Update 2 – DWSNZ ongoing bore requirements - September 2016
The purpose of these updates is to create a knowledge environment where all members can access the best information to plan, operate and maintain drinking water networks which takes account of any of the learnings from the Hawkes bay and generally accepted best practice.
As new information / knowledge from the Havelock North contamination become available, we’ll include it with these updates.
Ground water bores have a number of ongoing maintenance and compliance requirements as per the Drinking Water Standards New Zealand 2005 revised 2008 (DWSNZ), in addition to the normal E.coli and other monitoring.
If a supply is relying on the secure bore status and there is no microbiological treatment after the bore, then at a minimum frequency of every five years the following needs to carried out:
- The water age status needs to be carried out to confirm the water is older than one year to prove the water is not directly affected by surface or climatic influences, that a protozoa risk is not present.
- Each well needs to have a bore head inspection (sanitary survey)
- Water Safety Plan review
The drinking water supplier could determine that more frequent assessments are needed, based on a review- of the water catchment or in response to significant land use changes within the catchment and/or recharge zone. A change in water characteristics or the presence of E.coli at a well head could also be trigger to reassess the secure crtieria.
From the DWSNZ there is also the continued compliance bore water security criterion 3. Note that it is a minimum sample frequency table.
|Supply type||Population served 6||Minimum sampling frequency||Maximum days between samples|
|Bore waters with interim security, 1 bores 10 to 30 m deep, 2 the bore representing a bore ﬁeld, 3 provisionally secure bores 4||Up to 500 7||Weekly||13|
|501–10,000||Twice a week||5|
|More than 10,000||Daily||1|
|Secure bore water supplies 5||All||Monthly||45 (135)|
1. Monitoring requirements for bore water granted interim secure status may be reduced to one sample per month for the remaining nine months independent of population band (maximum of 45 days between samples) provided no E. coli has been detected during the ﬁrst three months (section 184.108.40.206).
2. Monitoring requirements for bores 10–30 m deep drawing from unconﬁned aquifers may be reduced to monthly (maximum of 45 days between samples) for the ﬁnal four years and nine months provided no E. coli has been detected during the ﬁrst three months. This is independent of population band (section 4.5.1).
3. Monitoring requirements for the bore representing a multiple bore ﬁeld may be reduced to monthly independent of population band (maximum of 45 days between samples) provided no E. coli has been detected during the ﬁrst three months (section 4.5.3). As a prerequisite, all bores drawing from the same ﬁeld must have no E. coli in three consecutive monthly samples.
4. Monitoring requirements for secure bore water that has been downgraded to provisionally secure may be reduced to one sample per month for the remaining nine months independent of population band (maximum of 45 days between samples) provided no E. coli has been detected during the ﬁrst three months (sections 220.127.116.11 and 18.104.22.168).
5. Monitoring requirements for secure bore water supplies may be reduced to one sample per quarter (maximum of 135 days between samples) after no E. coli has been detected in 12 consecutive months of sampling after the bore water has been granted fully secure status.
6. If the bore is not the sole source, determine the population band by agreement with the DWA.
7. Sampling frequencies for E. coli in participating supplies servicing fewer than 500 people are discussed in section 10.
8. If the bore is used irregularly, variations to the sampling frequency must be agreed with the DWA.
There are three methods of proving compliance with the age criteria as identified in the DWSNZ, in order of preference used they are:
- Residence time tests carried out by GNS (look for a future guideline on this).
- Constant composition where conductivity, chloride and nitrate results are shown to be within a tight constant tolerance throughout the period.
- A verified hydrogeological model
The most commonly selected is residence time testing.
Bore head inspection (Sanitary survey)
This is an onsite review that needs to take in to account the water source, facilities, equipment, operation, and maintenance of a public water system for the purpose of evaluating its ability to produce and distribute safe drinking-water. This is required to be carried out by a person recognised as an expert in the field. It’s a balcony view of all the potential risks each bore has.
There are the obvious checks such as (but not limited to):
- Stock prevented from getting within 5 m of the bore
- Vandal proof
- Sloping concrete apron surrounding the bore with a 2 m radii
- The seal between casing and concrete apron is intact and extends at least 3 m down the casing (construction drawing required)
- Air vents opening are vermin proofed, facing down and above the flood level
- Where dual casings exits and bentonite is used to seal between the inner and outer casings, the bentonite level should be at the right height, (does it get checked after a reasonable earthquake?)
- Backflow protection exists and is checked
- Reasonable condition (no significant corrosion, is in working order)
- Saline monitoring (where applicable)
And then there are the wider aspects that also need to be looked at like:
- Changing raw water conditions between surveys.
- What changes have occurred upstream of the bores i.e. new trade waste practises, sceptic tanks, structures with deep foundations. (What might be heading towards the bores). This will involve forming a good relationship with the relevant Regional Council.
- Staff turnover and training
- Standby power
Water Safety Plans
The bore head inspection and water safety plans (WSP, previously termed Public Health Risk Management Plans)) are linked in that any potential risk identified during the inspection, has a plan with how that issue will be controlled should it occur.
Section 22.214.171.124. of the DWSNZ (Bore Water Security) states “The supply’s water safety plan must address contaminant sources and contaminant migration pathways.”
The outputs from the plan could range from following existing emergency chlorine dosing procedures through to projects identified to rectify issue and Capex funding will be available in year X.
The relevant Ministry of Health ground water related WSP templates are attached to help guide you.
Surface and Ground water sources.
Ground water abstraction - bores and wells
Ground Water Abstraction Springs
Link to the Guidelines for Drinking-water Quality Management for New Zealand, chapter on source water.
A Relevant Ground Water document
Another useful document on the design and operation of bores also from the Ministry of Health website, is this link below; don’t be put off by the small drinking water supply title.
The following public forum has been created for members to provide comments and their experiences.
Water New Zealand