Sources and management of water colour in the river Tees

Abstract

Over recent decades, a wide range of rivers and lakes draining peat-dominated catchments across the UK have exhibited statistically-significant increases in water colour and dissolved organic carbon (DOC) concentration. This has implications for the carbon budget of those peatlands, and for the long-term viability of water treatment works which must remove the colour in the treatment process. Suggested causes for such increases include lower water tables in the peat, and changes in the peat chemistry through decreasing atmospheric acid deposition. One factor potentially affecting the peat water tables, and therefore possibly related to the increases in DOC, is the practice of peatland drainage, which may affect both the production of DOC and the hydrological behaviour of the peat. Drainage is no longer believed to be beneficial in increasing the amenity value of peat and so there is a need to understand whether blocking the drains will be an effective strategy in decreasing DOC export and mitigating the observed increases at water treatment works. This thesis presents the results of monitoring individual blocked drain, unblocked drain, and stream catchments. The results are used to construct detailed DOC export budgets and to compare the behaviour of the catchments. This enables identification of the extent to which drainage increases DOC export; of differences in behaviour between blocked and unblocked drains, and of whether drain blocking is likely to reverse any such increase in DOC export. Results from these individual small catchments are considered in the context of the overall DOC export across the larger scale catchment of a large water treatment works. DOC sources across the larger catchment were also monitored and, using novel statistical techniques, catchment export is related to catchment properties including the presence of drainage. These results are used to assess the likely benefits of a large scale drain blocking programme with respect to the DOC concentration observed at the water treatment works. Results are presented showing that drainage does substantially increase the DOC export from peat, with DOC export being highest from flat, extensively drained peat areas. Blocking does decrease the export from individual drain catchments, but not to pre-drainage levels, even a decade after blocking. The decreases due to blocking are shown to be due primarily to changes in the hydrological behaviour of the drains rather than changes in the production of DOC. Therefore a catchment-wide drain blocking programme is recommended as a strategy for reduction, but not total amelioration, of the increasing DOC trend that is observed at water treatment works. However, before this is implemented further understanding of the large scale changes in peatland hydrology that may follow blocking will be required, as the results do not indicate a reduction in DOC production.