River water quality in New Zealand: an introduction and overview.

Robert J. Davies-Colley National Institute of Water and Atmospheric Research (NIWA), PO Box 11-115, Hillcrest, Hamilton, New Zealand. ABSTRACT: River water quality is important to uses and values within rivers and also in water bodies downstream. Water quality here encompasses: the physico-chemical attributes important to aquatic life and that vary diurnally (e.g. dissolved oxygen, temperature); optical attributes related to transmission of light through water (e.g. visual clarity); the major nutrient elements, nitrogen and phosphorus, that promote plant growth including nuisance algae (nitrate-N and ammonia-N are also toxic to aquatic life); and faecal microbial contaminants that can cause illness to people consuming water or recreating in rivers or downstream waters. River water quality is affected by both point-source pollution (wastewater discharge) and diffuse pollution from land use. In New Zealand, improved wastewater treatment over several decades has resulted in water quality being dominated by diffuse sources. The three major categories of diffuse pollution are (1) fi ne sediment, causing reduced water clarity and sedimentation of river beds and downstream water bodies, (2) the major nutrients (N, P), promoting aquatic plant (particularly nuisance algae) growth, and (3) faecal microbes, representing a hazard to human users of water or consumers of contaminated shellfi sh. Toxic metals may contaminate river waters draining relatively small areas, nationally, of urban and mining-affected land. River water quality in New Zealand, compared with Europe, North America and Asia, may be described as ‘fairly good’, and is very good (i.e. supports most values including habitat for aquatic life) in rivers draining the conservation estate – lands reserved for ecological and recreation purposes. However, this must be qualifi ed by recognition of widespread diffuse pollution from developed land, particularly pastoral agriculture, with fi ne sediment causing reduced visual clarity, faecal microbial contamination, and nutrient enrichment. Furthermore, river water quality depends strongly on fl ow state, and even rivers such as the Motueka (Tasman District, northern South Island), which is generally of ‘good’ water quality, are typically turbid and faecally polluted in stormfl ows. Grazing of livestock (on 40% of New Zealand’s land area) mobilises all three major categories of diffuse pollutant, with the result that rivers draining pastoral catchments are moderately degraded. Intensifi cation of pastoral land use generally increases the severity of diffuse pollution. While the worst water quality, nationally, is to be found in (a relatively few) urbanor mine-affected rivers in which other contaminants such as toxic metals are elevated, other soiland vegetation-disturbing land uses also adversely affect river water quality. For example, cropping (on 1.5% of the land area) mobilises nutrients and sediment. Water quality of rivers in plantation forests (7% of the land area) is generally appreciably better than that of rivers in pasture, and approaches the quality of rivers in native vegetation cover except for periodic ‘disturbance’ associated with harvest of the tree crop with associated removal of canopy cover. Fine sediment mobilisation remains an issue for plantation forestry. Unfortunately the water quality of rivers in New Zealand has been declining for the last 25 years, despite the very large expenditure on improved treatment (or diversion from rivers) of city and factory wastewaters – a clean-up tracked by long-term monitoring showing the reduction in certain pollutants such as ammoniacal-N and biochemical oxygen demand. The gains from point pollution control have been negated by steadily increasing diffuse pollution, particularly nitrogen and phosphorus enrichment from intensifi cation of pastoral agriculture. Fortunately, there have been recent encouraging signs that the decline in river health can be arrested, or even reversed, with stabilisation or improvement in a few (polluted) rivers in certain catchments and regions where there has been major effort on improved land management (e.g. riparian fencing and planting) and nutrient controls. Diffuse pollution is much more diffi cult to manage than point pollution. It remains to be seen whether the recent freshwater reforms, particularly the national ‘bottom lines’ protecting secondary contact recreation and aquatic habitat, can provide the desired outcome in terms of improved water quality. Enduring challenges include improving river water quality despite continuing pressures towards pastoral agricultural intensifi cation and urban expansion, and with the additional pressure of global warming driving increases in river water temperatures and declining river fl ows in some areas. Some work has been done internationally on valuing rivers, but less on valuing their water quality, and little research has been done in New Zealand. Valuing river water quality seems likely to remain a formidable challenge because of the multiple values and uses of rivers convolved with the multiple dimensions and attributes of water quality that affect (or are affected by) those values.