Estimated Nitrogen and Phosphorus Input to Fish Creek Watershed, Teton County, Wyoming

Nutrients, such as nitrogen and phosphorus, are essential for plant and animal growth and nourishment, but the overabundance of bioavailable nitrogen and phosphorus in water can cause adverse health and ecological effects. It is generally accepted that major increases in the primary production of surface-water bodies due to high inputs of nutrients is now the most important polluting effect in surface water in the developed world. The Fish Creek watershed is located along the southwestern margin of the city of Jackson Hole. Fish Creek is an important water body because it is used for irrigation, fishing, recreation, and adds scenic value to the Jackson Hole properties it flows through. Recent U.S. Geological Survey (USGS) studies indicated there is a greater biovolume of aquatic plants in Fish Creek than is typically observed in streams of similar size. Studies by the USGS also indicated that (1) the amount of biovolume in Fish Creek was inversely correlated to nitrate concentration, suggesting that the aquatic vegetation was likely consuming most or all of the nutrients available to the plants and (2) land-use activities in the west bank area of the watershed can affect groundwater quality, which can then affect Fish Creek's water quality. The Fish Creek watershed has multiple natural and anthropogenic sources of nutrients (nitrogen and phosphorus species) that can eventually migrate into Fish Creek. These sources include (1) atmospheric deposition, (2) fertilizers applied to lawns, trees, and golf courses, (3) wastewater (septic systems and sewage treatment plans), (4) livestock, (5) surface-water diversions entering the watershed, and (6) avalanche explosives. The U.S. Geological Survey, in cooperation with the Teton Conservation District (TCD), completed a study to identify and quantify nitrogen and phosphorus sources and inputs into the Fish Creek watershed. Geospatial datasets, values from literature reviews, water-quality data, and questionnaires distributed by the TCD were used to identify locations of sources and to quantify nitrogen and phosphorus inputs. This study did not address the transformation and uptake of nitrogen species (ammonia, ammonium, nitrite, nitrate, nitrogen gas, organic nitrogen) and phosphorus species (orthophosphate, organic phosphorus), because complex hydrological and chemical modeling are required for this depth of understanding. Human activities in the watershed predominately occur in the valley, on the east-south-eastern part of the watershed, and that area shows the greatest input of nitrogen and phosphorus. To characterize spatial patterns of nutrient inputs to the Fish Creek watershed, a grid of 10-acre cells was created and overlaid on the study area. Nutrient inputs were then aggregated for each 10-acre cells. The largest 10-acre cell input values are generally associated with cells that coincide with sewage treatment plant injection sites, livestock, and distributed land use where septic systems and lawns are located. Annual nitrogen input ranged from 25 to over 4,000 pounds in a 10-acre cell and annual phosphorus input ranged from about 3 to about 1,200 pounds in a 10-acre cell. Atmospheric deposition represented the largest overall source of estimated nitrogen input (46 percent) into the watershed, and represented the second highest percentage (26 percent) of total phosphorus input into the watershed. It is noteworthy that much of nutrient input from atmospheric deposition are likely used by canopy vegetation before they reach Fish Creek. The next largest sources of nitrogen input associated with human activities are cattle and lawns, and the next largest phosphorus inputs are cattle and horses. Although cattle are not in the watershed for the entire year, the large number of cattle grazing on the land produced higher input for both nitrogen and phosphorus than many of the other sources. Lawns had higher application rates of nutrients and had larger acreages than other fertilized areas. Management of human waste in the watershed is accomplished by use of septic systems and water-treatment plants and both methods contribute to the total nutrient inputs into the watershed. Nitrogen contributions from water-treatment plants produced a high-input 10-acre cell; however, when the total input of nitrogen from water-treatment plants (both liquid waste and biosolids) is compared to the total nitrogen input into the watershed and to the input of individual septic systems in the watershed, the water-treatment plants contribute a relatively small amount of nitrogen. Results from this study provide important information regarding sources and quantity of nitrogen and phosphorus inputs to Fish Creek watershed. These data provide valuable insight regarding the relative effects of various human activities and can be used to assist resource managers seeking to improve the water quality of the Fish Creek watershed.