SELF-POTENTIAL SIGNATURES ASSOCIATED WITH IN-SITU BIOGEOCHEMICAL PROCESSES IN A BIOLOGICAL PERMEABLE REACTIVE BARRIER, PORTADOWN, NORTHERN IRELAND

The self-potential (SP) signatures associated with the in-situ biogeochemical response of a biological permeable reactive barrier (PRB) to an injection of ~50L of contaminated groundwater were investigated. Primary groundwater contaminants at the former gasworks site in Portadown, Northern Ireland, include sulfate, cyanide, ammonia, and hydrocarbons as a result of former gasworks operations, although the aerobic PRB was designed to degrade primarily hydrocarbon and ammonia. SP measurements were collected using lead-lead chloride non-polarizing electrodes on both a 4m by 5m surface grid over the PRB at 0.25m spacing, and down the 14 boreholes installed in the PRB. SP, Eh, and fluid conductivity measurements were collected before, during, and after the injection to monitor the temporal evolution of the contaminant pulse through the barrier. Both borehole and surface SP measurements reflect a decrease in SP above the ambient noise level and in excess of the expected transient streaming potential caused by the transient increase in hydraulic head in the injection borehole at the PRB inlet. We therefore interpret the SP changes to be a result of transient biogeochemical processes in response to the injection, which suggest the usefulness of the SP method in monitoring in-situ biogeochemical processes within a PRB.