Renwick Water Supply and Reticulation
The water supply demand versus aquifer level and demand versus reservoir level graphs will be posted into the Council web site twice a week again this summer. This will include the flow out of the treatment plant into Renwick. This is posted to help Renwick residents understand the townships water use and to help moderate water use during periods when demand exceeds the capacity of the aquifer or reservoir. The graphs will be updated on Monday and Friday.
Renwick Water Demand
If water demand reaches 55 litres per second residents are urged to reduce water use especially irrigation use for the following two hours. This will ensure the network continues to supply water at good pressure and to meet the requirements for firefighting.
The need to limit irrigation becomes particularly important during summer after very hot days. The aquifer level will drop as Wairau River flows get low, generally at flows below 20 cubic metres per second. As the aquifer level drops in reaction to low river flows we have to reduce the volume of water being taken from the wells. When this occurs water demand will not be able to exceed 50 litres per second during peak periods. If it does the reservoirs will probably run dry leaving Renwick with no water.
Renwick residents are doing a great job curbing their garden irrigation to ensure the water supply can meet demand during this very dry weather. Limiting water use is especially important in the period 6.00 pm until 8.30 pm when water use is at its highest throughout the household. The graphs are showing the aquifer and demand at very similar levels to this time last summer. If this trend continues it will be unnecessary to increase restrictions beyond the current voluntary restrictions. Keep up the good work. Let hope it rains soon to ease the situation.
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How do I know if I should irrigate?
During dry periods we will publish an alert on this web site to show if and when limits are exceeded. We will also publish this notification on the Antenno app. We would then encourage residents to look for this information during very hot afternoons before they commence irrigating or not.
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What is the Renwick Aquifer level graph showing?
This graph shows the aquifer level at well 3 in Terrace Road for last year (purple line) versus this year (green line). The water level is measured in millimetres above the top of the pump in the well.
The green line across the page at 1000 mm is the level at which the output from the well has to be reduced to protect the pump from being damaged by air which can be drawn into the pump when the water level is close to the pump intake.
Reducing the pumps output will elevate the aquifer level at the well when the pump is running. But when the output from the wells is reduced the volume of water available to Renwick is also reduced. At these times it is necessary for the community to reduce peak water demand. The demand has to be approximately equal to or less than the combined output from the bore pumps.
This is normally about 55 litres per second. The reduction can be voluntary or with the implementation of restrictions.
16 March 2021
The water level at the Renwick water supply bores is holding steady. Intermittent periods of rain in the head waters of the Wairau River are helping keep the river flows above the mean annual. This is helping recharging of the aquifer. With more rainfall forecast for the next couple of days levels should remain as they are for a while yet.
What does the reservoir versus demand show?
The reservoir level is shown with the blue line and water demand by the red line (in litres per second).
During normal demand the reservoir fills and empties in the range of 85% to 98 %. It fills when the well pumps are operating, providing 50 to 60 litres per second. This input flow is generally in excess of demand so the reservoir fills. If demand exceeds the input from the wells, the reservoir will continue to empty beyond the well pump start level of 85% until demand drops back below the output rate from the wells.
16 March 2021
History of Renwick's water supply
The public water supply to Renwick was first established in 1975. Water was abstracted from a well in Terrace Road and pumped to a storage reservoir at an elevated site close to Boyce Street, now in River Terrace. After basic treatment, the water was pumped into supply. Since then, two further bores have been installed and a tank farm of nine storage tanks added to the Boyce Street site. The water is aerated through a trickle aerator on the way to the tanks. The water is disinfected with chlorine before being pumped to the township.
The water does not meet the protozoal requirements of the Drinking-water Standards for New Zealand (DWSNZ).
Flow and pressure are monitored at the treatment/pump station and the two distribution pumps are automatically started and stopped to meet demand fluctuations. Around half of the reticulation is asbestos cement pipe; the remainder is split evenly between polyethylene and PVC.
Water demand varies from summer to winter. During the winter demand is generally 1,500 cubic metres per day, however; during summer this can increase to 3,500 cubic metres per day. Most of this increase is due to irrigation of gardens and lawns.
During recent summers, water restrictions have had to be implemented to reduce demand as the water level at the wells has fallen. When levels get too low and recharge cannot keep up with demand the volume of water that can be extracted has to be reduced. It's during these times there are water restrictions.
The following extract and minor additions help explain why there is a need for water restrictions; it is taken from the flyer produced by Renwick's Smart and Connected Water Working Group.
Renwick Smart and Connected Water Group - water information from flyer
During recent summers, there were many rumours about why we ran low on water. The group investigated the truth about Renwick's water supply.
The following information will dispel some of those myths and give you a greater insight into why we face water restrictions in the summer.
- The positioning of the three Renwick township wells was decided on for economic and practical grounds when the system was set up in the 1970s and has served the community well.
- The original water supply for Renwick was designed for a smaller population with smaller landscaped gardens and the wells can no longer cope with the demand in times of high summer use.
- We live in a thirsty' area due to the type of soil and climatic conditions (low rainfall, high sun and drying winds). Garden irrigation is by far the biggest draw on water for urban Renwick during the summer months, with approximately 70% of peak day water use being for this purpose.
- The wells cannot recharge at a fast enough rate to keep up with this demand.
- It is this slow recharge rate that is the primary problem rather than any issue with the aquifer itself, or the result of others using the water (e.g. vineyards, Blenheim, Woodbourne etc).
- The wells are at the edge of the Wairau Aquifer and the soils there are not as free flowing as those closer to the river. They have been drilled to the maximum possible depth and cannot be made more productive.
- Localised rainfall does not recharge the aquifer; it is the Wairau River (and Gibsons Creek) that recharges the aquifer
- As is common in older water supply networks, some water is lost from leaks in the Renwick water supply system. Council began a programme of work in 2014 to locate and repair leaks at the roadside and on private properties. This work is ongoing.
- There are three new wells on Conders Bend Road, which are closer to the river and expected to have a faster recharge rate than the current wells. Tests are being undertaken by Council to confirm this.
What's being done to address the summer water supply problem? Testing of new bores in Conders Bend Road was completed during the winter of 2016. The testing was necessary to prove the yields from the well are going to meet current and future demand. Council has a resource consent to use the water for Renwick urban water supply. The construction of a pipeline and pumping infrastructure to provide this new water to the Renwick Community is programmed for 2020/21.
In the meantime it will be necessary to implement water restrictions over summer when the current wells are unable to meet demand.
Use of asbestos cement pipe in water supplies
Asbestos cement pipes were used around the world for providing drinking water during the 1980s and 1990s.
There have been numerous studies to show there are no health issues associated with the use of this pipe for carrying drinking water. Their use has since been discontinued for a variety of reasons, including the fact that they fracture with earth movements such as earthquakes. Human health impacts only occur if asbestos fibre dust is inhaled directly into the lungs.
See the World Health Organisation Report on asbestos in drinking water:
Go to World Health Organisation Report
Marlborough District Council is replacing these pipes in Renwick and elsewhere as they age, or when repairing leaks becomes uneconomic. Asbestos cement pipes in High Street and Kowhai Drive have been replaced. Further pipe replacement is planned for 2020.
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Compliance with Drinking Water Standards NZ