After a spill of contaminants at the soil surface, from leaking underground oil tanks etc., the contaminants dissolves in the soil water (water from precipitation), which are located between the soil particles in the unsaturated zone above the groundwater. The contaminants are then transported with the soil water down through the soil matrix and can lead to contamination of the groundwater. In case of severe pollution with contaminants in liquid forms, the liquid can also penetrate between the soil particles as a free phase and contaminate the groundwater.

An important part of the risk assessment of groundwater contamination is to estimate the concentration of contaminants that seeps towards the groundwater. So far, the concentration in soil water samples for most contaminants has been estimated by analyzing soil samples and converting to the concentration in the soil water via equilibrium calculations, which are often relatively conservative. For example, PFAS and pesticides have a relatively high detection limit in soil analyses, which means that even a content of PFAS or pesticides in the soil below the detection limit can result in a corresponding concentration in the pore water with content above the criterias for groundwater.
Especially in the cases described above and at sites with a large unsaturated zone, soil water sampling is a particularly suitable method for directly quantifying the content of dissolved organic contaminants in the soil water in the unsaturated zone, which contributes to a more realistic risk assessment for groundwater. After the installation of suction cells for subsequent soil water sampling, it is also possible to take multiple samples at the same point and examine the temporal and possibly seasonal variation. There are several examples of our suction cells being functional for more than approximately 30 years after installation.
DMR has developed two methods for sampling pore water from the unsaturated zone:
- Screening of soil water near ground surface: The screening method can be used for soil water sampling of non-volatile compounds such as PFAS, pesticides, heavy metals, and nutrients (nitrogen and phosphorus) etc. The method can be applied to depths of approximately 5 meters below ground level but is easiest to handle if the soil water sample is to be extracted from 0-2 meters below ground level. The method does not work optimally in formations consisting of coarse deposits such as coarse sand or gravel. Additionally, the method cannot be used directly for the investigation of volatile compounds (such as chlorinated solvents or volatile hydrocarbons), as there is a risk that they will be stripped from the water due to the vacuum applied during sampling. In these cases, sampling will require the use of Sorbicells and DMR’s automated control unit for controlled sampling.
- Deep soil water sampling: The method can be used for soil water sampling at depths greater than 5 meters below ground level. For deep soil water sampling, an automated control unit is used, which has various options for controlled sampling, such as automatic vacuum maintenance, sampling at a constant flow, etc. The method ensures gentle soil water sampling. The automatic vacuum control also helps ensure that soil water samples can be extracted from low-yield formations, such as clay and coarser sand. It is also possible to extract soil water samples for volatile compounds via Sorbicells. DMR has also developed an optimized method for deep soil water sampling; the DualTube method. With this method, the suction cell for subsequent soil water sampling can be installed either via a conventional drilling rig or Direct Push. The suction cell is connected via an innovative double tube system to the automated control unit, which subsequently ensures sampling under controlled conditions. When using DMR’s automated control unit, sampling can be monitored online via DMR Cloud.
In 2018 DMR bought the company Prenart Equipment, which is a more than 30 year old quality brand producing equipment for soil water sampling, producing specialized equipment for soil water sampling. Combined with DMR’s conceptual knowledgebase regarding transport of pollutants in the unsaturated zone, we are now a marked leading company, when it comes to sampling of soil water and combining it with risk assessment of organic pollutants.
DMR can provide all phases of consulting on method selection, sampling strategy design, sampling, and risk assessment. Additionally, DMR supplies equipment for soil water sampling if you wish to conduct the sampling yourself: https://www.dmr.eu/shop/soil-water-sampling/