Linking Groundwater Age and Chemistry Data to Determine Redox Reaction Rates and Trends in Nitrate Concentrations in Agricultural Areas

Transcription

1 Linking Groundwater Age and Chemistry Data to Determine Redox Reaction Rates and Trends in Nitrate Concentrations in Agricultural Areas A.J. Tesoriero, J.H. Duff and L.J. Puckett U.S. Geological Survey

2 National Water-Quality Assessment Program -Status, trends and understanding of groundwater and surface water quality in the US. -Cycle 1 started in 1991, Cycle 2 in 2001, Cycle 3 in 2013

3 Nutrient transport and transformation in groundwater and streams -Flow System Studies: Groundwater to streams -Nutrient Enrichment GW/SW Interaction Studies: Input to streams and in-stream processes

4 Why Study Nutrients? -Nitrate is the common contaminant of groundwater and the most likely to exceed USEPA MCL. -Most prevalent stressor of biological condition in wadeable streams in the US (USEPA, 2006). -Major cause of hypoxia in receiving waters (e.g., Gulf Coast, Albemarle- Pamlico Sounds)

5 Fertilizer Nitrogen Use In The United States Since Fertilizer N (tons x 10 6 ) Ruddy et al., 2006; USGS SIR

6 Average Nitrogen Application Rates on Corn 180 Fertilizer N (kg/ha) Data from K.G. Cassman, Ambio 31, 132 (2002).

7 Questions How has the increased application of fertilizer affected nitrate concentrations in shallow groundwater? Reconstruct trends. Are downgradient waters (groundwater and streams) vulnerable to these legacy effects? Calculate reaction rates Nitrate N 2

8 Compilation of 20 Groundwater Transect Studies -Most are from USGS National Water-quality Assessment Program. -Transect from recharge to discharge in surficial aquifers. -About 20 wells per transect, with short screened intervals. -Age dating, major ion, nutrient and pesticide analyses. -Primarily in agricultural areas Acknowledgements J.K. Bohlke, Karen Burow, Judy Denver, Betsy Frick, Chris Green, Brian Hughes, Leon Kauffman, Bruce Lindsey, Pete McMahon, Ed Modica, Steve Phillips, David Saad, Gary Spieran, Greg Steele and others.

9 Chlorofluorocarbon and Sulfur Hexafluoride Based Age Dating

10 Reconstructing Nitrate Concentrations in Recharge -Use age dating to determine recharge date. -Determine nitrate concentration. -Determine amount of N 2 from denitrification. -Recharge Nitrate Conc. =[NO 3- ]+[N 2 from denitrif.] Nitrate (mg/l as N)

11 Nitrate Concentrations in Recharge Increasing at Most Sites Nitrate (mg/l as N) Central Valley CA Nitrate (mg/l as N) Glacial Aquifer WI Nitrate (mg/l as N) Coastal Plain NC Nitrate (mg/l as N) Coastal Plain GA Tesoriero et al., JCH, 2007

12 Nitrate Concentrations in Recharging Groundwater Increasing in Agricultural Areas Across the United States 50 Nitrate in Recharge (mg/l as N) Recharge Year

13 Likelihood of High Nitrate Concentrations (> 10 mg/l as N) in Recharge Increasing Since 1950 Percent Probability of Nitrate in Recharge > 10 mg/l as N Fertilizer Applied in U.S. Probability of Nitrate > 10 mg/l Recharge Year Recharge Year N Application nnnnnnnnn (kg x 10 9 ) % of samples in decile > 10 mg/l as N

14 Estimating Equivalent Percentage of Applied N That Reaches Groundwater % of N applied reaching groundwater is estimated using reconstructed nitrate concentrations, cropland acreage, recharge and N application data. Glacial Aquifer in Wisconsin Nitrate (mg/l as N) % of N Applied GW Data

15 Are downgradient groundwater supplies and streams vulnerable to these legacy effects? Nitrate could be removed by denitrification if sub-oxic conditions are encountered. Dissolved oxygen reduction rates determined by examining relation between DO and groundwater age help assess vulnerability to legacy nitrate. First order rate expression: C = C 0 exp(-kt) Zero-order rate expression: C= C 0 -kt Dissolved Oxygen (mg/l) C/C y = e x R 2 = Denitrification Occurs Apparent Groundwater Age (years) Apparent Groundwater Age (years)

16 Three Scenarios Identified: Fast O 2 reduction in the upland. Slow O 2 reduction in the upland, fast in the riparian zone. Slow O 2 reduction in upland and riparian zone.

