CULVERDEN URBAN WATER NETWORK

Transcription

1 Water & Sanitary Assessment Community of Interest CULVERDEN URBAN WATER NETWORK Public water supply Update/Revision #1 Dec 2004 May 2010 Upgrades, alterations & considerations after November 2004 New deep water bore on SH1, 2004 Problems with inadequate water storage for outage periods Failed water tests and points where contamination can enter Will need to comply with a PHRMP within next 3 5 years

2 CULVERDEN URBAN WATER Catchment/Description of Service Within a few months of adopting the Previous W&S Assessment, the new deep bore servicing Culverden was commissioned. The bore is 77m deep with a 1.5m closed sump at the base and 9m of screen from 66.35m to 75.35m. The water bearing zone is a mix of clean water bearing gravels and sections with boulders. The bore was tested at 40L/sec with a 30m drawdown. The un-pumped water was artesian above nominal ground level (0.45m 3L/sec). The consented take from this bore is 33.9L/sec with a maximum volume of 1200m3/day. The older 11.7m bore remains as a back up, with the consent covering takes from both sources, not exceeding 1200m3/day. The bore casing is 250NB and the screen also 250mm. Water quality from the bore (ph, hardness, mineral composition) has been excellent with much lower CO2 levels. Treatment The previous manual/pre-set chlorination system remains in place, so it can be switched back into the system, should the shallow bore be in use or following positive E Coli tests arising at the intake (plant). With reliance now on the deep bore, there is no other treatment in place or proposed for this network. Reservoir Capacity The current buffer storage is limited to the one 90m3/tank located between the bores and the surface boost pumps at the intake site. This level of storage is very low (around 1½ hours at peak demand times). While there have been a limited number of pump, control or bore related problems over the past 5½ years, water outage in the town is almost inevitable. With the daily flows peaking around 600m3/day, the reservoir (useful capacity to low level is 80m3) does not provide any significant storage so when the bore pumps cut out, the reticulation system typically runs out of water and is more problematic to refill with a lot of air and pipe debris arising from consumer taps when supply is re-instated. A proposal to consider increasing the storage was presented to the Culverden Water Committee in November 2009 to consider two water storage options. These being A. Purchasing additional land at the intake (land owner was willing to consider this) and installing a field of plastic tanks to supplement the existing concrete reservoir. A further 8 x 30m3 plastic tanks were considered, which would increase total storage capacity to around 330m3 (½ day peak demand) but with the pumps and storage remaining located 2km distant to the supply area. Cost, including land purchase estimate was around $95,000 (September 2009).

3 B. Relocating storage to the domain immediately southeast of the town, where storage would be closer to consumers and more flexible provided, should an emergency supply be linked to the neighbouring Amuri supply. The benefit here was having storage and boost pumping at the point of consumption, allowing greater control and eliminating the potential for supply line failures between the pumps and town (older AC pipe). This would also reduce line pressures on the AC, perhaps extending its useful life. Cost was estimated at $186,000 (September 2009). The preferred option by the committee was the lower cost option but the concept did not receive full support. It is proposed to put this into the 2011/12 LTCCP for consideration.

4 Adequacy of Water Quantity The deep bore has excellent yield and can provide for considerable demand growth (200% of current demand). The shallow bore is limited by the size of pump in place, but this is back-up only. Water volume available exceeds demand.

5 Culverden Monthly Average Intake Flows Dec 2004 : May Jun Jun Jun Jun Jun Jun 09 Datran has not recorded daily flows between 2004 and late This fault has not been picked up. The only available daily records are shown below, which provides only a glimpse of how the average monthly flows compare to actual daily flows. The zero flows in the chart below are signal/meter failures, not pump outages. The actual chart shows a flow variation between m3/day. The monthly averages (see chart above) indicate m3/day. Peak daily volumes are considerably higher than the monthly averages. It could be that peak demand is greater than 700m3/day on occasion. These flows are similar to the 2003/04 flow set in the original W & S assessment, but have not reached the highs of 1900m3/day prior to water meters being installed at consumer connections. 800 Culverden Flows Datran (2010 only) 700 m3/day Date 0 23 December January February March April May 2010

6 Water Quality Microbiological tests post November 2004 to May 2010 show - Intake 1890 samples taken 1863 comply (98.6%) 27 fail (1.4%) Reticulation 689 samples taken 670 comply (97.2%) 19 fail (2.8%) Concern existed as to how positive E Coli counts could arise from a bore screened below 66m, particularly when it has a positive artesian head. The water for the deep bore has not been dated and no estimates made on possible aquifer travel velocities. However, water contamination remains highly improbable in the deeper aquifer. Subsequent investigations surrounded the concrete reservoir and aeration tower (not used with the deep bore). The reservoir is fully sealed, so animal/bird activity cannot impact on water quality. However, there was small volume of silt on the tank base (1cm or so). The tank was partially de-silted in 2008 but equipment could not reach the other side of the tank floor, so cleaning was incomplete. The focus then turned to the aeration tower, which had plenty of evidence of bird activity inside, some very recent. This was seen as the most likely source of contamination as it was the only source of recent contact that the water had with organic material and bird/animal activity. This area was made bird proof and no further positive bacterial tests have arisen since this work was done. It is now planned to bypass the aeration tower permanently. Culverden Intake Turbidity - Dec 2004 : May NTU Date 0 1/12/2004 1/06/2005 1/12/2005 1/06/2006 1/12/2006 1/06/2007 1/12/2007 1/06/2008 1/12/2008 1/06/2009 1/12/2009

