Ohio State University Extension Bulletin

Mound Systems for On-site Wastewater Treatment

Siting, Design and Construction in Ohio

Bulletin 813

Mound System Design Example

Step 7. Select Basal Area Loading Rate.

The wastewater moves down through the sand layer by gravity. When the sand-treated wastewater reaches the sand/soil interface it cannot move as quickly into the soil as the sand above. The wastewater begins to spread out in the downslope direction. The area that the wastewater spreads out on the sand/soil interface is called the basal area. It is necessary to use the basal loading rate to determine the basal area. The nature of the surface soil horizon determines the basal loading rate, as shown in Table 2. If there is a less permeable layer within the 2 feet of soil below the mound, a more conservative basal loading rate may be selected.

Table 2. Estimated Basal Loading Rates (After Converse and Tyler, 1990.)

Surface horizon based on soil morphological conditions If YES, the basal
loading rate is
(gpd/ft²):

Is the horizon gravelly coarse sand or coarser?
0.0*

Is consistence stronger than firm or hard, or any cemented class?
0.0*

Is texture sandy clay, clay or silty clay of high clay content and structure massive or weak, or silt loam and structure massive?
0.0*

Is texture sandy clay loam, clay loam or silty clay loam and structure massive?
0.0*

Is texture sandy clay, clay or silty clay of low clay content and structure moderate or strong?
0.2

Is texture sandy clay loam, clay loam or silty clay loam and structure weak?
0.2

Is texture sandy clay loam, clay loam or silty clay loam and structure moderate or strong?
0.4

Is texture sandy loam, loam, or silt loam and structure weak?
0.4

Is texture sandy loam, loam, or silt loam and structure moderate or strong?
0.6

Is texture fine sand, very fine sand, loamy fine sand, or loamy very fine sand?
0.6

Is texture coarse sand with single grain structure?
0.8

* If the site surface soil horizon condition results in zero basal loading rate, STOP here. The selected site is not suitable for mound system.

Table 2. Estimated Basal Loading Rates (After Converse and Tyler, 1990.)
Surface horizon based on soil morphological conditions	If YES, the basal loading rate is (gpd/ft²):
Is the horizon gravelly coarse sand or coarser?	0.0*
Is consistence stronger than firm or hard, or any cemented class?	0.0*
Is texture sandy clay, clay or silty clay of high clay content and structure massive or weak, or silt loam and structure massive?	0.0*
Is texture sandy clay loam, clay loam or silty clay loam and structure massive?	0.0*
Is texture sandy clay, clay or silty clay of low clay content and structure moderate or strong?	0.2
Is texture sandy clay loam, clay loam or silty clay loam and structure weak?	0.2
Is texture sandy clay loam, clay loam or silty clay loam and structure moderate or strong?	0.4
Is texture sandy loam, loam, or silt loam and structure weak?	0.4
Is texture sandy loam, loam, or silt loam and structure moderate or strong?	0.6
Is texture fine sand, very fine sand, loamy fine sand, or loamy very fine sand?	0.6
Is texture coarse sand with single grain structure?	0.8
* If the site surface soil horizon condition results in zero basal loading rate, STOP here. The selected site is not suitable for mound system.

Basal Area Loading Rate:
The site soil evaluation revealed that silt loam soil with moderate structure was present in the surface horizon (top 10 inches). Found under item I, the design basal loading rate is 0.6 gpd/ft².

Back | Forward | Table of Contents