Tuesday, 11 July 2006

Impacts of Soil Use for Wastewater Renovation.

Walter E. Grube, Pennsylvania Department of Environmental Protection, 2 East Main Street, Norristown, PA 19401

The continuing growth of suburban housing units, both singly and on sites which include hundreds of individual homes, demands systems which accept domestic sewage for treatment and ultimate disposal of the wastewater. Environmental concerns include a restriction on open release of treated sewage effluent waters into local waterways, many of which are classified as high-value or excellent-quality, with the regulatory requirement to avoid any potential negative impacts from pollutant sources. Spray irrigation of sewage treatment plant effluent onto land, with chemical and biological renovation as it seeps through soil, was proven to be an efficient and effective process by intensive studies at Penn State University in the early 1960's. Effluent renovation includes the treatment and removal of constituents such as BOD, suspended solids, nitrogen, phosphorus, trace metals, microorganisms, and trace organic compounds from the wastewater after it has received tertiary treatment by a modern sewage treatment plant. This paper does not address the parallel environmental concerns associated with land disposal of sludges resulting from sewage treatment. Soils and their suitability for wastewater irrigation in the eastern U.S. vary widely. The Piedmont Region contains many deep and well-drained soils, although in many areas these are interspersed with soils shallow to bedrock, possessing a shallow water table, or showing redoximorphic features which indicate a seasonal high water table. Southeastern Pennsylvania, as well as many other unique sections of the United States, contains natural and native soils which are adequately deep and possess the hydraulic properties favorable to irrigation-application of sewage treatment plant effluent waters. Within a three-county area near Philadelphia we have scores of fields receiving these waters through either above-ground spray irrigation or subsurface drip dispersal systems. These effluent renovation areas now total over 500 hectares. The USEPA's Onsite Wastewater Treatment Systems Manual (latest edition, 2002) serves as the basis for many states' recommendations and regulations, and dedicates 8 pages to detailing soil investigations needed to assure adequate system performance. State regulations require that a qualified soil scientist examine each site proposed for effluent irrigation to determine the presence of any zone which limits the capacity of the soil to permit the passage of water, and to measure the rate of water movement through a selected soil profile. Regulations further prescribe minimum specifications for soil test pits which must be excavated to permit soil profile evaluations and documentation. Thus, intensive study and mapping of each proposed wastewater irrigation site is necessary to delineate only suitable areas. The authors present details of the site and soil characterization which we apply in evaluation of sewage system plans. Research has not yet determined the pedologic impact of intensified leaching which effluent irrigation will impose. Future generations of soil scientists will need to determine the extent to which these soil profiles are being modified.

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