A concept for estimating depths to the redox interface for catchment scale 

nitrate modelling in a till area Anne Lausten Hansen (alha@geus.dk)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

• 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

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

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

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 !

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

Application in Norsminde fjord catcment

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Topography Soil type Redox depthobservations

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

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)

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Calibration target‐ Nitrate arrival percentage (NAP) to Norsminde fjord ‐

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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

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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Redox interface and reduction maps

Estimated versus observed redox depths‐ point scale ‐

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Estimated versus observed redox depths‐ catchment scale ‐

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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

• 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

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

Work in progress‐ Application of redox concept on 20 geological models ‐

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Uncertainty on nitrate reduction at different aggregation scales

Thank you for your attention!

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

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