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Chronology of the Halle-Künsebeck landfill site

GEG Limited Liability Company

Landfill I: The oldest section of what later became the "Halle-Künsebeck Central Landfill" was filled with domestic and commercial waste from 1966 to 1982.

The Halle-Künsebeck I and II landfills are located in quarried limestone pits on the southern ridge of the Teutoburg Forest. To the north, east and west, the steeply quarried limestone cliffs form the edge of the landfill, and to the south were the exits for the limestone trolley railway. These exits were then used for the leachate pipes.

In 1965/66, the first construction phase was given a base seal. A layer of 12 cm thick bitumen/mineral mixture 0/35 was installed on a levelling layer. At the time, this seal was highlighted as a special safety feature. In addition, two or three seepage water collectors were installed on the base seal, which was also an innovation at the time. As a result, after a few years, the state office set up a laboratory container and carried out initial seepage water analyses.

Until 1975, the city of Halle/Westphalia was the operator, and after the regional reform, operation was taken over by the district of Gütersloh. From then on, a compactor was used for installation. The first expansion of the landfill base followed in 1977. The base was constructed on a 10 cm sand bedding with a 2 mm NPE foil and again with a 10 cm sand bedding as a protective layer. The steeply sloping limestone rock walls were covered with foils that were not welded. This was obviously done to prevent seepage water from entering the rock; at that time, no one had thought about gas extraction.

By 1982, landfill I had been filled to a height of 220 metres above sea level. After much back and forth, a surface seal with five combination wells for seepage water extraction and degassing was built between 1989 and 1991. The construction project was also accompanied by external monitoring by the authorities.

Structure of the surface sealing:

• Protective and recultivation layer d > 100 cm (overburden, limestone gravel, topsoil)
• PE fleece 450 g/m²
• Mineral surface filter 8/32 (d = 30 cm)
• PE fleece 450 g/m²
• Two-layer mineral seal d > 60 cm (k_f: 1 x 10^-9 m/s) • PE fleece 450 g/m²
• Gas drainage 4/32 (d > 30 cm)
• PE fleece 450 g/m²

A complex surface water collection system, separate from the drainage water collection system, was installed with separate volume evaluations and has been documented to date with extensive weather data. The entire drainage system functions like a 6-hectare lysimeter. The development and effectiveness of the recultivation layer could be scientifically evaluated here, but this is not required by the district government or GEG mbH. There were no plans to clean or inspect the seepage water collectors with free discharge into the canal, which were installed in 1966. When the five combination wells were also connected to the sewer in 1991, a number of problems arose. Later, a pipe was laid to the leachate basin of landfill II, as the municipal sewage treatment plant had no experience with landfill leachate.

In order to assess the overall situation, the Ruppert office in Braunschweig created a simulation of landfill I in 2000 using all the data collected with a "Help model". It was found that the surface sealing of landfill I, with approx. 4.5 ha of mineral surface sealing, allows an average of 5,000 m³ of water to infiltrate the landfill body each year, with only approx. 1,000 m³ being discharged as leachate. The remaining 4,000 m³ of leachate seeps into the aquifer.

Geology and hydrology at the Künsebeck site

The Teutoburg Forest was formed at around the same time as the Alps. The previously horizontal layers of earth were pushed upwards and, over the course of geological time, the Teutoburg Forest as we know it today was formed. It consists of three parallel mountain ranges between Osnabrück, Bielefeld and Detmold. The middle and highest range consists of red sandstone, while the northern and southern ranges consist of Upper Cretaceous limestone. Our landfill site is located in the southern mountain range, in a limestone quarry on the layers of mined Cenoman limestone and Cenoman Pläner. The Cenoman limestone is overlaid by Turon, a marlstone. For a long period of geological history, the surface of the Cenoman limestone was the land surface and was heavily weathered and sealed during this time before the Turonian marlstone built up on top of it. This dense layer has a major advantage for the landfill site, as the contaminated groundwater in the fissures of the Cenoman layers cannot flow through the Turon layers into the Quaternary aquifer.

