Pollution in the Piha Lagoon
- Auckland Council has now declared Piha Lagoon “Permanently Unsafe” and erected signs to that effect. They are not publishing their water quality test results for Piha Lagoons on their website.
- Recent testing by the R&R shows that the Piha Lagoon’s E.coli levels are directly affected by how much the tide can wash out the lagoon – this is evidenced by testing around the time of the big storm in September that brought levels way down.
- Now the mouth of the lagoon is closing up again and the levels are back on the rise.
- The Piha R&R agrees with a report by Tonkin Taylor that the flow levels need to be kept up – and the council needs to work on keeping the mouth of the lagoon open to facilitate this.
- It cannot be claimed that maintaining flow levels in the stream and lagoon will solve the pollution problem on it’s own, but it is clear that it is a key component that will significantly reduce the problem. We just need to agree how best this can be achieved.
The Piha R&R has strived for at least 25 years to achieve clean up of the pollution in the Lagoon which, over that time, has reached recorded Ecoli levels as high as 24,000 MPM/100mL(safe swim level 550). Over the last two/three years we have worked closely with the council to isolate the source of pollution and develop an urgent strategy to eliminate this health and safety risk. This resulted in a comprehensive study undertaken by Auckland Council consultant coastal engineers Tonkin and Taylor. Their solution is simply “A combination of focusing on reducing point source pollution and periodic excavation provides the most comprehensive approach to reducing contaminant levels within the Lagoon”
Over the last 5 months Dave Bryant and Graeme Carrie have, at Piha R&R expense, tested the Lagoon when it flows naturally from winter rain and large tidal/wave action. We confirmed that the T&T proposal (as quoted above) does indeed work. Keeping the water flowing minimises pollution. We recorded E coli levels as low as <10 MPM/100mL !!
Council strategies are being developed and implemented to fix septic and other point source pollution at the source. However nothing is being done to keep the Lagoon flowing yet this is a vital component of the solution noted by consultants Tonkin and Taylor . The water must be kept moving. This is a relatively simple and economical remedy. Fixing the septic and point source pollution will not alone fix the pollution problem.
Last year pollution closed the lagoon for the entire summer when 91% of the E coli tests were above the 550/100ml safe swim limit. This is significantly high when compared to the range of 27%-50% recorded from testing over the last 15 years.
WHY? Because the lagoon was blocked for most of the summer and denied tidal access and the consequent flushing twice every 24hours. The attached graph shows the negative effect for 2015 and the positive effect of regular flushing in 2016 as per David’s report (See below.)
Since the Council started their annual testing program at the beginning of November EColi levels, as reported by EcoMatters Trust, have been rising. 3/11 – 550; 10/11 – 480; 17/11 – 2800; 22/11 – 290; 29/11 – 770 and 7/12 – 450. We are keen to have surrounding conditions such as recent rain fall, tidal status, water temperature and lagoon depth recorded against water sample gathering to gain a more complete picture of what is affecting large variations in test results.
What we are advocating is for the Auckland Council to follow the advice received from their consultants, Tonkin and Taylor, and open the lagoon to flushing with sea water. This would be undertaken in conjunction with suitable high tides and/or rainfall when nature does not do the job naturally. It is envisaged that this may be required a maximum of three times over each summer but only if indicated by negative test results. If undertaken with attention to natural conditions, this should be achieved with a back-hoe or Bobcat type machine and any visible works would be obliterated naturally within a few tide cycles.
Thousands of visitors and tourists are now confronted with stagnant water and pollution notices, with the majestic Lion Rock behind. Small children are denied the pleasure of safe swimming. Rare and endangered native fish species die in the toxic de-oxygenated slime. We urgently need action. We face another summer of unacceptable, unsafe, unhealthy pollution at a destination marketed by New Zealand to the world as an example of one of our finest beaches.
Summary of the Piha Lagoon testing May to November 2016
David J. Bryant, Micro Biologist PhD (Sydney.)
Samples were taken at the following points.
Site 1 – lagoon outlet.
Site 2 – 10m down stream from Eel bridge beside the effluent disposal field in the dunes.
Site 3 – 5m down-stream from the storm water outlet from the Piha Road.
Generally the sampling was done at about mid ebbing tide.
All samples were tested semi quantitatively for nitrate, nitrite, phosphate, dissolved oxygen and pH using kits supplied by Wai Care. Flow rates and water turbidity were also noted.
Three Escherichia coli counts were also performed by Aqualab on samples collected by Mr. G. Carrie from each of the three sampling positions.
Two samples prior to the lagoon mouth being opened by heavy rain on and one sample after good tidal flushing has been established. The results are reported in bold type.
These analyses were paid for privately by the Piha R&R
27 May Site 2&3 PO4 detected O2 decreased. Water moderately turbid. No tidal flushing.
3 June Site 2&3 PO4 and NO3 detected O2 decreased. Water moderately turbid. No tidal flushing.
4 July Site 1 E. coli 340/100ml. Site 2 E. c. 420/100ml. Site 3 E. c. 440/100ml. No tidal flushing.
8 August Site 2 PO4 detected O2 decreased. Minimal tidal flushing.
28 August Sites 1,2,3 All clear. Good O2 levels. Minimal tidal flushing.
13 September Site 1 E. c. 340/100ml. Site 2 74/100ml. Site 3 320/100ml. No tidal flushing.
24/25 September High rainfall in the catchment, lagoon mouth scoured out and very good tidal flushing which has been maintained until November and should persist for some time.
28 September Site 1 E. c. 330/100ml. (possible sampling error – retest)
Site 2 E.c. 86/100ml. Site 3 E. c. 41/100ml.
19 October Site 1 E. c. less than 10/100ml. Enterococci less than 10/100ml. (The retest.)
26 October Sites 1,2,3 All clear. Good O2 levels.
10 November Site 1 NO3 detected. Site 2 clear, good O2 level. Site 3 NO3 detected. … Moderate O2 level. (Sampled at high tide.)
Prior to good tidal flushing being established, site two showed the presence of nitrate and phosphate and an occasional lowering of the dissolved oxygen concentration from the effluent disposal field of the South Piha sewage system.
Site three downstream of the Piha Road storm water outlet also showed the presence of nitrate and phosphate coming from Piha Road properties.
The most significant finding from this investigation is found in the Escherichia coli counts prior to and following the lagoon mouth being opened to tidal flush twice a day.
The lagoon was principally stagnant when the mouth entrance was closed to the sea and the input pre lagoon stream flow was causing the height of the lagoon and its area to increase markedly. After the lagoon mouth was opened by heavy rain the lagoon volume has returned to what it was prior to the Waitakere West Coast beaches sand accumulation of recent years. (King D. et al, 2006)
Tidal flushing helps to reduce the incidence of pollution quite dramatically. (Tonkin and Taylor 2015)
I do not suggest that this flushing is sufficient of its own. We still need to pursue the Community Board initiatives to improve our private and public sewage treatment systems and their disposal leach fields .We can ensure that the pollution load will be significantly lessened if we occasionally over the summer open the lagoon when E coli levels are unsafe. A trigger point can be established based on tidal rise and fall.
We need to continue sampling at the three points we have established and are applying for funding from a Community Organisation or the Waitakere Ranges Local Board to accomplish this.
King D. Nichol S. Hume T. (2006). Rapid onshore sand flux in a high energy littoral cell: Piha beach. N.Z. Journal of Coastal Research, 1360 – 1369, November 2006.
Tonkin and Taylor. Piha lagoon Flushing Enhancement Options. Preliminary Assessment. February 2015. Job no. 30363.vR1. Prepared for Auckland Council.