Mercury in rivers in NW England: from rural headwaters to the heartlands of the historic industrial base

Total mercury (T-Hg) concentrations in rivers are described across a rural to urban/industrial and agricultural landscape gradient in NW England. T-Hg ranges between 0.2 and 230 ng L-1. The regional median was 3.6 ng L-1 with individual river medians ranging between 1.9 and 8.3 ng L-1. Median T-Hg concentrations were sometimes moderately higher for the lowland areas and at higher flows. Our estimates suggest that the Ribble estuary receives 9.2 kg yr-1 and the Wyre estuary 0.7 kg yr-1. In order to examine regional inputs from urban/industrial components, regression analysis was undertaken by comparing three types of hydrochemical signature: suspended sediments (SS), which provides a measure of the particulate component, dissolved organic carbon (DOC), that provides an indication of humic/fulvic acids that are part of the organic colloids and strong chelating agents, and boron a marker of sewage effluents and population density. The results show high positive relationships of T-Hg with both SS and DOC, but no relationship with the urban/industrial signal. The regression analysis with T-Hg indicated on average a gradient of 0.33 ng mg-1 for DOC and 0.2 ng mg-1 for SS. They indicate the primary importance of a diffuse source of T-Hg. For the upland areas and cleaner river systems, the linkages between T-Hg and DOC were particularly strong, while for the lowland areas, the linkage with SS proved stronger. Analysis of a later subset of data that partitions the SS into organic and inorganic fractions indicated that the T-Hg was primarily linked with the organic fraction. Indeed, multiple regression of T-Hg with DOC and POM reveal gradients similar to other parts of the World