The fissured Cenomanian layers, in which the contaminated groundwater is located, dip towards the south and lie at a depth of 300 to 400 metres below the surface at a distance of

1 to 2 km away at a depth of 300 to 400 metres below the surface, where the water in the fissures is extremely salty and has a very high density. This is another advantage for the landfill site, as the contaminated groundwater has a much lower density and cannot penetrate deeper layers.

Fig. 1: Hydrogeological cross-section (source: Schmidt and Carstensen, consulting hydrologists)

Fig. 2: Hydrochemical monitoring (source: Schmidt and Carstensen, consulting hydrologists)

As in the entire Teutoburg Forest, the contaminated groundwater flows out of the spring horizons in the transverse valleys of the limestone ridges, in our case the area of the separation layer between Turon and Cenoman, in the valley of the Künsebecker Bach stream. This is also where the seepage water first emerged in the early 1980s, resulting in the death of fish in all the ponds below.

The contaminated spring area and the water were diverted into the canal using a pumping system. In the meantime, a total of 4 groundwater measuring points and a defence well continuously pump between 0.5 and 2.5 m³/hour. For around 10 years now, the water from the spring catchment has been clean enough to flow into the Künsebecker Bach. The spring water continues to be continuously monitored by conductivity measurement and, if the limit value is exceeded, pumps are immediately switched on to discharge the water into the canal.

Landfill II: The newer part of the "Halle-Künsebeck Central Landfill" (DK II) was started with the first construction phase in 1982 and was repeatedly expanded in sections at the base until 1990. The development stages of waste management legislation can also be traced here.

Stages of expansion of the base extension of Landfill II:

• In 1980/82, a sand bed was laid on the limestone, covered with a 2.5 mm HDPE membrane and a 30 cm thick layer of sand as a protective layer and drainage.
• In 1987/88, a 2-layer, 60 cm thick mineral seal (k_f 1 x 10^-9 m/s) was installed, covered with a 2.5 mm HDPE film and a 50 cm drainage layer (8/16 mm).
• In 1990, a 4-layer, 100 cm thick mineral seal (k_f 1 x ^10-10 m/s) was installed, followed by a 3.0 mm HDPE sealing membrane and a 50 cm drainage layer (8/16 mm), followed by a 70 cm protective layer of granulate.

The size of the sealed base is approx. 7 ha. Some areas have no base sealing and are filled exclusively with mineral DKI material. The total surface area is approx. 10 ha.

The entire area has a seepage water pipe network, of which approximately 2,000 m are still passable today. These pipes are cleaned once a year and inspected with a camera. Due to the rocky subsoil, there has never been a problem with settlement at the base of the landfill in Künsebeck.

Initially, the waste was simply piled up in a cone shape, but later it was compacted using a caterpillar and finally a compactor. However, the waste was never dumped directly against the limestone walls.

Fig. 3: Waste dumped a few metres in front of the rock face (source: GEG mbH)

When it became clear that a much larger landfill volume was needed, work began on constructing a complex 40-metre-high steep wall seal in front of the rock face. Stabilisation was achieved with a gabion wall. A 1-metre-wide clay seal was built between the rock face and the gabion, which included a fissure water drainage system with a drain to a control shaft. It can be assumed that the escaping fissure water seeps away again in other areas, so that no fissure water reaches the shaft at all. At least to date, no fissure water has been observed in the inspection shaft.

Fig. 4: Construction of the rock face seal with gabion in front (source: GEG mbH)

On 31 December 1999, the landfill site was closed and the freshly deposited household waste was left exposed across the entire area of almost 10 hectares. The amount of leachate produced reacted extremely quickly to precipitation and the storage capacity of the leachate storage tank was limited to 440 m³. The total volume amounted to approximately 45,000 m³/year. Part of this was transported to Westerwiehe, 35 km away, to the company's own leachate treatment plant, and the other part to a large sewage treatment plant 15 km away.

