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.
| Co= Vb Cb+ Vi Ci+ Vs Cs - Vp Cp / (Vb+ Vi+ Vs - Vp) |
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.