A concept for estimating depths to the for catchment scale...
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A concept for estimating depths to the redox interface for catchment scale
nitrate modelling in a till area Anne Lausten Hansen ([email protected])1,2
Christensen BSB3, Ernstsen V1, He X1 and Refsgaard JC1
(1) Geological Survey of Denmark and Greenland(2) Department of Geosciences and Natural Resource Management, University of Copenhagen (3) Rambøll
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• Nitrate can be naturally transformed by reduced compounds(OM, Fe+2, pyrite) in the sediments
• Transition from oxic to reduced conditions = redox interface
• Spatial variation in the redox interface and in water flow paths leads to nitrate sensitive and nitrate roboust areas
• Important to know the location of the redox interface to delineate these areas
Introduction
Nitrate sensitive area Nitrate roboust area
Introduction Background Methodology Application Evaluation Future
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Introduction• Location of the redox interface in till areas varies several meters within short distances
• The interface can only be determined by drilling boreholes => Limited data
Large uncertainty on the location of the redox interface
• Interesting to develop methodologies to inferthe location of the redox interface from othervariables
Introduction Background Methodology Application Evaluation Future
Objective of this study
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Redox interface development ‐ Hypothesis‐
• The reduced compounds (redox capacity) in the sediments is depleted by oxygen and nitrate
• The present location of the redox interface is the result of the cumulative flux of oxygen in recharging groundwater since the onset of Holocene (11.700 years)
• Development of the interface in parts of the unsaturated zone can have happened fast due to oxygen diffusion in the air phase. In a clay till, however, this is only important in the root zone
Introduction Background Methodology Application Evaluation Future
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Redox interface concept• Key principle: estimate the spatial pattern of the redox interface
from variability in groundwater recharge and sediment redoxcapacity
• Redox equation: the redox depth in grid i is estimated as:
Redox depthi = fluxi ∙ f + min. redox depth
flux: recharge flux estimated with hydrological model f: redox interface migration constant (m over 11.700 pr mm yearly recharge) min. redox depth: Upper part of UZ where redox capacity have been depleted fast
due to air phase diffusion
Additional parameters: Maximum redox depth Lower redox depth in riparian lowlands
Introduction Background Methodology Application Evaluation Future
Dependent on the sediment redox capacity !
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Step 1:Extraction of recharge flux from hydrological model (no drainage and pumping)
Step 2:Difference in redox capacity betweensediment types applied to recharge map
Step 3:Apply redox equation, define f for mainsediment type
Step 4:Run nitrate model with estimated redoxinterface => Simulated nitrate arrival (% of nitrate input, NAP) at catchment outlet
Step 5:Compare simulated and observed NAPIf sim >< obs => new constant f and min. redox depth
Redox interface concept
Introduction Background Methodology Application Evaluation Future
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Application in Norsminde fjord catcment
Introduction Background Methodology Application Evaluation Future
Topography Soil type Redox depthobservations
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Models• Geological model
– 11 hydrogeological units – Based on borehole data from Jupiter and
geophysical data from Mini‐SkyTEM
• Hydrological model– MIKE SHE/MIKE 11– All hydrological processes– Grid scale 100x100 m
• Nitrate model– Particle tracking (MIKE SHE)
Introduction Background Methodology Application Evaluation Future
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Nitrate model ‐ particle tracking
• Nitrate input: Daily N leaching from rootzone
N balance method combined with Daisy simulations (Thirup (2013), available at www.nitrat.dk)
• Redox interface implemented as registration zone => particle registreted if crossing interface
• Nitrate arrival: particles arriving in fjord without crossing redox interface
• The model is run 4 years with N input (2000‐2003) and then additional 4 yearsto get all nitrate out (flow recycled)
Distribution of particles at different sim. time(N added first 4 years)
Introduction Background Methodology Application Evaluation Future
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Calibration target‐ Nitrate arrival percentage (NAP) to Norsminde fjord ‐
Introduction Background Methodology Application Evaluation Future
