Nitrate in groundwater : a review of the issue with emphasis on the New Zealand situation

Groundwater is one of man's most important water supplies, having constant quality and being widely available. In New Zealand one quarter of the population derives its domestic water supply from this source. However, groundwater contamination looms as a major environmental issue of the 1980's. Dissolved nitrogen, in the form of nitrate, is presently the most common contaminant recognised. Natural resources, including groundwater, are used in both the processes of production and consumption. Co-ordination of these processes relies on the existence of a common measure of value and a detailed system of property rights. As yet, no exclusive transferable rights exist for groundwater in New Zealand. As a result, groups in society do not receive benefit for the maintenance and improvement of the resource, neither are all the costs of its use borne by those who use it. The process of contamination involves an interaction between two natural processes - the hydrologic and nitrogen cycles - and the physical and chemical nature of the compound itself. Human land use practices, principally intensive agriculture and effluent disposal, alter one or both cycles to cause groundwater contamination. The ingestion of nitrate-rich water has been associated with a number of health disorders, the most common being Infantile Methaemoglobinemia (blue-babies). Nitrate is also responsible for the eutrophication of surface waters and problems in the canning industry. Because of the nature of groundwater systems, cause and effect are separated in both time and space. Methods to reduce contamination either address the cause (source reduction) or the effect (water treatment, alternative water supplies, preventive medicine). The costs of these four types of response are fairly well known. However, the value of groundwater to society is not. This study outlines the cause and effect of contamination, and reviews the present situation with regards to New Zealand. A framework is then developed within which the benefits to groundwater protection can be measured/and the four response options can be assessed with the objective of maximising Social Welfare. This framework is an aid to decision-makers, it does not supplant political decision making, nor does it claim to make a morally correct decision.