Various land remediation methods are used in Stockholm Royal Seaport

Land remediation in Stockholm Royal Seaport is a prerequisite for building and developing the district. Various methods are employed depending on the type of contamination, the location of the pollutants, and the intended use of the site. Among these, several remediation methods have been tested, including a completely new method for Sweden that remediates the land while simultaneously stabilising it.

After many years of industrial activity in Hjorthagen and gas production, it is necessary to remediate and treat the land, groundwater, and sediment before building homes, preschools, business premises, parks, and squares. The soil is partly ancient seabed with thick clay layers. During the gasworks era, fill materials were deposited on the seabed to expand the industrial area. Consequently, today both the clay and the fill materials are contaminated with substances such as tar, benzene, and heavy metals.

Different methods are used to remediate the soil:

Excavation of contaminants

The most extensively used land remediation method within the project is excavation remediation, which involves removing the contaminated soil. This method is employed when the contaminants are not too deep and are above the groundwater level. Excavation remediation is particularly advantageous in areas where buildings are to be constructed, as the soil needs to be excavated regardless. This method has been used in the earlier phases Västra, Norra 1, Norra 2, and Brofästet, as well as partially in Ropsten/Kolkajen. The excavated materials are transported to the Mass Consolidation Centre in Stockholm Royal Seaport. At the centre, the materials are sorted so that the small soil particles are separated from the larger ones. Since the contaminants are mostly bound to the small soil particles, only these need to be sent to landfill, while the larger gravel, stones, and boulders can be reused for backfilling utility trenches and roads in the area.

In situ – remediating on site

In 2016-2017, an innovative procurement for the Kolkajen phase, was carried out for new ways to contract consultants and contractors for project planning, pilot tests and inspections were implemented. During the pilot tests, various in situ remediation methods were investigated, including chemical oxidation, biological degradation, and soil washing. Several of the methods showed promising results, forming the basis for the full-scale remediation in Kolkajen.

In the Kolkajen phase, contaminants are found both above and below the groundwater level. Deep land remediation is a complex process and can be carried out in several ways. In Kolkajen, the remediation involves excavating the top layer of soil, and transporting it to the Mass Consolidation Centre. After removing the top layer of soil, layers of clay—up to 15 meters thick in some areas—are found, followed by moraine before reaching the bedrock. Instead of the deep soil with these contaminants, chemical oxidation is used, which involves adding chemical substances that break down the contaminants into carbon dioxide and water. In certain areas, where the new alignment of Bobergsgatan is planned, the clay also needs to be reinforced to prevent settlement damage when it is built upon. This area is the largest in Europe where this method is used^*. The method is called ISS-ISCO; In Situ Stabilisation and Solidification – In Situ Chemical Oxidation. The stabilisation was carried out with a machine weighing over 100 tonnes, equipped with a mixing tool with a diameter of two metres. This tool stirs the clay with cement and oxidising agents, thereby stabilising the clay and breaking down the contaminants. An environmental control organisation conducted daily sampling to ensure that the set remediation targets for soil and groundwater are met. Measurements are also made to monitor odours and contaminant levels in the air. Environmental monitoring of the groundwater will continue for several years after the remediation is completed. In situ remediation of the moraine is also done through chemical oxidation. However, instead of adding the oxidising agent using a large machine with a mixing tool, the project has installed plastic pipes in the ground, known as TAM pipes. These pipes allow the injection agent to be added at various levels in the soil and at recurring intervals. A total of 1,500 TAM pipes have been placed in the ground, and so far, 30,000 cubic metres of moraine have been remediated. Within the work area, the TAM pipes protrude from the ground.

More about the remediation method:

1. Once the top layer of soil has been excavated and the area backfilled with cleaner materials, the area to be remediated is divided into hexagons, each section covering approximately 350 square metres.

2. In each section, about 20 TAM pipes are installed, extendingdown to the bedrock, starting at the lowest groundwater level. chemical oxidising agent, breaking down and cleansing the soil of contaminants.

3. Regular monitoring of contaminant levels is conducted through groundwater analyses. Since the pipes remain in the ground, it is easy to reintroduce the treatment agent if necessary.

4. The pipes are left in the ground for at least six months and are monitored through analysis at least three times.

"Thanks to our on-site remediation, we can avoid the transports that would otherwise be needed to excavate all the deep soil. The remediation process also reduces the spread of contaminants from the soil to Lilla Värtan. Investigation work and pilot tests have been conducted for many years to determine the appropriate measures, and it's exciting that work is underway," says Sofia Billersjö, Land Environmental Specialist at the City Development Administration.

Upcoming land remediation

The upcoming phases such as Loudden, Värtahamnen, and Frihamnen also require land remediation. In order to develop these areas, the contamination situation has been mapped and risk assessments have been conducted, highlighting the current risks. These investigations will form the basis for the development of new studies and determine the necessary remediation measures.

Film about land remediation in Ropsten

Sofia Billersjö, Land Environmental Specialist, Fredrik Bergman, Implementation Manager, and Christina Salmhofer, Sustainability Strategist at Stockholm City Development Administration, discuss the land remediation process in Ropsten and explain why multifunctional blue and green spaces are important. Andreas Huss, an architect and resident of Stockholm Royal Seaport, shares his experience living in the area.

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^{*Source: Sheeba Environmental Engineering AB, the project engineers on site.}

Article Published: 06/05/2024 Updated: 27/06/2024