Re-assessment of the Risks of Protozoa in New Zealand's Natural Waters

The publication of the 1995 and 2000 editions of the Drinking Water Standards New Zealand (DWSNZ) expanded the focus of drinking water treatment onto the risks of protozoa. A large portion of the costs of the upgrading work on New Zealand’s treatment plants since then has been in response to the addition of the protozoal requirements.

National baseline monitoring for protozoa in our natural waters has been going on since 2009, funded by the Ministry of Health and undertaken by Massey University. Over an 8.25 year period 28 sites across New Zealand were tested, including representative groundwater wells and springs, bush catchments, intermediate rivers and lowland rivers. The results show that:

None of the samples collected from shallow groundwater/spring sites have contained protozoa although 8% of samples contained E. coli. These sites were deliberately selected because they were shallow or not secure, and had a history of occasionally containing E. coli.

Less than 3% of bush catchment samples and less than 5% of intermediate river samples contained protozoa.

No supplies sourcing water from lowland rivers would be required to achieve more than 3-log removal of Cryptosporidium oocysts for protozoal compliance.

The Catchment Risk Categorisation approach in DWSNZ (for supplies serving up to a population of 10,000) requires that shallow groundwater/spring sources need to achieve 3 log credits, intermediate river samples need to achieve 3 or 4 log credits, and lowland rivers need to achieve 4 log credits. Although DWSNZ allows for a water supplier to collect and analyse 26 samples over the course of a year to determine their source’s specific protozoal risk, the $25,000 cost of this alternative approach can be a significant barrier for smaller water suppliers. The eight years of protozoal monitoring is showing that by using the Catchment Risk Categorisation approach, the risks of protozoa in a source water are likely to be overstated, particularly in groundwater.

The paper presents the results of the New Zealand monitoring for protozoa, considers this in the context of what has been found in the USA and elsewhere, discusses international legislative and best practice requirements and offers some provisional guidance on whether DWSNZ is too conservative. With DWSNZ likely to be revised as an outcome of the Havelock North Drinking Water Inquiry, this paper helps inform that revision process and may thereby reduce upgrading costs for smaller water supplies - particularly those that have groundwater as their source.