The Fate of Onsite Septic System Nitrogen Discharges in Groundwater of the Hood Canal Basin Julie Horowitz, Bryan Atieh, Garrett Leque, Mark Benjamin, Michael Brett Department of Civil and Environmental Engineering, University of Washington The Story… ¾Eutrophication and low dissolved oxygen ¾Hood Canal ¾Onsite Septic Systems (OSS) as a potential source of nitrogen loading ¾Denitrification – the key variable in determining the nitrogen load ¾Measuring denitrification in the Hood Canal basin ¾Substantial spatial and temporal variability in denitrification U.S. Coastal ‘Dead Zones’ Associated with Human Activity Date of Hypoxic event 1970s 1980s 1990s 2000 Source: America’s Oceans: Charting a Course for the Sea Change. Pew Ocean Commission report June, 2003 Hood Canal, Washington Eutrophication in Hood Canal Hood Canal is an estuary where OSS N loading may exacerbate eutrophication. HCDOP HCDOP HCDOP , pet waste, lawns Newton, UW-APL Loading from OSS to Hood Canal Denitrification rates X Census data Travel Distance Household Groundwater Trash output Traffic studies Septic inputs nitrogen velocity Seasonal Per capita Septic Nitrogen X X Population water use nitrogen - removal effluent Nitrogen load to Hood Canal Nitrogen Fate and Transport Drainfield Septic Tank + Organic N Æ NH4 2 N Drainfield and Vadose Zone + - NH4 Æ NO3 N Denitrification O - NO3 Æ N2 3 Groundwater Flow Denitrification ¾Denitrification is the primary N removal process. + - - ¾Organic N NH4 NO2 /NO3 N2 ¾Requirements: 1) Denitrifying microbial population 2) Anoxic conditions (DO < 0.2 mg/L) 3) Electron donor supply (carbon) Denitrification Rates 15 N = 25 median = 0.06 mg/L/day 10 Frequency 5 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 mg N removed/L/day Partial Regression Plot Partial Regression Plot Dependent Variable: LOADING Dependent Variable: LOADING 600 600 400 400 200 200 Model 0 0 - 200 - 200 Sensitivity - 400 - 400 - 8000 - 6000 -4000 - 2000 0 2000 4 000 6 000 -80 -60 -40 -20 0 20 40 60 80 POP FLOW Analysis Partial Regression Plot Partial Regression Plot Dependent Variable: LOADING Dependent Variable: LOADING 3000 600 400 2000 200 0 1000 - 200 - 400 0 Partial correlation - 600 - 1000 - 800 -100 0 100 200 300 400 500 -.2 -.1 0.0 .1 .2 .3 .4 .5 .6 CONC lo g(loss) r Population 0.092 Partial Regression Plot Dependent Variable: LOADING 600 Flow 0.315 400 200 Concentration 0.946 0 - 200 log(loss rate) -0.762 - 400 -.2 -.1 0.0 .1 .2 1/vel 1/velocity -0.089 Loading from OSS to Hood Canal Denitrification rates s ie X d tu Census data Travel Distance S ld Household Groundwater Trash output Traffic studies Septic inputs ie nitrogen F velocity Seasonal Per capita Septic Nitrogen X X Population water use nitrogen - removal effluent Nitrogen load to Hood Canal Field Site Locations HCDOP Site Spatial Variability Upland Near-shore and Riparian Shoreline Well Installation and Sampling Field Site Set Up Groundwater Monitoring Wells Drainfield Septic Tank + - Nitrogen Speciation: NH4 NO3 50 40 L N) 30 g/ m ( + 4 20 Septic NH 10 0 024681012 Upland Site - 50 NO3 (mg/L N) 40 Riparian Site ) L N 30 g/ m 20 ( + 4 Sampling 10 Wells NH 0 024681012 - NO3 (mg/L N) Modeling of OSS N Removal 70 100 60 50 Septic 40 50 30 TN (mg/L) 20 0 10 0 GW 0 20 40 60 80 100 120 Chloride (mg/L) Upland: Nitrogen Removal 0.6 0.5 0.4 Partial removal ~ 66% 0.3 Probability 0.2 0.1 Upland site – Year 2 0 <0 5 152535455565758595 0.6 Percent Removal of TN 0.5 Upland site – Year 1 y 0.4 Partial removal ~ 40% ilit b a 0.3 b o r P 0.2 0.1 0 <0 5 152535455565758595 Percent Removal of TN Riparian: Nitrogen Removal 0.6 0.5 0.4 y Extensive N removal ~ 92% ilit b a 0.3 b o r P 0.2 0.1 0 <0 5 152535455565758595 Percent Removal of TN Near-shore: Nitrogen Removal School Site 40 35 30 25 g/L) m 20 Off Site Wells ( N TD 15 10 B-6 5 0 0 5 10 15 20 25 30 Cl (mg/L) ¾ Complicating factors – extent of removal difficult to determine. ¾Nitrogen removal likely ~30 % Shoreline: Nitrogen Removal DIN Removal Probability for Wells Adjacent to Bulkhead 0.7 Statistics 0.6 Average of Medians 10.1 STDEV of Medians 20.3 0.5 yy litlit ii 0.4 Little removal ~ 30% obabobab 0.3 PrPr 0.2 0.1 0.0 -10 -20 -30 -40 -50 -60 -70 -80 -90 00 0 0 -1 1 20 30 40 50 60 70 80 90 DIN Removal % Challenges ¾ Groundwater accessibility ¾ Dilution ¾ Multiple OSS sources ¾ Complex geology and groundwater flow paths ¾ Heterogeneity Spatial N Removal Variability Upland Partial Removal Extensive Removal Near-shore and Riparian Little Removal Little Removal Shoreline Conclusions ¾ OSS ammonium is typically converted to nitrate ¾ Denitrification is occurring in some locations around Hood Canal ¾ The extent of denitrification varies spatially and temporally ¾ Further research can be conducted to explore denitrification limiting conditions ¾ Problems with scaling up and with rates ¾ Common challenges with denitrification research This research was funded by the Washington State Legislature, and managed by the Puget Sound Partnership. This research was conducted in conjunction with the Hood Canal Dissolved Oxygen Program. Thank you!.