Shallow aquifers, commonly the most important sources of ground water, are also those most susceptible to contamination. The mode of entry of contaminants to shallow aquifers is (1) directly, via wells or secondary openings in consolidated rocks, (2) percolation through the zone of aeration, (3) induced infiltration through the zone of saturation, and (4) interaquifer leakage or flow through open holes. Natural removal or degradation of contaminants is by filtration, dispersion, sorption, ion exchange, oxidation, and various biochemical processes. These phenomena are controlled by the physical environment, structure; mineralogy, and hydraulic characteristics of the earth materials contacted by the liquid wastes. When liquid wastes enter an aquifer directly, there is little or no natural treatment by filtration, sorption, or oxidation. Purification is only by those processes that operate within the aquifer under anaerobic conditions. Contaminants from natural sources that enter aquifers under saturated‐flow conditions are degraded primarily by dilution. The natural processes effective in reducing contamination from surface‐water sources depend on the hydraulic regimen involved, which vary with individual cases. Liquid wastes percolating through the zone of aeration are those most likely to be purified by natural environment processes. Natural processes, however, do not effectively remove or degrade all contaminants, especially some of the many highly stable compounds that have gained widespread use in recent years, such as synthetic detergents. Comprehensive interdisciplinary research into the ability of various earth materials to remove many types of contaminants under varying hydrologic conditions is needed.