CIVE 633 - ENVIRONMENTAL HYDROLOGY
NUTRIENT LOADS TO A WATERBODY

MAJOR PHOSPHOROUS AND NITROGEN SOURCES

Major sources can contribute nutrients to a waterbody.
External nutrient sources include effluents from municipal and industrial point sources and diffuse non-point sources such as land runoff and deposition from the atmosphere.
Internal nutrient sources include nutrient regeneration from the bottom sediments, and groundwater seepage.
Point sources
Nutrient load from municipal and industrial point sources includes the direct discharge from a treatment facility.
The use of detergents can result in increased P content.
Private waste disposal systems, urine tanks, manure stacks and runoff from silos.
Urban runoff through piped sewer systems.
Private septic tanks also exist in some urban areas.
Diffuse (Non-point) external sources
Nutrients are deposited via atmospheric inputs directly onto a lake or reservoir.
Atmospheric nutrient deposition include both particulate and soluble forms.
Rural sources include land drainage from agricultural and forested areas.
Animal feedlots, leaking manure stacks and urine tanks can contribute nutrients.
Inorganic fertilizers, animal manure, and night soil.
Fertilizers applied at excessive rates can cause elevated nutrient concentrations.
Intensive forest practices such as clearcutting can increase exposure of soil and increase nutrients washed by runoff.
Diffuse (Non-point) internal sources
Nitrate nitrogen is often found in large concentrations in groundwater.
Nutrient recycling is particularly important in shallow lakes.
Nitrogen fixation can be a potentially significant source of nitrogen for some lakes and reservoirs.
Nitrogen fixation is important in eutrophic lakes containing high P levels and large populations of blue-green algae, including nitrogen-fixing algae.
Biological availability of nutrients
The soluble fraction of P and N are easily utilized by algae.
This fraction is termed the "biologically available" fraction.
Only a portion of the particulate nutrients are biologically available.
Only one-third of the particulate sediments may be biologically available.
P in municipal treatment plants is largely bioavailable.
As eutrophication management becomes more refined, it will become important to consider the bioavailability of nutrients.
Nutrient control targets should focus on bioavailable nutrients.

QUANTIFYING THE NUTRIENT LOAD

Quantitative information on the annual total nitrogen and phosphorous load is often sufficient for developing eutrophication control programs.
It is desirable to measure particulate P, soluble orthophosphate, total nitrogen, ammonia, nitrate, and if possible, organic nitrogen.
Waterbodies that have rapid flushing rates require more detail than annual loads.
Direct measurement of nutrients loads provide the most accurate estimates.
Unit loads are used in lieu of measurements.
Unit load is the quantity per nutrient per unit area per unit time.
The greater the runoff, the greater the quantity of nutrients transported from the watershed to the waterbody.
Point source loads
Unit loads from point sources are available directly from the discharger.
Knowledge of similar plants or similar conditions elsewehere may help when there is no data.
Municipal wastewater treatment plants are estimated from per capita inputs.
Effective per capita P and N contributions can vary from location to location, depending on food habits.

Diffuse (non-point) external loads
Caused by drainage from urban, agriculture, and forested areas.
Estimates from direct measurements of unit loads.
Nutrient concentrations vary less than flow information.
Particulate quantities can vary significantly with the flow.
Measurements should be made during high runoff.
Large portion is contributed by short-term high-runoff events.
Flow and concentration measurements are taken over a year.
Techniques for calculation are:

The average short-term load
The product of the average short-term flow and average short-term concentration.
The product of the weighted mean concentration and the annual flow.

Under average hydrologic conditions, a given land-use activity will export a relatively constant load of nutrients per unit area.
Use of nutrient export coefficients should be done with caution.
Wet and dry fallout from the atmosphere add nutrients to a waterbody.
Inputs are proportional to lake surface area.
Large atmospheric inputs are associated with proximity to urban areas.
Volatilization of ammonia nitrogen from feedlots may add to N content of the atmosphere.

Diffuse (non-point) internal loads
Groundwater may be a potential source of nutrients.
Release of nutrients from bottom sediments can contribute significantly to the total nutrient load of a waterbody.
This occurs most commonly under anoxic conditions.
It can also occur under oxic conditions.
Reliable method for estimating P flux between sediments and water column involves detailed P mass-balance calculations made at monthly or smaller intervals.

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