Ditch cleaning versus wetland restoration – Effects on mercury in water

Last changed: 06 December 2021
Forest ditch. Photo.

This project evaluates how two opposing forest activities, ditch cleaning and wetlands restoration, effects the runoff concentrations and loads of total mercury and bioavailable methyl-mercury. The study includes both an intensively monitored and controlled experimental field site in Västerbotten (Trollberget) and a synoptic set-up of sites, along the north-east coastland, covering a range of catchment characteristics.

Background

Drained peatlands make up a substantial share of Sweden's productive forest land.

In order to keep the areas as productive forest land, the ditches must be cleaned to maintain effective drainage when vegetation and sediments accumulates in the ditch beds. However, ditch cleaning can affect soils, water quality and carbon balance. Increased runoff of total mercury and methyl-mercury has been identified as a potential risk after ditch cleaning.

One alternative is to reduce draining capacity and restore wetland conditions. Wetland restoration is encouraged from a political level to reduce flooding and droughts as well as greenhouse gas emissions. However, as wetland restoration cause flooding of peat soils, it may promote the microbial formation of methyl-mercury.

Aim

The project aims to answer the following research questions:

  • Will the concentrations and loads of total and methyl-mercury increase in ditch water after ditch cleaning and wetland restoration, and for how long will these potentially elevated values last?
  • Are potentially elevated concentrations of total and methyl-mercury after ditch cleaning mainly associated with particle or dissolved fractions of mercury?
  • How does forest machinery driving during ditch cleaning operations influence the hydrological connectivity in the near-ditch zone?
  • Will sediment traps/ponds intended to improve water quality downstream work as mercury methylation hotspots, and will methyl-mercury formation be particularly elevated in areas draining acid sulfate soils?
  • What catchments will release more or less total and methyl-mercury after ditch cleaning?

Finally, this project will synthesize the results from this study with other published literature, in order to evaluate risks of ditch cleaning on total and methyl-mercury and in what types of areas restoration is preferable. The synthesis will also consider forest productivity as ditch cleaning is done to support forest production.

Results from the project are useful for balancing pros and cons of ditch cleaning and wetland restoration and improve forest water protection guidelines.

Methods

Trollberget is a state-of-the-art research field site that is a part of the Krycklan research infrastructure.

We will sample total and methyl-mercury concentrations and loads from catchments that will either be forest harvested and ditch cleaned (n=2), only forest harvested (n=2), restored wetlands (n=2), pristine references (n=2) and historically drained in the 1930s and then left alone (n=2) using a Before–After Control-Impact (BACI) design.

The BACI design is a powerful experimental design for environmental impact studies to statistically isolate the effect of the various treatments from natural variability. The monitoring of the catchments started in the autumn of 2018 and the forest harvest and wetland restoration will take place in 2020 and ditch cleaning will be conducted in 2021. The sampling of mercury in Trollberget will continue until 2023.

The synoptic study will evaluate how catchments with different characteristics differ in their release of total and methyl-mercury in response to ditch cleaning. The study will involve around 50 catchments sampled on 3 occasions. The sampling setup will include recently and less recently cleaned ditches, low sulfur to acid sulfide soils and references, historical drained but not ditch cleaned.

Forest ditch. Photo.
Many drainage ditches function poorly due to vegetation and accumulated sediment in the ditch beds. This is one of the ditches in Trollberget that will be ditch cleaned in 2021. Photo: Eliza Maher Hasselquist
Wetland with sparse trees. Photo.
An alternative activity to ditch cleaning is to restore the land as non-productive wetland by further reducing the draining capacity of the ditches. This photo is from one of the areas that will be restored in Trollberget. The wetland restoration will take place in 2020. Concentrations and loads of total and methyl-mercury have been sampled since the autumn 2018 and sampling will continue until 2023, resulting in a powerful 2-yrs pre-treatment and >2 yrs post-treatment period. Photo: Eliza Maher Hasselquist
Triangular weir in a forest ditch. Photo.
Concentrations and loads of total and methyl-mercury have been sampled in ditches in Trollberget since autumn 2018 and sampling will continue until 2023. The ditch cleaning will take place in 2021 in two of the catchments. As we expect high concentrations and loads of total and methyl-mercury in close connection to the ditch cleaning activities, we will sample with high frequency during the first two weeks, and especially during the first 24 hours following ditch cleaning. Photo: Eliza Maher Hasselquist
Dam in a forest area. Photo.
There is a risk that sediment traps intended to improve the water quality downstream may work as Hg methylation hotspots. Anaerobic microorganisms capable of methylating mercury may establish in these aquatic environment with slow flowing waters, high organic carbon loads and reduced oxygen conditions. We hypothesis that sedimentation traps, especially those on acid sulfur soils, can increase the formation of methyl-mercury. In this project we will samples upstream and downstream of 15 sedimentation traps to evaluate if these act as so called hotspots for mercury methylation. Photo: Karin Eklöf
Clearcut area with driving tracks. Photo.
Soil compaction and soil disturbance during forest machinery driving can increase the mobilization of methyl-mercury by causing superficial flow paths. How does forest machinery driving during ditch cleaning operations influence the hydrological connectivity in the near-ditch zone? This will be tested by hydrological modelling (HydroGeoSpere) informed by high resolution topographic maps. Photo: Karin Eklöf

Facts:

Researchers involved:

Field work:

  • Johannes Tiwari (SLU)
  • Joel Segersten (SLU)

Funding: 

  • This study is funded by the Swedish Environmental Protection Agency in their call for research about wetlands ecosystem services.
  • The research infrastructure and basic water sampling in Trollberget are further funded by the County Administration Board and the Swedish Forestry Agency through EU Life funding of the project GRIP on LIFE IP.
  • The research infrastructure is also co-funded by the Kempe Foundation and Formas.