Nitrogen Budget and Hydrologic Calculator

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.

Calculator Inputs

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.

Calculator Outputs

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.

Information for Existing Residential Areas

Single Family Housing:

  • Total number of houses:  
  • Number of sewered houses:  
  • Average lawn area per house: sq. ft. 
  • Average paved area per house: sq. ft. 
  • Average roof area per house: sq. ft. 

Multi-Family Housing

  • Total number of multi-family units:  
  • Number of sewered units:  
  • Average lawn area per unit: sq. ft. 
  • Average paved area per unit: sq. ft. 
  • Average roof area per unit: sq. ft. 

Average number of people per house / unit:  

Average sewer/septic flow per person: gpd 

Nitrogen concentration in septic effluent: mg/L 

Lawn fertilizer application: lbs/1000 sq.ft./year 

Percentage of fertilizer leached:

 

Existing Commercial and Industrial Land Use

Commercial and Industrial Land

  • Total area: acres 
  • Roof area, as percentage of total area:
  • Paved area, as percentage of total area:

Commercial and Industrial Water Use:

  • Total water use: gpd 
  • % sewered:

Municipal Building and School Water Use:

  • Total water use: gpd 
  • % sewered:

Nitrogen concentration in septic effluent: mg/L 

 

Existing Farming: Crops

Recharge from crop irrigation, as % of irrigation water

Crop A

  • Total land area: acres 
  • Annual irrigation withdrawal: acre-feet/year 
  • Fertilizer (nitrogen) application rate: lbs/acre/year 
  • Fertilizer leached, as % of total applied:

Crop B

  • Total land area: acres 
  • Annual irrigation withdrawal: acre-feet/year 
  • Fertilizer (nitrogen) application rate: lbs/acre/year 
  • Fertilizer leached, as % of total applied:

Crop C

  • Total land area: acres 
  • Annual irrigation withdrawal: acre-feet/year 
  • Fertilizer (nitrogen) application rate: lbs/acre/year 
  • Fertilizer leached, as % of total applied:

Pasture

  • Total land area: acres 
  • Annual irrigation withdrawal: acre-feet/year 
  • Fertilizer (nitrogen) application rate: lbs/acre/year 
  • Fertilizer leached, as % of total applied:

 

Existing Farming: Animals

Dairy / Cattle

  • Number of animals:  
  • Nitrogen Production: lbs/animal/year 
  • Nitrogen leached, as % of total produced:

Horses

  • Number of animals:  
  • Nitrogen Production: lbs/animal/year 
  • Nitrogen leached, as % of total produced:

Fowl

  • Number of birds:  
  • Nitrogen Production: lbs/animal/year 
  • Nitrogen leached, as % of total produced:

 

Other Existing Nitrogen Sources

Landfill

  • Total landfill area: acres 
  • Nitrogen leaching rate: lbs/acre/year 

Golf courses and athletic fields:

  • Total fertilized / irrigated area: acres 
  • Fertilizer application rate: lbs/1000sq.ft./year 
  • Annual irrigation withdrawal: acre-feet/year 
  • Fertilizer leached, as % of total applied:

Sewage treatment plant discharging into the wellhead protection area:

  • Average daily discharge: gpd 
  • Nitrogen concentration in sewage effluent: mg/L 

 

Hydrologic Information

Total Wellhead Protection area: acres 

Road Area, as percentage of total WPA:

Approved public supply well withdrawals: M gpd 

Water induced from infiltration of surface water:

Nitrogen concentration in surface water: mg/L 

Average annual recharge rate: inches per year 

Precipitation infiltrated from impervious areas: inches per year 

Nitrogen concentration in precipitation: mg/L 

 

Future Development, Build-Out

The calculations for future development are based on the assumption that development occurs in greenfields. If this is not the case, and certain types of land use (e.g., agricultural) are converted to developed land, it would be more accurate to re-run the calculator and change the inputs to account for land use conversion.

Single Family Housing:

  • Additional number of houses:  
  • Additional sewered houses:  

Multi-Family Housing

  • Additional number of multi-family units:  
  • Additional sewered units:  

Additional commercial and industrial water use

  • Total additional wastewater: gpd 
  • Additional sewered wastewater: gpd 

New Nitrogen Point Sources

  • Flow: gpd 
  • Nitrogen concentration: mg/L 

Additional water withdrawals: gpd 

 

Nitrogen Budget and Hydrologic Results

Existing water sources (entering the WPA) Million gpd Percent
Septic systems
Wastewater treatment plant effluent
Sewer leakage
Agriculture / Irrigation
Golf courses / Athletic fields
Natural Recharge / Precipitation
Surface Water
Total
Existing water withdrawals Million gpd Percent
Wells
Agriculture / Irrigation
Golf courses / Athletic fields
Total
Existing nitrogen sources lbs/year Percent
Septic systems
Wastewater treatment plant effluent
Sewer leakage
Lawns
Agriculture / Irrigation
Golf courses / Athletic fields
Landfills
Precipitation
Surface Water
Total
Summary
Nitrogen concentration, under existing conditions: mg/L 
Water Balance (input - output), under existing conditions: Million gpd 
Nitrogen concentration, buildout: mg/L 
Water Balance (input - output), buildout: Million gpd