Tracing the fate of nutrients in agricultural catchments by stable - - PowerPoint PPT Presentation

Christoph Kludt, Alexandra Giber, Christine Kühbeck, Frank-Andreas Weber, Christoph Schüth, Kay Knöller

Tracing the fate of nutrients in agricultural catchments by stable isotope techniques Part I: The Hessian Ried Story

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Outline

Background The nitrate problem in Europe The monitoring/analytical/experimental approach Results Part I: The Hessian Ried Story: Denitrification in groundwater Part II: The Selke Story: Denitrification hot spot in a river stretch Part III: The Bode Story: Denitrification potential in a mesoscale river catchment Conclusions

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Nitrogen surplus in Europe in kg/ ha (2005)

20 40 60 80 100 120 140 1940 1950 1960 1970 1980 1990 2000 2010 2020

kg N/(ha a)

BRD BRD

(1) (2)

Zielwert 2010 80 kgN/ha

Germany

T arget value 80 kg/ ha

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SW NO

Q N c   440

3

Nitrogen surplus and nitrate in groundwater

cNO3

• =

Nitrate in leachate (mg/ l) N = Nitrogen excess (kg/ h) QSW = Groundwater recharge (mm/a)

What are the resulting nitrate concentrations in leachate water considering nitrogen surplus ? C-NO3: 200 mg/ L C-NO3: 260 mg/ L C-NO3: 340mg/ L

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Nitrate in groundwater - Germany

Situation About 27% of all GW bodies in Germany are in a poor chemical state due to high nitrate concentrations.

Distribution of nitrate concentrations in network monitoring wells Number of monitoring wells N=723

Concentration classes mg/ L nitrate percentage

Groundwater body – good status Groundwater body – poor status

Status of groundwater bodies with respect to nitrate

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Potential solutions for the nitrate problem

Sustainable solution: Reduction of N-surplus by good agricultural practice.

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Potential solutions for the nitrate problem

Sustainable solution: Reduction of N-surplus by good agricultural practice. Non-sustainable solution: Natural attenuation concept

Natural Attenuation “Variety of physical, chemical, or biological processes that are at work in-situ, under favourable conditions, acting without human intervention to reduce the mass, toxicity, mobility, volume or concentration of contaminants” (NRC 2000) M ain processes “Nitrate Attenuation”:

• Assimilation and conversion to

biomass (soil)

• M icrobial nitrate reduction

(denitrification)

• Dilution (mixing, diffusion,

dispersion)

• Sorption

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Potential solutions for the nitrate problem

Sustainable solution: Reduction of N-surplus by good agricultural practice. Non-sustainable solution: Natural attenuation concept

Natural Attenuation “Variety of physical, chemical, or biological processes that are at work in-situ, under favourable conditions, acting without human intervention to reduce the mass, toxicity, mobility, volume or concentration of contaminants” (NRC 2000) M ain processes “Nitrate Attenuation”:

• Assimilation and conversion to

biomass (soil)

• M icrobial nitrate reduction

(denitrification)

Denitrification - Reactions:

4 NO3

• + 5 CH2O  2 N2 + 4 HCO3
• + CO2

14 NO3

• + 5 FeS

2 + 4 H+  7 N2 + 10 SO4 2- + 5 Fe2+

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The field site: Hessian Ried

Alluvial plain in the upper Rhine valley between the rivers M ain (north), Rhine (west), Neckar (south), and the Odenwald mountains (east). Thick sedimentary fill of the graben structure  great aquifer. Covers an area of approx. 1200 km2. 33% of the area is intensively used for agriculture. Serves as a major drinking water source for the Frankfurt metropolitan area

http://www.ginkgomaps.com/de/rl3c_de_deutschland_landkarte_illdtmcolgw30scut_ja_mres.jpg

Chemical status Hessian Ried

http://wrrl.hessen.de

Frankfurt a. M.

Darmstadt

Heppenheim

Main good > 50 mg/l NO3-

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The experimental/ monitoring approach

• Estimation of the denitrification

potential using sediments from drilling cores

 Sulfide- / Disulfide sulphur (CRS- M ethod, extern)  Organic carbon (Liquitoc II, TUD)

Eschollbrücken Jägersburger Wald

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The experimental/ monitoring approach

• Estimation of the denitrification

potential using sediments from drilling cores

 Sulfide- / Disulfide sulphur (CRS- M ethod, extern)  Organic carbon (Liquitoc II, TUD)

• Determination of prevailing

denitrification pathways

 Batch experiments  Column experiments

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Nitrate reduction potential in drilling cores

U,t

EB

50 100 150 10 20 30 40 50 60 2000 4000 6000 Teufe u GOK (m) TOC - concentration (mg/kg) TOC Sulfid-/Disulfid S Di-/ sulfide - concentration (mg/kg)

JW

50 100 150 5 10 15 20 25 30 35 40 1000 2000 3000 4000 Teufe u GOK (m) TOC- concentration (mg/kg) TOC Sulfid-/Disulfid S Di-/sulfide- concentration (mg/kg)

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Column experiments Batch experiments Batch experiments

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1 2

2 4 6 8 10 12

[mmol/l] Pore volumen Nitrat Nitrit Sulfat

Adaption phase Begin nitrite formation Transition phase Highest reduction kinetics Denitrification potential exhausted

Autotrophic heterotrophic

Column experiments

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t0 C0

d15N concentration time

d15N(NO3)-0

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Isotope effects during denitrification enrichment of heavy N and O isotopes in residual nitrate (Rayleigh fractionation)