17 Flow System Study Scenario 1: Fast O 2 reduction in Upland: Coastal Plain of North Carolina Confined Animal Feeding Operation (CAFO) Blue Ridge/ Valley and Ridge Piedmont VIRGINIA NORTH CAROLINA Inner Coastal Plain Albemarle-Pamlico Drainage Basin (ALBE) Outer Coastal Plain Manure Spray Fields

18 1: Thin Oxic Zone in Upland, Sub-oxic Near Streams High DO reduction rates in upland and in discharge areas. Examples: Otter Tail, MN; Lizzie, NC. C/C C/C k=0.13 k=0.13 k= Apparent Groundwater Age (years) Upland Riparian

19 Fast Dissolved Oxygen Reduction Rates at Coastal Plain of NC Site -Nitrate is lost quickly after DO is consumed. -Other sites with similar rates: Otter Tail and Battle Creek, MN Dissolved Oxygen (mg/l) Sandy Run Watershed, NC Redox Process O2 NO3 Mn(IV) Fe(III)/SO4 Sandy Run, NC CH Apparent Groundwater Age (years) Apparent Groundwater Age (years) Redox process determined using program by Jurgens et. al

20 Scenario 1: Thin Oxic Zone in Upland and Sub-oxic Near Streams Streams and much of the upland are not vulnerable to legacy nitrate. Oxic GW Flow Mn/Fe/Sulfate Reducing Nitrate Reducing Tesoriero et al., WRR, 2005

21 Scenario 2: Oxic Upland, Sub-oxic Near Streams Low DO reduction rates in upland but high rates in discharge area. Examples: Fishtrap Creek, WA; Morgan Creek. MD C/C C/C C/C 0 = exp(-kt) k=0.13 k=0.03 k=0.03 yr -1 Slow k=0.13 Fast Apparent Groundwater Age (years) Upland Riparian

22 Scenario 2: Slow Dissolved Oxygen Reduction Rates in Upland But Fast at Riparian Zone Example: Abbotsford-Sumas Aquifer, BC-WA -DO remain high in upland aquifer but once groundwater enters riparian zone DO and other electron acceptors are consumed. -Site with similar behavior: Morgan Creek, MD Dissolved Oxygen (mg/l) Abbotsford-Sumas Aquifer British Columbia, Canada- Washington State Redox Process O2 NO3 Mn(IV) Fe(III)/SO4 Abbotsford-Sumas Aquifer, WA and BC CH Apparent Groundwater Age (years) Apparent Groundwater Age (years)

23 Scenario 2. Oxic Upland, Sub-oxic Near Streams. Upland groundwater is vulnerable to legacy nitrate but streams are not. Fishtrap Creek, WA Sub-oxic GW Flow Oxic Tesoriero et al., WRR, 2000

24 Scenario 2: Slow DO Reduction Rates in Upland, Fast in Riparian Zone. Trans-boundary Study, Abbotsford-Sumas Aquifer, BC-WA

25 3: Oxic in Upland and Near Streams Low DO reduction rates in upland and in discharge areas C/C 0 C/C k=0.13 k=0.03 Examples: Tomorrow R., WI, Chesterville Branch, MD Apparent Groundwater Age (years) Upland Riparian

26 Scenario 3: Slow Rates in Upland and in Riparian Zone -Glacial aquifer in Wisconsin -Both groundwater and streams vulnerable to nitrate Dissolved Oxygen (mg/l) 10 1 Glacial Aquifer Polonia, WI Redox Process O2 NO3 Mn(IV) Fe(III)/SO4 CH4 Glacial Aquifer Polonia, WI Apparent Groundwater Age (years) Apparent Groundwater Age (years)

27 Scenario 3: Oxic in Upland and Near Streams Upland groundwater system and streams are vulnerable to legacy nitrate. Oxic GW Flow D.A. Saad, JEQ, 2008.

