Sources of nitrogen and phosphorus in the Waikato and Waipa rivers, 2003–12

Report: TR 2014/56

Author: Bill Vant

About this report

It is convenient to divide the catchment of the Waikato and Waipa rivers into three roughly-equal sections: Upper Waikato, from the outflow of Lake Taupo to the Karapiro dam; Lower Waikato, from the Karapiro dam to the sea; and Waipa, the catchment of this major tributary of the Waikato River. Routine water quality and flow monitoring data were used to determine the mass flows or loads of the plant nutrients nitrogen and phosphorus at several locations in each sub-catchment during 2003–12. Each of the three sub-catchments contributed similar proportions—about one-third—of the mass flows of each nutrient. The combined mass flow of nitrogen was similar to that observed during the 1990s, but that of phosphorus was about 12% lower (due to the lower average river flow during 2003–12).

Some 19 moderate-to-large consented point sources discharge a variety of contaminants—including nitrogen and phosphorus—to waterbodies in the catchment. These sources include 11 sewage treatment plants of widely-varying size, and 8 industrial discharges—dairy factories, meatworks, power stations and a pulp and paper mill. Consent monitoring data were used to calculate the mass flows of nitrogen and phosphorus from these operations during 2003–12. Wastewater flows and nutrient concentrations varied widely, depending on the size and nature of the operations. For example, the pulp and paper mill discharged a large volume of low-nutrient wastewater, while the two meatworks discharged much smaller volumes that contained relatively high nutrient concentrations.

More than half of the combined mass flow of nitrogen from these point sources was contributed by just three operations, namely Hamilton sewage (26% of the total), Kinleith pulp and paper mill (20%) and Horotiu meatworks (12%). And nearly half of the combined mass flow of phosphorus was contributed by just two of these, namely Hamilton sewage (37%) and the Kinleith mill (11%).

The mass flows of nitrogen and phosphorus discharged from several of the point sources fell over the decade. The available data showed a drop of about 30% in the combined mass flow of phosphorus, and a 7% drop in nitrogen. These reductions were mostly due to ongoing improvements in wastewater treatment at the sites.

Altogether, the 19 point sources contributed about 7% of the mass flow of nitrogen carried to the sea by the Waikato and Waipa Rivers during 2003–12. They also contributed about 18% of the mass flow of phosphorus. The greatest concentration of point sources is found in the Lower Waikato sub-catchment, where they contributed 12% of the nitrogen and 31% of the phosphorus.

Naturally-occurring processes within the catchments also contribute to the nutrient mass flows in the rivers; and these processes would have operated prior to human development of the catchments. About one-third of the current mass flows of both nutrients is estimated to be due to these natural or “background” processes. A proportion of the mass flows from land that has been developed by people can be regarded as natural and essentially un-manageable.

Subtracting the contributions from point and natural sources from the overall mass flows carried by the rivers provides an estimate of the nutrient loads caused by development of the land—and in these catchments this largely means development of the land for pastoral farming. These diffuse, man-made sources contributed about 61% of the mass flow of nitrogen carried to the sea by the Waikato and Waipa rivers during 2003–12, and about 45% of the phosphorus.

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