What is driving nitrogen concentration changes over time in US stream? Results from the U.S. EPA National Rivers and Streams Assessments (2000-2014)

Uncertainty about how land use practice changes will quantitatively improve water quality and how long it will take to see improvements hinder efforts to reduce nitrogen (N) in streams. We used existing USEPA program datasets to understand the changes in stream water quality over time and how land management can affect stream total N concentration ([TN]) in the contiguous US. Specifically, we used the USEPA’s national inventories of N inputs (2002-2012) for the contiguous US, stream TN concentration from the National Rivers and Streams Assessment (NRSA, yrs. 2000-2004, 2008-2009, and 2013-2014), and land characteristics variables derived from the EPA Stream-Catchment dataset (StreamCat). Between NRSA survey 1 (2000-2004) and survey 2 (2008-2009), there were proportionally more sites showing decreasing [TN] trends than increasing trends for the contiguous US (48% vs. 45%), and there were more sites with increasing [TN] (49%) than decreasing TN (42%) between survey 2 and the later survey 3 (2013-2014). However, the national stream [TN] did not change significantly between 2000 and 2014, consistent with the lack of a national trend in N inputs. Streams in the Central Plains had significantly greater [TN] than streams in the West and Appalachians, reflecting the pattern in N inputs for those areas. Sites with both the greatest increase or decrease in TN concentration (>10X) through time were located in the temperate plains where inputs were generally high. Preliminary results show that changes in total landscape N input or surplus at the HUC8 level are not reflected in changes in stream [TN] over time, indicating potential lag time in N management and the importance of other watershed characteristics. The results will be refined using downscaled N inventory (NHDPlus catchment level), StreamCat landscape variables, and isotopic data to improve our understanding of the processes and factors controlling stream TN concentration.

Impact/Purpose

The US EPA National Rivers and Streams Assessment (NRSA) surveys allow for the examination of large-scale changes in stream nutrient concentrations over time. EPA staff have also been preparing an inventory of N inputs to HUC8 watersheds across the continental US in 2002, 2007 and 2012. This inventory showed that while nitrogen (N) input to the entire continental US was similar in 2002 and 2012, N inputs increased in much of the Central Plains, decreased in the eastern US and showed little change across most of the West. WED-FEB staff examined temporal and spatial variations in total nitrogen (TN) concentrations in a spatially-balanced survey of over 3,000 wadeable streams in the NRSA between 2000 and 2014. The results indicate that despite substantial effort to improve N management, streams N concentration and N inputs for the entire US have not changed significantly over the time period.

Citation

Lin, J., R. Hill, R. Sabo, A. Herlihy, M. Weber, B. Pickard, S. LeDuc, Jacqueline Brooks, C. Clark, S. Leibowitz, J. Stoddard, AND J. Compton. What is driving nitrogen concentration changes over time in US stream? Results from the U.S. EPA National Rivers and Streams Assessments (2000-2014). AGU, San Francisco, CA, December 09 - 13, 2019.