The gully pot operates under two distinct regimes: dry weather and wet weather. During dry weather, rapid drops in dissolved oxygen concentration result in the establishment of anaerobic conditions. This leads to the release of oxygen demanding soluble organics, ammonium and possibly sulphides. Complex changes also occur to the phase and bio-availability of a range of heavy metals. Concurrently, some material previously held in suspension in the inflow settles to the base of the pot to form a bed whilst significant amounts of smaller material remain in suspension. The bed formed under the liquor seems to undergo an aging process. In wet weather, incoming runoff rapidly displaces the standing liquor, which can represent a significant fraction of the total flow volume and pollutant load contributing to a first foul flush. The solids trapping efficiency of the pot is high for larger particles, but poor for smaller particles which carry proportionately more of the pollutant load. Organics and smaller solids can be re-entrained, but there is little evidence of large-scale overturning of the bed.