The Relationship 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 typical 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. Phosphorus (P) and nitrogen (N) are important to all living organisms; however, excessive nutrients stimulate growth of bacteria and algae that consume oxygen and other biological resources thereby negatively affecting water quality for general beneficial uses. In aquatic ecosystems the P to N ratio normally dos not exceed 1 P to every 16 N. The main reason for the difference between the two is the relative availability of nitrogen, which is abundant in the atmosphere and the relative immobility of P. However, if nitrogen is found more in its gaseous phase, phosphorus will be relatively plentiful. By studying how these ratios change, the limiting nutrient can be determined and consequently, this information can be used to improve water quality management. In order to properly mange non-point pollution, it is critical to consider the impacts of different land-use scenarios. Therefore, 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 land-use. This 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 model used runoff water volume and the expected mean concentration (EMC) to estimate the annual loading of nutrients and pesticides. The study was conducted in the Snake River Basin in Idaho and it included a comparison between the L-THIA modeling results and quality control data. The results of the study showed that phosphorus is the most common water qualitylimiting nutrient in the basin, as it is in most freshwater ecosystems. The results of this study may be useful in developing and calculating water quality indices, which require information about several water quality parameters that may not be readily available for many watersheds. In addition, the method will help provide a consistent description of general water quality conditions and long-term trends within a given watershed. 1 Graduate Student and Professor, CEE Dept., Utah State University, Logan, UT 84321 2 Advisory Scientist/Engineer, Integrated Environmental Analysis Department, Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho 83415-2213.