Wyoming Administrative Code › Agency 020 - Environmental Quality, Dept. of › Sub-Agency 0011 - Water Quality › Chapter 23 - MINIMUM STANDARDS FOR SUBDIVISION APPLICATIONS › Appendix B - Minimum Isolation Distance Analysis

Current through July 13, 2024
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