Solute dynamics in the bed sediments of a stormwater infiltration basin

Abstract

Constructed stormwater infiltration basins have been used extensively in urban areas to dispose of stormwater and compensate for reduced groundwater recharge. Even though stormwater basins are designed to retain suspended solids, stormwater sediments accumulating in the infiltration beds may act as a source of dissolved contaminants. Contaminant concentrations in the bed sediments of a 30-year old basin have been determined and those constituents identified that were most likely to be eluted during present infiltration of stormwater. Bed sediments had high concentrations of organic carbon (100 g kg −1 sediment dry weight), nitrogen (6.2 g N kg −1 sed. DW), phosphorus (6.2 g P 2O 5 kg −1 sed. DW), total hydrocarbons (2791 mg kg −1 sed. DW), and heavy metals (3341 mg kg −1 sed. DW). A total of 33 organic compounds were detected including 15 polycyclic aromatic hydrocarbons (52.9 mg kg −1 sed. DW,) and 10 polychlorinated biphenyls (547 μg kg −1 sed. DW). Chemical analysis of pore water stored in the infiltration bed and results of slow filtration column experiments showed that oxidation of organic carbon led to almost permanent anoxic conditions and resulted in the release of ammonium, phosphates and dissolved organic carbon during dry-weather periods. Hydrocarbons and heavy metals were rarely detected in pore water despite their high concentrations in the sediment. During rainfall events, the preferential flow of water through highly permeable areas of the infiltration bed with little sediment accumulation tended to separate stormwater sediment from infiltrating water, thereby reducing the leaching of nutrients. It is concluded that there can be a discrepancy between the contamination potential evaluated from the analysis of nutrient concentrations in inflow stormwater and the environmental risk resulting from the percolation of inflow water through a layer of permeable sediment.