Mobility: How high is the risk of a chemical reaching the groundwater?

If a chemical is to be authorised in Europe, one of the requirements is to assess whether it poses a risk to the environment. In addition to its toxicity and effects on environmental organisms, this assessment also examines how the chemical spreads in the environment if it enters it, either intentionally or unintentionally.

In the environment, a chemical does not move independently, but always in conjunction with a transport medium, usually air or water. As non-volatile chemicals in most cases end up in soil or water bodies, it is particularly important to understand how the chemical moves through the soil with water. Soil plays a special protective role here, as it can filter chemicals dissolved in water out of the water as it seeps through, for example when rainwater infiltrates. Thanks to this filtering function of the soil, most chemicals do not enter the groundwater, which is crucial for supplying people with clean drinking water.

Whether a chemical is filtered out of water by soil effectively or less effectively can be described using the so-called ‘sorption coefficient’. Whilst chemicals with high sorption coefficients are retained very strongly by soil, chemicals with low sorption coefficients are a potential hazard to groundwater. The category “mobile (M)” or “very mobile (vM)” is therefore particularly important in chemical regulation.

Determination of the sorption coefficient using laboratory methods and computer models
 

Various methods are available for determining the sorption coefficient, ranging from a fully computer-based estimation based on molecular structure (“QSAR”) to complex experimental investigations in natural soil cores using radioactively labelled chemicals. In between, there are laboratory methods of varying complexity for estimating mobility. In chemical regulation, data from laboratory methods are used initially. Studies using soil cores are now only carried out in exceptional cases, as these can now be well simulated using computer models provided that relevant data from laboratory experiments are available.

The laboratory methods commonly used today to determine mobility were originally developed primarily for the investigation of plant protection products. These are generally less mobile, and the measurement range of current laboratory methods is consequently optimised for this moderate mobility. However, with the REACH chemicals regulation, the focus has now shifted to entirely different chemicals. These include many chemicals that are highly mobile in soil, such as ionisable organic chemicals or highly polar substances. For these chemicals, established laboratory methods no longer function reliably, leading to significant uncertainty regarding their transport behaviour. Even though the tests do indicate that these chemicals will be mobile in soil, a numerical determination of the sorption coefficient is often not possible. As regulatory assessments are based on these numerical values of the sorption coefficient, this poses a problem.

Various approaches to determining mobility

Whilst current laboratory methods are based on the sorption of chemicals onto the organic soil matrix (‘humus’), it is possible that ionic chemicals interact primarily with partially charged soil minerals (e.g. clay minerals). To investigate this, scientists in the Department of Ecological Chemistry at Fraunhofer IME are pursuing two different approaches:

The soil column method

In a project funded by the industry association CEFIC, the mobility of highly polar organic chemicals in soil material from a bank filtration system is being investigated. To do this, the material is packed into soil columns and the chemical, dissolved in water flowing through the column at a constant rate, is ‘drawn’ through the column. Due to the interactions between the chemical and the soil material, this results in a delayed arrival of the chemical at the column outlet. Using this delay, the sorption coefficient can be calculated with a tailored computer model. What is special about the soil material is the near-total absence of organic carbon, meaning that the observed retention can be attributed almost entirely to the interaction with the mineral soil matrix. As with all new methods, there are initially some unexpected challenges to overcome, such as the unexpectedly rapid degradation of one of the selected test substances during column passage or the interference of the inorganic tracer (KBr) during the subsequent chemical analysis of the column eluate. However, by using a radioactively labelled test substance, the sorption coefficient was successfully determined. The data generated from this experiment are being used to further optimise the column test and to determine experimental sorption coefficients for the other test substances as well. Following successful development, the aim is to establish the soil column method as a reference method for determining the mobility of highly mobile organic chemicals. An integral part of the project is the adaptation of models that derive reliable numerical sorption coefficients from the column data. This work is being carried out in cooperation with scientists from the Technical University of Denmark (DTU).

In a second project initiated by the UK Environment Agency, a screening method is being developed to estimate the sorption coefficient with relatively little experimental effort, based on the retention of the chemical in a commercially available analytical HPLC separation column. Such a method was developed in the 1990s by Fraunhofer IME for moderately mobile chemicals (OECD test guideline 121) but has limitations for highly mobile chemicals. By adapting the column materials used, attempts are currently being made to better represent the relevant interactions of mobile chemicals in soils with low humus content. A reference method is urgently needed for this purpose and is currently being developed within the CEFIC project described above.

The high relevance of this work became clear at the Mobility Workshop held at the Royal Society of Chemistry library in London in November 2025 ("The Mobility of Polar and Ionisable Substances in the Environment"). In addition to representatives from industry, numerous representatives from the chemical regulatory sector were also present. In particular, the urgency of the work was emphasised on several occasions, as chemicals are already being classified according to their mobility based on data from current testing guidelines, which are not at all suitable for testing mobile chemicals and do not provide reliable data.

The Leaching Calculator

During the workshop, the Leaching Calculator – a development by the IME’s Modelling and Bioinformatic Department – was presented. Using the sorption coefficient and the biodegradation rate, it can provide a significantly better assessment of a substance’s transport behaviour than has previously been possible. Both parameters are interrelated, and strong retention in the upper soil layer – which is biologically much more active than the aquifer – means that biodegradable chemicals are retained there long enough to be completely eliminated. This is now taken into account in the Leaching Calculator.