41 – 49 % of the nitrate leaching arrives in Norsminde fjord
Input period 1998‐2005 1998‐2004 2000‐2003 2000‐2003 2000 ‐ 2005 2000 ‐ 2004Obs period 1998‐2005 1999 ‐ 2005 2000‐2003 2001‐2004 2000 ‐ 2005 2001 ‐ 2005Avg. N leaching input [t/yr] 365 368 281 281 279 267Avg. obs N flux to fjord [t/yr] 157 151 127 135 131 130NAP [N leaching/N flux] 0.43 0.41 0.45 0.48 0.47 0.49
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Redox scenarios and calibration• Redox scenarios (based on sensitivity analysis)
– Scenario 1: Recharge flux layer 2 (3 ‐ 4 m.b.s) Redox depth in riparian lowlands 1.5 m
– Scenario 2: Recharge flux layer 1 (0 ‐ 3 m.b.s)Redox depth in riparian lowlands 1.5 m
– Scenario 3: Recharge flux layer 2 (3 ‐ 4 m.b.s) No riparian lowlands
• Calibration– All 3 scenarios was calibrated to NAP = 45%
Introduction Background Methodology Application Evaluation Future
Scenario Constant f Min. redox depthScenario 1 0.025 2.65Scenario 2 0.0155 1.5Scenario 3 0.025 2.5
Calibrated parameter valuesNorsminde redox data (clay till)Avg. redox capacity: 418 meq‐e/kgO2 conc.: 11.4 mg/l (10oC)=> Constant f = 0.025
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Introduction Background Methodology Application Evaluation Future
Redox interface and reduction maps
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Estimated versus observed redox depths‐ point scale ‐
Introduction Background Methodology Application Evaluation Future
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Estimated versus observed redox depths‐ catchment scale ‐
Introduction Background Methodology Application Evaluation Future
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Evaluation of Results
• The model is able to simulate observed nitrate arrival (NAP)to Norsminde fjord
• All 3 scenarios can be cailbrated to NAP = 45% => equifinality
• Redox depth observations not sufficient to choose between scenarios
• Cumulative distribution of redox depths close to observed
• Site‐specific redox depths is not well estimated
• Results okay on cathment scale, but not on small scale
Introduction Background Methodology Application Evaluation Future
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• Recharge flux– Constant flux– Only vertical component of flux
• Migration constant f– Uniform migration constant f within sediment type– Variation in sediment type with depth not included
• Scale issue (Model grid scale 100x100 m)– Affects estimated redox depths due to averaging– Affects compariosn of estimated vs. obseved redox depths
• Nitrate data– N leaching– N flux to Norsminde fjord
• Geological and hydrological model– Flow paths correct ?
Factors affecting the results
Introduction Background Methodology Application Evaluation Future
Norsminde dataRedox capacity (clay till)Avgerage: 418 meq‐e/kgSt.dev.: 150 meq‐e/kg
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Conclusions
• The concept is capable of estimating the general location of the redox interface, but not at grid scale
• The model is therefore not able to accurately simulate nitrate reduction at grid scale
• The uncertainty on the reduction potential maps needs to be evaluated
Introduction Background Methodology Application Evaluation Future
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Work in progress‐ Application of redox concept on 20 geological models ‐
Introduction Background Methodology Application Evaluation Future
Uncertainty on nitrate reduction at different aggregation scales
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Thank you for your attention!
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Redox depth observations
• Log‐normal distributed (p‐value 0.8) with a mean of ln(redox) = 1.6 m => redox = 4.7 m
• No correlation to other variables (elevation, distance to stream, min. water table)
• A varioagram analysis showed spatial correlation with a correlation length of 289 m
Only resolved by a few clusters ofboreholes and not the entire data set=> not representative for whole area
Introduction Background Methodology Application Evaluation Future
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Redox interface development ‐Mass balance example ‐
Borehole profile from Lillebæk (LOOP4)Meq e‐/kg : milli‐electron‐equivalentsper kg sedimentData from Ernstsen (2013), available at www.nitrat.dk
Introduction Background Methodology Application Evaluation Future
1. Redox capacity (reactive): 450 meq e‐/kg2. Bulk density: 1590 kg/m3
3. Electron use (O2 reduction): 4 e‐/mole= 0.125 meq e‐/mg
4. O2 conc. (10 oC): 11.4 mg/l5. Recharge rate: 273 mm/yr
Migration pr. year (3*4*5)/(1*2): 5.4e‐4 m/yrTotal migration (11.700 years): 6.4 m (below root zone)
Migration pr. mm yearly recharge: 0.023 m/(mm*yr‐1)
Migration constant f‐ Independent on recharge flux‐ Very dependent on redox capacity