By Rob Davies-Colley
The quality of water in rivers and downstream lakes and estuaries strongly affects how we can use it and what value we place on it. And to define water quality, it is imperative to make long-term, national measurements.
Much information on the water quality of New Zealand rivers at national scale has come from monitoring at 77 sites in the National Rivers Water Quality Network (NRWQN) operated by NIWA for 25 years. A much larger number of water quality sites is operated by regional councils, though almost all have been running for shorter periods.
Important attributes that together define water quality include:
- constituents important to aquatic life that vary over a 24-hr cycle (e.g., dissolved oxygen);
- optical properties related to transmission of light through water (e.g., visual clarity);
- the major nutrient elements, nitrogen (N) and phosphorus (P), which 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.
‘Point’ pollution from wastewater discharges (entering receiving waters at a point) needs to be distinguished from ‘diffuse’ pollution arising from land use. Improved wastewater treatment over several decades has resulted in water quality in New Zealand being dominated by the diffuse sources – which are much more difficult to manage. There are three major categories of diffuse pollution: fine sediment, the major nutrients (N, P), and faecal microbes. Toxic metals may contaminate a small number of river waters within urban centres or downstream of mines.
Compared with Europe, North America and Asia, river water quality in New Zealand is ‘fairly good’ overall, though conditions vary greatly from place to place depending on land use. Water quality is very good (i.e. supports most values including habitat for aquatic life) in rivers draining conservation lands. Conversely, there is widespread diffuse pollution from developed land, particularly pastoral agriculture which degrades rivers with fine sediment (reducing visual clarity), faecal microbial contamination, and nutrient enrichment.
Water quality in rivers can also vary greatly from one day to the next. Even rivers that have good water quality most of the time may be turbid and polluted by faecal microbes during floods or high flows. Typically, diffuse pollutants move mainly during storm flows, in sharp contrast to contaminants from wastewater which are highest at low flow when dilution in receiving rivers is least.
A relatively few urban- and mine-affected rivers in NZ probably have the worst water quality because of mobilisation of toxic contaminants such as heavy metals as well as severe habitat modification.
All three major categories of diffuse pollutant are mobilized on grazed livestock pasture (40% of NZ’s land area), resulting in degraded water quality in rivers draining pastoral catchments. As might be expected, diffuse pollution from pastoral agriculture tends to increase with land use intensification. Cropland (about 1.5% of area) yields somewhat degraded water quality owing to mobilisation of nutrients and fine sediment. Plantation forests (7% of land area) generally yield appreciably better water quality than pasture, and rivers draining plantations sometimes approach the quality of rivers in native vegetation cover, although periodic harvest operations can mobilise fine sediment.
The water quality of numerous rivers in New Zealand has generally been declining over the last 25 years, despite a very large expenditure on improved treatment (or diversion from rivers) of city and factory wastewaters. However, the gains from this point pollution control have been outweighed by steadily increasing diffuse pollution, particularly nitrogen and phosphorus enrichment from intensification of pastoral agriculture.
Fortunately, there have been encouraging signs of late that river water quality declines can be arrested, or even reversed. Water quality appears to have recently improved 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), soil conservation and nutrient controls.
Continued improvements in river water quality are expected to be an enduring challenge while intensification of pastoral agriculture and urban expansion also continue. This will be further exacerbated by global warming driving increases in river water temperatures and declining river flows in some areas.
Dr Rob Davies-Colley is a principal scientist for water quality at NIWA.
Ballantine, D.; and Davies-Colley, R.J. (2013 in press). Water quality trends in New Zealand Rivers: 1989-2009. Environmental monitoring and assessment. (Journal code – EMAS-D-13-01488)
Davies-Colley, R. J. (2009). Land use and water quality in New Zealand – an overview. Water 162: 32-35.
Davies-Colley, R. J.; Smith, D. G.; Ward, R.; Bryers, G. G.; McBride, G. B.; Quinn, J. M.; and Scarsbrook, M. R. (2011). Twenty years of New Zealand’s National Rivers Water Quality Network: benefits of careful design and consistent operation. Journal of the American Water Resources Association 47:750-771.
Howard-Williams, C.; Davies-Colley, R.; Rutherford, K.; Wilcock, R. (2011). Diffuse pollution and freshwater degradation: New Zealand Perspectives. Water 172, 56-66