Reviews and syntheses: Understanding the impacts of peatland catchment management on dissolved organic matter concentration and treatability

Abstract. In the UK, most large reservoirs constructed for public water supply are in upland areas. Many are situated in catchments characterised by organic-rich soils, including peatlands. Although these soils naturally leach large amounts of dissolved organic matter (DOM) to water, the widespread degradation of upland peat in the UK is believed to have exacerbated rates of DOM loss. High and rising DOM concentrations in these regions raise treatment challenges for the water industry. In the UK, water companies are increasingly considering whether upland-catchment peat restoration measures can slow down or even reverse rising source water DOM concentrations and, thus, reduce the need for more costly and complex engineering solutions. There remains considerable uncertainty around the effectiveness of such measures, and a comprehensive overview of the research in this area remains lacking. Here, we review the peer-reviewed evidence of the effectiveness of four catchment management options in controlling DOM release from peat soils: ditch blocking, revegetation, reducing forest cover and cessation of managed burning. Results of plot-scale investigations into the effects of ditch blocking on DOM leaching are currently largely equivocal, while there is a paucity of information regarding impacts at spatial scales of more direct relevance to water managers. There is some, although limited, evidence that the terrestrial vegetation type may influence DOM concentrations and treatability. The presence of plantation forestry on peat soils is generally associated with elevated DOM concentrations, although reducing forest cover appears to have little short-term benefit, and associated disturbance may even increase concentrations further. Catchment management measures have rarely been monitored with downstream water quality as the focus. 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.