What controls the depth to the redox-interface?

Transcription

1 What controls the depth to the redox-interface? Anne Lausten Hansen University of Copenhagen Geological Survey of Denmark and Greenland (GEUS) Co-authors: Vibeke Ernstsen (GEUS) Jens Christian Refsgaard (GEUS) Rene Therrien (Uni. Laval) Xin He (GEUS) Karsten Høgh Jensen (Uni. Copenhagen)

2 Nitrate Why do we care? Excess nitrate from agriculture causes eutrophication of surface waters Nitrate leaching is a major water resources management problem in Denmark According to the EU Water Framework Directive Denmark must lower nitrate load to surface waters considerable A lot of research on how to change agricultural management to lower nitrate leaching have been done. It has had an effect, but not enough. Berlingske newspaper 15. sept Danish waters suffer from fish death

3 NiCA project Nitrate reduction in geologically heterogeneous catchments Nitrate can be natural degraded in the subsurface by denitrification Denitrification in the saturated zone removes up to 50% of nitrate leaching from the root zone in Danish catchments At present this natural degradation is not taken into account. The current management strategy towards nitrate leaching is a uniform regulation for all areas NiCA objectives Delineate areas with large and small denitrification capacity in subsurface Identify smallest potential scale at which a numerical model can have predictive capability Implementation of differentiated agricultural management

4 Denitrification in groundwater Denitrification occur under anaerobic/reduced conditions The amount of and where denitrification in a catchment occurs depends on: Local-scale water flow patterns Location of redox-interface Focus

5 Department of Geography and Geology Redox-interface What is the problem? Location of the redox-interface varies several meters within short distances The depth to the interface can only be determined by drilling boreholes Sediment color Redox capacity (total amount of reducing components: OM, pyrite, Fe(II)) Data are sparse uncertainty on interface uncertainty in nitrate models Brown/yellow grey /black Increase in reducing comp.

6 Lillebæk catchment Existing redox data from the Danish borehole database

7 Lillebæk catchment Existing redox data from the Danish borehole database Redox depth [m] Variogram (lag distance 300 m) Average 5.6 Stv. 4.1 Varians 17.2

8 Lillebæk catchment New data Department of Geosciences and Natural Resource Management

9 Lillebæk catchment New data Redox depth Average [m] 4.5 Stv. 1.8 Varians 3.1 Variogram (lag distance 2.5 m) Variogram (lag distance 50 m)

10 Study: Modeling the location of the redox-interface Objectives To test what controls the location and variation of the redox-interface by reproducing the observed geostatistics on redox depths from Lillebæk Asses easy obtainable variable to estimate the location of the redox-interface (fx. recharge rate) Assumptions The redox interface was located at ground surface after last glaciation The interface started to move downward at the beginning of Holocene (~ BP) in conjunction with soil development The interface has moved downward due to consumption of the redox capacity in the sediments by oxygen in infiltrating water The redox capacity in unsaturated zone has been consumed relatively quickly diffusion in air phase neglected

11 Modeling approach Numerical model: HydroGeoSphere Study area: Lillebæk Model set up: Only upper till unit 2D transect Length: 400 m ΔX: 0.5 m ΔZ: Upper 12 m 15 varying layers (0.3 1 m) Below 12 m 5 uniform layers Processes: Overland flow Saturated flow Unsaturated flow Evapotranspiration

12 Flow model Climate data: Daily data from one year is repeated Lower BC: Hydraulic head from hydraulic head from catchment model

13 Flow model No observation data at transect Flow model is calibrated against hydraulic head from catchment model No comprehensive calibration, goal is to get a reasonable water balance

14 Redox capacity depletion new feature in HGS Requirements for transport simulation Reduction of O 2 in infiltrating water as a function of redox capacity in sediment Simultaneous reduction of inflowing O 2 and of redox capacity in sediment New feature: Instantaneous reaction approach (Borden and Bedient, 1986) O(t + 1) = O(t) R(t)/F R(t + 1) = R(t) O(t) * F O(t + 1) = 0 if R(t) > O(t) * F R(t + 1) = 0 if O(t) > R(t)/F Assumption: Reaction rate > groundwater flow Reduction of dioxygen O e - => 2 H 2 O 1 mm O 2 uses 4 meq e - => 32 mg O 2 uses 1 meq e - => 1 mg O 2 uses meq e -

15 Redox capacity depletion new feature in HGS Testing of feature Simple synthetic 1D setup Compare with analytical solution (simple hand calculations) Qin C(O 2 )in Testing of O 2 multiplication factor Increase input concentration to speed up simulation Testing of what factor can be applied without error on the result Initial redox capacity giving to much Qout = Qin

16 Model scenarios Who much complexity is needed? Homogeneous clay Heterogeneous clay Random field generated with FGEN Heterogeneous clay with sand lenses Dual continuum (fractures)

17 Thank you for your attention!