An overview of water quality in relation to land use in New Zealand
Much information on the water quality of New Zealand rivers at national scale has come from monitoring at the 77 sites of the National Rivers Water Quality Network (NRWQN) operated by NIWA for 25 years.
A much larger number of water quality sites are operated by regional councils, although almost all have been running for shorter periods and there are difficulties with aggregating the regional data to provide a coherent national picture.
Four main categories of attributes that are measured in the NRWQN together define water quality:
- dissolved oxygen, temperature
- optical properties (e.g. visual clarity – Figure 1)
- the major nutrient elements, nitrogen (N) and phosphorus (P) (Figure 2), which promote plant growth including nuisance algae (nitrate-N and ammoniacal-N are also toxic to aquatic life)
- faecal microbial contaminants indicating the possible presence of infectious disease agents.
Sources of pollutants arising from discharge at a point may be usefully distinguished from ‘diffuse’ pollution arising from land use. Improved point-source (wastewater) treatment over several decades has resulted in New Zealand water quality issues being related predominantly to diffuse sources – which are much more difficult to manage than point sources.
Dissolved oxygen and visual clarity are fairly high in New Zealand rivers, indicating good water quality overall. Water quality is very high at ‘reference’ sites on rivers draining conservation lands. Conversely, there is widespread diffuse pollution from developed land, particularly pastoral agriculture. which mobilises fine sediment, faecal microbial contaminants, and nutrients. These diffuse pollutants all tend to increase with pastoral intensification.
River water quality can vary greatly from day to day. Even rivers that have good water quality most of the time may be turbid and polluted by faecal microbes during floods. Typically, diffuse pollutants move mainly during storm flows, whereas contaminants from wastewater are highest at low flow when dilution in receiving rivers is least.
Gradual improvements in the visual clarity of New Zealand rivers over 25 years possibly reflects soil conservation work and exclusion of livestock from channels by riparian fencing. However, a strong increase in nutrients, particularly N, has occurred over the same period in many rivers, reflecting intensification of pastoral agriculture.
Fortunately, there have been encouraging signs that decline in river water quality can be reversed. Nutrient enrichment appears to have slowed or decreased in recent years in certain catchments and regions of New Zealand where there has been major effort improved land management (e.g. riparian fencing and planting), soil conservation and nutrient controls.
Rob Davies-Colley — NIWA-Hamilton
P: 07 856 1725 E: davis-colley@niwa.co.nz
Figure 1. Measuring visual water clarity at a river site in the NRWQN. The technician is using an underwater periscope (black item in his left hand) fitted with a 45 degree mirror to view horizontally under the water surface. Visual clarity is the distance (measured by tape measure) at which the visual target (black disc on the left of the image) disappears. The black disc is fixed to a steel pole driven into the river gravel. Image - Graham Timpany.
Figure 2. Measuring nitrogen and phosphorus in river water samples from the NRWQN on the Flow Injection Analyser (FIA) in NIWA Hamilton’s water quality laboratory. Image - Janice Meadow.