28 Why do dissolved oxygen reduction rates vary? Electrons Needed for O2 Reduction: O 2 + 4H+ + 4e - 2H 2 O Electron Donors : CH 2 O + H 2 O CO 2 + 4H + + 4e HS H 2 O 0.5 SO H + + 4e - Corr. Coeff. Between Fe, DOC or Sulfate and GW Age Chest. Branch HPGW Tomorrow R. Otter Tail Battle Br. Lapine Morgan Ck. Sandy Run DOC Sulfate Fe DO Reduction Rate Constant (yr -1 ) Fishtrap Ck.

29 Nutrient Enrichment Effects (NEET) Groundwater-Surface Water Interaction Studies: -Whole stream response to nitrate loading in three streams draining agricultural landscapes. JEQ, v. 37, Identifying nutrient pathways and processes affecting nitrate and OP inputs to streams in agricultural watersheds. JEQ, v. 38, In progress: Influence of ground water legacy effects on stream nitrate in WI stream.

30 GW/SW Work at the Tomorrow River FPS Site, Portage County, WI

31 NEET Studies: Quantify groundwater flux with bromide tracer.

32 Nitrate Concentrations in Streambed are High and Variable Resulting in Sharp Increase in Stream Nitrate Concentrations. Nitrate-N in Streambed (mg/l) Meters From Top of Reach Nitrate-N or Bromide in Stream (mg/l)

33 Concentration of Nitrate Discharging to Stream Estimated Using Tracer and Nitrate Concentration Data 35 Nitrate-N in Streambed (mg/l) Calculated [NO 3- ] in GW Discharge Meters From Top of Reach

34 Legacy Nitrogen: Polonia, WI Large increase in nitrate concentration predicted for vulnerable stream in Wisconsin Nitrate (mg/l as N) Streambed Samples 2009 Streambed Samples 2019? Stream in 2009 Stream in 2019? Nitrate Conc. in Stream in 2009: 3 mg./l Nitrate Conc. in Stream in 2019: 5 mg./l?

35 Key Findings: Nitrate in Recharge in Agricultural Areas Reconstructed trends suggest that nitrate concentrations in recharge have increased markedly in the last 40 years. Likelihood of concentrations exceeding MCL is high (>40%) in recently recharged water. Increase in nitrate typically coincides with increase in fertilizer application. Equivalent percentage of fertilizer reaching groundwater varies widely between sites, with a median of 20%.

36 Key Findings: Nitrate Transport to Deeper Groundwater and Streams Dissolved oxygen reduction rates are often low in uplands. As a result, as groundwater moves deeper into aquifers we can expect to see nitrate concentrations increase in most cases. Low dissolved oxygen reduction rates in some riparian zones make these streams susceptible to legacy nitrate.

37 Next Step: Simulation and Forecasting Nitrate in recharge as a function of application rate. Use application rate scenarios for past and future N loadings to the water table. Use reaction rate data to predict transformations. Simulate watershed impacts for each scenario. Nitrate in Recharge (mg/l as N) Nitrate in Recharge (mg/l as N) Annual 1980 Nitrogen 1990 Fertilizer Application Rate (kg/ha) C/C ?? ? Apparent Groundwater Age (years) 1.9

38 Advantages: Reconstructing Trends Can establish trends with one sampling event. Trends during key period of intensive applications can be estimated. Key Limitation: Must be able to account for degradates. Future Work: Scale up findings to larger study areas. Couple results with groundwater modeling. Resample networks every 10 years.