Roadside gully pots (catch basins) have been identified as potential sources that can make significant contributions to stormwater pollutant loadings. Between storm events the gully pot sediment and liquor undergo changes in composition as a result of biochemical reactions. Sediment maturation and acidic dissolution processes release pollutants from the contaminated chamber sediments and interstitial pore waters into the relatively clean gully pot liquor. Dissolved pollutant profiles for gully pot outflows therefore show substantial early contributions from gully pot liquor and interstitial waters reflecting microbial and geochemical degradation processes which act upon the trapped chamber sediments both during and between storm events.

The majority of dissolved organic carbon is washed out in the gully pot outflow in the early low flow stages, suggesting that the main contributing source is the supernatant gully pot liquor. Small additional releases coincide with, and indicate additional releases of, soluble organics from the interstitial waters as the basal sediments are disturbed. Conductivity changes show that dissolved inorganics also exhibit efficient removal during the low flow stages of storms, with the absence of delayed peaks indicating a negligible contribution from the settled gully pot sediments.

During storm events, low runoff rates produce marked decreases in pH levels from the initial gully pot liquor value of pH 6.0-7.1 to a value approaching typical rainfall levels (average rainfall pH = 4.1). This lowering of the pH indicates that the dissolved buffering agents initially present on the road surface and in the gully pot liquor have become depleted and exhaustion of dissolved Ca clearly illustrates this effect. Initial decreases in dissolved oxygen concentrations and redox potential are indicative of exposure of the reduced basal sediments as overlying supernatant liquor is washed out. A subsequent secondary decrease in redox potential, coinciding with increasing flows, is due to the additional release of reduced interstitial waters as the gully pot basal sediments are disturbed. Finally, dissolved oxygen levels return to normal as the oxygenated surface waters become predominant in the outflow waters.