Understanding the impacts of peatland catchment management on DOM concentration and treatability

Abstract. In the UK most large reservoirs constructed for public water supply are in upland areas and situated in catchments that contain at least some organic-rich soils. Dissolved organic matter (DOM) leaching from these soils imparts a brownish colour to water and raises treatment challenges for the water industry since excessive post-treatment concentrations result in the generation of potentially harmful disinfection by-products in drinking water. The primary method for maintaining sufficiently low pre-disinfection DOM concentrations is chemical coagulation, but in the past 15 years water companies have increasingly considered the capacity for catchment interventions to improve raw water quality at source, reducing the need for costly and complex engineering solutions in treatment works. There remains considerable uncertainty around the effectiveness of these catchment engineering-based measures and a comprehensive overview of the research in this area remains lacking. Here we review the peer-reviewed evidence for the effectiveness of four management options for upland organic soil-dominated catchments that are being considered by the water industry as options for controlling DOM releases. These are ditch blocking, revegetation, reducing forest cover, and cessation of managed burning. Results of plot scale investigations into effects of ditch blocking on ditch-blocking are available but largely equivocal, while there is a paucity of information regarding impacts at spatial scales of more direct relevance to water managers. The presence of plantation forestry on peat soils is generally associated with increasing DOM concentrations, although canopy removal has little short-term benefit and can even further increase concentrations. Although not widely studied, the available evidence suggests that Sphagnum mosses produce DOM that is more easily removed via conventional treatment processes compared to vascular plants such as heather and grass species. We found surprisingly little published research around the extent to which manipulation of in-reservoir processes might be used to mitigate or exacerbate changes in inflowing DOM as part of a catchment management approach. This review concluded that catchment management measures have rarely been monitored with downstream water quality as the focus, and that restoration impacts vary across sites. To mitigate the uncertainty surrounding restoration effects on DOM, measures should be undertaken on a site-specific basis, where the scale, effect size and duration of the intervention are considered in relation to subsequent biogeochemical processing that occurs in the reservoir, the treatment capacity of the water treatment works and future projected DOM trends.