Tracing the fate of nutrients in agricultural catchments by stable isotope techniques Part I: The Hessian Ried Story Christoph Kludt, Alexandra Giber, Christine Kühbeck, Frank-Andreas Weber, Christoph Schüth, Kay Knöller
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 SEITE 2
SEITE 3
Nitrogen surplus in Europe in kg/ ha (2005) 140 Germany (1) BRD (2) BRD 120 100 kg N/(ha a) T arget value 80 Zielwert 2010 80 kg/ ha 80 kgN/ha 60 40 20 0 1940 1950 1960 1970 1980 1990 2000 2010 2020 SEITE 4
Nitrogen surplus and nitrate in groundwater What are the resulting nitrate concentrations in leachate water considering nitrogen surplus ? 440 N c NO 3 Q SW - = c NO3 Nitrate in leachate (mg/ l) N = Nitrogen excess (kg/ h) Q SW = C-NO 3 : Groundwater recharge (mm/a) 340mg/ L C-NO 3 : 200 mg/ L C-NO 3 : 260 mg/ L SEITE 5
Nitrate in groundwater - Germany Status of groundwater bodies with respect to nitrate 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 percentage Groundwater body – good status Concentration classes mg/ L nitrate Groundwater body – poor status SEITE 6
Potential solutions for the nitrate problem Sustainable solution: Reduction of N-surplus by good agricultural practice. SEITE 7
Potential solutions for the nitrate problem Natural Attenuation “Variety of physical, chemical, or biological Sustainable solution: processes that are at work in-situ, under Reduction of N-surplus by favourable conditions, acting without human intervention to reduce the mass, toxicity, good agricultural practice. mobility, volume or concentration of contaminants” (NRC 2000) M ain processes “Nitrate Attenuation”: Non-sustainable solution: Assimilation and conversion to Natural attenuation concept biomass (soil) M icrobial nitrate reduction (denitrification) Dilution (mixing, diffusion, dispersion) Sorption SEITE 8
Potential solutions for the nitrate problem Natural Attenuation “Variety of physical, chemical, or biological Sustainable solution: Denitrification - Reactions: processes that are at work in-situ, under Reduction of N-surplus by favourable conditions, acting without human intervention to reduce the mass, toxicity, good agricultural practice. - + 5 CH 2 O 2 N 2 + 4 HCO 3 - + CO 2 4 NO 3 mobility, volume or concentration of contaminants” (NRC 2000) 2 + 4 H + 7 N 2 + 10 SO 4 - + 5 FeS 2- + 5 Fe 2+ 14 NO 3 M ain processes “Nitrate Attenuation”: Non-sustainable solution: Assimilation and conversion to Natural attenuation concept biomass (soil) M icrobial nitrate reduction (denitrification) SEITE 9 SEITE 9
The field site: Hessian Ried Alluvial plain in the upper Rhine http://www.ginkgomaps.com/de/rl3c_de_deutschland_landkarte_illdtmcolgw30scut_ja_mres.jpg valley between the rivers M ain (north), Rhine (west), Neckar Frankfurt a. M. (south), and the Odenwald Main mountains (east). Thick sedimentary fill of the graben structure great Chemical status Darmstadt aquifer. Hessian Ried http://wrrl.hessen.de Covers an area of approx. 1200 km 2 . 33% of the area is intensively good Heppenheim used for agriculture. > 50 mg/l NO 3- Serves as a major drinking water source for the Frankfurt metropolitan area SEITE 10
The experimental/ monitoring approach Eschollbrücken • Estimation of the denitrification potential using sediments from drilling Jägersburger cores Wald Sulfide- / Disulfide sulphur (CRS- M ethod, extern) Organic carbon (Liquitoc II, TUD) SEITE 11 SEITE 11
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 SEITE 12 SEITE 12
Nitrate reduction potential in drilling cores TOC- concentration (mg/kg) TOC - concentration (mg/kg) JW EB 0 1000 2000 3000 4000 0 2000 4000 6000 0 0 TOC TOC 5 Sulfid-/Disulfid S 10 10 Sulfid-/Disulfid S 20 15 Batch experiments Teufe u GOK (m) Teufe u GOK (m) Batch 20 Column 30 experiments experiments 25 40 30 U,t 50 35 40 60 0 50 100 150 0 50 100 150 Di-/ sulfide - concentration (mg/kg) Di-/sulfide- concentration (mg/kg) 7 SEITE 13
