Relation of Land-use to Total Nitrogen/ Phosphorus in Streams

Comprehensive watershed management requires the determination of both point and non-point sources of pollution within a watershed. The primary non-point source pollutants in a watershed are nutrients, sediment, animal wastes, salts, and pesticides. Nutrients are mainly nitrogen and phosphorous. Agricultural activities and wastewater treatment plants are the principal sources of nitrogen in surface waters. Phosphorus enters the environment from human or animal wastes, fertilizers, soaps, industrial wastes, and plants. Excessive nutrients consume other biological resources and negatively affect water quality for general beneficial uses. Phosphorus (P) and nitrogen (N) are important to all living organisms. In aquatic ecosystems the ratios normally do not exceed 1 P to every 16 N. The main reason for the difference between the two is their relative availability of nitrogen, which is abundant in the atmosphere. However, if nitrogen is found more in its gaseous phase, phosphorus will be relatively plentiful. By studying how the ratios change, the limiting nutrient can be determined and consequently, this information can be used to improve water quality management. The objectives of this study were to estimate the Total Nitrogen/Total Phosphorus (TN:TP) ratios in streams and to establish the relationships between these parameters and the land-use. It is critical to consider the impact on water quality of different land-use scenarios. The study used the GIS version of the L-THIA (Long-Term Hydrologic Impact Assessment) model developed by Purdue University, which provides results of non point source pollution. The calculation used runoff water volume and the expected mean concentration (EMC) to provide the annual loading of nutrients and pesticides. The study was conducted in the Snake River Basin in Idaho and a comparison between the results obtained from the model and the quality control data was performed. The study showed that phosphorus to be the most common limiting nutrient in the studied basin as it is in most freshwater ecosystems. The study may be useful in developing and calculating water quality indices, which require information about several water quality parameters that may not be available for many watersheds. In addition, the method will help in providing a consistent description of general water quality conditions and long-term trends.