HBRC monitors a range of physical and chemical attributes. Some of these can’t be managed directly, such as iron, manganese and hardness – seen in groundwater due to the geology of the aquifer. Iron, manganese and hardness affect water taste and colour, and can affect pipes and pumps. Levels of these attributes depend on the aquifer material - sand, gravel and rock - and the length of time groundwater spends in the aquifer. Other attributes - pathogens, pesticides and nutrients - can enter groundwater as a result of land use or discharge activities.
Humans are sensitive to dissolved nutrient levels, such as nitrate, but ecosystem health is much more sensitive. The threshold for (human) drinking water is 11.3 mg/L nitrate to Nitrogen, while ecosystems rarely have natural nitrate concentrations (annual median) higher than 1.0mg/L nitrate to Nitrogen.
Karamu Stream gains flow from groundwater springs, as do the Raupare and Irongate streams.
Groundwater in the Heretaunga Plains contributes to surface waters in the Karamū, Raupare and Irongate streams. The degree that ecosystem values are affected by nutrients depends on concentrations and the volume of flows to surface water. HBRC is in the process of modelling water flows and sources of contaminants, with more information to come on this.
The Group noted several bores showing slightly elevated nitrate concentrations in the unconfined aquifer, where localised land uses are likely to be the cause. The Group also noted occasional traces of E. coli in some monitoring bores and sought more information about phosphorous trends and levels. Phosphorus in deeper groundwater results from geology and travel time through the aquifer. Phosphorus is not normally expected in shallower groundwater - it generally binds to soil particles and doesn’t move into groundwater.
HBRC’s Regional Policy Statement (RPS) and Regional Resource Management Plan (RRMP) already have objectives, policies and rules in relation to groundwater management, especially for the Heretaunga Plains. These generally serve the outcomes sought by the group as it seeks to maintain groundwater quality across the Plains. However, specific attention will be given to the management of storm water discharges to groundwater. The Stormwater Working Group has yet to report back on this.
TANK Group will develop specific objectives relating to:
- Nitrogen – details to come as modelling informs on concentration and flows. Levels will be based on NOF and ANZECC guidelines
- based on The NZ Drinking Water StandardsE. coli –
- Pesticides – based on NZ Drinking Water Standards
The Group identified that further monitoring and investigation is needed to examine reasons and possible trends relating to phosphorus, isolated nitrate levels and occasional E. coli measurements.
Barry Lynch, HBRC Land Scientist, reminded the Group about the importance of reducing sediment loss from productive farmland. Sediment reduces the state of some freshwater attributes, it affects water values, especially ecosystems, in estuaries and coastal waters.
Some erosion is natural and occurs regardless of human intervention. HBRC’s SedNet model - introduced to the Group at meeting 23 - predicted if the catchments were fully forested with no human land use, sediment load would still be about 30% of what is happening now.
Under a pastoral system some sediment loss still occurs, but mitigation measures will reduce the amount. Measures include good vegetation management, including trees on highly erodible land. Stock exclusion from waterways will also reduce sediment, nutrients and e. coli in freshwater.
The Group received information about how sediment loss might be reduced and the relative success of different approaches, in Table 1.