Page 253 - Hydrogeology Principles and Practice

HYDC06 12/5/05 5:34 PM Page 236

236 Chapter Six

Fig. 6.22 Landfill leachate contamination of groundwater in a lacustrine sand aquifer at Villa Farm, West Midlands, showing (a) the
distribution of chloride concentration along a vertical cross-section of the pollution plume as determined by borehole (BH) sampling and
(b) a schematic diagram showing the transition from oxidizing conditions in the background, uncontaminated groundwater to the heavily
polluted and highly reducing zone near the lagoons at the base of the aquifer. After Williams et al. (1991).

ated sediments of the drain field supplies oxygen for septic systems due to a lack of labile organic carbon in
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aerobic oxidation of organic carbon and NH and natural settings. Without natural attenuation by deni-
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a consequent decrease in waste water alkalinity. The trification, it is quite likely that in unconfined sand
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nitrate formed by NH oxidation in this zone is the aquifers common in North America, the typical mini-
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primary and generally unavoidable adverse impact mum permissible distance between a well and septic
of septic systems at most sites (Fig. 6.24). In the third tank (25–35 m) will not be sufficient to provide pro-
zone, nitrate is reduced to N by the anaerobic pro- tection against nitrate contamination by dispersive
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cess of denitrification, although rarely found below dilution alone (Robertson et al. 1991).