The nitrogen budget and hydrologic calculator presented here has been developed for guidance in the protection of groundwater quality from land uses within wellhead protection areas (WHPA) surrounding public supply wells in Washington. The purpose of the calculator is to estimate nitrogen loads and concentrations for ground water supplies under both current and future conditions of land development, and to allow the user to determine if the water budget is balanced for the WHPA.
Mass-Balance Modeling Approach
Nitrogen in ground water originates from a variety of natural and anthropogenic sources (e.g., sewage, fertilizers, farm animal wastes, precipitation, and landfills). Nitrogen can occur in several different forms in water near its sources: organic nitrogen compounds, ammonium, nitrite, and nitrate. Generally, all of these are oxidized to nitrate by the time the water reaches a public supply well. Therefore, for the purposes of this calculator, all of these species of nitrogen are assumed to have been converted to nitrate in the well water.
Nitrogen is a conservative compound in ground water and, in most cases, does not react after it enters the aquifer system. It does not decay, enter into processes which cause it to be precipitated or volatilized, enter into ion exchange, or become sorbed to aquifer materials. In groundwater, nitrogen obeys the laws of conservation of mass, because it is neither lost nor gained in the water. Therefore, only the laws of dilution need be applied to determine its concentration. Because of these characteristics, a mass-balance approach may be used to accurately predict the concentration of nitrogen in ground water.
To use this calculator, you should be prepared to enter the following information within the WHPA:
- Residential Land Use;
- Existing Commercial and Industrial Land Use;
- Existing Farming Data, Including Crops and Animals;
- Landfill, Golf Courses, and Sewage Treatment Plant Information;
- Hydrologic Information, such as Recharge Rates; and
- Future Development, Build-Out Information.
Once all the relevant inputs have been entered, the Calculator estimates the following outputs:
- Water entering the WHPA and contributing to recharge;
- Water withdrawals exiting the WHPA and contributing to aquifer depletion;
- Nitrogen sources within the WHPA;
- Nitrogen concentrations, under existing conditions and future build-out; and
- Water budget / balance, under existing conditions and future build-out.