Sediment – a carrier of nasties
The focus of the erosion and sediment control industry has always been to minimise soil loss and sediment export from site. Did you know that by reducing the sediment export from site you will also significantly reduce a lot of other nasties entering our waterways?
A misconception that our sediment basins only manage sediment, linked with misguided blanketed concerns relating to the use of Aluminium based coagulant, has led some to believe that the risks associated with enhancing sediment basin performance with coagulants is higher than the risk associated with sediment laden runoff discharge in overdesign rainfall events from traditional sediment basins. Although we should undertake due diligence on what coagulants or flocculants we use for each site, our Number 1 goal should be to limit as much fine sediment leaving site as possible. A review of available data by authors in the Houshmand et.al. – A Review of Urban Sediment Contamination paper found that concentrations of some contaminants can be directly linked to particle size. As shown below, silt and clay particles are transporters for heavy metals, hydrocarbons, phosphorus, pesticides and even nitrogen. Topo has also been involved in projects where we found that the majority of E. coli was also removed from runoff with the use of a coagulant in a sediment basin. The ability to remove E. coli simply in a flow-through sediment basin reduced the need for other treatment methods typically used to remove E. coli.
It is our thinking that as ESC practitioners our fundamental goal should be to limit soil mobilisation through erosion control techniques where possible, but secondly to do everything possible to limit fine sediment leaving site via our designed sediment controls. This emphasises on removal of fine sediment directs us to use the best available controls rather than the overuse of token Type 2 and 3 sediment controls widely used on sites. Although they have their place in the ESC toolkit, the use of Type 2 and 3 sediment controls such as rock filter dams and sediment fence that have limited effect in managing fine sediment will have little ability of managing the other nasties in site runoff attached to the sediment particles. When preparing Erosion and Sediment Control Plans (ESCP), practitioners should always look for strategies to manage fine sediment effectively so that we can not only efficiently remove sediment, but all other contaminants that bind to these particles.
