Transport, Reactions and Dynamics of Heavy Metals in Contaminated Marine Sediments

Transcription

1 Transport, Reactions and Dynamics of Heavy Metals in Contaminated Marine Sediments 4. Rostocker Baggergutseminar By Dr. Ole Larsen

2 Sediment management Sediment management is important for several reasons, industry, environment, flood control etc. 2,, m³/year is dredged in Europe Dredging is a cost factor - dredging costs range from few to hundred / m³ depending on treatment The cheapest solution is to deposit the sediments in the environment Management is easy!!

3 Questions? What happens when a contaminated sediment is deposited in a pristine environment? What happens when a sediment is kept without any treatment? What happens when the sediment is deposited at a controlled site? Or: What is the turn-over of heavy metals in sediments??

4 Bulk analyses what do the tell us?,6,4,2 Depth Area SedGT Salinity "NAO" N-load P-load CalmDays Cu Pb Cd Zn S-W TotSp CrustSp GastSp PolySp OthTaxSp BorSp SurfSp HerbSp OthGuilSp SDSp, -,2 -,4 Estuaries (6 data sets); 1 PLS component Depth > 3 m X-load, Y-weights -,6,6,4,2, -,2 -,4 Estuaries (6 data sets); 2 PLS component Depth > 3 m X-load, vægte Y- Depth Area Salinity SedGT "NAO" N-load P-load CalmDays S-W TotSp Cu Pb Cd Zn CrustSp GastSp PolySp OthTaxSp BorSp SurfSp HerbSp OthGuilSp SDSp -,6 Heavy metal PLS axis 52% of total species richness could be explained by 3 PLS components. SedOrg Cu, Pb,Cd 2% 14% 25% 41% Sample area Depth, salinity N&P-Load In Estuaries depth, salinity (and sampled area) accounted for 41% of explained species richness, Nutrient load accounted for 25% and sediment concentration of metals (Cu) accounted for 2% of explained richness

5 Cu one main problem 5 4 Estuaries Århus Bay Wadden Sea Other areas Cu mg/ kg DW Cu = 1.91*X +.42; ±5% Pred.Int Loss on ignition - % DW

6 Temporal release of metals on sediment reduction and oxidation anoxic oxic Ar/CO 2 air/co 2 from org from + MnO 2 FeS Removal to sulphides Removal to oxides 62 hours 5 1 hours Hamilton-Taylor et al, Aq. Sci, 1996

7 The problem can be separated into a: Physical (transport) Biogeochemical (reactions) Biological (dynamics) And a management problem

8 Oxygen is the key parameter Oxygen is consumed through biogeochemical processes (organic matter diagenesis; ; oxidation of transition metals) Oxygen enter the sediment either dissolved in water or produced by the benthic community

9 Transport of oxygen into sediments Diffusion Bioturbation / Bioirrigation Porewater flows (Advection)

10 Diffusion solute transport dominant transport process in muddy sediments transport by random movement Fick s first law of diffusion

11 Diffusion Fick s 1st law: J = porosity J = -D s dc/dz

12 Biological Processes The benthic fauna turbates the sediment: digs burrows creates topography bulldozes the sediment eats the sediment mixes the upper 1-15 cm Ventilates the sediment Wiebke Ziebis

13 Bioirrigation The benthic fauna irrigates the sediment... by passive ventilation by piston pumping to detoxify their burrows to filter food out of the water column

14 Bioirrigation water exchange across the sediment-water interface transports mainly solutes creates diffusive gradients locally very effective

15 Pore water transport advective pore water flow convective pore water flow (density-driven) driven) wave pumping (Riedl( 1972) tidal pumping (beach filling and drainage) resuspension (release and trapping of pore water) gas seeps rapid permeable sediments directional over longer distances than diffusion

16 3 min

17 Oxygen measurements: Diffusion No current: diffusion oxygen penetration depth: ~3 mm

18 Oxygen measurements: Advection Topography + oscillating current: intrusion of oxygen-rich water into the sediment (ripple trough) release of oxygen-depleted pore water at the ripple crest horizontal oxygen concentration gradients 19 x 24 mm [4x4 µm/pixel], 7 min, 1 frame/min

19 Oxygen measurements: Advection 87 min (1min/frame) mobile ripples: mobile anoxic zones Precht et al. 24

20 Oxygen measurements: Advection

21 Advection vs. Diffusion

22 Advection vs. Diffusion

23 Carbon dioxide ALGAE Oxygen organic Material O 2 H 2 O NO - 3 N 2 MnO 2 Mn 2+ FeOOH Fe 2+ SO 2-4 H 2 S + + CO 2 NH 4 + CH 4

24 Mn and Fe in pore waters sub-oxic conditions cm scale O 2 NO 3 - CH 2 O + O 2 CO 2 + H 2 O NH O 2 NO 3- + H 2 O + 2H + CH 2 O + NO 3- CO 2 + N 2 + H 2 O Mn CH 2 O + MnO 2 Mn 2+ + CO 2 + H 2 O Fe CH 2 O + FeOOH Fe 2+ + CO 2 + H 2 O unproductive deep ocean Froelich et al, GCA, 1979

25 Major sources and sinks of trace metals in sediments Org matter Fe/Mn oxides M M release from org. matter Fe/Mn oxides formed in situ M Release from Fe/Mn oxides M General adsorption solids S 2- Removal as metal sulfides

