An evaluation of the sources and controls of nitrate contamination in the Lake Horowhenua area

Abstract

Very high and steady levels of nitrate have been recorded in the Lake Horowhenua area. The nitrate concentrations have been slowly increasing over the last decade. This indicates that the quality of groundwater in the area is slowly degrading. The study area is underlain by two distinct strata; the Otaki sandstone formation of the last interglacial, and fluvial gravels from the last glacial. The two strata have permeabilities that vary by an order of magnitude. The sandstone has a much lower porosity, hydraulic conductivity and groundwater velocity. It therefore has a reduced ability to dilute contaminants. Nitrate concentrations of groundwater from bores in the area range from negligible amounts (0.1 mg/L) to over twice the New Zealand drinking water standard of 11.3 mg/L. The spatial distribution of bores with high nitrate concentrations, and data from surface waters, indicate the contamination is mostly in the north and east of the study area. This area is underlain by the Otaki sandstone. A lack of correlation between chloride and nitrate rules out septic tanks as a source of contamination. High levels of phosphate throughout the study area imply a combination of artificial nitrate sources such as fertilisers, and natural sources, such as sewage or animal wastes, of contamination. CFC-dating of groundwater from bores in the sandstone suggests that the contamination is not only a recent phenomenon but has a historical component as well. This implies past and ongoing land use practices as the main source of nitrate contamination in this area. Nitrate levels in bores to the west of Lake Horowhenua are low (<5 mg/L). Levels of other ions such as chloride and ammonia, however, are high. Occasional high numbers of faecal coliform bacteria were also noted. The main source of contamination west of the lake is likely to be animal wastes from the intensive farming in this area. Lake Horowhenua itself is extremely eutrophic. Nutrient rich groundwater enters the lake through several discrete springs on the eastern shore of the lake. The testing of core samples of sediment from the lake found nitrate and ammonia levels to be twice as high on the eastern side of the lake than on the western side. This implies groundwater has a major effect on the nutrient balance of the lake. The levels of nitrate were highest in the centre of the profiles indicating that (a) the contamination is 'historic' as old sediments contain high levels, and (b) reduction in nitrate concentration in the sediment is slow therefore there is an ongoing source of contamination even if 'primary' sources are cleaned up.