Winchester Site
Virginia Polytechnic Institute and State University
On-site Wastewater Treatment and Disposal Systems
Summary
The Winchester low pressure distribution system was designed to treat 450 gpd through three subsystems. Subsystem 1 was based on the Virginia Department of Health regulations for a 50 min/in percolation rate which resulted in a 0.5 gpd/ft2 trench bottom loading rate. Subsystem 2 was based on Wisconsin recommendations and was designed at a 0.72 gpd/ft2 trench bottom loading rate. The third system was based on North Carolina recommendations and was designed for a loading rate of 0.25 gpd/ft2.
This site had a failing conventional system on a 10% slope and the soil was restricted due to shallow depth to bedrock. The percolation rate was estimated at 120 min/inch.
North Carolina Subsystem
Within a few months of operation there was evidence that the NC subsystem had failed. Effluent was continually ponding in the trenches to a depth of 6 to 8 inches and the subsystem failed hydraulically. Nitrate levels were greater than 10 mg per Liter 12 inches below the bottom of the NC subsystem. There were also large reductions in NH4 concentrations at the 12 inch depth compared to effluent concentrations. These data indicate that even though effluent was ponding some nitrification was taking place.
Wisconsin Subsystem
In this design, NO3-N concentrations were below 10 mg per Liter at all sampling depths. Ammonium-N concentrations decreased with soil depth, indicating nitrification. The NH4-N levels from the applied effluent. The soil matric potentials were low (dry conditions) at both depths under the trenches for each seasonal category. The pH decreased with depth indicating a reducing effect on soil solution by the effluent. The PO4-P levels at the 12 inch depth reflect a 99% reduction from effluent levels which demonstrates the strong affinity for P by the soil.
Virginia Subsystem
This subsystem maintained the lowest NO3-N concentrations (3.69-4.9 mg per Liter) overall soil depth. Ammonium levels were 11.76 and 9.21 mg per Liter at 12 and 18 inches under the trench bottoms. These relatively high levels might signify low nitrification and insufficient aeration of the soil profile, however the NH4-N levels reflected a 89% reduction from the effluent NH4-N concentrations. The large reduction in the inorganic N/Cl ratio (85%) indicated that large quantities of inorganic N were being removed from the effluent as it moved through the soil profile. Phosphate levels significantly decreased as the effluent percolated below the trenches signifying soil adsorption of PO4-P. Unlike the pH measurements, Cl- and EC concentrations did not change with soil depth. The decrease in pH levels are probably more reflective of the strong acidic buffering capacity of the soil while the EC and Cl- data signified that the effluent continued to affect the soil solution down to 18 inches below the VA subsystem. The soil matric potentials showed unsaturated conditions under the VA subsystem and indicated relatively dry conditions at all depths measured below the trenches for each seasonal category.
URL http://fbox.vt.edu:10021/cals/cses/reneau/projects/winchester.html
Last Updated: 5/26/98
Created By Katie Corbin
Pictures of the Winchester Site