Analysis of Water Quality and Flow Data for the Addington Catchment (2015-2016)
Addington Brook is a stormwater-influenced Christchurch waterway. Substantial instream surface water quality monitoring has shown elevated solids, metal and nutrient concentrations along the brook, which is thought to be a major contributor of these pollutants to the Avon River/Ōtākaro system and downstream estuary. Longitudinal and spatial patterns of water quality within the catchment were investigated from four wet weather events sampled in the spring-summer of 2015-2016. Analysis included assessment of treatment performance of the KiwiRail Stormwater Ponds (Matipo Street ponds) at the top of the catchment. Sites throughout the catchment responded in similar magnitudes to rainfall, with higher water levels measured from stormwater runoff shortly after rainfall was recorded. Similar inflow and outflow water levels in the Matipo ponds indicate that the detention system is mitigating peak inflows to some extent. Large amounts of TSS enter Addington Brook downstream just before it converges with the Avon River/Ōtākaro so targeting pollutant mitigation at this point could substantially reduce the amount of solids entering the Avon River/Ōtākaro. Dissolved zinc is consistently and highly elevated above the relevant Land and Water Regional Plan (LWRP) in-stream guideline, despite the pond’s ability to retain large portions of it and should be targeted at the Deans Ave location where its concentration is consistently highest. Dissolved copper was effectively removed within the pond system and is not an apparent cause for concern further down-stream in this catchment. More metals were removed in the ponds, in all sampling years, when concentrations were greatest indicating that the capacity of the system does not appear to be exhausted. Given the ubiquitous nature of dissolved zinc in Christchurch urban waterways, it would be wise to implement source-control of this highly bioavailable metal, especially given dissolved fractions are much more difficult to remove than particulate fractions. Concentrations of TN (predominantly TKN) and of TP (predominantly particulate) between sampling locations are generally similar. Some nutrients are retained within the Matipo pond system although inorganic nitrate/nitrate concentrations in the outflow were higher during larger rainfall durations and depths, suggesting that nitrogen in the pond was converted from organic to inorganic fractions. TN and nitrate removal efficiencies seem to have decreased compared to estimates in 2008. Despite the ability of the ponds to significantly reduce net phosphorus overall, DRP concentrations consistently exceeded the LWRP guideline. Although solids concentrations were often very high in the stormwater, the ponds were quite effective at reducing these concentrations, which is most likely attributed to adequate settling potential within the ponds. Dissolved metal loads for the downstream site at Riccarton Ave measured during the four sampled events, were very similar to modelled loads. Overall, 2.3-2.8 kg dissolved Zn/event originate from impermeable surfaces (which comprise 79% roofs, 12% carparks, 9% roads) within the catchment while 45-84 g dissolved Cu/event originate from impermeable surfaces (39% carparks, 30% roads, 31% roofs).
SubjectsField of Research::09 - Engineering::0905 - Civil Engineering::090508 - Water Quality Engineering
- Engineering: Reports