Contaminants within urban waterways sediment and the implications for flood protectionp rojects

Urban waterways throughout the world are often neglected and underappreciated stretches of land which receive untreated urban run-off from a variety of land uses. They once were a valued source of food for Iwi, but now due to our increased understanding of urban contaminants and how they may impact on the food cycle, this tradition is often prevented from occurring. Increasingly we see pollution warning signs due to transient biological pollutants but residing in the sediment on the bed of our urban rivers, several other contaminants, which have been accumulating for well over a century, present other issues to the health of our waterways and a financial burden for those looking to remove the sediment.

Following the Canterbury earthquake sequence in 2010-2011, the land in several areas of Christchurch dropped in elevation and combined with an influx of liquefaction derived sand and silt together with damaged stormwater drains all resulted in several significant flooding events along the Ōpāwaho Heathcote River. Christchurch City Council (CCC) have completed several initial flood protection works along the Heathcote River including dredging and reprofiling of the Woolston Cut.

The paper describes sediment assessments undertaken in 2017-2018, the assessment methodology and the analytical results with regards to the National Environmental Standards (NES) Regulations and also the Asbestos Regulations (2016). Due to the presence of elevated concentrations of asbestos, heavy metals and polycyclic aromatic hydrocarbons within the sediment additional controls were required during the dredging works. This paper outlines the contaminants identified in the sediment along an approximately 3 km stretch of the Heathcote River and the potential impact on the aquatic ecosystem and human health of those in contact with it. It also discusses the elutriation analytical technique designed to assess the release of contaminants into the water column due to sediment disturbance. Site management implications and cost implications on the CCC dredging programme are also discussed as is the need for an urban ambient asbestos concentration in soil.