26 The TREAD approach

27 The different experiments Deposition on sand [A]: Harbour sediment was added to the sand to simulate dumping of polluted sediments in a region with higher bottom water currents and less polluted sediments. One control (pristine sand from Sylt) ) flume [B] One control (undisturbed harbour sediment) [C] One similar flume added a suspension of harbour sediment [D]

28 Investigated perturbations cover: Sediment treatment (homogenization and deposition on sand) Cohesive and non-cohesive sediments Importance of fauna Salinity Availability of organic matter

29 Experimental methods Chamber incubations Microsensors 2D Optodes Microscopy Bulk Chemistry Rate measurements (radiotracers) DET DGT

30 DET Diffusive Equilibrium in Thin-films Gel (95% water) filter 1 mm Elute in acid 1.8 cm 1mm ICP-MS Equilibrate in sediment C gel = C pw slicing Constrained probe DET measures porewater concentration

31 DGT (Diffusive Gradients in Thin-films) Principles in sediments C DGT Depends on: R e s i n g e l Diffusive gel g C DGT diffusion C soln C bulk pw sediment k -1 C solid 1 2 mm C bulk pw k -1 [C solid ] diffusion C DGT = M g/(dat) Flux = M/(At)

32 Deployment and Measurement Probe Deployment Scanning Slicing LA-ICP-MS 3mm x 3mm Measure at 2 µm intervals combined AgI & Chelex 1 day AgI gel + S(-II) Elute in acid ICP-MS Ag2S

33 2D Image of metals distribution from DGT Measurements D G T F e (p p m ) D G T C o (p p b ) D G T N i ( p p b ) Fe Co Ni X D a t a X D a ta X D a ta Depth (mm)

34 A DGT Measurements B Fe (µm) Co (nm) Ni (nm) Cu (nm) Cd (nm) Pb (nm) Total dissolved Sulphides Fe (µm) Co (nm) Ni (nm) Cu (nm) Cd (nm) Pb (nm) Total dissolved sulphides fauna OCTOBER 23 Depth (mm) Fe Co Ni Cu Cd Pb S OCTOBER 23 Depth (mm) JANUARY 24 Depth (mm) JANUARY 24 Depth (mm) algae fauna MAY 24 Depth (mm) MAY 24 Depth (mm) lower salinity OCTOBER 24 Depth (mm) OCTOBER 24 Depth (mm) APRIL 25 Depth (mm) APRIL 25 Depth (mm)

35 Sediment flux, October 24 (after 3 treatments), Flume D (harbour sediment, homogenised) Fe Co Ni Cu Cd Fe (pmolcm -2 s -1 ) Co (fmolcm -2 s -1 ) Ni (fmolcm -2 s -1 ) Cu (fmolcm -2 s -1 ) Cd (fmolcm -2 s -1 ) Depth (mm)

36 DGT image (3x5 cm) of Cu mobilisation in the surface layers of the sediment Vertical Distance (mm) Vertical Distance (mm) Horizontal Distance (mm) Cu Horizontal Distance (mm)

37 Laser ablation measurements required to build the image Vertical distance Distance (mm) (mm) Horizontal distance Distance (mm) (mm)

38 Oxygen 18:39

39 Cohesive and non-cohesive sediments Pore water concentrations of contaminants measured with DET are similar in silt (contaminant) and sandy sediments (pristine) The bulk concentration of contaminants are 1 fold higher in silt than in sand The contaminants remobilize much faster in sandy sediments than in silty sediments

40 Importance of fauna Fauna helps mixing the sediments. Silt deposited on the surface of a sand is rapidly mixed into the sediment (days) Fauna oxidize the sediments and draw the sulphide concentration down The effect of fauna is more pronounced in sand than in silt

41 Availability of organic matter In sandy sediments addition of organic matter leads to a pronounced formation of sulphide the sulphide traps the contaminants Due to the low degree of mixing organic matter added to a silty sediment reduces the iron oxides in the top layer and thereby some contaminants (Co, Ni) are released

42 Sediment treatment Any treatment of the sediment increase the release of contaminants (for a period of time) The contaminant distribution in Silty sediments is controlled by diagenesis Sandy sediments are much more heterogeneous and probably controlled by transport Any resuspension lead to loss of metals

43 Conclusions I 2D mapping showed patchy structure for trace metal fluxes; The same general features emerge irrespective of manipulation; Metals can be remobilized in very tight vertical zones; Steep gradients of Fe, Mn,, Co and Ni within 1cm of the interface, yet no release; Substantial fluxes of release for Cu and Cd because they are mobilised so close to interface; Scale is important for understanding processes; Chemistry of metals in sediments is very dynamic.

44 Conclusions II Pore water and bulk concentrations of contaminants are not related Regulation based on pore water concentrations is under protective Regulation based on bulk concentrations is over protective The effect organic matter and fauna depend on the grain size of the sediments (site specific) The influence of heavy metal on bacterial diversity is overshadowed by the organic matter

45 Thanks for your attention! Ole Larsen DHI Wasser & Umwelt und MPI für marine Mikrobiologie

46 Major contributions from: Delft University of Technology the Netherlands Ann-Charlotte Toes Maria Daleke Jeanine Geelhoed Gerard Muyzer Marine Biological Laboratory Helsingör Copenhagen University Denmark Henrik Staal Anders Tengberg Ronnie Glud Lancaster University, UK Max-Planck Institute for Marine Microbiology Bremen, Germany Sophie Tankere-Muller Kent Warken Hao Zhang Bill Davison Niko Finke Kyriakos Vamvakopulos Elimar Precht Ole Larsen TREAD was financed by the European Commission EVK-CT-22-81