Disease-causing bacteria, such as Campylobacter and Salmonella, can be transmitted through drinking water, but monitoring of specific pathogens is not required by the DWSNZ. Instead, the indicator E. coli is monitored (see section 3.1.1a). The assumption made is that if the disinfection process has reduced the E. coli concentration to less than one organism per 100 mL, the concentrations of any pathogens originally in the water will also have been reduced to an acceptable level.
The frequency at which E. coli samples must be taken depends on several factors:
the nature of the disinfection process(es) in use: chlorination, treatment with chlorine dioxide, ozone or UV light, or no disinfection at all
whether the disinfectant concentration in the water is monitored continuously or non-continuously
the number of people being supplied with water from the treatment plant (if the disinfectant is chlorine or chlorine dioxide and is not continuously monitored)15
the “secure” status of a groundwater source.
Direct E. coli monitoring is required at all treatment plants, except where treatment is by continuously monitored chlorine or continuously monitored chlorine dioxide. To show compliance with the DWSNZ in these treatment plants, the water supplier can monitor performance parameters that show the treatment process is operating properly, if they wish, or monitor E. coli if they prefer.
5.1.1 Secure groundwater
The microbiological quality of groundwaters is often better than that of surface waters because concentrations of microbes in the water are reduced as the water percolates into the ground and moves through the aquifer. This improvement in water quality occurs because of processes such as filtration, adsorption and natural die-off of the organisms as they are carried through the soil, sands and gravels underground.
Where there is little opportunity for these processes to improve the water quality (eg, where groundwater is shallow), the microbial quality of the water may vary in response to weather events at the surface in much the same way as surface water quality may vary. For monitoring purposes, these shallow groundwaters are treated as surface waters.
Groundwaters that are isolated from events at the surface, because of their depth or protection by impermeable overlying strata, are of constant, high microbial quality. These groundwater sources are termed “secure”. A secure status allows a marked reduction in monitoring requirements for E. coli (see DWSNZ sections 4.3.10 and 4.5).
Treatment of secure groundwater is often not undertaken (eg, Christchurch City) because of the protection afforded by overlying strata and the further reduction of contaminants by natural processes, such as adsorption, filtration and in the case of micro-organisms, die-off. The time the water travels underground is a key factor in improving the microbial quality of the water as longer times allow for larger numbers of micro-organisms to die. Die-off does not occur with chemical contaminants; as a result a “secure” status is only an indicator of good microbial, not chemical, water quality.
NES note: When the potential effects of a proposed activity are being considered in relation to a secure groundwater, the following must be taken into account:
Disturbance of the substrata, such as might happen during quarrying or well drilling, could affect the secure status of the supply by opening the aquifer to more direct influence from the surface. Rigorous hydrogeological assessment of the potential effect of the activity is therefore important.
Where an activity is unlikely to disturb substrata, increased release of animal or human faeces into the environment may not affect the quality of a secure groundwater. However, it is still desirable that the level of pollution by the activity is kept to a minimum so that the natural treatment processes occurring underground are not overwhelmed.
Disinfection effectiveness depends on:
residual disinfectant concentration – the disinfectant concentration that remains after the disinfectant has reacted with any contaminants in the water
contact time – the minimum time an adequate disinfection residual is in the water
acidity/alkalinity (pH, for chlorine and chlorine dioxide-treated systems)
turbidity – microbes can stick to the surface of particles in the water that cause turbidity, and by doing so are protected from the disinfectant
flow rate through the treatment plant (for ozone- and UV-treated systems).
Problems controlling these factors can result in inadequate disinfection of the water, and E. coli being detected in the treated water.
Although all these factors are controlled at the treatment plant, contaminants in the source water can affect the turbidity (see ESR Report FW0778 section 126.96.36.199) and the residual disinfectant concentration by reacting with the disinfectant.
NES note: New catchment activities that lead to high levels of organic matter or turbidity in the source water, especially if combined with substantial variability in these contaminant levels, may lead to inadequate disinfection and thus to E. coli being present after treatment.
15 Without continuous monitoring there is the possibility of a system failure. The additional requirement of E. coli monitoring provides another check on the water quality. As the number of people supplied increases, the consequences of a treatment failure increase (ie, more people get sick), therefore there is a need to increase checks on water quality.