A groundwater quality survey of an unsewered, semi‐rural area

Abstract

A baseline groundwater quality survey of 120 household wells in an unsewered area at Yaldhurst, near Christchurch, was carried out between September and December, 1976. Ammonia-N levels were below the detection limit (0.02 g m-3) in all samples. Nitrate-N, conductivity, and chloride levels tended to vary according to well depth and location, being lower in the deeper wells and in wells located in an area of shallow, stony soils. Approximately 33% of the wells contained coliform bacteria, faecal coliform bacteria or faecal streptococci. A subset of 25 wells was sampled fortnightly from January to August 1977. From January to June, mean nitrate-N and conductivity levels remained stable at approximately 1.5 g m-3 and 11.2 mS m-1 respectively. An increase in these levels to 3.3 g m-3 and 15.8 mS m-1 in July-August corresponded with a rise in the water table. Leaching losses from soils probably accounted for most of the nitrate entering the Yaldhurst aquifers, the contribution by septic tank systems being estimated at 20-30%. No direct evidence of contamination of the wells by septic tank effluent was found, although 23 of the 25 wells exhibited intermittent contamination by indicator bacteria throughout the 8 month sampling programme. Although a localised health hazard may exist, septic tank systems in the area were not considered likely to constitute a health threat to the confined aquifers underlying Christchurch city.

Modeling the Impact of Clustered Septic Tank Systems on Groundwater Quality

“…Septic tank systems were considered to be the major source of fecal contamination. Subsequently, 25 wells were selected in 1977 for monitoring of fecal coliforms and NO 3 − (Sinton, 1982). These 25 wells had indicator bacteria present and high levels of electrical conductivity in the 1976 survey.…”

“…The impact of clustered septic tank systems on groundwater quality in New Zealand has previously been investigated in two field studies performed at a site near Christchurch, in 1977 (Sinton, 1982) and in 1986 (Close et al, 1989). Elevated NO 3 − levels and locally high fecal coliform levels in groundwater were found in both surveys.…”

“…Elevated NO 3 − levels and locally high fecal coliform levels in groundwater were found in both surveys. The 1977 survey (Sinton, 1982) also found a clear trend of increasing NO 3 − concentrations in the downgradient groundwater as the number of upgradient septic tank systems increased, but this trend was not found for fecal coliforms. The aim of our study was to establish a contaminant transport model that can predict the effects of clustered disposal systems on groundwater quality, using the same field site as Sinton (1982) as an example.…”

Transport and attenuation of bacteria and bacteriophages in an alluvial gravel aquifer

“…The presence of faecal micro-organisms in these aquifers has been demonstrated in surveys of household wells in unsewered areas (Sinton 1982;Ayrey & Noonan 1983), and in studies of ground water quality near effluent irrigation schemes and septic tank systems (Martin & Noonan 1977;Sinton 1986a). In addition, microbial tracing studies have been undertaken to determine the rates of movement of microbes in saturated gravels (Noonan & McNabb 1979;Pyle 1979;Sinton 1979Sinton ,1980aSinton ,b, 1986aSinton & Close 1983).…”

Distinguishing human from animal faecal contamination in water: A review

“…In New Zealand, groundwater quality surveys of some unsewered communities have shown that up to 33% of household drinking water bores contain faecal bacteria (Sinton 1982;Ayrey & Noonan 1983). Although septic tank effluent was the suspected source of this contamination, animal sources could not be discounted because of uncertainties over groundwater flow directions and velocities.…”