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 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, two of which are in the East Coast Complex 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, South Marlborough 2007-2015
Overall, the two monitoring sites do not show strong fluctuations in water quality over the last two decades. The Waima River has superior water quality compared to the Flaxbourne, which is listed in the pMEP as degraded or at risk of degradation. The table and graph on this page show the historical water quality in this FMU has fluctuated but is slightly improved. Note this graph is historic; only the Waima/Ure River and Flaxbourne River are part of this FMU area.
The 2016 surface water SOE report provides insight as to the sharp dip in WQI around 2011-2013 for the Waima River and a wider trend around pH and nitrogen in the waterbodies:
“A unique phenomenon for the larger rivers in [the South Marlborough] group is the occurrence of consistently elevated pH values as a result of limestone deposits in the catchments. The Waima River catchment contains the largest area of pure limestone, which manifests in the highest pH values of any river currently monitored. Subsequently, exceedances of the pH guideline account for a large part of the reduction in the Water Quality Index for this waterway. The unusually high soluble inorganic nitrogen concentrations in the Waima River, however, are not a natural phenomenon. Concentrations are often higher than in the Flaxbourne River, which has a significantly larger proportion of pasture in the catchment. It is unknown what is causing the high nitrogen concentrations with leachate from large piles of organic material or direct discharges of organic waste being only two of the possible sources. This would need to be investigated further, particularly, if the Water Quality Index for the Waima River declines into the ‘marginal’ category as it has done in the past.”
The below table shows the historical WQI indices for the ECC FMU monitoring sites.
East Coast Complex SOE monitoring sites – historic quality measures | ||||||
|---|---|---|---|---|---|---|
Monitoring site | 2007-2009 | 2010-2012 | 2011-2013 | 2012-2014 | 2014-2016 | 2016-2018 |
Flaxbourne River | Fair | Marginal | Marginal | Marginal | Marginal | Marginal |
Waima/Ure River | Fair | Fair | Marginal | Fair | Good | Good |
The historic WQI for these two monitoring sites are summarised into the below chart, which shows that 50% of the WQI was marginal from 2007-2018. Only 17% of the WQI were in the good category, and 33% were classed as fair. There were no excellent, poor, or very poor results over that time. The latest WQI summary up to 2022 is in the Awatere Current Freshwater State page.
Ground Water Quality
In this FMU, groundwater and surface water are essentially the same within the alluvial gravels, and flows move between the ground and the surface depending on local conditions.
Local groundwater resources in the Blind River catchment are limited. A thin, discontinuous shallow riparian aquifer exists associated with alluvium overlying the papa-rock mudstone basement. This small natural reservoir provides baseflow for wetlands, Blind River, and freshwater flow into the coastal estuary. Where the alluvium is thin, the water appears as the river. In thicker parts, the river flow is lost to groundwater and the river dries up. This is frequent during summer months and explains why the flow is intermittent along its length.
No large aquifers have been found for the Flaxbourne catchment, which is mostly formed of bedrock with limited alluvium to store water. Even less is known about the Waima/Ure catchment. Recharge of these alluvial aquifers is from rainfall. While there is continual flow in the headwaters, flow becomes ephemeral in the middle and lower reaches where the natural steep grades of these rivers means that water drains more quickly than it is replenished.
Water quantity
Surface Water quantity
Due to the low amount of rainfall in the area, the associated mean flow in the Flaxbourne River is considerably less than northern rivers despite having a much larger catchment size.
Historically, ECC surface water was mainly used for domestic and stock water purposes. It was not used for large scale irrigation, frost fighting, or private damming until much more recently. The significant increases in irrigation quantity coincided with the rise in viticulture development.
In the northern part of this FMU, most crop irrigation water is either imported from the neighbouring Awatere River surface water. In other areas, irrigation water represents spring runoff that has been intercepted and captured in earth dams. Consented takes for irrigation of groundwater have existed in the lower reaches of these catchments for some years. Recent irrigation-related investigations have, however, identified a moderate groundwater resource in the coastal reach of these catchments.
Surface water quantity is measured through river flow rates (m3/sec) which are measured continuously. In the ECC FMU, this has been measured at the Flaxbourne River at Corrie Downs site since 2003. Given the length of recording, this site gives a good representation of water quantity over time.
East Coast Complex 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 |
Flaxbourne at Corrie Downs | 70km² | 0.47 m³/sec | 0.01m³/sec | 26.6 m3/sec | 0.00 m3/sec | 133 m3/sec |
The mean flow at the Corrie Downs site is 0.47 m³/sec, which is a mean over the entire time the site has been measured. Comparison of this mean flow to those of northern rivers highlights the relatively smaller volume of water in the Flaxbourne, such as the Rai River (Te Hoiere/Pelorus) at 11.4 m³/sec, the Branch River (Wairau FMU) at 19.6 m³/sec, or the nearby Awatere at 14 m³/sec.
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.
Groundwater quantity
Groundwater quantity at Ward has been monitored at the Needles Creek Gravels Aquifer, well P29w/0169 since 2003. As discussed above, the surface water and groundwater in this FMU are considered one system due to the natural hydrology. Accordingly, the well is also used as a measure of the surface flow. The well is part of the SOE groundwater monitoring programme, measuring the level of water (mm) above mean sea level. The well is not monitored for water quality.
The well was significantly damaged in the 2016 Kaikōura earthquake and subsequently repaired in 2018. Historically, Needles Creek lost surface flow in dry periods. Following the Kaikōura earthquake, horizontal and vertical ground displacement impacted water levels which are now generally much higher than they were pre-earthquake.
This is illustrated in the below graph from the 2019 Needles Creek Well Report prepared by OPUS, where the vertical red line signifies the Kaikōura earthquake and the dashed horizontal line is the existing minimum water level for water take resource consents, below which restrictions are put in place for those water users. The blue line represents the original Needles well, where the data after the horizontal line was impacted by the physical well damage. The replacement Needles well is the orange line and is at a much higher level. This points to a change in the hydrology of the system which has in all likelihood occurred due to natural factors.
Water level data at Needles Creek pre-Kaikōura earthquake well (blue) and post Kaikōura earthquake well (orange).
Source: OPUS, 2019
