Natural electrical potential difference affects water transport in clay

Dutch Researcher Katja Heister investigated how electrical potential differences in clay layers influence the transport of salt and water through these. The outcomes of her research have important implications for new models of water transport, for example, those which predict the distribution of substances from waste deposits.

The transport of water and its solutes through clay plays an important role, for example, in the intrusion of seawater into the groundwater of coastal areas, the distribution of substances from polluted sludge or waste sites and the storage of radioactive or toxic waste in deep clay layers.

Until now, computer models for water transport did not consider transport due to electrical potential differences. However the effects of an electrical potential difference across different clay layers cannot be ignored and must be included in these models. The data and parameters obtained by Heister during her research can be used for this purpose.

Water transport in the soil is not only caused by hydrostatic pressure but also by differences in salt concentration and electrical potential difference. These processes are called chemical osmosis and electro-osmosis, respectively. Both a difference in water pressure and in salt concentration can give rise to an electrical potential difference over the clay layer that affects the transport of water (streaming potential) and solutes (membrane potential) through it. The potential differences arise because the clay layer acts like a semi-permeable membrane, in a similar manner to the wall of a biological cell.

In laboratory experiments, Heister examined the effect of streaming and membrane potentials on the transport of water and dissolved salts through a dense clay layer. She used different types of clay such as a commercially available Wyoming bentonite, Boom Clay from Belgium and Calais Clay from the polder Groot Mijdrecht in the Netherlands.

Heister observed significant streaming and membrane potentials in both the Wyoming bentonite and the Boom Clay. These potential differences give rise to a counterflow of water and salts through the clay. The Calais Clay strongly acidified in the laboratory to form an acid sulphate soil, in Dutch ’katteklei’. Heister could not observe any electrical potentials in this.