The water quality of the River Wear, north-east England

Abstract

A 1-year detailed study of water quality in the River Wear in combination with longer (decadal) term Environment Agency data (Harmonised Monitoring Scheme) show the influences of historic lead–zinc and coal mining and sewage inputs. The water quality for many determinands, such as sodium, chloride, boron, nitrate, and soluble reactive phosphorus, varies seasonally due to changing flow conditions. For most dissolved determinands, concentrations decrease with increasing flow in response to dilution of point and diffuse sources by rainfall. However, concentrations increase with increasing flow for dissolved organic carbon, aluminium, lead, iron, yttrium, and the lanthanides and actinides. This increase probably reflects two processes. Firstly, trace element-enriched runoff occurs from the acidic moorland areas of the catchment when it wets up. Secondly, at high flows, increased production of transition metals bearing microparticulate material occurs; these pass through conventional filters used to separate dissolved from particulate materials. For the particulate components, iron, manganese, aluminium, and some trace transition metals are present above the analytical detection limits and concentrations increase with increasing flow in response to increasing suspended sediment levels. However, particulate metal concentrations are relatively low for the Wear compared to the other eastern UK rivers, even though historic lead–zinc mining activity in the upper portions of the Wear catchment has led to extensive spoil contamination of the land. This feature probably reflects the lack of a significant flood plain area, where trace contaminants can accumulate within the sediments. The importance of inputs of pumped water from former coalmines is highlighted; increased manganese and dissolved carbon dioxide concentrations and reduced pH result. Examination of the Harmonised Monitoring Scheme data indicate no clear long-term changes in water quality for all but one determinand, although analysis is hampered by changes in sampling frequency for all determinands and changes in detection limits for certain trace metals. The only clear long-term change is a reduction in the micro-organic insecticide, aldrin. Nonetheless, the long-term data, when plotted as a function of month, follow the same pattern as the LOIS data. For aldrin, the higher values during the early period of record occurred in the latter half of each year, presumably as a consequence of seasonal application and enhanced leaching during the autumn.