A methodology to assess the potential impact of swales on groundwater quality

An important design criterion for sustainable urban drainage systems (SUDS) should be to support the management of water quality in receiving waterbodies and to minimise poor treatment performance for both acute short term and chronic long term situations. The impact assessment procedure described here for discharges from swales to receiving waters follows that which has been previously published (Ellis et al., 2012) and which is based on the quantification of pollution hazard indices (PIs) for differing land uses in a runoff catchment area. The procedure employs a simple semi-quantitative approach for the impact assessment of residual water quality that is carried forward to the final receptor body following treatment within a swale. The procedure is now applied to groundwater, as well as surface receiving waters, and is illustrated by reference to standard dry swale systems through consideration of both volumetric and pollutant distributions within the treatment system. Application in respect of TSS, nitrate, chloride, heavy metals (Cd, Cu, Pb, Zn) and hydrocarbons is demonstrated for a swale receiving highway runoff. The analysis shows that for TSS, metals and hydrocarbons between 22% and 34% of the total influent pollutant load is likely to be directed towards the underlying soils whereas only between 4% and 16% of chloride and nitrate might pass forward to sub-surface infiltration. The likelihood of breaching groundwater quality standards is assessed as being minimal or negligible although caution must be exercised where swale infiltration may be located within a sensitive groundwater zone.