In general, the quality of Canterbury's groundwater and surface waters is extremely high. However, industrial, agricultural, and urban development has had varying effects on the quality of all the region's water bodies. Generally, water quality declines from the mountains to the sea, reflecting the increasing intensity of land use, changes in catchment geology, and the accumulation of contaminants.
Over the past 20 years, the quality of some of the most degraded surface waters in Canterbury has improved owing to a decrease in the impact of many point-source discharges to surface waters. However, there has been a subsequent increase in discharges to land, increasing the risk of groundwater contamination. Less attention has been paid to non-point source discharges, and these are now generally having a greater effect on water quality across Canterbury than point source discharges.
The following discussion summarises the state of water quality in this region, and describes some of the main influencing factors. The water resources have been divided into 4 main groups, based largely on their geography. The discussion on the water bodies of the Plains has been further subdivided, reflecting the diversity of water types in this area.
Lakes in the high country have the highest water quality of any surface water body in the Canterbury Region. However, all the small high country streams, small lakes, wetlands and tarns are vulnerable to contamination owing to their size.
Lake Alexandrina is the only high country lake recorded as being permanently enriched (i.e., having high nutrient concentrations) and is subject to periodic algal blooms. Lake Emma (Ashburton River catchment) and Lake Grassmere (Waimakariri River catchment) are also subject to nutrient enrichment.
High stock densities in the catchments of the small lakes and tarns, stock access to water bodies, grazing in lake margins and marginal wetlands, removal of riparian vegetation, high numbers of water birds, and drainage and/or over-sowing and top-dressing of wetlands will all adversely affect water quality and habitat values of high country water bodies.
Microbiological water quality in the foothills tends to be lower than in the High Country and Intermontane Basin zones owing to the more intensive land use and a general absence of dense riparian vegetation. The upper Selwyn River is an example, with sites around Glentunnel at times exceeding microbiological water quality guidelines for contact recreation.
There is a high risk of microbiological contamination of Banks Peninsula streams owing to stock having access to streams and defecating in or near them. Incidents of stream and spring contamination by pesticides have also been recorded. Surface water contamination is of particular concern on Banks Peninsula because stream and spring water provide the main source of domestic water supply.
Lake Forsyth (Wairewa) is eutrophic and the lake is subject to frequent toxic algal blooms.
Sediment from accelerated erosion on the Banks Peninsula hills discolours coastal waters. Concern has been expressed over the possible effects of sedimentation on the local coastal fisheries and other coastal ecosystems and on the clarity of the water for diving.
The impacts of human activity on water quality are of greatest concern in the smaller surface water bodies of the Plains, and in areas with high recreational use.
The Plains sections of the foothill-sourced rivers and groundwater-fed streams tend to be particularly vulnerable to contamination owing to the intensive land uses in their catchments combined with natural low flows (e.g., parts of the Opihi River system). The groundwater-fed streams often have nitrate concentrations, reflecting the quality of the groundwater. Many of the water bodies at times also support high populations of water birds that contribute microbiological contaminants (e.g., the Ashburton River).
All the water bodies in this area receive agricultural runoff, and some also have inputs from urban drainage systems. Most streams and the drains and stock water races that flow into them, are unfenced and stock generally have unrestricted access. Environment Canterbury is currently trying to identify the type and extent of contaminants in these systems and the need to manage them.
Concern has been expressed over present and future water contamination resulting from increases in dairying, especially in small catchments and where it is combined with border-dyke irrigation (e.g., North Waitaki floodplain streams, such as the Waikakahi Stream).
Low concentrations of pesticides (e.g., simazine and terbuthylazine,) have been found in a number of water bodies in the greater Christchurch area and other parts of the region. These concentrations fall well below the water quality guidelines to protect aquatic ecosystems;. However, for one river (the L2) two different herbicides were present in concentrations exceeding water quality guidelines for irrigation use.
Reduced water quality in the lower reaches of the Waimakariri River and the smaller foothill sourced braided rivers (the Selwyn, Ashburton and Opihi) is frequently a result of point sources of contamination, such as community sewage discharges, and, in the case of the Waimakariri River, meat processing wastes. However, these impacts are compounded by the cumulative effects of upstream non-point source activities, such as runoff from adjacent land and the effects of large numbers of water birds.
Lowland lakes, coastal lagoons, and estuaries, including Lakes Ellesmere (Te Waihora), Forsyth (Te Wairewa) and Wainono Lagoon, have the highest nutrient status of any water bodies in the region owing to their location at the lower reaches of their catchments where they act as a sink for contaminants from upstream. These water bodies also receive contaminants from runoff from surrounding intensive agricultural land uses.
The main water quality issues in urban water bodies result from contaminants (sediment, nutrients and hazardous substances) carried through the stormwater systems. Stormwater is contaminated predominantly from runoff from industrial and urban areas, and from runoff from roads and paved areas. Earthworks associated with residential developments can result in substantial increases in sediment discharge, particularly in hill catchments.
Although groundwater quality within the Plains aquifers is generally very high, there are exceptions:
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