Processes Causing Attenuation in Aquifers

Two zones are considered the most relevant physical media controlling the movement of water and pollutants: the unsaturated (vadose) zone and saturated depths. Contaminants arising from both point and non-point sources, such as municipal and hazardous waste landfills, suburban septic systems, mining and petroleum production, and agriculture, all filter through the vadose zone, where resident microbes, plant roots (rhizosphere), macrofauna (earthworms, ants, and termites), and organized colloids (humic matter, minerals) reduce their concentrations to levels acceptable by sanitary authorities. The principles of the vadose zone’s functioning are not completely understood because of the presence of the many subsystems and their chemico-physical and biological interactions. The vadose zone (through the many microbiological and chemico-physical processes taking place) is potentially capable of blocking or/and degrading any natural or artificially introduced contaminants flowing in deep waters. Soil overlying the water table provides the primary protection against groundwater pollution. Bacteria, sediment, and other insoluble forms of contamination become trapped within the soil pores. Some chemicals are absorbed or react chemically with various soil constituents, thereby preventing or slowing the migration of these pollutants into the groundwater. In addition, plants and soil microorganisms use some potential pollutants, such as nitrogen, as nutrients for growth, thereby depleting the amount that reaches the groundwater. In soils and sediments, the microbial mediated attenuation processes (natural or engineered) usually work through coupling electron acceptors such as nitrate, sulfate, and carbon dioxide with electron donors such as organic compounds