The Puget Sound Basin study team compiled historical nutrient data to evaluate the transport of nutrients in the major rivers of the Puget Sound Basin study unit of the National Water Quality Assessment program. The rivers included in this retrospective analysis carry an average nutrient load of about 11,000 tons of inorganic nitrogen, 9,900 tons of organic nitrogen, and 2,100 tons of total phosphorus to Puget Sound and its adjacent marine waters each year. Rivers with the largest watersheds and streamflows transport the greatest nutrient loads.
Nutrient transport from basins was normalized by drainage areas and expressed as a nutrient yield in tons per square mile per year. The smallest yields are from rivers in the Olympic Mountains. These rivers generally yield less than 1 ton per square mile per year of inorganic nitrogen and less than 0.1 ton per square mile per year of phosphorus. The largest yields are from rivers draining the east side of the Puget Sound Basin. These rivers generally yield more than 1 ton per square mile per year of inorganic nitrogen and more than 0.1 ton per square mile per year of phosphorus.
Estimates of nutrient-source loading rates to the watersheds help explain, in part, stream loads and river basin yields. Because they are among the least developed and populous watersheds, annual loading rates to western Puget Sound Basin watersheds from animal manures, agricultural fertilizers, and precipitation are small (typically less than 1 ton per square mile). To the more densely populated and agricultural eastern Puget Sound Basin watersheds, the loading rates are generally greater than 2 tons per square mile, which correspond to the higher east-side stream nutrient loads and basin yields.To eight of the major watersheds, more than half of the annual nitrogen loading is from animal manures. Agricultural fertilizers make up from 9 to 33 percent of the nitrogen loading to these eight basins. Each year, animal manures, agricultural fertilizers, and precipitation contri- bute 10 tons of nitrogen per square mile to the Samish River Basin and 9 tons per square mile to the Nooksack River Basin. In response, nutrient yields from these two basins (up to 2.8 tons of nitrogen per square mile per year and up to 0.3 ton of phosphorus per square mile per year) are among the highest rates for watersheds of the Puget Sound Basin. Fertilizer is the principal nutrient source to the Samish River Basin, which might account for the relatively low phosphorus yield compared to the yield from the Nooksack River Basin, where animal manures are the principal source.
Nutrients from precipitation and domestic-applied fertilizers are of interest for the populous Green, Lake Washington, and Snohomish River Basins. Precipitation is estimated to contribute from 1 to 2 tons of nitrogen per square mile each year and from 0.10 to 0.2 ton of phosphorus per square mile each year to these three basins. In addition, if it is assumed that one-eighth of the urban area is fertilized on an annual basis, a watershed the size of Lake Washington might receive nearly 3 tons of nitrogen per square mile. The total annual nitrogen loading rate to a watershed similar to that of Lake Washington would nearly double with the additional contribution from urban fertilizer applications.