Each year, the Hahei Wastewater Treatment Plant (WWTP) is independently monitored to assess its impact on the health of Wigmore Stream. This includes testing water quality and surveying stream life both upstream (before the discharge point) and downstream (after it).

Key Findings: (refer Section 5 Discussion and Conclusions of the report)

• No measurable harm from the treatment plant: Monitoring shows that the treated wastewater discharged into the stream does not cause ecological harm. Conditions upstream and downstream of the discharge point were broadly similar.
• Nutrient levels: While some nutrient levels (such as nitrogen and phosphorus) were higher downstream, the increases were modest and did not appear to affect aquatic life.
• Bacteria levels (E. coli): A key finding was that bacteria levels in the stream were significantly higher than in the treated water coming from the WWTP. In fact, the treated wastewater was cleaner in this regard than the stream itself, as measured both upstream and downstream from the WWTP discharge. This points strongly to contamination from either land use or  particularly from septic tanks on properties adjacent to the stream—as the likely source, a view that has been commonly expressed in the community
• Stream life: The variety and abundance of aquatic species in the stream remains limited, mostly due to natural factors such as tidal influence and soft sediment, rather than any effect from the WWTP.

Community Implications:

These results support growing concern among Hahei residents that existing properties close to the Wigmore Stream should be prioritised for connection to the WWTP, to reduce contamination risks and protect stream and beach health. The existing treatment plant has sufficient spare capacity to service all properties adjacent to the stream and these properties should be connected as a priority. It is the view of many in the community —and one we share—that the existing WWTP needs to be upgraded (capacity increased) to enable all residential properties in Hahei (including an allowance for future development) to be connected to the same with the first priority connecting those with ageing septic systems

What’s next?

A new water quality monitoring point has been added on a side stream (upstream of the WWTP outlet) to help trace other potential pollution sources. The report also recommends that Thames-Coromandel District Council review public health signage, given the health risks posed by elevated bacteria levels—unrelated to the WWTP—especially for swimming and shellfish collection in the estuary area.

Read the full report here