Ground‐Water Contamination from Two Small Septic Systems on Sand Aquifers

Abstract

Distinct plumes of septic system‐impacted ground water at two single‐family homes located on shallow unconfined sand aquifers in Ontario showed elevated levels of Cl−, NO3−, Na+, Ca2+, K+, alkalinity, and dissolved organic carbon and depressed levels of pH and dissolved oxygen. At the Cambridge site, in use 12 years, the plume had sharp lateral and vertical boundaries and was more than 130 m in length with a uniform width of about 10 m. As a result of low transverse dispersion in the aquifer, mobile plume solutes such as NO3− and Na+ occurred at more than 50 percent of the source concentrations 130 m downgradient from the septic system. At the Muskoka site, in use three years, the plume also had discrete boundaries reflecting low transverse dispersion. After 1.5 years of system operation, the Muskoka plume began discharging to a river located 20 m from the tile field. Almost complete NOs attenuation was observed within the last 2 m of the plume flowpath before discharge to the river. This was attributed to denitrification occurring within organic matter‐enriched riverbed sediments. The very weakly dispersive nature of the two aquifers was consistent with the results of recently reported natural‐gradient tracer tests in sands. Therefore, for many unconfined sand aquifers, the minimum distance‐to‐well regulations for permitting septic systems in most parts of North America should not be expected to be adequately protective of well‐water quality in situations where mobile contaminants such as NOs are not attenuated by chemical or microbiological processes.

Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

“…Since the source of DOC is similar, this difference in DOC reactivity could reflect more extensive degradation in the unsaturated zone at Muskoka. The depth of the unsaturated zone at Cambridge (2 m) is somewhat shallower than that at Muskoka (3 m) (Robertson et al, 1991). The reactivity of the less labile fraction, DOC 2 , is similar at both sites (kf ox2 = 0.1 yr − 1 ).…”

“…Robertson et al, 1991;Wilhelm et al, 1994aWilhelm et al, ,b, 1996Robertson et al, 1998;Zanini et al, 1998;Robertson, 2003). Here, we test our model by using field data from two well-studied domestic effluent plumes (Cambridge and Muskoka; Fig.…”

“…This distance is about 2-3 m at the Cambridge and Muskoka sites. Both flow fields in the saturated zone are largely horizontal and characterized by sharp upper and lower plume boundaries which indicate low lateral and vertical dispersion (Robertson et al, 1991). Thus, a 1D approximation is adopted to represent the major processes occurring along the narrow plume core, assuming the effluent source to be below the water table.…”

“…Most studies on nutrients in groundwater systems focus on tracking nutrient concentrations along flowpaths (Robertson et al, 1991;Harman et al, 1996;Ptacek, 1998;Corbett et al, 2002;Griggs et al, 2003). Nitrogen dynamics in groundwater are relatively well-studied.…”

“…The model is applied to field data for two oxic septic plumes propagating in the saturated zone of two contrasting aquifers, one rich in calcite, the other in Fe oxides (Robertson et al, 1991;Robertson et al, 1998;Robertson, 2003). Given the low concentrations of dissolved P and associated high analytical uncertainties in the measurements, in particular in the saturated zone of the Fe oxide-rich aquifer, the aims of this modelling exercise are to capture the trends in nutrient concentrations in two contrasting aquifers using a process-oriented model, to predict their evolution in time and to assess their sensitivity to changes in P input.…”