THE GULLY POT AS A BIOCHEMICAL REACTOR

Roadside gully pots (catch basins) have been identified as potential s ources that can make significant contributions to stormwater pollutant loadings. Between storm events the gully pot sediment and liquor unde rgo changes in composition as a result of biochemical reactions. Sedim ent maturation and acidic dissolution processes release pollutants fro m the contaminated chamber sediments and interstitial pore waters into the relatively clean gully pot liquor. Dissolved pollutant profiles f or gully pot outflows therefore show substantial early contributions f rom gully pot liquor and interstitial waters reflecting microbial and geochemical degradation processes which act upon the trapped chamber s ediments both during and between storm events. The majority of dissolv ed organic carbon is washed out in the gully pot outflow in the early low flow stages, suggesting that the main contributing source is the s upernatant gully pot liquor. Small additional releases coincide with, and indicate additional releases of, soluble organics from the interst itial waters as the basal sediments are disturbed. Conductivity change s show that dissolved inorganics also exhibit efficient removal during the low flow stages of storms, with the absence of delayed peaks indi cating 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 valu e approaching typical rainfall levels (average rainfall pH = 4.1). Thi s lowering of the pH indicates that the dissolved buffering agents ini tially present on the road surface and in the gully pot liquor have be come 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 a s overlying supernatant liquor is washed out. A subsequent secondary d ecrease in redox potential, coinciding with increasing flows, is due t o the additional release of reduced interstitial waters as the gully p ot basal sediments are disturbed. Finally, dissolved oxygen levels ret urn to normal as the oxygenated surface waters become predominant in t he outflow waters.