Decadal Impact of Clean Air Act policies on US stream nitrogen concentrations.
US Clean Air Act regulations have resulted in decreased nitrogen (N) deposition in the US over the last two decades. To explore whether wadeable streams responded to declining deposition, we combined US EPA’s National Nutrient Inventory with the National Rivers and Streams Assessment (NRSA) from three surveys spanning between 2000 and 2014. The inventory allowed identification of NRSA watersheds where deposition is the largest N input source. Weighted change analysis indicated that stream total N (TN) concentrations declined significantly in the Appalachian region, consistent with reductions in N input rates. Similar regional reductions in stream TN concentrations or inputs were not observed in the Western and Plains regions. Among the stream population we examined that had deposition as the largest N source, the ecoregional percent length of eastern (Appalachian region) streams with TN > 0.7 mg N/L (the TN threshold for oligotrophic/mesotrophic streams) decreased from 18% to 8% between the first and third NRSA surveys, while the ecoregional value increased from 25% to36 % during the same timeframe for streams in the Plains. Conditions of streams we studied in the Western region appeared to stay relatively constant regarding TN concentrations, which was due to the lack to change in the regional N input rate. The variations in regional stream response to reduced deposition N can be attributed to many factors, such as lag time, weather/climate, nutrient legacy pool, and more. The systematic reduction in N concentrations of wadeable streams in the eastern US where deposition is the largest N source supports the effectiveness of air and water quality regulations.