Identifying sources of dissolved organic carbon on the River Swale, Yorkshire

A dissolved organic carbon (DOC) sampling study was carried out on the River Swale in Yorkshire as part of the NERC LOIS programme. Loads of DOC were calculated from the river and its tributaries. For the River Swale calculated loads varied between 1900 and 9000 kg/day and for the tributaries between 1 and 8500 kg/day. No relationship was found between the amount of organic carbon in the soil of the catchments and the DOC loads, perhaps because of the preceding dry summer. DOC loads were also estimated for sewage point sources and ranged between 7 and 287 kg/day. Some of the catchments draining to the River Swale respond with high DOC loads during high flow conditions, others do not. Some catchments are dominated by sewage point sources during low flows and diffuse sources during high flows. A mass balance for DOC on the River Swale was calculated, however, loads from inputs did not balance with outputs because of a lack of data and errors inherent in some of the approaches used. Despite this, results indicate in-stream losses of approximately 20%. A highly detailed measurement study of a stretch of river is recommended to identify all the sources and sinks of DOC, the relative size of their contribution and how they respond in differing conditions.