C Filtering in the upper soil layer

Many factors, including the soil's physical and chemical characteristics and the environment in the soil (table 3-8) affect the removal of fecal bacteria in the soil and prevent their movement into ground water. The primary factors are filtration, adsorption, and die-off in the soil.

Bacteria passing through the soil matrix can be filtered as a result of three processes acting independently or in combination. These processes are:

• physical filtration or straining by the soil matrix

• sedimentation of bacteria in the soil pores

• "bridging," whereby previously filtered bacteria block or reduce the size of pores through which other bacteria would normally pass

Soil texture, structure, and pore size vary considerably among soils and influence the effectiveness of the filtering process. Adsorption of micro-organisms onto clay particles and organic material effectively removes bacteria from liquids. Filtration and adsorption can remove over 90 percent of the bacteria applied in effluent in the first half inch of soil. Almost total removal can be accomplished in the first 2 inches of fine-textured soils.

Table 3-8 Soil factors affecting infiltration and movement (leaching) of bacteria in soil

Physical characteristics


Particle size distribution Clay type & content Organic matter type

& content Pore size distribution Temperature Moisture content Fragipan (hardpan) Surface compaction

Environmental & chemical factors

Cation-exchange capacity Chemical makeup of ions & their concentrations Bacterial density and dimensions Nature of organic matter in waste effluent solution (concentration & size) pH

Some soils have a tremendous capacity to remove bacteria and protect the ground water resource. However, coarse-textured or disturbed soils do not provide the same level of treatment as undisturbed, fine-textured soils. In addition, overloading or constant saturation of the soil can greatly reduce its ability to remove bacteria.

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