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WELL OPEN TO MOST OF THE SATURATED ~ THICKNESS "'" SEPTIC. SYSTEM EFFLUENT WATER TABLE '" / SATURATED ZONE ~ CONTAMINANT . PLUME ~ GENERAL GROUND.WATER FLOW SHALE BEOROCK Figure 2. Diagram of a septic-system installation, the general direction of ground-water flow, and the configuration of a plume of ground water that has been degraded by septic-system effluent. POTENTIAL SOURCES OF CONTAMINANTS TO THE GROUND WATER Human-related activities that could potentially contribute contaminants to the ground water include agri- culture, animal husbandry, lawn and garden maintenance, and the use of scptic systems. Potential sources of contam- inants to the ground water on the Mesa include fertilizers applied to lawns_ gardens, and farmlands; animal waste from cattlc, horses, and other livestock; and septic-system effluent. Water that recharges the ground water and that has been in contact with inorganic fertilizers could be enriched in nitrate or ammonia; water that has been in contact with organic fertilizers could be enriched in nitrate, ammonia, or DOC; and water that has been in contact with animal waste or manure could be enriched in nitrate, ammonia, DOC, and fecal-indicator bacteria. Septic-system emnent could be enriched in nitrate, ammonia, DOC, detergents and other household chemicals, fecal-indicator bacteria, and caffeine. In addition, soils arc a potential source of total coliform bacteria. The evaluation of possible human-induced elfects on ground-water quality is the broad focus of this report, but many residents of the Mesa are concerned about the effects of septic-system effluent on ground-water quality. There- fore, a discussion of the geochemical processes affecting septic-system ef11uent follows to provide residents with an understanding of the processes that occur to decrease the concentrations of contaminants in septic-system effluent. Although this discussion focuses on chemicals and bacteria in septic-system effluent, the geochemical processes work . in similar fashion to decrease the concentrations of poten- tial contaminants originating from animal wastes Of lawn, garden, and agricultural chemicals. REMOVAL OF CHEMICAL AND BIOLOGICAL CONSTITUENTS FROM SEPTIC-SYSTEM EFFLUENT IN THE SUBSOIL Several processes occur in the subsoil (unsaturated zone above the water table and the saturated zone below the water table) to decrease the concentrations of chemical and biological constituents in septic-system effluent (fig. 2). Most of the potential contaminants in septic-system effluent are removed in the unsaturated zone below the leach field and above the water table by oxidation or filtration (Wilhelm, Schiff, and Cherry, 1994). When effluent reaches the unsaturated zone above the water table, it flows through the pores between the particles, such as sand and clays, that make up the subsoil. Large particles and bacteria in the effluent can be filtered by the subsoil, leaving mostly dissolved compounds in the effluent. As the effluent flows through the subsoil, ammonia is oxidized to form nitrate. . When nitrate reaches the water table, and if DOC is present and dissolved-oxygen concentrations are low, the nitrate and DOC may be consumed by denitrifying bacteria to produce nitrogen gas. Thus, the concentration of nitrate increases beyond the leach field but then decreases as it travels through the saturated zone (Robertson and others, 1989). Catfeine and MBAS can be degraded to other compounds by bacteria in the saturated zone in the vicinity of the leach field where the compounds originated, or they can persist in ground water if bacteria are not present or the efficiency of treatment within the saturated zone has been reduced. Biological constituents in septic-system effluent that can cause disease (pathogenic organisms) include bacteria and viruses. These microorganisms have ditferent survival rates and transp0l1 properties in the saturated and unsatur- ated zones below a leach tield. Total coliform and E. coli bacteria can be removed from septic-system effluent by filtration as the effluent nows through the unsaturated zone (Viraraghavan and Warnock, 1976). However, if the water table becomes closer to the land surface, the unsaturated zone thins and more of the bacteria in the effluent can poten- . tially reach the ground water (Canter and Knox, 1985). 2