Chemical Quality Status of Rivers for the Water Framework Directive: A Case Study of Toxic Metals in North West England

Resumen

This paper provides data from two years of monitoring of the chemical quality of rivers and streams in North West England from the clean headwaters to polluted rivers just above the tidal reach and covers 26 sites including the Ribble, Wyre and the tributary rivers of the Calder and Douglas. Across the basins that include areas of rural, urban and industrial typologies, data is presented for three of the priority substances in the Water Framework Directive i.e., nickel (Ni), cadmium (Cd), and lead (Pb). Average concentrations are low and well below the Environmental Quality Standards values for all three of these substances. Cadmium and Pb appear in approximately equal proportions in the dissolved (<0.45 µm) and in the acid available particulate fractions (>0.45 µm) whilst Ni occurs predominantly in the dissolved form (92%). Regional inputs of these metals arise mostly from diffuse sources as the storm-flow concentrations are generally greater than at base-flow condition. Greater concentrations of Ni are transported at the headwaters and smaller tributary sites under storm flow condition than for the main stream of the Ribble. For Ni, amounts increase as the river proceeds from its headwaters down towards the Ribble and Wyre estuaries, whilst Cd and Pb show consistent values throughout the catchment. There is annual cycling of dissolved concentrations of Cd, Pb and Ni for the clean headwater streams that gives maxima during the latter half of the year when the river flow is greater. For the impacted sites the pattern is less distinct or absent. Our estimates suggest that the Ribble estuary receives 550 t y-1 of dissolved Ni, 16 t y-1 of dissolved Cd and 240 t y-1 of dissolved Pb.