Nutrient Transport Background
Declining water quality in rivers and streams has been linked to excessive losses of nutrients from soil within their watersheds, particularly nitrogen and phosphorus. Excess nutrients are a major problem for water managers because they can cause algal blooms that increase costs to treat drinking water, limit recreational activities, and be toxic to humans and wildlife. Nutrient over-enrichment can also lead to eutrophication in downstream water bodies, and deplete them of oxygen (hypoxia) that can threaten fish and other aquatic animals.
- Agriculture: Animal manure, excess fertilizer applied to crops and fields, and soil erosion make agriculture one of the largest sources of nitrogen and phosphorus pollution in the country.
- Stormwater: When precipitation falls on our cities and towns, it runs across hard surfaces – like rooftops, sidewalks and roads – and carries pollutants, including nitrogen and phosphorus, into local waterways.
- Wastewater: Our sewer and septic systems are responsible for treating large quantities of waste, and these systems do not always operate properly or remove enough nitrogen and phosphorus before discharging into waterways.
- Fossil Fuels: Electric power generation, industry, transportation and agriculture have increased the amount of nitrogen in the air through use of fossil fuels.
- In and Around the Home: Fertilizers, yard and pet waste, and certain soaps and detergents contain nitrogen and phosphorus, and can contribute to nutrient pollution if not properly used or disposed of. The amount of hard surfaces and type of landscaping can also increase the runoff of nitrogen and phosphorus during wet weather.
(See the U.S. EPA website for more information)
The Main Culprit – Phosphorus
Phosphorus is an essential element for plant growth. However, immobilization of soil phosphorus in inorganic and organic forms unavailable for crop uptake necessitates phosphorus amendments as fertilizer or animal manure to achieve desired crop yield goals. Although phosphorus is not directly toxic, the continued application of phosphorus to agricultural land and its subsequent movement to surface waters in runoff can accelerate eutrophication. This can impair water use for industry, recreation, drinking, and fisheries, due to the increased growth of undesirable algae and aquatic weeds. Although nitrogen and carbon are also associated with accelerated eutrophication, most attention has focused on phosphorus, due to the difficulty in controlling the exchange of nitrogen and carbon between the atmosphere and a water body, and fixation of atmospheric nitrogen by some blue-green algae. Thus, phosphorus is often the limiting element and its control is of prime importance in reducing the accelerated eutrophication of surface waters.
What’s Being Done
Most of the modeling done in the past through the Great Lakes Tributary Modeling Program has focused on the transport of sediment from tributaries to the Great Lakes, but the link between sediment and nutrients is one that cannot be ignored. Many of the modeling efforts of the past can be re-examined or rejuvenated to help answer nutrient transport questions. There is a long way to go in the area of nutrient management, but with working with other agencies and groups, the Army Corps of Engineers is always striving to develop new methods and models that can assist with answering how to reduce nutrients ending up in the Great Lakes.