Academic journal article Journal of Environmental Health

Transport of E. Coli in a Sandy Soil as Impacted by Depth to Water Table

Academic journal article Journal of Environmental Health

Transport of E. Coli in a Sandy Soil as Impacted by Depth to Water Table

Article excerpt


Approximately 20% of the U.S. and 50% of the North Carolina households use septic systems for managing their sewage (U.S. Census Bureau, 2004, 2011). In order to properly treat wastewater containing an array of disease-causing organisms (e.g., Salmonella spp., E. coli) (Meschke & Sobsey, 1998), the soil in the treatment area of any septic system (known as drainfield) must remain aerobic and unsaturated (Paul, Rose, Jiang, Kellogg, & Shinn, 1995). The relatively nutrient-poor unsaturated soil is an undesirable environment for anaerobic bacteria in nutrient-rich septic tank effluent. Also, aerobic soil contains a large number of indigenous bacteria, protozoa, and nematodes that can prey on bacteria present in the wastewater.

According to North Carolina regulations (North Carolina Department of Environment and Natural Resources [NCDENR], 2007), sites for individual septic systems must contain an adequate area with a minimum of 45 cm (18 in.) of suitable unsaturated soil between the bottom of the system's trenches and any unsuitable layer (based on soil structure, mineralogy, wetness, and the presence of a restrictive layer or bedrock) in sandy soils, and a minimum of 30 cm (12 in.) of suitable unsaturated soil in coarse loamy, fine loamy, and clayey soil groups. The regulations regarding this separation distance vary among states (for example, Delaware Department of Natural Resources and Environmental Control, 2005; Idaho Department of Environmental Quality, 2011; Indiana State Department of Health, 1990).

The seasonal high water table (SHWT), identified using morphological properties with a chroma 2 or less (i.e., redoximorphic features) (Vepraskas, 1992), or by direct monitoring, is considered a restrictive layer. Severson and co-authors (2008), however, showed that the SHWT may rise above the redoximorphic features of chroma 2 or less at certain times during the year. The capillary fringe (CF), the zone immediately above the water table, is nearly saturated even though it is considered to be part of the vadose (unsaturated) zone. Misinterpretation of the redoximorphic features or not recognizing the presence of CF may result in an overestimation of the actual thickness of the unsaturated soil needed for the removal of microbes in wastewater.

The fate and transport of E. coli and other bacteria through soil have been studied for decades (Ausland, Stevik, Hanssen, Kohler, & Jenssen, 2002; Bolster & Abit, 2012; Butler, Orlob, & McGauhey, 1954; Faust, 1982; Gagliardi & Karns, 2000; Hagedorn, McCoy, & Rahe, 1981; Jiang, Morgan, & Doyle, 2002; Karathanasis, Mueller, Boone, & Thompson, 2006; Rahe, Hagedorn, McCoy, & Kling, 1978; Smith, Thomas, White, & Ritonga, 1985; Stiles & Crohurst, 1923; Tate, 1978). Despite these studies, evidence regarding the critical distance for treatment and removal of enteric bacteria in soils between the bottom of the septic system trenches and the SHWT remains inconclusive. Caldwell (1937) and Caldwell and Parr (1937) recommended 150 to 200 cm of unsaturated soil for effectively removing all enteric bacteria from sewage. Cogger and co-authors (1988) found that 30 cm of unsaturated coastal plain soils of North Carolina was not adequate for removing bacteria, but 60 cm may be the critical threshold for bacterial treatment.

Mathematical models have been used to describe transport of bacteria through soil (Abu-Ashour, Joy, Lee, Whiteley, & Zelin, 1994; Hijnen, Brouwer-Hanzens, Charles, & Medema, 2005; Hornberger, Mills, & Herman, 1992; McGechan & Vinten, 2003). In recent years, the HYDRUS model (Simunek, van Genuchten, & Sejna, 2006) has been used to simulate water flow in the drainfield of septic systems (Finch, Radcliffe, & West, 2008; Heatwole & McCray, 2007; Radcliffe & West, 2009). These models require specific information about various soil properties that may be difficult to determine. …

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