Stormwater Nutrient Concentrations

Temporal variation in riverine nutrient concentrations and the impact of short term storm events on nutrient loading in Hood Canal

General Research Statement:

As part of the Hood Canal Dissolved Oxygen Program Integrated Assessment and Modeling study (HCDOP-IAM), this study seeks to help constrain riverine nutrient export estimates into the Lower Hood Canal region in order to determine the factors contributing to low Dissolved Oxygen levels in Hood Canal, Washington. The current model utilizes nutrient data from monthly samplings of the 43 streams that drain into Hood Canal; however, this study proposes that there is substantial temporal variability in river nutrient concentrations that must be considered. Samples were collected during several major autumn and winter storm events from the Skokomish, Union, and Tahuya Rivers in the Lower Hood Canal region. Dissolved nutrient data suggests that large amounts of nutrients accumulate in soils during dry summer months, and are mobilized during large storm events in autumn and early winter; the result is a high correlation between river discharge and dissolved nutrient concentrations, meaning nutrient export can be significantly higher during a storm event than would be estimated by using a monthly average nutrient concentration. By mid-winter this response is less marked, suggesting that this reservoir of accumulated nutrients is exhausted. This study reveals that storm events may have a large influence on total nutrient export into the Lower Hood Canal region, which may be underestimated in current models.

Methods

  • Sampling: ISCO autosamplers, capable of autonomously collecting 24 one liter samples upon activation, were setup on the Union, Tahuya, and Skokomish Rivers. The autosamplers were programmed to automatically collect samples at 3 or 4 hour intervals, depending on the predicted storm length. Samples were collected during significant storm events from October 2008 through October 2009.
  • Analysis: Total Suspended Sediment analyses are performed. Dissolved nutrient (nitrate, nitrite, ammonium, phosphate, and silicate), Dissolved Organic Carbon (DOC), and Total Dissolved Nitrogen (TDN) concentrations are measured by Kathy Krogslund's marine chemistry lab in the UW Oceanography department. Particulate Carbon and Nitrogen analyses are performed by the Stable Isotope Facility at UC Davis. Also a pilot study to measure lignin phenols-organic biomarkers-is in progress.
  • River Discharge Data: Real-time river discharge data for the Skokomish and Tahuya Rivers are obtained from USGS (http://waterdata.usgs.gov/nwis/rt). Union River discharge data is provided by the Hood Canal Salmon Enhancement Group (HCSEG). Under several circumstances Union River discharge data was not available, so Huge Creek data from USGS was normalized to Union River values for this time period. Huge Creek is within close proximity to Union River.

 
Results

Results from storm samplings show that nutrient concentrations are closely coupled with river discharge in early autumn storms. The figures on the right show the correlation between river discharge and dissolved organic carbon, total dissolved nitrogen, and total suspended sediment concentrations on short timescales (days). Furthermore Carbon to Nitrogen and Nitrogen to Phosphorus (not shown) ratios increase with river discharge indicating that during rapid shallow flow (i.e. storm runoff) C is more readily mobilized from soils than N and both C and N are mobilized more readily than P. 

Winter and spring samplings do not show the same pronounced increase in nutrient concentrations with increased river discharge, indicating that nutrients accumulate in the drainage basin soils during dry periods (i.e. summer) and are mobilized and exhausted by early and late autumn storms. During the sampled January 2009 storm, a ~150% increase in river discharge only results in a ~7% increase in TDN concentration, whereas during the sampled November 2008 storm, a ~167% increase in river discharge results in a ~167% increase in TDN concentration. Furthermore, regression analysis suggests that "accumulated nutrients" are exhausted from soils more rapidly in the Skokomish River basin relative to the Union River likely because the much larger Skokomish River represents an integration of small, Union-like streams.

Rough calculations suggest that properly accounting for storm-induced N surges can make a profound difference in total N export estimations depending on when monthly samples were taken. Relating this storm data to 2007 monthly stream data suggests that October 2007 export estimates may be ~10% too low, and November estimates may be as much as ~100% too low for the Skokomish River. Furthermore, sampling during non-average conditions can greatly influence export estimates (e.g. Oct. 2007 monthly samples were taken during a period of higher than average river discharge, meaning the "average" value obtained was likely far in excess of the actual monthly average). Sampling will continue until Fall 2009.

Publication Status

Ward, N.D., J.E. Richey, and R.G. Keil. 2011. Temporal variation in river nutrient and dissolved lignin phenol concentrations and the impact of storm events on nutrient loading to Hood Canal, Washington, USA. Biogeochemistry. DOI: 10.1007/s10533-012-9700-9

Steinberg, P.D., M.T. Brett, J.S. Bechtold, J.E. Richey, L.M. Porensky, and S.N. Smith. 2010. The influence of watershed characteristics on nitrogen export to and marine fate in Hood Canal, Washington, USA. Biogeochemistry. DOI 10.1007/s10533-010-9521-7.