Historic Freshwater State

Water quality

Surface Water Quality – WQI measures

Surface water, being above ground waterways, are easily recognizable across the Marlborough landscape – such as our iconic braided rivers. Surface water courses gain their flow through catchment areas as well as underground springs in some cases. A catchment is the area of land from which all rainfall and streams flows out into a river, a lake or the sea. The catchment of large rivers, such as the Te Hoiere/Pelorus River, can be divided into smaller sub-catchments, which usually represent the tributaries.

The Surface Water Quality State of the Environment (SOE) monitoring has been operational in Marlborough since 2007. The programme measures 56 sites regionally, six of which are in the Te Hoiere/Pelorus FMU.

The Water Quality Index (WQI) serves as a consolidated score reflecting the quality of river water. It is derived from nine chemical and physical parameters measured on a monthly basis. The WQI ranges from 0 to 100 with higher values indicating better water quality. The WQI simplifies comparisons of water quality across various waterways and serves as a valuable tool for identifying degraded waterways and prioritising improvement actions.

*WQI graph, Te Hoiere/Pelorus 2007-2015*

The five index classes represent water quality, with “marginal” and “poor” categories requiring improvement. The higher the index number, the better the water quality and accordingly the closer to “excellent” WQI rating. Read more about our surface water SOE monitoring programme below:

Go to the SOE monitoring reporting page

The below table shows the historical WQI indices for the Te Hoiere/Pelorus FMU monitoring sites. This data is also shown on the graph right and summarised into the table and chart below, which show the water quality in this FMU generally fluctuated but also improved across many of the sites from 2007-2018.

Te Hoiere/Pelorus WQI 2008-2018
Monitoring site	2007-2009	2010-2012	2011-2013	2012-2014	2014-2016	2016-2018
Ronga River	Not measured	Marginal	Marginal	Marginal	Marginal	Fair
Opouri River	Fair	Fair	Fair	Fair	Fair	Good
Rai River	Marginal	Fair	Fair	Fair	Fair	Good
Mid Pelorus	Good	Good	Good	Good	Good	Excellent
Wakamarina River	Good	Good	Good	Good	Good	Good
Lower Pelorus	Marginal	Marginal	Fair	Fair	Fair	Good

Recreational Water Quality

The recreational water quality programme evaluates the health risk to swimmers at 16 popular swimming location within the region, 8 of which are river sites, through the monitoring of Enterococci and E. coli levels. To assess the risk to water users, the council collects weekly water samples from the most frequented beaches and river sites during the summer months. Measuring the concentrations of all harmful microorganisms, or pathogens, in these samples is both complicated and costly. Instead, samples are analysed for indicator bacteria, which are easier to measure and typically coexist with harmful microorganisms. E. coli serves as the indicator bacteria for river samples. The results are reported each year and provide quality trends over time.

Today, two types of reporting are used for recreational water quality monitoring: NPS-FM 2020 attribute bands and Suitability for Contact Recreation Grades (SFR grades). Prior to the NPSFM 2020, only SFR grades were used. Determining the SFR Grade for a swimming site is done in two parts. The first part is an assessment of the health risk from all potential faecal sources within the catchment. Based on the assessment, sites are assigned one of five Sanitary Inspection Categories, SICs. The second part of the SFR grading is the Microbiological Assessment Category (MAC). The MAC is calculated from bacteria concentrations in samples taken at a site over a period of five years. You can read more about the process for SFR grades on our Recreational Water Quality Report page:

Go to the Recreational Water Quality Report page

The grades can be summarised as follows:

Three monitoring sites are in this FMU: Rai River at Rai Falls, and two Pelorus River sites – at the Pelorus Bridge and further downstream at Totara Flat, as shown on the map right.

The Rai River site was discontinued for monitoring in 2017 due to the very low numbers of users at the site. However, due to the NPS-FM 2020 requirements, this site was recently re-established for monitoring.

Due to the length of time necessary to derive a MAC result, there are several years without SFR grades where the data was either not collected or insufficient to assign a grade. The Rai Falls site has been regraded as of 2022/2023 season.

