Stormwater quality management of highway runoff is a confusing industry. Design standards are not consistent and may be applied differently with great discretion. This can result in projects being non-compliant or, potentially more troubling, compliant with design standards but likely to result in substantial worsening of the existing environment.
The typical Australian approach to stormwater quality management of highways (and typically any urban catchment), is to target reductions across three “surrogate” pollutants: Total Suspended Solids (TSS), Total Phosphorous (TP) and Total Nitrogen (TN). These reductions are reported as a reduction from the mitigated design condition to the unmitigated design condition and do not provide any comparison to the pre-development loads. Furthermore, the forms of TSS present both before and after a highway is constructed may differ greatly, comparison of total annual loads for each scenario may be comparing apples with oranges.
There is also doubt over the suitability of TSS, TP and TN to reflect the types of pollutants coming from highways catchments. Monitoring of highway runoff reports on metals and hydrocarbons more often than the adopted surrogates with a growing concern regarding microplastics.
These issues combined, cause confusion and additional modelling scenarios when undertaking sustainability assessments which require demonstration of a ‘no-worsening’ or ‘no net increase’ in pollutants to receiving waters.
Understanding the pollutants of concern is also vital to inform the design of suitable mitigation measures.
Whilst most presentations on water quality management apply to development-scale or urban retrofits, this presentation will shine a light on the highway industry through a stormwater management lens. Highlighting multiple global literature reviews, discussions with global leaders and advocates in this space, as well as the presenter’s personal experiences over 20 years, this presentation will put forth recommendations at all scales from policy change down to interdisciplinary interactions on projects.
This presentation will discuss the major questions causing uncertainty including:
- Are TSS, TP and TN suitable surrogates for pollutants likely to present in highway runoff?
- Would a flow-based approach to stormwater quality management be more appropriate than reductions of individual pollutants loads?
- Are the sustainability ratings from Infrastructure Sustainability Council (“Dis-1”) suitable to assess water quality impact?
- Are our modelling methods able to adequately represent highway pollutant generation?
