Appendix B - Minimum Isolation Distance Analysis
VERTICAL TRAVEL TIME CALCULATION:
The following equation is used to determine the vertical travel time (t1) from the leach field to the water table:
Given: t1 = d * [THETA] ÷ 0.5([ALPHA])
Where:
t1 = vertical travel time (years)
[ALPHA] is total recharge (effluent + precipitation in cm/yr)
[THETA] is volumetric soil moisture in a sandy clay (mL/cm3)
d is the depth to groundwater (cm)
0.5 is the infiltration factor (assumes 50% of precipitation will infiltrate the soil)
(Note: [ALPHA], [THETA], and d will change from site to site.)
Example calculation of [ALPHA]:
Assuming that 100 gallons per day of wastewater will be generated per bedroom, a typical 3 bedroom house is expected to discharge an average of 300 gallons per day.
Total yearly effluent discharge = 300 gallons/day * 365 days/year * 1 ft3/7.48 gallons
= 14,639 ft3
Next, calculate the required soil absorption surface area (square feet) by referencing Figure 7 of Chapter 25 of Water Quality Rules and Regulations. The sizing of a leach field is dependant upon the soil percolation rate and the average daily effluent discharge. For this example use the median soil percolation rate of 15 minutes per inch. Also, use the previously calculated 300 gallons per day effluent discharge per house.
Required soil absorption infiltrative area = 300 gallons/day * 1ft2 day/0.52 gallons = 577 ft2
Next, calculate the pro-rated inches per year of effluent recharge to the aquifer:
14,639 ft3 * (577 ft2)-1 = 25.4 ft * 12 inches/ft = 304 inches/year
To obtain TOTAL average annual recharge to the aquifer (effluent + precipitation) add the average annual precipitation for the area (15 inches/year) to the pro-rated effluent recharge, which equates to 319 inches/year or 810 cm/year ([ALPHA]). Note the average annual precipitation for an area can be obtained from Chapter 17, Wyoming Water Quality Rules and Regulations (Figure 2) or from the USDA. National Resources Conservation Service.
Volumetric Soil Moisture Content at Field Capacity
| Soil Type | Volumetric Soil Moisture Content @ Field Capacity, [THETA], mL/cm3 |
| Cobble sand | 0.045 |
| Sand | 0.062 |
| Sandy loam | 0.190 |
| Loam | 0.232 |
| Silty loam | 0.284 |
| Sandy clay loam | 0.244 |
| Clay loam | 0.310 |
| Silty clay loam | 0.342 |
| Sandy clay | 0.321 (default) |
| Silty clay | 0.371 |
| Clay | 0.378 |
* Source: WDEQ Water Quality Rules and Regulations, Chapter 17
HORIZONTAL TRAVEL TIME CALCULATION:
The following equation is used to determine, based upon the advective flow equation, the distance at which leachate in groundwater will reach a downgradient well or surface water body, or migrate beyond the property boundary over a travel-time period of two years*.
The distance (x) which leachate will migrate vertically (t1) and horizontally (t2) over a travel-time period of two years (t) is calculated as follows:
Calculate horizontal travel time (t2):
(t2) = 2 years - (t1)
Where:
(t1) = vertical travel time
Calculate the horizontal linear velocity:
Given: v = ki/n
Where:
v = average groundwater linear velocity
k = hydraulic conductivity of the saturated aquifer (feet/day)
i = hydraulic gradient (feet/feet)
n = effective porosity (dimensionless)
Next, calculate the horizontal travel distance, x:
x = t2 * v
t2 = horizontal time of travel
x = the minimum allowable isolation distance
* If the calculated vertical travel time is 2 years or greater the horizontal travel time analysis does not need to be completed.
Fetter, C.W. 1994. Applied Hydrogeology, 3rd Edition, Prentice-Hall, New Jersey, 691 pp