Coastal Water Quality: Hauraki Rivers and Southern Firth of Thames, 2000-09

Several rivers in the Hauraki area flow in a generally northerly direction into the Southern Firth of Thames. Information from recent studies of the water quality of these rivers and the Southern Firth was collated and analysed. Information is also available for the 22 moderate-to-large discharges of sewage or industrial wastewaters that enter the rivers. The quality of the water in the Kauaeranga River was mostly excellent, being well oxygenated and clear, and containing low concentrations of total nitrogen (N) and total phosphorus (P), and moderate concentrations of Escherichia coli bacteria. However, this river accounted for just 8% of the combined flow of the Hauraki rivers. Conversely, the water quality of the Piako (10% of the combined flow) and Waitoa (7%) Rivers was generally poor, being somewhat oxygen-depleted and murky, and with particularly high concentrations of total N and total P (5–7 times higher than guideline values). Concentrations of E. coli were also high (6 times higher than guideline values). The water quality of the largest river, the Waihou (75% of the combined flow), was intermediate: at the most downstream site on the main-stem the water was often well oxygenated, but was murky, and concentrations of total N, total P and E. coli were 2–3 times higher than guideline values. The water quality of its main tributary the Ohinemuri River, however, was generally good. Many (67%) of the records of river water quality at the 17 sites have been reasonably stable over the past 20 years. Some 18% of the records showed important improvements, while 15% showed deteriorations. Several of the improvements have resulted from improved treatment of the wastewaters that are discharged to the rivers. In some cases these appear to have offset the effects of more intensive farming on the concentrations of total N and total P that are found in the rivers. At other sites, however, concentrations of total N have increased, probably as a result of the overall intensification of farming in the catchments of these rivers. Moderately-high concentrations of algal chlorophyll a were found in the estuarine section of the lower Waihou River, near the zone of the “estuarine turbidity maximum”. However, the species found here were mostly coastal, and so were unlikely to be actively-growing as the water was fresh and highly turbid; furthermore, there was no evidence of any biological uptake of N or P. Naturally-occurring processes associated with the mixing of fresh and saline waters can retain and concentrate small particles, and are probably responsible for the high concentrations of both suspended solids and chlorophyll found in this section of the river. Mass flows of total N and total P during 2000–09 were determined at 11 sites on the Hauraki rivers. Altogether the rivers carried about 3360 t/yr of nitrogen and 270 t/yr of phosphorus to the Southern Firth of Thames. The Waihou River carried 50–60% of the combined mass flows while the Kauaeranga River carried 2–4%; the Piako and Waitoa Rivers each carried about 20%. The 22 point source discharges contributed about 8% of the nitrogen and 25% of the phosphorus that was carried by the rivers. Background sources were estimated to contribute about 23% of the combined mass flow of total N and 28% of the total P. The remaining 70% of the combined mass flow of total N and 46% of the total P is likely to have come from diffuse agricultural sources in the rivers’ catchments. During 2000–09 the combined mass flow of nitrogen carried by the rivers increased at a rate of about 1% per year, while the combined mass flow of phosphorus decreased at a rate of about 5% per year. The water quality of the Southern Firth of Thames was generally good. The water there was mainly seawater, indicating that the freshwater inflows from the Hauraki rivers were generally highly-diluted. The water column was weakly-stratified and well oxygenated. Although murky, it was probably sufficiently well-lit to support algal growth. The water contained moderate concentrations of N and P, and low-to moderate concentrations of algal chlorophyll a, with values being substantially lower than those found in nutrient-enriched coastal waters elsewhere (e.g. Auckland’s Manukau Harbour). Purpose: The Waikato Regional Council operates a routine river water quality monitoring programme that includes a number of sites on the Hauraki rivers. It also issues the resource consents that permit the discharge of treated wastewaters to these rivers; consent holders are required to monitor the flow and water quality of these discharges and to provide the information to the council.

This report collates, analyses and interprets the available information and considers several aspects of water quality in the Hauraki Rivers, including: •the current water quality of the Hauraki rivers, •changes in water quality over the past 20 years, • current mass flows of N and P in the Hauraki rivers into the Southern Firth of Thames, • main sources of mass flows of N and P, and importance of consented discharges of wastewaters, •current water quality of the Southern Firth of Thames, in particular the concentrations of N and P, and the level of algal biomass suppported by these nutrients.

There are four moderately-large rivers that flow into the southern part of the Firth: the Kauaeranga, Piako, Waitoa and Waihou Rivers. The Waitoa joins the Piako River about 40 km upstream of its mouth. A fifth river, the Ohinemuri, is a major tributary of the Waihou River. In this report these rivers are collectively called the “Hauraki rivers”.

2000-2009.

Varied. Frequency of collection: Water quality has been monitored monthly at 17 sites on the Hauraki rivers, beginning during 1990–94. The water quality of the Southern Firth of Thames was studied during 2006–07, while that of the extensive estuarine section of one of the rivers was studied during 2009–10.

Gaps in collection: Discussed in detail in the report. Data quality: Varied.

Report available from Waikato Regional Council in digital or hard copy form.

Sensitivity/Confidentiality: None

Ongoing