Effects of septic tank effluent on chemical quality of alluvial gravel aquifers

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Abstract
The effects of septic tank effluent on groundwater quality were monitored for 18 months near 2 disposal systems: a 5.5 m deep boulder pit in an unconfined aquifer, and an 18 m deep injection well in a confined aquifer, both in the Canterbury Plains, New Zealand. Partial sealing of the bottom metre of the boulder pit gave rise to 2 flow mechanisms: rapid flow through highly permeable pathways which resulted in marked diurnal fluctuations of chemical concentrations in some of the nearby wells, and slower seepage through the “sealed” layer. Concentrations of various chemical constituents increased in some of the monitoring wells. Dispersion‐adjusted concentrations of ammonium, total Kjeldahl nitrogen, phosphorus, biochemical oxygen demand and methylene‐blue active substances decreased rapidly with distance through the boulder pit and groundwater strata. Levels of nitrate increased because of transformation of ammonium, and sodium, potassium and fluorescent whitening agents (FWA) decreased slightly. The study indicated that FWA would be useful in identifying septic tank contamination in domestic wells.