Wyoming Administrative Code
Agency 020 - Environmental Quality, Dept. of
Sub-Agency 0011 - Water Quality
Chapter 23 - MINIMUM STANDARDS FOR SUBDIVISION APPLICATIONS
Appendix A - Cumulative Nitrate Loading Analysis

In order to calculate the nitrate concentration from multiple septic systems at the down gradient property boundary of the proposed subdivision the following nitrogen mass balance equation (Wehrmann Model) is used. If Co exceeds 10 mg/L NO3- as N, WDEQ recommend to the County Commission that the subdivision not be approved unless modified to achieve the standards within this chapter.

Where:

Co = diluted concentration of NO3 as N leaving the subdivision

Vb = volume of ground water entering the subdivision from upgradient area

Cb = ambient concentration of NO3- as N contained in the ground water entering the subdivision

Vi = volume of precipitation infiltrating beneath the subdivision

Ci = concentration of NO3- as N contained in the infiltrating precipitation

Vs = volume of septic effluent introduced beneath the subdivision

Cs = concentration of NO3- as N contained in the septic effluent (assume 40 mg/L for conventional septic systems, and manufacturer specifications (mg/L) for enhanced treatment systems)

Vp = volume of ground water pumped by wells beneath the subdivision (use only if same aquifer as Vs)

Cp = concentration of nitrate-nitrogen contained in the pumped ground water

Wehrmann, H.A. 1984. Managing Ground Water Nitrate Quality by Mass Balance Modeling in the Rockton-Roscoe Area, Illinois. In Proceedings of the NWWA Eastern Regional Conference on Ground Water Management, National Water Well Association, Dublin, Ohio, pp. 558-587.