The Chemical Potential of Water in Soils and Sediments

Abstract

Core Ideas Measurements of the true chemical potential of water in soils are presented. The thermodynamic state of water is not uniform in up to 80% of the pore spaces of soils and sediments. Application of the chemical potential of water in biogeochemical studies of soils is discussed. The chemical potential of water characterizes the thermodynamic state of the aqueous environment in soils and sediments. In the presence of clay minerals and other fines, such as organic matter and silt, it is lower than at standard state (less than −237.18 kJ mol‐1). As a result, chemical reactions may proceed rather differently than in an ideal aqueous solution. This is currently not considered in biogeochemical models of soils and sediments. Therefore, we conducted a series of quantitative measurements of water's chemical potential in soils of various clay content. We can show that the amount of pore water in soils traditionally determined gravimetrically from water loss after drying at 105°C, overestimates the pore water that is thermodynamically at standard state. We discuss implications for biogeochemical processes in soils and sediments and give evidence that clay content is a good predictor for an estimate of chemical potential. Although the accurate chemical potential of water has so far received little attention, it may emerge as a useful tool in soils sciences, geochemistry, and other related disciplines.