Executive Summary
This report provides a summary of the current status of groundwater quality in New Zealand, and an assessment of changes over the period from 1995 to 2006. Data used in this study were sourced from the state of the environment monitoring programmes operated by 15 regional authorities, and from the National Groundwater Monitoring Programme run by GNS Science.
The study identified two major national-scale groundwater quality issues:
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contamination with nitrate and/or microbial pathogens, especially in shallow wells in unconfined aquifers
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naturally elevated concentrations of iron, manganese, arsenic and/or ammonia, especially in deeper wells in confined aquifers.
At about a third of the monitoring sites considered in this study the groundwater shows some level of human influence, with nitrate and/or sulphate concentrations in excess of natural background levels. At another third of the sites the groundwater shows little or no evidence of human influence, but due to high levels of oxygen in the aquifer any introduced nitrate or sulphate will persist and accumulate. At the remaining third of the sites the groundwater is oxygen-poor and is not likely to accumulate significant nitrate, but due to natural processes it commonly accumulates concentrations of iron, manganese, arsenic and/or ammonia that exceed water-quality guidelines.
The health-related guideline values for nitrate and indicator bacteria are exceeded at 5% and 20% of the monitoring sites for which indicator data were available, respectively.
At about two-thirds of the sites considered in this study groundwater quality is either constant over time or changing slowly, probably due to the natural process of water-rock interaction. Changes in groundwater quality over time are more rapid at the remaining sites, with patterns that suggest human influence. At 6% of all sites a dilution trend, possibly related to changes in pumping regime, is indicated by rapid and concurrent decreases in the concentrations of several major elements. Time trends in parameters such as nitrate and sulphate suggest increasing (12% of sites) or decreasing (10% of sites) levels of human or agricultural impact.
Elevated levels of nitrates and faecal bacteria in groundwater are almost certainly the result of human activities on the land. This conclusion is supported by the fact that elevated nitrate and bacteria levels are more often observed in shallow, unconfined aquifers (ie, those that tend to lie under flat expanses of urban and rural land) than deep, confined aquifers. However, this study has not been able to prove the relationship between different land use types (and intensity of land uses) and the magnitude or pattern of their impacts on groundwater quality. This is in fact a common result that has been observed in several previous studies in New Zealand and overseas, for a number of reasons. Firstly, an overwhelming majority of groundwater quality monitoring wells are located in developed catchments and it is therefore hard to make robust comparisons with groundwater in undeveloped areas (ie, pristine groundwater). Secondly, the quality of groundwater drawn from a monitoring well does not necessarily reflect the land use directly surrounding that well; water may have travelled from an area dominated by a different type/intensity of land use. Third, groundwater that has been impacted by land use might not have had sufficient time to move all the way from the aquifer’s recharge area to the monitoring well. In order to better understand relationships between groundwater quality and different types of land use, accurate knowledge of the age and source of the groundwater being monitored is required. This is presently a major information gap in New Zealand.
