Risöfladan experimental field

The need for field experiments

In the PRECIKEM projects, focus has been on treating fields in active agricultural use in areas where acid sulfate soils are predominant. The goal has been to raise the pH of the drainage water, to precipitate metals in solution, and to slow down the continued oxidation of sulfides together with the concomitant acidification of the partially oxidized layer of acid sulfate soil underneath the plough layer. Since liming of the plough layer has been shown to not affect the underlying soil layer, the overarching idea in the PRECIKEM projects has been to use subsurface drainpipes in combination with the structure of interconnected cracks that develop in a marine sediment with a high clay content as it is drained and dried. Suspensions of suitable chemicals (such as ultrafine-grained limestone, slaked lime, precipitated calcium carbonate, and peat) are pumped through the drainpipes and into the surrounding soil layer via these hydrologically very active cracks. The drainpipes are connected to a control well. The basis of the PRECIKEM technique is thus controlled drainage with subirrigation where part of the irrigation water is treatment suspensions (Figure 1).

To test the technique on a practical scale, an experimental field has been built as part of the PRECIKEM projects. In order to be able to simultaneously test the effects of several different treatments on drainage water and to make comparisons with drainage water from untreated soils, the field has been divided into a number of subfields. The place chosen for the experimental field was a field at Risöfladan owned by the Vocational College of Ostrobothnia.

 

Risöfladan

Risöfladan lies in Vaasa in Ostrobothnia (Figure 2) on the Finnish west coast and is the bottom of an old bay that previously functioned as the estuary of a local river. The area a is bilingual, and the river is known by several names: Toby å in Swedish, and Laihianjoki or Tuovilanjoki in Finnish. Today the river flows south of Risöfladan and the river mouth together with the new estuary lies west of Risöfladan. The isostatic land uplift due to postglacial rebound in the area is about 8–9 millimetres per year and the area was in the 1940s still under water with a vegetation dominated by reeds. In the 1950s, the area was surrounded by embankments and drained using a pumping station by the river. Open ditches were used to begin with and the agricultural school of Korsholm (today’s Vocational College of Ostrobothnia) began cultivating cereal crops on the land after liming the severely acidified top layer of the soil. The first subsurface drainage system was installed in 1968. The ground level is still about half a metre below sea level and the pumping station is thus still needed to maintain a proper drainage of the area. Today the soil material underneath the well-maintained plough layer consists of a typical acid sulfate soil and the drainage water is both acidic and rich in metals.

The experimental field is constructed in 2011

In 2011 an experimental field consisting of nine subfields was constructed on land at Risöfladan belonging to the Vocational College of Ostrobothnia (Figure 3). The construction was part of the PRECIKEM project and thus funded by the project funders (see separate page describing the PRECIKEM project). The field was planned by Rainer Rosendahl at ProAgria Österbottens Svenska Lantbrukssällskap, who also oversaw its construction. The construction work itself was done by Nybacks Gräv. The subfields were one hectare each and a separate drainage system was built at about 110 centimetres below ground level for each subfield. The drainage system consisted of three drainpipes 80 millimetres in diameter and equipped with flush pipes. The distance between the drainpipes was 26 metres, and these were connected to a collector pipe 100 millimetres in diameter, leading to a control well.

 

Every subfield is surrounded by a plastic sheet extending from about 0.4 metres below the ground level down to a depth of 1.9 metres (Figure 4). At this depth the soil material consists of a very dense clay with little or no hydrological conductivity. This sheet provides hydrological isolation between the subfields and between the subfields and the open ditches surrounding the experimental field. The sheet prevents treatments from affecting neighbouring subfields and also helps in storing groundwater in the subfields by preventing a direct flow of groundwater to the open ditches. The only exception is subfield 2 where the plastic sheet between the subfield and the open ditch was not installed in order to provide information about the extent of the direct hydrological communication between the ditch and the subfield.

A water pipe was built from the nearby river allowing river water to be pumped to each control well.

 

The experimental field is extended in 2016

With the help of separate funding awarded by Drainage Foundation (Salaojituksen Tukisäätiö) the experimental field could be extended in 2016. Subfields 10–12 were constructed with a new type of drainage system that allows for a circulating subirrigation (Figure 5).

In the evaluation of the field experiments performed 2012–2014, it was concluded that the spreading of the treatment suspensions in the soil was most effective close to the control well where flow and pressure were the largest, while the spreading diminished further out along the drainpipes as the flow in the drainage approached zero and some sedimentation in the pipes started to occur. The need for a drainage system where flow and pressure are the same in all parts of the system, and where cul-de-sacs in the form of conventional drainpipes are avoided, was evident. A new type of drainage system that allows for a circulating subirrigation was planned by Rainer Rosendahl at ProAgria Österbottens Svenska Lantbrukssällskap (Figure 6). 

A feed pipe was connected to the drainpipes and the control well was modified to allow for a circulating subirrigation water/treatment suspension (Figure 7). The drainpipes and collector pipe work as a normal drainage system when not subirrigating. The new drainage system is expected to be of interest also in conventional subirrigation.

 

The experimental field is supplemented in 2020

In a third stage the experimental field was supplemented in two subfields (6 and 9) where a separate irrigation pipe above the drainage system (Figure 8) was added.

The supplement was funded by Svensk-Österbottniska Samfundet. Planning and oversight of the construction was provided by Mikael Blomqvist ProAgria Österbottens Svenska Lantbrukssällskap. The irrigation pipe was installed by Nybacks Gräv. This pipe is in the form of a loop and lies in upper part of the acidified soil layer about 60 centimetres below ground level and thus about 50 centimetres above the drainage system. The irrigation pipe is connected to its own well allowing for a circulating subirrigation water/treatment suspension and thus avoiding problems with sedimentation in the pipe. Both the outgoing part and the returning part of the loop lies between drainpipes (Figure 9, animation). When a treatment suspension is injected using the new irrigation pipe, it flows through cracks and other macropores downward and to the sides towards the drainpipes in the lower part of the acidified soil layer. The intention is to treat a larger soil volume this way.

Link to animation

 

*Picture 2b. Creative Commons Attribution 4.0 International License.