Figure 3 . Well humified organic matter has a very large adsorptive capacity for both organic and inorganic compounds, including most pollutants. For most soils, organic matter is concentrated in the topsoil. Maintaining an active organic component in the topsoil through good soil and crop management enhances the soil's capacity to serve as a filter. Another beneficial effect of organic matter is its improvement of soil structure.
Structure refers to the massing of individual soil particles into larger aggregates.
Organic matter tends to promote desirable aggregation and thus increases the sorptive capacity of the soil. Soil structure can be affected by land use and management. For example, tilling the soil under the proper moisture condition and incorporating crop residues enhance soil structure. The potential for groundwater pollution is influenced not only by the soil but also by characteristics of the various contaminants. This section describes how the specific characteristics of nitrate, pesticides, and biological contaminants affect their potential for contaminating groundwater.