Historical SFR grades in this FMU back to the start of the monitoring programme are summarised below:

Historical SFR grades for Te Hoiere/Pelorus FMU river monitoring sites, by summer season
Site	09/10	2010/11	2011/12	2012/13	2013/14	2014/15	2015/16	2016/17	2017/18	2018/19
Rai Falls	V. Poor	V. Poor	V. Poor	V. Poor	V. Poor	V. Poor	V. Poor	Poor	N/A	N/A
Pelorus Bridge	Fair	Fair	Good	Good	Good	Good	Good	Good	Good	Good
Totara Flat	V. Poor	V. Poor	V. Poor	V. Poor	Poor	Poor	Poor	Fair	Fair	Fair

The above table and chart show improvement in SFR grades over the ten years however the Rai Falls and Totara Flat sites had several years with Very Poor or Poor water quality. On the other hand, the Pelorus Bridge site has been shown over time to have the best water quality of the three and some of the best in the region. A key factor to this is that the Bridge site is upstream of the Rai River and accordingly is not influenced by water quality from the Rai. The Totara Flat site on the other hand is downstream of the confluence and so is influenced by the Rai River water quality.

Rainfall has a direct impact on E. Coli concentrations and therefore the quality of the site for recreational use. Generally, the highest E. Coli readings occur in the days after the highest rainfall events. Accordingly, the ‘unsafe’ concentrations were usually caused by runoff because of rainfall. This is one water quality factor that the extensive Te Hoiere/Pelorus project launched in 2019 has aims to improve water quality in this area. The project is further discussed below:

Go to the Te Hoiere Project website

Ground Water Quality

The scale of groundwater resources in this FMU are small due to the mountainous topography of the area.

Groundwater systems are riparian-type aquifers, formed by the alluvial gravels that fill the base of the river valleys. Due to their narrow extent, they have limited storage and rely on continual recharge from the associated rivers or streams and rainfall. It is likely that water moves back and forth between the aquifers and the rivers depending on the nature of the sediments and the gradient of the river reach.

The Council has operated a groundwater quality monitoring well at Rai Valley (#10323) since 2012, following an exploratory well investigation. This is a relatively recent extension to the overall groundwater quality network which is predominantly within the Wairau FMU. This site has now been monitored quarterly for 11 years, as part of the State of the Environment Groundwater monitoring programme, for a range of quality indicators shown in the table below together with their 5-year median measures:

A focus of this monitoring well is to assess land use impacts on groundwater. Council are particularly interested in how groundwater in this area interacts with streams and the contribution of drainage water from land on the terrace towards the Rai River. Well #10323 taps an unconfined aquifer on a terrace of the Rai River and is relatively shallow being screened between 6 and 9 metres below the surface.

Water quantity

Surface water quantity

Due to the high amount of rainfall in the area, the associated mean flow in the Rai and Kaituna Rivers is more than southern rivers despite having a smaller catchment size.

Historically, these surface waters were mainly used for domestic and stock water purposes. It was not used for irrigation until much more recently in the area’s history. Irrigation (primarily dairy irrigation) did not commence until the early 2000s.

Surface water quantity is measured through river flow rates (m3/sec) which are measured continuously. There are two river flow monitoring site the Te Hoiere/Pelorus FMU on the Rai River at Rai Falls and the Kaituna River at Higgins Bridge. The Rai site has been measured for water stage and flow since 1979. The Kaituna site has been measured since 1989. Given the length of recording, these sites give an excellent representation of water quantity over time.

Te Hoiere/Pelorus surface flow sites – water quantity
Monitoring site	Catchment area (monitoring site only)	Mean flow	7-day MALF (mean annual low flow)	Mean annual flood flow	Lowest recorded	Highest recorded
Rai Valley at Rai Falls	211 km²	11.4 m³/sec	1.25 m³/sec	345 m³/sec	0.58 m³/sec	827 m³/sec
Kaituna River at Higgins Bridge	135 km²	3.96 m³/sec	0.42 m³/sec	123 m³/sec	0.15 m³/sec	175 m³/sec

The mean flow at the Rai River site is 11.4 m³/sec, which is a mean over the entire time the site has been measured. When compared rivers further south in the region, the Rai River has a much lower mean flow – the Wairau at Tuamarina has a mean flow of 100 m³/sec, significantly higher than both waterways in this FMU however this is to be expected. The Branch River at Weir Intake has mean flow of 19.6 m³/sec, also in the Wairau FMU and a larger flow than the Rai. Interestingly, the Rai River is closer to the Awatere River terms of mean flow, being 14 m³/sec. These comparisons assist with considerations of scale, particularly in comparing the Rai River to the Wairau in terms of the volume of water in the waterway.

The 7 day mean annual low flow (MALF) is an important measure, as this shows the mean flow during an average summer dry period. Irrigators should consider this flow rate in the context of their irrigation consents and associated flow cut off conditions, as this flow rate is occurring during the summer days when irrigation requirements are high.