Rewetting peatlands: A potential consequence for surface water quality

Sammanfattning

Wetland restoration is widely promoted as a strategy to mitigate climate change challenges. Although the advantages are well known, the impacts on surface water following restorations remain uncertain. Consequently, restoration can release, for instance, more dissolved organic carbon (DOC), which can increase water colour and thereby affect downstream water quality. Additionally, precipitation can further increase DOC concentration and discolouration from these areas. This can create challenges for drinking water treatment if chlorine is used in the clarification stage, as it can form carcinogenic byproducts known as trihalomethanes (THMs) if combined with higher amounts of DOC. The existing research indicates inconsistent DOC and water colour responses following rewetting, but the majority show an increase in the concentrations. Therefore, the aim is to assess whether rewetting and precipitation increase DOC concentrations and water colour. This study examined three management types, drained, manually rewetted, and naturally rewetted, across three Swedish test sites (Trollberget, Skogaryd and Plönninge), as well as how precipitation impacts DOC concentrations and water colour before and after rain events. Water colour was measured through absorbance at 420 nm (Abs420). Data for Trollberget and Skogaryd were collected from their respective databases, and Plönninge’s data were collected through field sampling. The findings indicated a significant difference solely in Trollberget, where manually rewetted and naturally rewetted areas showed higher DOC and absorbance in the runoff than from the drained sites. In contrast, Skogaryd and Plönninge indicated no difference between the chosen management types. When examined at the site level, all three locations differed from each other. An observed trend was that DOC and absorbance increased the more south the sites were located. Additionally, DOC and absorbance did not differ significantly before and after precipitation. These findings do not answer whether restoration should be conducted in drinking water catchments; rather, they emphasize the complexity of rewetting a peatland after years of drainage. One of the findings shows that the average temperature plays a role regarding enhanced DOC and absorbance release, probably due to intensified microbial activity, thus increasing decomposition of organic matter. Therefore, it is recommended to conduct a site-specific evaluation before restoration to ensure socio-economic impacts remain acceptable. Further long-term research is needed to clarify the true impacts of how DOC responses depend on current site conditions.