Publications: Research reports and publications

Sharland and Packer Creek study: Water chemistry and microbiology

1 January, 2005
Cawthron Report 1048. Prepared for Nelson City Council.

EXECUTIVE SUMMARY

A recent review of ecological data from the Maitai River and its tributaries (Crowe et al., 2004) found that Sharland Creek, a large tributary of the Maitai River, had very high conductivity and nitrate-N concentrations compared with those found in the Maitai main stem. Both parameters appeared to have increased significantly over the last 20 years.

This study was undertaken in response to the review, which identified investigation of the factors causing elevated nitrate-N concentrations and conductivity in Sharland Creek as an important area for future research. Although nitrate-N concentrations and conductivity were the primary focus of this study, a wider range of chemical, physical and microbiological parameters (pH; turbidity; black disc; and E. coli) were measured concurrently, and are also reported here.

Sampling was carried out on three occasions: 18 August 2004; 6 October 2004; and 24 February 2005, each with different flows and flow histories. Fifteen sites were sampled, spread throughout Sharland Creek, Packer Creek, two small tributaries of Sharland Creek and the Maitai River (upstream and downstream of the Sharland Creek confluence). A variety of land uses were identified throughout the catchment including mature exotic forest, young (recently harvested) exotic forest, native forest, agriculture and the rifle range on Packer Creek.

Nitrate-N occurred at very high concentrations throughout the Sharland Creek catchment, with concentrations in both Sharland and Packer creeks considerably higher than that usually found in low elevation, exotic forest rivers throughout New Zealand. Conductivity values in Sharland and Packer creeks were within the range of conductivities commonly found at low elevation exotic forestry sites throughout New Zealand, but generally were higher than those at the Maitai River sites. E. coli concentrations were at the low end of the range generally found in low elevation exotic forest streams throughout New Zealand, and were consistently below guideline levels for contact recreation. Site means for water clarity (black disc) in this study generally were similar to values for lowland exotic forestry streams throughout New Zealand.

Nitrate-N concentrations, conductivity and pH tended to be highest, and water clarity poorest, in the upper and mid reaches and tributaries of Sharland Creek, upstream of the confluence with Packer Creek. Packer Creek generally had better water quality than the upper and mid reaches of Sharland Creek, with relatively low nitrate-N concentrations; lower conductivities; slightly lower pH values; and relatively good water clarity, even at high flow. Since water from Packer Creek tended to be of better quality than that from Sharland Creek upstream of the confluence, and since Packer Creek made a relatively large contribution to the flow downstream, water quality parameters in Sharland Creek downstream of Packer Creek tended to improve to some degree, due to dilution effects.

Thus, nitrate-N concentrations, conductivity and pH were all lower in the lower reaches of Sharland Creek than they were upstream, and water clarity was slightly higher. Most parameters showed some variation between sampling occasions, probably due to seasonal factors and the different flow regimes during and preceding sampling.

Inputs of nitrogen and the compounds causing high conductivity water appear to come from diffuse sources throughout the catchment, but in particular, from around the upper and mid reaches of Sharland Creek. Possible sources, such as inputs from extensive tracts of recently logged / young exotic forest and inputs from underlying bedrock, were considered, but no single source was apparent. Tracts of young (recently harvested) pines, and associated forestry roads and skid sites, were likely to be responsible for a large proportion of sediment inputs, particularly in the upper and mid reaches of Sharland Creek.

Of the water quality parameters measured in this study, inputs of nitrate-N and poor clarity water appeared to be having the most marked impact on water quality in the Maitai River downstream. Maitai River nitrate-N concentrations were consistently higher downstream of Sharland Creek, and nitrate-N loads from Sharland Creek were at least double those coming from the entire Maitai catchment upstream, resulting in dramatic increases in loads downstream. The high turbidity / poor clarity water entering the Maitai River from Sharland Creek reduced water clarity in the Maitai River and is also likely to have increased loads of fine sediment in the substrate downstream, particularly at high flow. In contrast, any impacts on conductivity and pH were minor and relatively uncertain, and there was no evidence of any impact on E. coli concentrations.