The source of trace elements in groundwater in sandy aquifers

1. The source of trace elements in groundwater in sandy aquifers Marc J.M. Vissers Faculty of geosciences

2. Why trace elements in groundwater • Geochemistry • Redistribution of trace elements (ore and natural anomalies) • Global biogeochemical cycle • Environmental science • Atmospheric pollution / acidification • Agricultural pollution / acidification • Consumption (direct and indirect)

3. This talk: Environmental geochemistry • Study area and processes • Present a 3-step approach for interpretation: • 1: Equilibrium modeling approach • 2: Coprecipitation- codissolution approach • 3: New: Steady-state input approach

4. Sandy, unconsolidated aquifer, with ice-pushed ridge in the east Mainly Agricultural land use, eastern part cultivated in the 1920’s 10 Borings, total of 244 mini screens Study area and processesMap of the study area

5. Study area and processesCross-section of the study area • Filtrated over 0.45μm, analyzed on ICP-MS • Sampled in 1989 (no trace elements), 1996 (½), and 2002 (all) • Randomly analyzed on > 70 (mostly inorganic) parameters

6. 70 elements for 10 wells x 25 screens 1: Equilibrium modeling approach 2: Codissolution-coprecipitation approach 3: New: Steady-state input approach

7. 1: Equilibrium modeling Theory and Assumptions • Using CHEAQS and WATEQP • Al3+(aq) + 3OH-(aq)  AlOH3(s) Solid phase • Al3+(aq) + F-(aq)  AlF2-(aq) Speciation Equilibrium modeling assumes • chemical equilibrium (also redox and pH) • pure phases • transport in dissolved phase only

8. 1: Equilibrium modelingResults Pure phase saturation explains: • Sulfate: Barium (barite) • Carbonate: Calcium and apparently iron and manganese in reduced zone • Hydroxides: Aluminum, manganese in acid zone • Iron / Calcium / pH: Phosphorous (vivianite and apatite) • Phosphates: REY in acid water • Pure phase: Uranium (uraninite) in reduced water • Depending on local conditions!

9. 1: Equilibrium modeling Summary Not many elements are controlled by saturation, so one may conclude: Source-term limitation Source-term limitation may be sedimentary and / or input-determined.

10. 2: Coprecipitation-codissolutionAssumptions and theory Codissolution: Ca(1-x)SrxCO3(1-x)Ca2+ + xSr2+ + CO32- • Congruent, and main source • Where x is the fraction of a TRACE ELEMENT in a MAJOR ELEMENT PHASE • Can (and should be) verified using mineral data Coprecipitation: When saturation of a major element phase is reached through increasing concentrations or changing redox or pH conditions, the “opposite” reaction may occur

11. 2: Coprecipitation-codissolution Bulk sediment geochemistry

12. Significant aluminosilicate weathering 2: CodissolutionExample 1

13. 2: Codissolution / Coprecipitation Example 2 • Al-Be and Al-Ga (also Al-REE): is observed codissolution real dissolution? water Dutch soil

14. 2: CoprecipitationExample Different source, but relation

15. 2: Coprecipitation-codissolutionResults Codissolution: • Ca – Sr (carbonates and feldspar, and clay) • K – Rb (from clay mineral as identified from observed ratios) • Fe – As (iron (oxy-) hydroxides) • Mn – Mo (manganese hydroxides?) • Clay (Ca-Mg-Sr) – Cd-Tl (maybe Pb) • Al – Ga / Be / REY • Zr – Hf Coprecipitation: • Fe – Mn • Al – REY / Be? • Fe/S – As

16. 3: A novel approach • But what about the ‘normal’ background (e.g. Cu, Pb, Li, etc) and unexplained anomalies (e.g. Zn, Co).  INPUT SOURCE LIMITATION

17. 3: Steady-state input approachAssumptions • Atmospheric deposition has been relatively constant in the Holocene, and the sediments have become “saturated” with these TE • Concentrations should be constant with depth • Differences in evaporative concentration  ratio TE/CE should be constant with depth X-Na+ + Me+(aq)  X-Me+ + Na+(aq) seemingly conservative behavior! • The start of the “Anthropocene” has caused changes! • Geochemical processes cause changes!

18. Sorption, depending on pH Evaporative concentration 3: Steady-state input approachResults: Absolute concentrations match + Evap. Rain Salland Rain Sweden Seawater

19. 3: Steady-state input approachResults: Absolute concentrations match + Evap.

20. Boron

21. Age 3: Steady-state input approachLithium normalizing on Sodium (Na) 2 log units 2 log units

22. 3: Steady-state input approachLithium, Cobalt, Nickel, Rubidium, and Copper

23. Vissers, M.J.M., 2005, Patterns of groundwater quality, NGS335

24. Conclusions • The steady-state input approach significantly increases the understanding of trace element behavior in the subsurface • Anomalies can be identified • Anomalously high weathering releasing Be, Cd, Tl, Ga, Co, Ni • Kinetic incongruent “dissolution”, releasing Li, Rb, Cs • Mobilization in specific redox environments, Zn, Co, Ni • Diffuse atmospheric / agricultural pollution • The true baseline concentrations can be predicted! m.vissers@geo.uu.nl

25. Conclusions • For many elements rain is the main source. • Apart from breakthrough of K and Rb, also Cu, Pb and many other elements are observed to be anthropogenically enriched in groundwater • “Groundwater enrichment factors” of many trace elements vary from 1 (Lithium) to more than 100 (Co, Ni, Zn) m.vissers@geo.uu.nl

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