Nitrogen Trends in the Puget Sound Watershed

Nitrogen from Major Puget Sound Rivers is Generally Decreasing

Ecology analyzed trends in nitrogen concentrations in rivers between 1995-2007 in the 2009 River and Stream Water Quality Monitoring Report. This original trend analysis has now been updated to incorporate more recent data. This extended trend analysis is in the 2013 River and Stream Water Quality Monitoring Report.

Trends in nitrogen can be summarized in different ways, but the summary presented here is focused on flow-adjusted trends. Since streamflows and nitrogen concentrations are often correlated, a flow-adjustment allows us to see if a trend in nitrogen concentration is there for reasons other than changes in streamflow.

The table below summarizes the results of both the original 1995-2007 trends and the updated 1995-2013 trends. Trends are summarized across all seasons (annually) as well as during the summer, and color-coded with symbols as described in the legend below:


The updated trends show a continuation of decreasing nitrogen concentrations in most of the 13 major rivers within the Puget Sound watershed between 1995 through 2013. Decreasing trends across most rivers and seasons are stronger for total nitrogen than for nitrate + nitrite.

There are, however, some rivers that show an increasing trend. These include the Deschutes and Elwha River, which show an increase in nitrate + nitrite concentrations across all seasons and during the summer, though these trends are not statistically significant. These two rivers also show a non-significant increasing trend in total nitrogen concentrations across all seasons. The Puyallup River has a significantly decreasing trend in total nitrogen, but a non-significant decreasing trend in nitrate + nitrite.

We don’t yet know all the watershed processes in individual watersheds that might be contributing to decreasing nitrogen concentrations. However, these data help us synthesize large-scale patterns and allow us to identify the kinds of research questions we need to ask and explore. Continued long-term monitoring will allow us to track these patterns into the future to see when and if changes to these trends occur over time.

Nitrogen Inputs from Municipal Wastewater May Change in the Future

Monitoring at large plants that treat municipal wastewater shows reductions in flows over time, mostly due to water conservation efforts that have reduced per capita wastewater use. However, the volume of wastewater flow is expected to rise as the population of Puget Sound continues to grow. An increase in wastewater flows translates into an increase in nitrogen loads, unless treatment technologies that result in higher nitrogen removal are implemented.

Since National Pollutant Discharge Elimination System (NPDES) permits rarely require nitrogen limits or reporting of nitrogen concentrations in their effluent, we don’t have sufficient long-term wastewater-treatment-plant effluent nitrogen concentration data to evaluate trends in nitrogen concentrations or loads over time.

The LOTT wastewater treatment plant in Olympia is currently the only facility in the Puget Sound region that has nitrogen concentration and load limits in their NPDES permit. However, a few wastewater facilities in the Puget Sound region are moving to higher nitrogen removal and/or are becoming more interested in wastewater reuse—both of which can result in lower nitrogen loads entering Puget Sound from wastewater effluent in the future.

Marine Trends discusses if levels of nitrogen measured within Puget Sound are changing over time.