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Suitability of Limestone-derived Soils for On-site Wastewater Disposal

出版	Virginia Polytechnic Institute and State University, 1986
URL	http://books.google.com.hk/books?id=qhtPAQAAIAAJ&hl=&source=gbs_api

註釋Limestone-derived soils comprises large partes of the Great Valley of Virginia as well as other parts of the United States and world. Previous researchers proposed relatively low application rates for septic tank effluent (STE) to clayey subsoils of glacial origin but much higher rates of applicatio have been noted to be suitable in some but not all clayey soils developed in limestone residuum. Additional questions heve also been reised about the potential for pollution of groundwaters by nitrate (NO3 -N) formation from de ammonium by the Virginia State Board of Health in 1982 provided for effluent loading rates to be determined either by the soil texture or the percolation rate but did not take into account soil structure or the protential for leaching of NO3 -N to groundwaters. The porpuse of this work was to (1) test the hypothesis that clayey, well drained soils are suitable for septic tank subsurface absorption systems (ST -SAS) if low pressure distribution (LPD) of STE is used; (2) determine approximate loading rates for soils with different grade, size, and type of structure, and depths to restrictive layers; (3) determine the extent of nitrification in clayey limestone-derived soils below ST -SAS in which effluent was (a) ponded and (b) not ponded. Results of this study indicate that clayey soils developed in limestone residuum are suitable for disposal of STE. Where the soil structure is weak blocky to massive but soil colors indicate god drainage, flux densities of 0,5 cm d-1 may be suitable with LPD. ST -SAS installed in fine textured soils with 15 to 30 cm of soil with moderate to strong blocky structure beneath them may be dosed at 1 cm d -1. Where more than 60 cm of soil with strong structure but no other apparent restrictions underlies a ST -SAS, loading rates of 2 to 3 cm -1 may be suitable. Presence of>25% coarse fragments in otherwise well structured soils may result in lower effluent acceptance rates, hence a maximun rate of 1 to 2 cm c -1 may be suitable iln these soils depending on other soil properties. If effluent is continually ponded in ST -SAS, nitrification may be minimal. However, nitrification was predominant below sT -SAS which were not continously ponded and below trenches which were rested. Concentrations>60 mg L -1 of NO-3 may be present in water leaving the immediate trench bottom area below ST -SAS. The density of ST -SAS placement and loading rate should be determined in part by the area required to dilute NO3 concentrations to de U.S. Public Health Service standards of 10 mg L-1.