Appendix c - Public Water Supply Analysis
The following equations are used to calculate the nitrate concentration at a public drinking water supply well from drainfields that fall within the capture zone of the well, as defined below. If Cp exceeds 10 mg/L NO3 - as N, WDEQ will recommend to the County Commission that the subdivision not be approved unless modified to achieve the standards within this chapter.
First, calculate the capture zone for the water supply well using the following formula:
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Where:
CFR = Calculated fixed radius of pumping well
Q = average annual pumping rate (gallons/year)
t = time of travel (years); assume 2 years unless a steady-state capture zone is calculated.
7.48 = conversion factor (ft3 to gallons)
n = effective porosity
H = length of well screen (feet)
pi = 3.14
Next, the volume of aquifer water flowing beneath a drainfield located within the capture zone is calculated using the following formula:
Qleach = (KiA)7.48*z
Where:
Qleach = volume of water flowing beneath the drainfield (gallons/day)
K = hydraulic conductivity (feet/day)
i = hydraulic gradient
A = cross-sectional dimensions of drainfield (length x depth); assumed to be 75 feet
long by 30 feet deep.
z = number of wastewater treatment systems within the CFR
Next, the nitrate concentration beneath the drainfield is calculated by mixing the septic leachate with the aquifer water flowing beneath the drainfield (Qleach) using the following formula:
Cr = ((Qseptic *Cseptic) + (Qleach * Cleach)) / (Qseptic + Qleach)
Where:
Cr = diluted nitrate concentration beneath the septic drainfield (mg/L)
Qseptic = discharge of septic (gallons/day) = (300 *z)
Cseptic = average nitrate concentration of septic leachate (assume 40 mg/L for conventional septic systems; manufacturer specifications (mg/L) for enhanced treatment systems)
Qleach = volume of water flowing beneath the drainfield (gallons/day)
Cleach = ambient nitrate concentration in ground water (mg/L)
300 = average daily leachate discharge for a 3 bedroom home
z = number of wastewater treatment systems within the CFR
Next, the volume of water contained within the well cylinder (from pumping) is calculated using the following equation:
Qwell = n((pi)(HCFR2))*7.48 / 730
Where:
Qwell = well discharge (gallons/day)
Finally, the concentration of nitrates within the well cylinder is calculated using the following equation:
Cp = ((Qleach + Qseptic*Cr)+(Qwell*Cwell)) / (Qleach + Qseptic + Qwell)
Where:
Cp = Concentration of nitrates within the pumping well (mg/L)
Cwell = ambient nitrate concentration in ground water (mg/L) = Cleach