Almost Everywhere: Naturally Occurring Arsenic in Wisconsin’s Aquifers

1. Almost Everywhere: Naturally Occurring Arsenic in Wisconsin’s Aquifers Madeline Gotkowitz Wisconsin Geological and Natural History Survey

2. Arsenic concentrations in Wisconsin % samples > 5 ppb > 10 % 2 to 10% <2 % 1 sample > 5ppb insufficient samples

3. Study area: eastern Wisconsin > 10 ppb contour

4. <2 ppm As 4 ppm As 133 ppm As

5. Eastern Wisconsin Stratigraphy

6. Quarry on Leonard Pt Road dolomite Sulfide-cement horizon sandstone

8. Pre-development Pumped system confined

9. Pumped system Managed system

10. Mandatory well casing depth based on regional hydrogeology

11. Geochemistry indicates sulfide oxidation at high-arsenic wells, FeOH reduction at others Fe SO4 A = < 2 mg/L B = 2-10 mg/L C = 10-100 mg/L D = >100 mg/L pH High Fe High SO4 Low pH Schreiber et al. 2003

12. Without pumping, the well becomes strongly reducing ORP DO Gotkowitz et al. 2004

13. Arsenic, iron and sulfate within a well, non-pumping conditions

14. Effect of pumping rate and volume on redox 200 gallons every hour 100 gallons every 8 hours Gotkowitz et al. 2004

15. Residences transferred to public water supply

16. In situ disinfection

17. In situ disinfection treatments Gotkowitz et al. 2008

18. Arsenic cycling following sulfide exposure to DO, chlorine As-bearing Pyrite Cl2 Cl2 Primary reservoir O2 (minor) O2 (minor) Fe(II) SO4 As species Release from primary reservoir Fe(III) As(V) HFOs w/adsorbed As Secondary reservoir Release from secondary reservoir Desorption of As (pH) or reductive dissolution of HFOs (microbially mediated) West et al. In Review

19. Arsenic in bedrock aquifers • High concentrations of solid-phase arsenic are associated with sulfide minerals • Arsenic mobilized under oxiding conditions but can become sequestered on iron-oxide minerals • Iron-oxide minerals become a secondary source of arsenic to groundwater under reducing conditions • Complex cycling of arsenic is affected by the water table, pumping rates and volumes, and well disinfection

20. Southeastern Wisconsin Arsenic > 10 µg/l in 10% of wells Arsenic > 10 µg/l in 20% of wells

21. Discontinuous sand and gravel lenses provide groundwater; these may be overlain by low-conductivity glacial tills aquifer Low As Moderate As High As Root et al. 2009

22. Organic carbon triggers arsenic-iron-oxide dissolution Low As Moderate As High As Arsenic, mg/kg Organic matter, % Root et al. 2009

23. Arsenic in glacial deposits • Low concentrations of solid-phase arsenic is associated with iron and manganese oxide minerals , within all stratigraphic units • Solubility of the solid-phase arsenic results from reducing conditions along deep groundwater flow paths; driven in part by organic carbon leading to reductive dissolution of Fe- and Mn- oxides

24. Implications of arsenic cycling for managing exposure to arsenic… • Groundwater extraction affects arsenic mobilization • Alters flowpaths • Alters redox conditions • Rate and frequency of pumping impacts biogeochemistry • Strategies must control redox conditions • Well construction: prevent introduction of oxygen, controls flowpath • Extend community water supplies (avoids well disinfection, provides routine testing and /or treatment)

25. Q&A