7 Culverden Reticulation Turbidity - Dec 2004 : May NTU Date 0 1/12/2004 1/06/2005 1/12/2005 1/06/2006 1/12/2006 1/06/2007 1/12/2007 1/06/2008 1/12/2008 1/06/2009 1/12/2009 Both intake and reticulation turbidity readings are very high. No turbidity samples have been taken pre reservoir, so it is difficult to determine whether the cause is primarily bore derived silts or tank sediment, re-suspended when tank content is low. Given that turbidity peaks during summer months, more than winter months (visual assessment of charts above) these also co-incide with times of peak demand when the reservoir can get very low with a lot more agitation in the shallow water occurring than when the tank remains mostly full between pump runs (winter months). There are three take-off points where water can be sampled for the plant (intake). The first is at the base of the aeration tower (between bore and reservoir), another post boost pumping and the third is 100m (approx) down from the pump-house in the AC main. There is no record of where each is taken so determinations of bore vs reservoir for sediment loads cannot be made. A full tank clean-out may provide the answer for this, along with some bore water turbidity readings. The proportion of samples above 1NTU & 4NTU (Dec 2004 May 2010) are- >1NTU >4NTU Intake 190/371 51% 59/371 16% Reticulation 169/275 61% 55/275 20% The average turbidity readings for the same period above are- Intake 2.45NTU Reticulation 2.95NTU

8 Peak turbidity values are in excess of 25NTU for both sets. There is only one water sample set for the new bore water, with all historical sets being for the shallow (11.9m bore). The sample set was collected post bore development. The only query must be the high iron reading, which may be related to initial casing corrosion, as there has been no evidence of iron problems to date. ph 9.7 iron 0.66g/m3 nitrate N <.1g/m3 turbidity 3.4NTU hardness 20g/m3 sulphate 0.9g/m3 conductivity 10mS/m CULVERDEN WASTEWATER There have been no changes to the position noted in the previous assessment. PLANNING FOR GROWTH AND DEMAND Water Further evaluation of bore water turbidity to address the high NTU readings within the system Full cleaning of the existing reservoir and checks made on all access points Presentation of the reservoir upgrade (and possible relocation) for the 2011/12 LTCCP UPDATED NETWORK RISK ASSESSMENT 1. Source Biological Contamination low risk with the more recent deep supply bore (60+m depth from ground level to the intake screen) but much higher with the back-up shallow bore deep bore artesian to above ground level but shallow bore not artesian and within shallow unconfined aquifer surface contamination risk must be considered as low with the primary deep bore but high (requiring disinfections) with the shallow back-up bore water are in the deep bore has not been established storage reservoir is covered, locked with raised weather and vermin/bird proof lids system under pressure so contamination eventuating from within the network is low and well managed flushing of lines post repairs to minimise re-contamination

9 2. Mineral Contamination from Source There are no elements of note for the deeper bore Higher turbidity and dissolved CO2 is a risk with the shallow bore which if not aerated, can corrode metal fittings within the network there are no indications (to date) of metal or chemical contaminants present at MAV levels or higher deep water is not considered corrosive in terms of plumbo-solvency so leaching of metal fittings under the priority water source is low. 3. Surface Derived Contamination risk of agro-chemical is low to moderate for shallow gallery deep bore not influenced by localised surface activities and contamination of nearby waterways 4. Particle Contamination Turbidity readings indicate high particle loadings, whether from the bore and/or supply is yet to be established Assisted settlement or filtration may be required to meet DWSNZ in the future 5. Flood Intrusion occurrence has been zero and the probability of surface water surging the intake site is very low protection is afforded with bore casing seals and sealed head-works risk of contamination is therefore low but risk of outage due to power faults and limited access during inundation is higher but not at a level warranting specific attention both bores are alongside sealed major traffic routes, off public land 6. Seawater Intrusion Not considered an issue with >50km to the coast and 175m asl. 7. Earthquake with the deeper casing, differential movement of sub-strata during earthquake may occur, but well logs do not show base-rock to fine zone changes, so shear failure is not thought to be high shallow back-up bore already has dislodged concrete casing sections, so further movement or collapse of the well casing during a severe ground shake is moderate to high. failure due to power outage or reticulation failure during ground shaking will be higher risk issues to attend to 8. Land Slump, Erosion & Subsidence Both bores are located on flat land with a consolidated alluvial sub-strata, so are not anticipated to settle or slump at any stage

10 river erosion does not pose any risk to either of the bores 9. Climatic Influences the water sources and reticulation are not at any measurable risk from wind, temperature or direct precipitation impacts. 10. Global Warming impact of changing rainfall intensity is limited to flood damage changes to water recharge from altering weather patterns is difficult to assess, given a lack of understanding of aquifer recharge patterns for the deeper of the two bores. 11. Man Influenced Impacts impacts on above ground assets are possibly limited to vandalism, with the intake, shed and reservoir being adjacent to SH8 vandalism occurrence has been nil to date, however unintentional damage of services within the reticulation area have been rare, with limited development within the urban area impact on the bores and water quality are negligible water theft and leakage due to unauthorised access to the network is low act of terror or sabotage is possible, as the network is not routinely policed nor is it tamper proof, but risk of occurrence is not high, as Culverden is not a significant or strategic location in a national context. 12. Water Treatment Plant Failure treatment is limited to manual set chlorine dosing only with the back-up shallow bore source only no correction for ph, water hardness or iron is currently in place for this supply, so failure of treatment hardware has little or no real significance to this network, unless water is drawn from the gallery SUMMARY OF BARRIERS TO CONTAMINATION a. Prevention at Source & Catchment Controls none for either of the bores b. Treatment for Micro-organisms & Chemical Contaminants Manual set chlorine rate for shallow back-up bore no treatment for deep bores c. Prevention of Re-infection in Reticulation none provided