Hydraulic conductivity of onsite constructed wetlands

Hydraulic conductivities were determined for four onsite constructed wetlands systems located in northeast Alabama. These systems have operated continuously since 1992 or 1993. Water levels were measured for relatively steady flow rates that represented potential instantaneous maximum flows. Hydraulic conductivity of the gravel in each cell was calculated using Darcy's Law. Calculated values ranged from 340 to 8,400 m/day (1, 100 to 27,700 ft/day) for pea gravel (size 8-9) to slightly larger gravel (size 6-7). The low value reflects substantial clogging and the high value reflects minor clogging. Clogging varies with distance down the wetland cell in response to wastewater loading. It is greatest in the inlet area and least in the outlet. Variability between the systems was large, but representative hydraulic conductivity values for relatively long and narrow cells (length to width ratio of 4 to 1) appear to be about 900 m/day (3,000 ft/day) for the inlet area, 3,000 m/day (10,000 ft/day) near the inlet quarter point, and about 12,000 m/day (40,000 ft/day) for the outlet half of the cell. Additional studies at other sites are needed to determine whether these values are actually typical. Use of 260 m/day (850 ft/day) for design of systems based on TVA guidelines is still considered a conservative, simplified approach with significant advantages and few drawbacks, Since the design will be hydraulically conservative, the approach minimizes the difference in water levels between inlet and outlet for a cell with a flat surface and bottom. If additional flexibility is needed for adapting a system to a particular site, design values could be justified that are an order of magnitude higher (i.e. 2,600 to 3,000 m/day, 8,500 to 10,000 ft/day). Designs should also routinely specify septic tank Filters to minimize suspended solids load on the inlet of the wetlands cell.