## Groundwater Mounding

During the application period the applied wastewater percolates through the soil profile and reaches either a slowly permeable layer or the groundwater table. When the water cannot flow vertically, it tends to form a temporary mound at the interface with the groundwater before it can move laterally with the ground-water. If the magnitude of the mound amounts to 1 to 2 ft of overall rise, the mounding is of little consequence. If the mound rises to within 2 ft of the basin surface, however, the treatment performance and rate of infiltration will diminish. Groundwater mounding equations (Reed and Crites, 1984) and nomographs (USEPA, 1981) can be used to determine the impact of groundwater mounding.

Example 8.4. Process Design for Soil Aquifer Treatment Calculate the field area required for a soil aquifer treatment system that treats 0.75 mgd of municipal wastewater. Preapplication treatment in an aerated pond reduces the BOD to 120 mg/L and the total nitrogen to 25 mg/L. The soils on the site have a moderate variability, and the minimum infiltration rate, using basin infiltration tests, is 2.5 in./hr. Upgradient groundwater has an average nitrate nitrogen concentration of 8 mg/L, which limits the downgradient groundwater quality to 8 mg/L.

Solution

1. Calculate the hydraulic loading rate (using Equation 8.3) and a 7% design factor from Table 8.21:

Lw = 0.07(24 hr/d)(365 d/yr)(2.5 in./hr)(1 ft/12 in.) = 128 ft/yr

2. Compare the hydraulic loading to the acceptable nitrogen loading rate using Equation 8.14:

Because the calculated 23.7 lb/ac-d is well below the 60-lb/ac-d limit, nitrogen loading is not limiting for the field area determination.

3. Calculate the BOD loading in lb/d:

4. Calculate the required field area on the basis of hydraulic loading rate: A = 3.06(0.75)(365)/128 = 6.5 ac

5. Calculate the field area needed to keep the BOD loading below 300 lb/ac-d:

6. Determine the field area required. Because the area needed for the hydraulic loading is greater than the area required for the BOD loading rate, the limiting field area is 6.5 ac.