Evaluation of the effects of absorption field product architecture type on effluent dispersal and soil chemical properties in a profile-limited soil

On-site wastewater treatment systems (OWTSs) are commonly used by households in areas of low population density to treat household wastewater and recycle it back to the environment. The traditional absorption field product used in OWTSs is a pipe-and-gravel architecture type, which has been used for decades and is the basis for most design regulations. However, new products of differing architecture types including chambers, polystyreneaggregate, pipe-and-tire-chip, and gravel-less-pipe systems have recently become available. A three-year field study was conducted in Bethel Heights, AR to assess the performance of several newer products and to compare different architecture types to the traditional pipe-and-gravel design under wet and dry soil conditions. Thirteen products of four different architecture types were installed at the Bethel Heights Wastewater Treatment Facility (BHWTF) in a Captina siltloam soil (fine-silty, siliceous, active, mesic Typic Fragiudult) with redoximorphic features indicating a seasonal water table at various depths. Loading rates were determined using soil morphological indicators and the maximum allowable rate under State of Arkansas regulations. Product performance was evaluated based on the height of in-trench solution storage, measured approximately weekly from January 2009 through January 2012. Between May 2010 and January 2012, the thickness of any biomat formation was also measured approximately weekly. Soil samples were collected at the time of installation in 2008 and after > 2 years of effluent dosing in 2011 at the infiltrative surface (i.e., 45to 55-cm depth) and were analyzed for pH, electrical conductivity (EC), and Mehlich-3 extractable nutrients. No products exhibited signs of failure (i.e., surface ponding) throughout the 3-yr duration of this study. Architecture type alone did not affect (P > 0.05) product performance. However, the performance of individual products differed under wet and dry soil conditions (P < 0.05). There was a correlation between percent coarse fragments at the infiltrative surface and mean height of solution stored (P < 0.05), but no other correlation was observed between solution storage and several soil properties (P > 0.05). The presence of a biomat ranged from 0 to 98 % of the time among the 13 products. When present, biomat thickness differed significantly (P < 0.05) among all four architecture types, ranging from 1.4 to 6.2 cm thick on average in the pipe-and-aggregate and polystyrene-aggregate types, respectively. Regression analyses showed that biomat thickness increased within three products, did not change within nine products, and decreased within one product over the 3-year duration of this study. There was no effect (P > 0.05) of architecture type on soil chemical properties at the infiltrative surface except for copper (Cu) concentration, but there were significant differences (P < 0.05) in pH, EC, and extractable phosphorus (P), magnesium (Mg), sodium (Na), iron (Fe), manganese (Mn), zinc (Zn), and Cu over time when averaged among all products. Soil pH and extractable Mg and Cu decreased, while EC and extractable P, Na, Fe, Mn, and Zn concentrations increased over time. Results showed that there are several currently approved alternative products with in-trench solution storage heights less than or equal to the traditional pipe-and-gravel system, but that there are also several alternative products with intrench solution storage heights greater than the traditional pipe-and-gravel system. The soil morphology approach appears to be sufficiently adequate and appropriately environmentally conservative for assigning OWTS loading rates to alternative products and to the traditional pipe-and-gravel system. This thesis is approved for recommendation to the Graduate Council.