The groundwater system and nitrate contamination of the unconfined aquifer, Manakau, Horowhenua

Abstract

A groundwater nitrate contamination problem was identified in the Manakau area, south of Levin, late in 1994. Manakau Village itself consists of approximately 60 households with another 60 in the surrounding area. Many households get their water supply from domestic bores and all have septic tank systems. The predominant land uses are dairying and other farming (sheep, cattle and deer). There are also areas of market gardens, nurseries, and forestry. Nitrate concentrations in the bores range from negligible amounts (<0.5mg/L NO3-N) up to levels over three times the New Zealand drinking water standard of 11.3mg/L NO3-N. The spatial distribution of bores with high nitrate concentrations appeared to be random over the township. This pattern did not support a diffuse nitrate source, such as stocked pasture or fertilizer use. The low nitrate levels in local streams (<1mg/L NO3-N) also did not support a diffuse source. The possibility of point source contamination from septic tank or dairy shed effluent, however, was not confirmed with a strong correlation between nitrate concentrations and incidences of faecal coliform bacteria contamination. Expected correlations between nitrate concentrations and concentrations of chloride, ammonia, and phosphorus were also not found. However, a mutually exclusive relationship between high iron and nitrate concentrations was evident, suggesting a variability in redox conditions within the aquifer. High iron concentrations indicate a reducing groundwater environment where denitrification has taken place. In these conditions, nitrogen will no longer he in its oxidized NO3 form. It is thought that the redox variability in the aquifer produces the random spatial distribution of bores with high nitrate concentrations rather than simply the presence or absence of contamination. The unconfined aquifer is composed of two distinct strata - poorly sorted fluvial gravels from the last glacial and the relatively homogeneous Otaki sandstone formation of the last interglacial. Permeabilities of the two strata vary by an order of magnitude. This affects the groundwater flow direction on a local scale, with the preferred flow paths being through the more permeable gravels. An average hydraulic gradient of 0.018 was calculated and groundwater velocities estimated at 7.3m/day for the gravels and only 0.15m/day for the sandstone. The majority of bores with high nitrate concentrations were associated with the Otaki sandstone formation. Those sandstone bores that did not show high nitrate levels had either high iron, ammonia, or phosphorus concentrations, indicating that contamination may still be present but not in the form of nitrate. The sandstone is therefore contaminated because of its poor dilution capacity resulting from its low permeability and slow groundwater velocity. The main area of contamination is in Manakau Village which is underlain by the Otaki sandstone. All bores tap into this stratum. The lack of a significant up-gradient source area indicates that the source must come from within the sandstone. In the village this source is likely to be septic tank effluent. The density of tanks in the village is very high, at 1.4 per hectare. Outside the village nitrate nitrogen and oxygen isotope results suggest that the nitrate, formed from nitrification of ammonia, is being reduced and that the source of the ammonia is natural rather than artificial. This excludes fertilizers as a source, but the results do not differentiate between human or animal waste sources. Alternative water supplies available in Manakau require treatment (e.g. iron and manganese rich deep groundwater) or are not always reliable (e.g. rain water collection from rooftops). The shallow groundwater provides the most reliable water supply in terms of quantity, however, its quality is now compromised. The recommendation of this study is to concentrate on the reduction of contamination in the shallow groundwater, rather than ignoring this and relying on inadequate alternative water supplies. The feasibility of a sewage scheme for the growing Manakau population should be explored, along with nitrogen loading investigations in areas outside the village.