Molecular weight spectra of dissolved organic carbon in a rewetted Welsh peatland and possible implications for water quality

Demand for water from catchments dominated by upland peat as a source of drinking water supplies in the UK is likely to increase in the future as demand per capita continues to rise (Thomsen 1990) and/or summer droughts increase in frequency (Arnell 1992). Concern has been expressed in recent years over rising colour levels (related to dissolved organic carbon (DOC) and iron (Fe)) from such catchments (e.g. Kay et al. 1989) causing reduced drinking water quality. One of the major causes of increased DOC concentrations is rewetting following periods of relative drought (Mitchell & McDonald 1992). Experimental rewetting of a naturally drained wetland in Mid-Wales over four years was found to substantially increase the concentrations of DOC, and Fe in the pore-water, with peak values of >60 mg dm -3 (Fe) and >300 mg dm -3 (DOC) after rewetting, compared with typical values of 5000 to 90 000 to 200 000 AMW material. The AMW spectrum of DOC in the experimental wetland changed with season, and the >90 000 to <200000 AMW fraction could only be seen in spring, representing a transient pool of carbon that is rapidly transformed in or transported from the wetland. These findings suggest that rewetting of peatland following drought (e.g. due to climate change) has the potential to reduce water quality, Moreover, recent interest in restoration (rewetting) of drained peatlands (Wheeler & Shaw 1995) could create an additional source of DOC rich water.

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