Sources and storm loading variations of metal species in a gullypot catchment

The temporal variations of metal species in the outflow from a single gullypot catchment are discussed for two storms with differing hydrological characteristics, and the major contributing sources within the system are identified from an analysis of six discrete storm events. Under low flow conditions the dissolved metals are complexed by organics present in the gullypot liquor and interstitial sediment waters, and the strongly bound fraction predominates in the gullypot outflow. As the flow increases the Chelex removable fraction becomes important due to acid washing initially of road-surface sediments and subsequently of the mobilized gullypot sediments. The particulate-associated metals exhibit distinct temporal similarities to the storm hydrograph and chemograph for suspended solids with the exchangeable fraction being dominant. Hydrodynamic sorting mechanisms on the road surface produce the earlier contributions for this fraction, with later inputs, particularly for Cd and Pb, from chemical exchange processes within overturned gullypot sediment. This basal sediment also provides important inputs of the carbonate and hydrous oxide fraction for Cu and Zn as the gullypot contents become oxygenated later in the storm. The consequences of these findings are discussed in terms of gullypot maintenance and the water quality of stormwater outfalls.