Soil-water interactions

The composition of water flowing through and out of soils and rocks reflects the composition of the material that it has passed through. The chemical processes involved are mainly relatively rapid cation exchange reactions with soil colloidal materials, and slow acid hydrolysis reactions which decompose primary minerals. Examples are given of the results of these processes during flow through acid podzolic soils into surface waters. This pattern is modified during periods of high water discharge. The proportion of flow that takes different routes through the land surface, and the contact time with soil or rock, are then important for water composition. Sources of acidity can be generated within soil, when chemically reduced material in anaerobic environments is exposed to air by changes in the soil water regime. It is suggested that only detailed soil-water investigations in the field can satisfactorily explain lake and stream water composition at any specific site.