Solving A Mercury-Contaminated Fiber Sediment Problem in The Baltic Sea




Project Info

Client
Municipality of Luleå, Sweden

Location
Sweden

Description

Mercury-contaminated pulp mill fibers deposited in the Lule River, which flows into the Baltic Sea close to the Swedish city of Luleå, have been migrating into the surrounding water, with potential for uptake by aquatic species – posing an environmental risk. Golder completed an analysis of various remediation technologies to resolve this problem.

The contamination goes back to the 1950s, when a pulp mill in Karlshäll used phenyl-mercury-acetate to prevent mold growth in pulp and discharged untreated effluent water and pulp fiber into the Lule River.

Unfortunately, Karlshäll is one of many hundreds of former pulp mills in Sweden that have discharged untreated effluent water, causing similar contaminated fiber banks and fiber-rich sediments. This is a problem that extends beyond Sweden, and the lessons learned at Karlshäll will serve as a basis for future complex remediation.

Since 2011, Golder has worked with the Municipality of Luleå to find a sustainable solution to deal with the shallow mercury-contaminated fiber bank, located only 50 cm below the water’s surface. This fiber bank consists of organic waste material containing mercury from the former plant’s waste stream. Some of it is also mixed with the natural sediment, forming fiber-rich sediment.

Because the river was used extensively to transport timber (prior to the 1980s), remediation is made more difficult by many logs resting on the riverbed and embedded in the sediment, as well as vertical timbers left from structures built to help move floating logs for processing.

Investigating possible solutions for remediating the sediment

One option Golder considered was suction dredging. This method is limited, mainly due to the need to treat a much larger volume of water brought to the surface along with the sediment, compared to other methods.

About fiber banks:
  • Fiber banks were accumulated at the bottom of the river when production-related wood and cellulose fibers were discharged in the water outside the factory.
  • Fiber banks are toxic to the water due to the unnatural substrates, oxygen deficiency and gas formation due to digestion of organic matter.
  • Most often, the fiber banks are also contaminated with persistent organic pollutants or mercury.
  • Due to the uplift of land, the fiber banks rise closer to the water surface every year and will eventually resuspend the contamination.

Golder also considered “freeze dredging,” in which metal plates are lowered by cables onto the riverbed and cooled to below freezing. Frozen sediment adhering to the plates can then be lifted to surface for disposal. This option, while it limits wider dispersal of sediment, was found to be expensive due to high electrical costs for a large-scale application over the entire site.

A third option involved capping the mercury-contaminated sediment with an impervious barrier, but there were limitations due to cost, the need to monitor the cap’s integrity long-term, and the fact that there is a natural uplift of the riverbed of about 1 cm (half an inch) per year, which might impact the cap’s ability to withstand erosion by ice, currents and rough water/waves.

Golder also considered options for safe disposal of the organic sediment, if it is dredged up and brought to shore. One option considered was having the organic material incinerated after dredging and dewatering.

The team also considered landfill disposal. However, Swedish and European Union standard practice is to avoid disposing of organic materials in landfills, so few commercial landfills are permitted to accept organic matter such as the fiber bank at Karlshäll. Golder completed a permit application and due to the remediation complexity, permission to build a specifically designed landfill was granted.

The method for remediation and disposal will be decided in a procurement process where the most cost efficient and sustainable method is chosen.

Next Steps

Golder’s investigations were carried out between 2011 and 2017. In early 2017, Golder participated in a court hearing to seek an environmental permit for the remediation work. The permit was granted, and since then Golder has been working with the Municipality of Luleå to arrange funding for the project from the Swedish Environmental Protection Agency (EPA).

In October 2018, funding for cleanup of the Karlshäll site of 160.5 million Swedish Krona (17.7 million USD) was announced by the EPA, with an expected project start date of 2020. The intervening time is being used for choosing remediation technologies, design and procurement, leading to a public tendering process. Golder’s involvement is expected to include detailed design, technical support during construction, and environmental monitoring.

Surrounding the fiber bank itself is a larger area of fiber-rich sediments and soils. Golder is also investigating if these areas will pose a risk after the fiber bank has been remediated. The main problem with fiber-rich sediments is spreading, and an advanced wave and current model has been set up for the site. For the area above sea level, which has been formed due to the regional uplift of a former part of the bay, uptake in biota and subsequently animals and humans is the focus, along with how climate change and the regional uplift will affect the risks.

Given the many instances in which mercury-impacted sediment from former pulp plants lies offshore along the Swedish coast – and elsewhere in the world – the lessons learned at Karlshäll are anticipated to aid in identification of sustainable solutions elsewhere.


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