Community groundwater supply source protection

Report: TR 2000/10

Author: John Hadfield

Community groundwater supply source protection makes good public health, economic and environmental sense. Groundwater resources are vulnerable to contamination. Cleaning up contaminated groundwater is complicated, costly, and sometimes may be practically impossible. By comparison source water protection costs very little. It is in the interests of the community, Environment Waikato, health and other local government authorities, to work together for more effective groundwater supply source protection.

Community groundwater supplies in the Waikato region include about 90 school and 28 district council managed supplies, as well as numerous motor camp water supplies. Nearly 90% of the registered school supplies in the Health Waikato area draw water from the ground. In 1998, only 1% of non-urban school supplies complied with the drinking water standards mainly as a result of insufficient monitoring.

There are many potential sources of groundwater contamination including human and animal effluent, industrial wastewater, landfill leachate, fertilisers and pesticides, leakage from pipelines and underground storage tanks. Contamination of groundwater usually happens gradually and can go unnoticed for some time. Shallow, water table aquifers with thin soil cover are most vulnerable to contamination, which generally enters from the ground surface. This highlights the need for proper well-head completion by sealing around the top of well casing with concrete to avoid direct access of contaminants e.g. micro-organisms.

The vulnerability of individual community groundwater supplies to contamination from the ground surface was assessed, where possible, using DRASTIC. This is an evaluation system for assessing the hydrogeologic setting and provides an index of relative vulnerability based on seven factors including depth and soil cover. About 25% of the school sites assessed were considered vulnerable (DRASTIC index > 150) with 17% being less than 10 m deep. Only four of the 28 district water supply wells had DRASTIC scores above 150. There was insufficient information to assess many of the supplies.

School groundwater supplies were sampled at source for routine water quality analyses in 2000. Eight supplies (9%) were found to have determinands which transgressed drinking water guidelines. Five of the transgressions were for manganese. Other transgressions were for nitrate, boron and copper. A further ten sites had determinands (mostly nitrate) over 50% of the drinking water guideline. Many sites had chemical concentrations that exceeded aesthetic guidelines, which relate to nuisance rather than health concerns. Common examples were high iron and manganese concentrations. The pH was also often outside aesthetic guidelines.

There is a plethora of legislation relating to drinking water management with health authorities, district councils and Environment Waikato all having responsibilities for aspects of community groundwater supply protection. Supply owners most importantly have responsibility to comply with requirements for potable water supply and a vested interest to ensure the wellbeing of supply users. Management and monitoring of the quality of a community supply is generally the supplier's responsibility. Environment Waikato's responsibilities relate to source water protection.

Community groundwater supplies should be registered with the Ministry of Health and monitored for compliance with drinking water standards. This is currently voluntary but likely to become mandatory with imminent changes in health legislation. The drinking water standards for New Zealand list maximum acceptable values for microbial (priority 1), chemical (priority 2) and aesthetic contaminants. Monitoring regimes are stipulated and are less onerous for supplies which can be demonstrated to be secure. Many community water supplies currently can not demonstrate compliance with drinking water standards.

Public health risk management plans are likely to be required by the Ministry of Health to address not only source protection but wider aspects such as treatment and reticulation. Well-head protection areas (WHPAs) are suggested in this report as an effective mechanism for groundwater supply source protection. Indicative WHPAs are included for some supplies to promote discussion.

Environment Waikato manages groundwater resources to maintain or enhance quality using a range of methods including: regulation of discharges; controlling well construction; working with industry in guideline development; quality monitoring; issue investigation and environmental education.

Opportunities exist in the protection of community water supply sources for mutual benefit through further co-operation and partnership. It is in the interests of health and environmental agencies as well as the communities to work together for safe drinking water supplies. Benefits of co-operation include information sharing; better understanding of contamination prevention; more effective protection measures and educational opportunities. Water quality information from school and district supplies required to demonstrate compliance with health standards is, for example, also useful as an environmental indicator.