Evolution of fluid electrical conductivity (FEC) profiles associated with a contaminant plume in a horizontal single-plane fractured rock aquifer system

Abstract

Fluid electrical conductivity (FEC) profiling is a simple and efficient technique used to determine properties such as flow rate, salinity and hydraulic characteristics such as transmissivity. The method is also commonly used to identify and locate high inflow zones intersected by a wellbore, from which groundwater samples can be collected for the purpose of water quality monitoring. Moreover, the identified inflow zones may be targeted for transport and hydraulic tests which may assist in the understanding of groundwater flow and solute mass transport properties of the subsurface. The method primarily involves profiling the FEC with depth in a borehole under either natural or stressed conditions, using a downhole Temperature Level Conductivity probe. Once the FEC tests are conducted and graphs are obtained, observations may be derived from the profiles. Zones where fluid flows into the borehole displays anomalies in the FEC profiles, which may be analysed to infer inflow rate and salinity of the individual fractures. The current challenge with the use of this method is that its application has not yet been studied in a controlled laboratory aquifer environment, in order to understand the typical FEC profile responses in aquifers of different structures and groundwater qualities. Furthermore, no guidelines have been developed to assist in the interpretation of FEC profiles under different hydrogeological conditions. In this study laboratory tests were conducted with the use of a physical model to investigate the evolution of FEC profiles associated with a contaminant plume, in a horizontal single-plane fractured rock aquifer system. To achieve this, two groundwater flow and transport conditions were simulated; one with freshwater flow and the other with saline (contaminated) water. Generally, two distinct profiles associated with a contaminant plume in a borehole drilled in a horizontal single-plane fractured rock aquifer were identified and conceptualised as (1) the conceptual background profile and (2) the conceptual elevated FEC profile. Essentially, within the conceptual background profile three distinct segments (zones) were noted, each which responded differently; the upper segment, transition zone and the lower segment. The conceptual elevated FEC profile was observed when a contaminant associated with increased FEC values reached the monitoring borehole; its anomaly is more or less pronounced (at the fracture position) depending on the stage of the contaminant plume within the system. These profiles were observed under a controlled laboratory environment and were also verified in the field. The profiles which were obtained in the field were comparable to those observed in the laboratory experiment thus increasing the confidence in the accuracy of the laboratory results.