order to develop and test a methodology for incorporating time-lapse electrical resistivity imaging (ERI) into the monitoring of salt-affected soil and groundwater, a multifaceted study including time-lapse electrical resistivity imaging, push tool conductivity (PTC), and core analysis was conducted to monitor the movement of a saline contaminant plume over the span of 3 years. The survey was done on a field site containing salt-affected soils and groundwater to depths of over 7 m. The site contained a tile drain system at approximately 2 m below ground level. Temperature and saturation changes were accounted for in electrical conductivity (EC) measurements to isolate changes in electrical conductivity due to changes in salt distribution. ERI inversion parameters were selected so that the inverse models gave the best match to PTC depth profiles and the best correlation with core EC data. A strong correlation between the core data and the ERI results was observed. Time-lapse ERI difference images showed that the subsurface EC distribution was consistent with preferential solute removal above the tile drains in some locations. The ERI-delineated reduction in solute concentration is consistent with nonuniform flushing due to depression-focused recharge. The addition of time-lapse ERI to the study allowed delineation of details of solute redistribution that would not have been possible with point measurements alone. Citation: Hayley, K., L. R. Bentley, and M. Gharibi (2009), Time-lapse electrical resistivity monitoring of salt-affected soil and groundwater, Water Resour. Res., 45, W07425, doi:10.1029/2008WR007616. 1. Introduction [2] Petroleum reservoirs typically produce a mixture of petroleum hydrocarbons and saline water. Coproduced saline water is generally disposed of by injection into appropriate geologic formations. However, because of legacy contamination, accidental spills, or pipeline breaks, salt-affected soils are perhaps the single most common environmental problem faced by the upstream hydrocar- bon industry [Harris, 1998; Cook et al., 2002]. Salt- affected soils can be remediated by flushing salt from the soil, but the remediation progress needs to be monitored. Traditional hydrogeological monitoring consists of sparse point measurements from expensive well installations. The addition of geophysical methods allows for spatially exten- sive mapping and monitoring, which can be used to interpo- late or extrapolate the results of the hard data measurements. In the following, we present the results of 3 years of 3-D time- lapse electrical resistivity imaging that has been used as part of a monitoring program over a salt-affected site with an underlying tile drain system. [3] The electrical conductivities of rocks and soils are dependent on several parameters, including soil type, water saturation, pore water ionic concentration, and temperature. Variations in electrical conductivity (EC) have been used in the time-lapse electrical resistivity imaging (ERI) studies to track tracer migration [Daily et al., 1992; Slater and Sandberg, 2000; Kemna et al., 2002], to monitor infiltration and soil moisture changes [Barker and Moore, 1998; Binley et al., 2002; French and Binley , 2004; Jayawickreme et al., 2008], and to monitor contaminant transport and remedia- tion. Hauck [2002] used time-lapse ERI to monitor frozen ground and related the observed differences to changes in liquid water saturation and temperature. [4] In this study, a methodology is described for imaging subtle changes in salinity using time-lapse ERI. We illus- trate how the addition of time-lapse ERI provides a unique picture of the redistribution of salts and insight into the hydrologic processes acting on a site. 2. Site Description [5] The field site is located in central Alberta, Canada. The near-surface material at the site is composed of glacial till approximately 20 m thick. The glacial till is a hetero- geneous mix of material with an average grain size distri- bution of 5% clay, 55% silt, and 40% sand. There are sporadic cobbles and small sand lenses within the matrix. The soils on the site contained brines and hydrocarbons as a result of a variety of oil field activities through the 1970s and 1980s and a pipeline break in the 1980s. The hydro- carbon has been removed by bioremediation using a land farming technique; however, much of the salt remains. Elevated salt concentration can be found to depths of 1 Department of Geoscience, University of Calgary, Calgary, Alberta, Canada. 2 Formerly at Department of Geoscience, University of Calgary, Calgary, Alberta, Canada. 3 Now at Quantec Geoscience Limited, Toronto, Ontario, Canada. Copyright 2009 by the American Geophysical Union. 0043-1397/09/2008WR007616$09.00 W07425 WATER RESOURCES RESEARCH, VOL. 45, W07425, doi:10.1029/2008WR007616, 2009 Click Here for Full Articl e 1 of 14