Tuesday’s Soil Ecosystem Service – Water filtration and purification
Definition: Soil represents the largest natural filter on the planet. Soil filters mechanical particles from precipitating waters and cleans it from chemicals. As water moves through the soil profile, pollutants (i.e. heavy metals, oils, pesticide residues, viruses, and mineral particles) are retained, neutralised and decomposed by the soil structure and system of voids. Primary soil properties involved in the filtering/purification process are soil structure, texture, organic matter content, biota and soil depth. The water filtration and purification soil ecosystem service provide drinking water for human and biota. Filtering and purification capacity of the soil are finite. When exceeded, the soil turns from sink for pollutants to source of pollutants.
Relevant effects, processes and controlling soil properties: The ability of a soil to mechanically filter particles is largely dependent upon the size of particles and voids and how fast water flows through soil, where chemical purification of precipitating water depends on interaction with active clay minerals, organic matter and living organisms. Water flows faster through coarse textured sandy soils because of the large voids between sand grains. The shorter time the water has to interact with the soil particles combined with the smaller surface area results in water that is not as clean as the water that flows through the fine texture/clayey soil. The fate and transport of polluting substances strongly depend on the neutralising, buffering and binding capacity of the soil, i.e. the specific ion adsorption, chemical degradation and leaching processes they are subjected to. Clay particles and organic matter have charges that attract some chemicals and keep them from moving to groundwater. Transport of pollutants adsorbed to soil particles may occur through soil erosion, colloid transport in macrospores and deposition downstream. Also, metabolites resulting from degradation may behave differently than the original substances; for example, they may be very soluble and therefore leach out to the groundwater or surface water. The soil can retain applied nutrients or pesticides, organic and inorganic compounds, which can originate from agricultural, municipal or industrial by-products. Soil compaction, contamination, organic matter decline or alteration of the composition of organisms, deplete soil purification capacities and accelerate the movement of polluted water through the soil into groundwater – important drinking water supplies.
Relation to the entire ecosystem: Filtration and purification of water are one of the most important services that soils play in our ecosystem and especially for humans. Even if soils largely provide this service, vegetation can support it in different ways, e.g. as an additional filter above the soils or in the technical use of phytoremediation. Healthy soils are critical to ensuring clean water for consumption, crop production, recreation, and more.
Relations to other services: All water cleaning processes are connected with the amount and residence time of percolating water and thus with the services soil water retention and soil water storage. Furthermore, soil organic matter, as well as biological activity (habitat provision), plays a crucial role thereby.
Land use impacts: The capacity of soils to deliver this service is strongly influenced by land use and (unsustainable) soil management as they affect soil properties such as soil pH, soil organic matter content and biological activity, and induce soil degradation such as erosion, compaction and contamination.
Climate change impacts: In the course of climate change higher temperatures and longer dry periods may affect the water uptake capacity from soils and are likely to decrease their organic matter content, which will lead to a quantitative and qualitative impact on water filtration and purification capacity of the soil.
Demand aspects: Due to environmental problems and the growing world’s population, the societal demand for water filtration and purification will remain high, even if there is no clear link to climate change.
- Output: Clean ground and spring water, provision of drinking water.
- Provision: Undesirable substances are removed from water by infiltrating into and percolating through the soil. Physical, chemical or biological processes, which are controlled by soil depth, soil structure, particle size distribution, amount and quality of organic matter, soil pH, and the biological activity, contribute to this cleaning process.
- Demand: As the world’s population grows, the demand for clean water for personal uses (e.g. drinking, domestic use) or agricultural and industrial use will also grow.
- Threat: The degradation of soils (i.e. heavy soil pollution, erosion, compaction, organic matter decline, acidification, salinisation) threatens water filtration and purification.