Sources of nutrients in rivers

Waikato Regional Council  does not have a specific monitoring programme to identify and distinguish nutrient sources. Instead, we collate information from existing monitoring programmes, and from specific records of nutrient loads from moderate-to-large point sources. The existing programmes are:

• Waikato River water quality monitoring programme.
• Regional Rivers water quality monitoring programme (which covers rivers throughout the Region, other than the Waikato River).
• Surface water flow programmes.

Water quality is usually monitored at monthly intervals, while flows are monitored continuously. Information is also obtained from other organisations including the National Institute of Water and Atmospheric Research (NIWA). Information on nutrient loads from point sources is usually obtained from:

1. Resource consent applications - the assessment of environmental effects.
2. Consent monitoring.

Monitoring sites

This indicator is based on information from seven river monitoring sites and 40 moderate-to-large point source discharges (refer to table below).

Monitoring frequency

Both Waikato Regional Council's and NIWA’s river monitoring information used here comes from monthly sampling. River flows are monitored continuously. The frequency of point source discharge monitoring can vary from daily monitoring at some sites, to a few estimates of effluent quality and quantity at others.

Monitoring history

• Phosphorus monitoring at Mercer in the Waikato River started in 1987.
• Nitrogen monitoring at Mercer in the Waikato River started in 1990.
• The Hauraki rivers (Waihou, Piako and Waitoa) monitoring started in 1993.

Measurement technique

We determine river flows from records of water level and the relevant ratings. We either measure or estimate effluent flows and use laboratory analysis to determine effluent quality. We get the water quality results from grab samples, which are analysed for:

• total nitrogen (as the sum of total Kjeldahl nitrogen and nitrate plus nitrite nitrogen)
• total phosphorus.

We determined the mass flows of total nitrogen and phosphorus for the Waikato River during 2003-12 and for the Waihou, Piako/ Waitoa rivers during 2000 - 09. We calculated the mass flows for the river sites from records of river flow and nutrient concentration. In each case, we applied relationships between flow and nutrient concentration to the continuous records of river flow. We obtained the nutrient mass flows from moderate-to-large point sources from unpublished consent monitoring records. We did not include small point sources – such as discharges from farm dairy effluent ponds – in the point source category. We regarded small point sources as contributing to the general nutrient loads from developed land.

We estimated the background (natural) mass flows from the specific nutrient yields for undeveloped areas (native forest) in the catchments. The mass flow from developed land was calculated by difference. We converted the nutrient loads into area-based values (in kg/ha/y), so that we could compare nutrient loads between different catchments. We got these values by dividing the nutrient load by the area of its catchment. Nutrient loads from background (natural) sources are generally relatively low. Usually, most of the nutrient load comes from people’s activities - either from direct discharges of wastewater, or from runoff and leaching from areas of developed land. Nitrogen loads from point sources are relatively low in all four rivers. But the non-point source loads from areas of developed land are high, particularly in the intensively farmed catchments of the Waihou, Waitoa and Piako Rivers. The loads from developed land tend to be highest in areas of intensive dairy farming. Loads of phosphorus from areas of developed land are moderate. Usually, the loads from point sources are low-to-moderate. However in the period prior to 2008, the point source load of phosphorus to the Waitoa sub-catchment was high.

It can be difficult to obtain the necessary information, particularly for the point source discharges. We also know that there have been changes to the nutrient loads discharged from some point sources since the period of study (or that changes are planned for the near future). As a result, this information is likely to become out of date fairly quickly.

Beard, S. 2010: Regional Rivers Water Quality Monitoring Programme: Data Report 2009. Environment Waikato Technical Report 2010/11, Waikato Regional Council, Hamilton.

The graphs show the amounts of nitrogen and phosphorus in four major Waikato rivers that come from background sources, point sources, or developed land.