By Lesley Bolton-Ritchie
The quality of the water in an estuary influences the health, abundance and survival of the plants and animals that live in or pass through it and the suitability of estuary water for contact recreation. For the plants and animals it is the concentration of toxicants and oxygen in the water that can affect the survival of species and excessive nutrient concentrations can affect the growth of nuisance macroalgae, phytoplankton and microphytobenthos. For contact recreation it is the concentration of faecal indicator bacteria and hence the likely presence of pathogens that can affect human health.
The following is a case study on water quality in the Estuary of the Heathcote and Avon Rivers/Ihutai in the south-east of Christchurch. The Canterbury Regional Coastal Environment Plan has assigned two water quality classes to this estuary. The red shaded areas in the map are designated as Coastal AE water, i.e. for the maintenance of aquatic ecosystems. The remainder of the estuary is designated as coastal CR water, i.e. for contact recreation and the maintenance of aquatic ecosystems.
For many years Christchurch tertiary treated wastewater was discharged into this estuary. This wastewater was a source of ammonia nitrogen, phosphorus, faecal indicator bacteria and pathogens to estuary water. Ammonia nitrogen often occurred at potentially toxic (to marine life) concentrations at sites in the vicinity of the wastewater discharge point. When the Christchurch City Council applied to Canterbury Regional Council to renew its’ resource consent to discharge this wastewater into the estuary, it was declined. On 4 March 2010 the wastewater discharge was diverted away from the estuary; the wastewater is now discharged into Pegasus Bay some 3 km from shore.
Within months of the diversion of the wastewater there was up to a 90% decrease in ammonia nitrogen and phosphorus concentrations in estuary water. This decrease was interrupted by the 2010-2011 earthquake sequence when raw sewage was discharged to the rivers and directly into the estuary because of broken infrastructure (raw sewage was discharged directly into the estuary in the Penguin Street area).
The dissolved inorganic nutrients in the wastewater also meant more than enough nutrients in estuary water to allow for the prolific growth of the nuisance algae sea lettuce and the red algae Gracilaria chilensis.
While there are still post-earthquake issues with infrastructure, in the main the quality of the river water flowing into the estuary now has the largest influence on nutrient concentrations in estuary water. Both rivers arise from springs that are fed from groundwater in the shallow aquifers. Notable concentrations of nitrate occur in the spring water (PDF). It is the nitrates in the spring water that now have the greatest influence on dissolved inorganic nitrogen concentrations in estuary water. However, there are other nutrients sources to the river and directly into the estuary including stormwater (at least 67 outlets into the estuary), point source discharges from industrial sites, infrequent sewage overflows, catchment geology and the presence of large numbers of waterfowl (PDF).
When wastewater was discharged into the estuary the suitability for recreation grade at all estuary sites was Poor or Very Poor. With the removal of the wastewater sites now have either a Poor or Good grade http://ecan.govt.nz/services/online-services/monitoring/swimming-water-quality/Pages/Default.aspx. The three sites that still have a Poor grade are within the area classified as coastal AE water. The concentration of faecal indicator bacteria at these sites is primarily influenced by faecal indicator bacteria loads in river water (from waterfowl and dogs; PDF) and one area supports an abundance of waterfowl.
Future improvements in nutrient and faecal indicator bacteria concentrations in estuary water can be achieved by improved stormwater quality and reducing the number of industrial point source discharges. It is unlikely that waterfowl or dog numbers will decrease. As this is an urban estuary there will always be human influences on water quality. However, the aim is to minimise the impact on aquatic ecosystems and to allow people to be able to use the estuary for contact recreation without having their health compromised.
Lesley Bolton-Ritchie is a coastal water quality and ecology scientist at the Canterbury Regional Council.