Microbial contamination of alluvial gravel aquifers by septic tank effluent

Abstract

The effects of two methods of septic tank effluent disposal on the microbial quality of alluvial gravel aquifers were investigated at an experimental site in the Canterbury Plains, New Zealand. The movement of faecal coliform bacteria 9 m from a 5.5 m deep soakage pit into an unconfined aquifer, and 42 m from an 18 m deep injection bore into a confined aquifer was recorded. Partial sealing of the soakage pit sidewalls was evident, but approximately 80% of the effluent appeared to percolate rapidly into the unconfined groundwater through a permeable pathway in the unsaturated zone. There was evidence of groundwater mounding beneath the soakage pit and around the injection bore and the consequent radial spread of leachate from both disposal structures. In both the confined and unconfined aquifers, the most heavily contaminated bores exhibited marked diurnal fluctuations in faecal coliform concentrations in response to periods of effluent discharge. First arrival velocities of a rifampycin-resistantEscherichia coli tracer of approximately 15 m day⁻¹ in the unconfined groundwater and 151 m day⁻¹ in the confined groundwater were recorded. Implications of the findings for the monitoring and management of groundwater quality beneath unsewered communities on alluvial gravel formations are briefly discussed.