Water quality of lowland, permeable Chalk rivers: the Frome and Piddle catchments, west Dorset, UK

This paper provides a baseline interpretation of extensive hydrochemical characteristics of UK Chalk catchments based on the Frome/Piddle (west Dorset), a major component of the LOCAR (Lowland Catchment Research) programme. It examines a wide range of chemical determinands using data from two long-term (1979–2000) river water monitoring sites. The water quality of the Frome and Piddle is very similar to that of agricultural catchments associated with Chalk aquifers. Waters are of a calcium bicarbonate type and nutrient bearing. In general, there are poor correlations between major ion determinands and between chemical concentrations and flow due to the large groundwater storage which buffers chemical variability. Hence, there are no major chemical gradients within the aquifer to provide distinct water end-member chemistries for contrasting hydrochemical environments to allow chemical hydrograph separation on the basis of flow. NO₃-N and soluble reactive phosphorus (SRP) are the dominant species of dissolved N and P, respectively. NO₃-N sources both within the aquifer and within the soils contribute to relatively high concentrations, with average values close to those of other agriculturally impacted catchments. SRP concentrations are generally lower than for high agriculturally, urban and industrially impacted catchments. High correlations between Al, Cu, Fe, Mn and Zn occur due to the association of these trace metals with the soil-forming oxide and hydroxide ions present in suspended solids. Biological process influence SiO₂, pH and EpCO₂ levels. Average levels of EpCO₂ at 4.15 and 5.52 times atmospheric pressure in the Frome and Piddle, respectively, indicates that the river waters have not fully degassed to equilibrium with the atmosphere, suggesting a short residence time of spring waters in the river channel.