Dry Weather Discharges from a Monitored Stormwater Catchment

Stormwater is known to be a major source of contaminants to aquatic environments within urban areas. Research and monitoring over the past two decades has provided a good understanding of the types of contaminants and the concentrations of these from different land uses and land covers, such as roading, roofing and residential areas. This monitoring is the basis of many models in use in New Zealand. However, there is somewhat limited information on some land use types, including stormwater from industrial catchments. The available information suggests the stormwater from these catchments can have very high contaminant concentrations and loads. In the Auckland region there exist a high number of stormwater outflows that discharge directly to the aquatic receiving environment without any treatment. These aquatic environments are often low energy, fragile estuarine areas that are susceptible to environmental degradation. Auckland Council conducted an 11-month study (stage one of the Whau Catchment Contaminant Study) that was aimed at identifying characteristics of stormwater runoff during rain events and, as part of this, supported the collection of a continuous time-series of hydrology and basic water quality parameters. An unexpected observation from continuous monitoring was numerous dry weather flow events were measured during the project that would have otherwise gone unmonitored. Dry weather discharges have been identified as a contributing issue to environmental degradation. Often dry weather discharges are associated with a combination of illicit discharges and failing infrastructure. Illicit discharge of contaminants to stormwater networks can occur at any time, and water quality measurements targeted to storm events may not capture such activities. Information into frequency and quality of dry weather discharges is unknown and goes unmonitored.