Blooms of cyanobacteria (blue-green algae) in New Zealand’s freshwaters have increased in frequency over recent years. The toxins (cyanotoxins) produced by organisms forming these blooms have potentially severe health effects (eg, damage to the liver and the central nervous system). To address the increased threat to health posed by these two factors, the DWSNZ 2005 contain a separate section that specifies what steps have to be taken to show compliance with respect to cyanotoxins.
7.2 Characteristics of cyanotoxins
Cyanotoxins are chemicals of biological origin. The monitoring requirements, however, are different from other chemicals because cyanotoxins:
are present in water irregularly, or seasonally
are present at concentrations of health concern for only short periods, so that monitoring throughout the year is unnecessary
have health effects that are acute and potentially fatal at low concentrations; even in the absence of acute effects they may cause long-term damage
can increase rapidly in concentration (as do the numbers of the cyanobacteria producing them), hence sampling frequencies need to be higher than those required for other chemical contaminants when toxins are present.
Preventing algal bloom formation is the best defence against the presence of this type of contaminant in drinking water. Catchment control to prevent bloom formation, however, is beyond the control of water suppliers. Water suppliers depend on controlling levels of nutrients in water bodies as the primary barrier against the presence of these contaminants in their supplies. The NES could help ensure that activities in the catchment do not introduce large quantities of nutrients into waters that are susceptible to algal blooms.
When algal blooms occur, care in abstracting water from the source can minimise the cell numbers and toxin concentrations entering the treatment plant. When water entering a treatment plant contains cyanobacteria or cyanotoxins, or both, producing safe water can be difficult.
Cyanotoxins are present within cyanobacterial cells as well as in the surrounding water. The rupture of cells (lysis) during treatment can result in further release of toxin into the water. The water supplier therefore faces the difficulty of trying to remove cells whole without further damage to them. The destruction or removal of any free toxin that is present in the water also presents a difficulty. The efficacy of activated carbon in adsorbing toxins depends on the toxin; the efficacy of chemical oxidation by disinfectants (chlorine, chlorine dioxide, or ozone) depends on the treatment process and the toxin(s) in question.
NES note: The difficulty in treating water to remove cyanotoxins places the emphasis on good management of the catchment as the first defence against water becoming unsafe due to these contaminants. Warm temperatures, high light levels and high nutrient concentrations encourage the growth of cyanobacteria. The first two factors cannot be controlled, but when assessing the effects of a new catchment activity, any increase in nutrient loading of the source that might result from the activity must be taken into account. This is particularly important if the source is a lake or reservoir. An activity that decreases the water flow or water level in a source may also predispose it to algal blooms.
7.3 DWSNZ approach to cyanotoxins
Cyanotoxin measurement is expensive; for some toxins, any methods that can measure their concentration below their MAVs are unavailable. The DWSNZ encourages water suppliers to use surrogate measurements, or observations, to warn of an impending increase in the concentration of cyanobacteria cells – and therefore cyanotoxin concentrations – in the source water. The surrogate measurements used could include: algal cell counts, chlorophyll concentration, nutrient concentrations, water temperature, and the appearance of scum on the water surface. Collection of information about surrogates associated with a source is required if a source water has experienced algal blooms in the past, or a drinking-water assessor considers the source to be at risk of a bloom.
The nature of the surrogate measurements and the frequency at which these measurements have to be made by the water supplier are not specified in the DWSNZ. Water suppliers should identify which surrogate, or group of surrogates, they believe will be the most valuable for indicating bloom formation in their source water. Several surrogates may be monitored over a period of years to determine which surrogate, or combination of surrogates, is most helpful in predicting when a bloom is imminent.
Surrogate measures cannot provide a reliable estimate of toxin concentrations. Therefore, direct cyanotoxin measurements by the water supplier in the source water are required to establish the level of health risk. These measurements start when the surrogate reaches a level considered likely to signal elevated toxin concentrations in the source water. This action level is determined by the drinking-water assessor in conjunction with the water supplier using data collected by the water supplier, and is situation-specific. Toxin monitoring in treated water must also be undertaken by the water suppliers if the toxin concentration in the source water approaches 50% of its MAV.