Column experiments Autotrophic heterotrophic Adaption phase Begin nitrite Highest reduction Denitrification potential formation kinetics exhausted Transition phase 2 [mmol/l] 1 0 0 2 4 6 8 10 12 Pore volumen Nitrat Nitrit Sulfat SEITE 14
Column experiments C 0 Isotope effects during denitrification concentration d 15 N enrichment of heavy N and O isotopes in residual nitrate d 15 N (NO 3 )-0 (Rayleigh fractionation) d e + d ln f NO NO initial 3 3 (f = C/C 0 ) time t 0 SEITE 15 SEITE 15 SEITE 15
Column experiments Isotope effects during denitrification enrichment of heavy N and O isotopes in residual nitrate (Rayleigh fractionation) d e + d ln f NO NO initial 3 3 (f = C/C 0 ) SEITE 16 SEITE 16 SEITE 16 SEITE 16
Batch experiments 5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO 3 : 100 mg/ l, SO 4 : 125 mg/ L) nitrate concentrations SEITE 17
Batch experiments 5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO 3 : 100 mg/ l, SO 4 : 125 mg/ L) nitrate isotopes SEITE 18
Batch experiments 5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO 3 : 100 mg/ l, SO 4 : 125 mg/ L) sulfate concentrations SEITE 19
Batch experiments 5 identical batches with aquifer sediment from two different drilling cores 140 g sediment + 177 mL synthetic groundwater (NO 3 : 100 mg/ l, SO 4 : 125 mg/ L) sulfate concentrations and isotopes d 34 S-SO 4 -background: +5… +8 ‰ , d 34 S-SO 4 from sulfide oxidation: -15..-20 ‰ SEITE 20
Insights from isotopic composition of sulfate SO 4 background SO 4 from sulfide oxidation Autotrophic denitrification seems to be the prevailing pathway during experiments despite TOC >> FeS 2 SEITE 21
Insights from isotopic composition of sulfate SO 4 background SO 4 from sulfide oxidation Autotrophic denitrification seems to be of major importance in the aquifer despite TOC >> FeS 2 SEITE 22 SEITE 22
Implication for Water Framework Directive 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 Leachate screens, concentrations will be high for decades to travel times (a) > 15 come or will even increas due to exhausted reduction 9-15 3-9 potential on a flowpath. 1-3 0-1 Travel times for leachate through the unsaturated zone M odell: M IKE-SHE (BGS Umwelt) SEITE 23
Tracing the fate of nutrients in agricultural catchments by stable isotope techniques Part II: The Selke Story N. Trauth et al. (UFZ Department Hydrogeology), K. Knöller
Impact of groundwater/ surface water exchange on the fate of nitrate in groundwater SEITE 25
SEITE 26
Installations Surface water monitoring Groundwater wells in riparian zone Groundwater wells in regional groundwater from Nico Trauth et al. 2015 SEITE 27 27
Hydrochemistry - Nitrate Low nitrate conc. next to stream M ixing and/ or denitrification? >100 mg/ l < 5 mg/ l >100 mg/ l SEITE 28 28
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 SEITE 29
Nitrate isotope monitoring Selke-site 2014/15 Temporal variation of nitrogen isotope signatures in nitrate river water 15 N tracer experiment ??? SEITE 30
Nitrate isotope monitoring Selke-site 2014/15 Temporal variation of nitrogen isotope signatures in nitrate ground water SEITE 31
Nitrate isotope monitoring Selke-site 2014/15 Temporal variation of nitrogen isotope signatures in nitrate ground water SEITE 32
Nitrate isotope monitoring Selke-site 2014/15 Validation of denitrification in biogeochemical hot spots: Expected parameters: Nitrogen isotope Slope in the dual isotope enrichment factor: plot: -15 to -10 ‰ 0.5 to 1.0 SEITE 33
Recommend
More recommend