At that time, there was still no approved concept for how to proceed; the ideas about surface sealing were completely confused among the operator, politicians and the approval authority. In 2000, the Ruppert office managed to develop an approvable solution with a summary report on the existing approvals and amendment applications. The 10 hectares were divided into an old area of 3.5 hectares with relatively low settlement and a new area of 6.5 hectares with expected high settlement. The existing permit for final sealing with a multi-layer mineral seal and a KDB with a REKU layer made of locally available material continued to apply to the old area. The new area was to receive temporary surface sealing for the time being in order to reduce the amount of seepage water.

In 2000/2001, the following structure was created:

To create the surface shape with a suitable gradient (max. 1:3), the pre-profiling was partially constructed using soil and construction waste (load up to DKI). A 30 cm thick layer of HMV slag was then applied to this layer as a base and levelling layer, which was prepared on site over many weeks under the supervision of the University of Münster.

The actual temporary seal, a 2.0 mm thick KDB of the type Carbofol 406 with a surface structure on both sides, was then applied to a sand protection layer. Old tyres secured with rope chains in a 5 x 5 m grid served as wind protection.

Then came the time for observation and planning, and the following questions arose.

1. How does the settlement behave?
2. How will the amount and concentration of seepage water develop?
3. Will the groundwater pollution change?
4. How will the spring at Künsebecker Bach develop?
5. How will the gas volume and quality develop?
6. How does the temporary KDB behave in relation to settlement and weather conditions?
7. How are the surface water volumes of the two landfills developing?
8. How is surface water infiltration working?
9. How can we optimise the analysis effort for the approx. 55 groundwater measuring points and 5 defence wells?
10. Can the temporary KDB sealing be integrated into the final sealing?
11. What do we do with the temporary KDB when the final sealing is constructed?
12. Can the previously approved final sealing be optimised?
13. How will we build?

2015-2017 Construction of surface seals BA1 and BA2

After the settlement had subsided, the first construction phase (old area) of the final surface sealing was completed in 2015, covering an area of approx. 26,000 m².

The sealing system consists of a LAGA-approved bentonite mat of type Bentofix NSP 4900 LAGA, a BAM plastic sealing membrane Carbofol MegaFriction/MegaFriction and a protective and drainage mat of type Secudrain RZ 331 WDZ 701 RZ 201, also with BAM approval, so that the sealing system fully complies with the landfill ordinance.

The selected surface sealing system, with geosynthetics from NAUE GmbH & Co. KG, was already known to GEG mbH from the previously recultivated Westerwiehe 1 and 2 landfills as a cost-effective, quick-to-install, approved sealing system. The second construction phase was then put out to tender in winter 2015 after completion of the first phase using the same system, but with an area of 63,000 m². However, the temporary sealing had to be removed first in order to be able to profile the areas of the second construction phase.

GEG mbH had the 15-year-old, still fully functional temporary plastic sealing membrane of type Carbofol 406 2.0 mm f/f cut out at the weld seams and sold for another temporary waterproofing measure at a landfill site.

The profiling work could now be carried out with GPS-controlled bulldozers.

The sealing system was then installed on the base and levelling layer with the components Bentofix NSP 4900 LAGA, Carbofol 507 2.5 mm MegaFriction / MegaFriction BAM and Secudrain RZ 331 WDZ 701 RZ 201.

In order to integrate the recultivated Halle-Künsebeck landfill site into the landscape of the Teutoburg Forest with its typical poor limestone plains, the Detmold district government approved a regional, limestone-containing cover soil from the neighbouring Müller quarry, on top of a previously installed 20 cm thick layer of sand.

The erosion-resistant cover soil has so far withstood all heavy rainfall events and requires only minimal maintenance, so that the aftercare costs remain manageable.

Fig. 5: Temporary sealing with old tyre ballast

Fig. 6: Removal of the temporary waterproofing

Fig. 7: Sealing work in the second construction phase

The Künsebeck 1 and 2 landfills are now entering the aftercare phase, and local politicians are developing ideas for their subsequent use.