Aspects of solute movement in the British uplands

Abstract

The aim of this research is to determine the process by which runoff draining parts of the UK uplands becomes discoloured. Recent increases in colour are perceived as a problem due to the frequency with which EC surface water directives are breached, to the increased cost of meeting standards, to an increase in consumer complaints, and due to the possible deleterious health effects of consuming coloured potable water. The work was conducted over the last four years, largely in the Upper Burn and Nidd valleys, north Yorkshire, and at the University of Leeds. Existing knowledge on the chemical nature and environmental genesis of coloured water is reviewed, and related to the chemical nature of coloured waters examined in this study. Analysis of water quality archives and direct monitoring of selected catchments was conducted enabling the spatial and temporal distribution of coloured waters to be determined. A tentative process theory was produced, and tested by controlled laboratory investigations. Coloured runoff is derived from Winter Hill soil, raw acid oligo-fibrous peat, and is most intense from areas with slopes < 5% and high drainage densities. The seasonal pattern of colour is highly regular, and is a product of rainfall and soil moisture deficit. Colour is strongly associated with discharge, and demonstrates the operation of a catchment flushing mechanism. Runoff is discoloured in a three phase process; colour production, solubilisation and removal. Colour is a product of organic decomposition, dependent upon microbiological activity, moisture deficit, temperature and peat structure. Solubilisation of colour is dependent upon solution pH, temperature, the chemical structure of organic molecules and the availability of iron. The removal of decomposition products, as colour, is strongly dependent on the moisture status of peat, and its ability to rewet after drying. The implications of these findings for future colour levels and for catchment and catchwater management are discussed