Self potential monitoring of induced saline intrusion in a beach aquifer

Increased groundwater abstractions, combined with possible reductions in recharge rates, are likely to be detrimental to the long-term viability of coastal groundwater resources due to saltwater intrusion (SI). The key processes governing SI have been long understood but monitoring the ingress of saline water into coastal aquifers, and especially its risk to groundwater sources, is a complex and costly exercise. Previous studies have indicated that self-potential (SP) could be used to remotely track the movement of saline-freshwater interfaces associated with SI. The work reported here describes SP response in monitoring wells during pumping tests conducted on a beach aquifer at Benone Strand, on the northern tip of Northern Ireland, UK. In addition to SP, measurements of water level, temperature, fluid electrical conductivity and salinity were obtained from a set of monitoring wells along with wider spatial changes in salinity measured using time-dependent electrical resistance tomography (ERT).

Self potential voltages arise from subsurface pressure and concentration gradients. These potentials (typically in the millivolt range) can be detected and logged in the field using installed electrodes. There are two main types of SP; electro-kinetic potential (V_EK), due to differential flow velocities, and exclusion-diffusion potential (V_ED), due to ion concentration gradients with different mobilities. To utilize SP as an early warning system for SI, the relative contributions from the two sources (V_EK, V_ED) need to be determined. This was done through detailed monitoring before, during and after a pumping test. Comparing SP and ERT responses with well water levels indicate that the observed and progressive drop in SP during pumping is primarily due to the induced movement of a body of saline water in the upper part of the aquifer downgradient of the pumping wells. The result provides further evidence that SP monitoring could be an important geophysical method for remotely sensing SI in the vicinity of monitoring and abstraction wells and could function as an early warning system for saline ingress into groundwater sources.