Current Freshwater State

Overall, there are mixed results for the NPS-FM attribute measures. A sole E grade for E. Coli in Doctor’s Creek is the poorest reported attribute band in the whole FMU, which wsas unchanged between the 2020 and 2023 reports. There are a considerable number of B attributes, showing that there is room for improvement but attributes are not below national bottom lines. Nitrate and Ammonia NPSFM attributes are almost all in the A band across both reports.

The main causes of degraded water quality in the FMU are livestock access to waterways and erosion, leaching from cattle pastures, cropping, and potentially sediment from the establishment of new vineyards. Urban and semi-urban areas are also a source of contaminants through damaged sewage infrastructure.

E. coli grades are variable across sites, with Goulter and Branch Rivers showing A grades. These rivers have catchments that are predominantly native vegetation with low sources of faecal contamination. This was unchanged at the 2023 report. At the other end of the scale are sites that are below the national bottom lines—Mill Creek and Taylor River with D grades and Doctor’s Creek with an E grade. At the 2023 report, E. coli concentrations improved in numerous rivers and streams, including Doctors Creek and the Taylor River however this was not a large enough improvement to move up a reporting band.

Similarly, attributes of Dissolved Reactive Phosphorus (DRP) , Macroinvertebrate Community Index (MCI), and Macroinvertebrate Average Score per Metric (APSM) also show variability in results across monitoring sites, but there are several sites in the Wairau FMU with D grades for these measures.

In particular, the 2023 report shows the MCI attribute has five D bands reported in Tuamarina River, Mid Ōpaoa, Doctor’s Creek, Murphys Creek, and Spring Creek. These are unchanged from the 2020 report. However, we can see one improvement at the Lower Wairau monitoring site which improved from a D to C band for MCI. It is important to note that some waterways naturally exhibit lower MCI scores, such as spring-fed streams like Murphys Creek and Spring Creek. However, in other waterways, low MCI scores are primarily attributed to pastoral land use practices. Trend analysis reveals changes in MCI scores for several sites, primarily showing reductions. However, it is essential to note that a reduction of 1 MCI score per year is also observed for waterways almost exclusively covered in native vegetation, such as the Goulter River. This suggests that many of the seemingly degrading trends, particularly in the wider Wairau catchment, are likely a result of natural variability.

There is a lack of any sites within the A-band for the MCI attribute across the region, including rivers in catchments that are almost completely covered in native vegetation, such as the Goulter River. This not only applies to the NPS-FM state, which is based on the median score but also to MCI values for individual samples, which rarely fall within the A-band. This suggests that MCI scores within the B- band represent comparatively good river health.

Goulter, Branch, Ohinemahuta, and Lower Wairau River sites are all in the ‘Good’ measure for WQI and have been for successive monitoring years. The Upper Wairau site improved between 2020 and 2022 to reach a ‘Good’ measure too.

The 2020 SOE surface water report discussed the importance of the Goulter site, as this river is one of the very few larger tributaries of the Wairau River that remains predominantly in native vegetation. Measurements for this river will therefore give an excellent indication of natural water quality in a large river. The 2019/2020 monitoring data shows no exceedances of any of the guideline values, resulting in the highest possible Water Quality Index of 100. This is encouraging, as it confirms that measurements below guideline values can regularly be achieved by un-modified waterways.

In the upper and mid Wairau catchment, turbidity was identified as the main cause of reduced WQI. The highest turbidity was measured in the Waihopai River and was the result of the combination of frequent thunderstorms in the catchment at the time, as well as high mudstone content in the underlying geology in some areas. Localised high intensity rainfall events in the upper catchment commonly cause slips, which wash away slowly, causing elevated turbidity for weeks or sometimes months after the event. Although these slips occur upstream of the Mid Waihopai sampling site, trend analysis only shows an increasing trend for the Lower Waihopai River. This indicates human activity may be further increasing turbidity in the lower parts of the Waihopai River.

At the 2023 SOE report, the rivers with the lowest turbidity levels were the Goulter River and Black Birch Stream, both characterised by very small areas of non-native vegetation within their catchments. In contrast, the Upper Wairau River and Branch River, despite having similarly high proportions of native vegetation cover, exhibited higher turbidity levels. These elevated turbidity levels are likely attributed to geological factors similar to those found in the nearby Waihopai River catchment. Groundwater-dominated streams, such as Murphys Creek and Spring Creek, typically maintain low turbidity levels. However, Mill Creek stands as an anomaly, with activities in the surface catchment causing turbidity significantly higher than expected from a spring-fed stream.

In the 2023 report, Mill Creek, a groundwater-fed spring located west of the Wairau Valley township, stood out with the highest DIN concentration. Naturally, DIN concentrations are very low. This can be seen for rivers with a high percentage of native vegetation cover within their catchment such as the Goulter River and Branch River, both of which show good results across the WQI and NPSFM reporting. Higher DIN concentrations are also found in an area of springs within the Blenheim area, and include Murphys Creek, Doctors Creek and the Taylor River. Other waterways with elevated DIN concentrations are found in catchments with significant areas of pasture and higher rainfall, such as the Rai River.

In the 2019 NPS-FM Draft, attribute limits for DIN were proposed, though they were ultimately not included in the final release. Nevertheless, these limits are still valuable for contextualising DIN concentrations. The majority of sites regionally are within the A and B Draft NPS-FM bands, while the higher concentrations in two of the spring-fed streams put them into the D band.

DIN concentrations in the Blenheim springs have been decreasing over the past 14 years, with most of this change occurring more than 10 years ago. These springs are supplied by water from the Wairau Aquifer, and changes within the broader Wairau catchment are the primary drivers. These changes are likely closely linked to the extensive conversion of pasture into vineyards. Established vineyards tend to leach significantly less nitrogen compared to pasture.