The effect of urban runoff on the water quality of the Sweetbriar Brook, Ampthill, UK

Abstract: The Sweetbriar Brook, a small watercourse in Ampthill, Bedfordshire, in a mainly residential catchment, was studied. The scope was to characterise the water quality during low flow conditions and storms, by chemical analyses of dissolved oxygen, turbidity, metals, nutrients, oil and thermotolerant coliform bacteria, among others. A baseline series of analyses from five sampling points along the stream was undertaken during spring-summer 2005. One storm was sampled in short time intervals at the most downstream sampling point. Rainfall volumes and intensities were recorded nearby. The water quality at baseline conditions was generally good. One exception was thermotolerant coliforms, which were occasionally very high, posing a health risk to children playing in or near the water. Another exception was nutrients, especially phosphorus was exceeding guideline values at some locations. Iron occurred in elevated levels in the groundwater and seemed to dissolve in standing water with oxygen depletion. The runoff did generally not carry higher pollutant concentrations than expected for this catchment, except for coliforms, which were dramatically high in some samples. An interesting observation was that not only one first flush, carrying most pollutants, occurred, but subcatchments with different times of concentration gave rise to individual pollutant peaks. Flooding was a problem in this catchment, retro-fitting a sustainable urban drainage system is hence recommended. Sediment analyses of total zinc and thermotolerant coliforms were also undertaken, as a complement to the water analyses, showing relatively high zinc concentrations, increasing with depth. Coliforms were fewer than expected from the numbers in the water and demonstrated no consistency in sampling points or depths. No statistical tests were undertaken, as more sampling points and variables were prioritised over replicate samples. This is however a drawback in the interpretation and reliability of the results. More sampling, both chemical and biological, is needed to verify the results.