(f = C/C0) initial NO NO

3 3

f ln



d + e  d Column experiments

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Isotope effects during denitrification enrichment of heavy N and O isotopes in residual nitrate (Rayleigh fractionation)

(f = C/C0) initial NO NO

3 3

f ln



d + e  d Column experiments

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Batch experiments

5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO3: 100 mg/ l, SO4: 125 mg/ L)

nitrate concentrations

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Batch experiments

5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO3: 100 mg/ l, SO4: 125 mg/ L)

nitrate isotopes

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Batch experiments

5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO3: 100 mg/ l, SO4: 125 mg/ L)

sulfate concentrations

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Batch experiments

5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO3: 100 mg/ l, SO4: 125 mg/ L)

sulfate concentrations and isotopes

d34S-SO4-background:

+5… +8 ‰ ,

d34S-SO4 from sulfide oxidation:

• 15..-20 ‰

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Insights from isotopic composition of sulfate

SO4 from sulfide oxidation SO4 background Autotrophic denitrification seems to be the prevailing pathway during experiments despite TOC >> FeS

2

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Insights from isotopic composition of sulfate

SO4 from sulfide oxidation SO4 background Autotrophic denitrification seems to be of major importance in the aquifer despite TOC >> FeS

2

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M eeting the targets of the WFD (good chemical status of groundwater) at the groundwater surface:

Time frame is dependent on (i) time needed to change agricultural practices plus (ii) travel time of leachate through the unsaturated zone. In the Hessian Ried targets could be met at the groundwater surface e.g. for 17% of the area within 3 years after changing agricultural practices and for 10 % in more than 15 years. For some monitoring wells of the WFD with deeper screens, concentrations will be high for decades to come or will even increas due to exhausted reduction potential on a flowpath.

Travel times for leachate through the unsaturated zone

M odell: M IKE-SHE (BGS Umwelt)

Leachate travel times (a)

> 15 9-15 3-9 1-3 0-1

Implication for Water Framework Directive

• N. Trauth et al. (UFZ Department Hydrogeology), K. Knöller

Tracing the fate of nutrients in agricultural catchments by stable isotope techniques Part II: The Selke Story

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Impact of groundwater/ surface water exchange on the fate of nitrate in groundwater

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• Surface water

monitoring

• Groundwater wells in

riparian zone

• Groundwater wells in

regional groundwater

Installations

27 from Nico Trauth et al. 2015

>100 mg/ l < 5 mg/ l >100 mg/ l Low nitrate conc. next to stream

 M ixing and/ or denitrification?

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Hydrochemistry - Nitrate

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Nitrate isotope monitoring Selke-site 2014/15

Inventory: Background isotope signatures No obvious difference between river nitrate and groundwater background any change in isotope signatures related to biogeochemistry

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Nitrate isotope monitoring Selke-site 2014/15

Temporal variation of nitrogen isotope signatures in nitrate

15N tracer experiment

river water

???

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Nitrate isotope monitoring Selke-site 2014/15

Temporal variation of nitrogen isotope signatures in nitrate

ground water

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Nitrate isotope monitoring Selke-site 2014/15

Temporal variation of nitrogen isotope signatures in nitrate

ground water

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Nitrate isotope monitoring Selke-site 2014/15

Validation of denitrification in biogeochemical hot spots:

Nitrogen isotope enrichment factor:

• 15 to -10 ‰

Slope in the dual isotope plot: 0.5 to 1.0 Expected parameters:

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Nitrate isotope monitoring Selke-site 2014/15

Interference with nitrification and/or dilution

Validation of denitrification in biogeochemical hot spots:

Nitrogen isotope enrichment factor:

• 15 to -10 ‰

Slope in the dual isotope plot: 0.5 to 1.0 Expected parameters:

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hot spot cold spot

Christin M üller, M atthias Zink, Luis Samaniego, Ronald Krieg, M ichael Rode, Ralf M erz, Kay Knöller

Tracing the fate of nutrients in agricultural catchments by stable isotope techniques Part III: The Bode Story

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133 surface water sampling sites seasonal sampling 25 intensive sites monthly sampling

Isotope monitoring in the Bode river catchment

sub-catchments sampling sites

NH4 in fertilizer and rain Soil nitrogen M anure and septic waste Uptake and denitrification

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Nitrate isotopes along the main river

Spatial distribution

Typical nitrate isotopic signatures after Kendall (1998) and Kendall et al. (2007) Analytical data modified after M ueller et al. (2015) Mueller et al. (2016, ES&T)

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Large variation in agricultural tributaries in the northern sub-basins due to seasonal fertilizer application

=

10 20 30 40 50 0,0 0,2 0,4 0,6 0,8 1,0

coefficient of variation cv (cNO3) [%] portion of agricultural landuse

Spatial distribution cv Land cover

Mueller et al. (2016, ES&T)

Nitrate isotope fingerprinting in the mesoscale Bode River catchment:

temporal variability of nitrate concentrations

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0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

• 5

5 10 15 20 25

Aagri/ Ages (%)

δ15N- NO3 -> (AIR) ‰ )

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Subcatchment Aagri ratio vs. δ15NNO3

Nitrate isotope fingerprinting in the mesoscale Bode River catchment:

Regional relevance and variability of N-sources and sinks

Seasonal variability of denitrification impact?

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Potential denitrification vectors No indication that denitrification has a significant impact on regional scale

Nitrate isotope fingerprinting in the mesoscale Bode River catchment:

Regional vs. local impact of denitrification

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Thank you for your attention!

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Denitrification pathways in the aquifer

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Denitrification pathways in the aquifer