Arsenic in Minnesota Groundwater: Occurrence and Geochemical Mobilization Mechanisms

1. Arsenic in Minnesota Groundwater: Occurrence and Geochemical Mobilization Mechanisms Mindy L. Erickson, Ph.D, P.E. US Geological Survey November 9, 2011 Funding provided by U of MN (CURA and WRC), MDH, and USGS

2. Why Arsenic? Why Now? • At 50 ug/l in drinking water, lifetime cancer risk is about 2 in 100 (similar to second-hand smoke) • MCL change to 10 ug/l affected bout 250 upper Midwest Public Water Systems • Treatment plants for arsenic are expensive • Beginning in 2008, all new MN wells tested for As

3. Goals • Characterize arsenic occurrence • Characterize arsenic temporal variability • Test potential geochemical mechanisms and geological controls • Provide interested parties with results

4. Completed Work • Database compilation • Sediment • Geochemistry: metals, organic carbon • Sequential extraction • Scanning electron microscopy • Ground water • Geochemistry: trace elements, arsenic species organic carbon, ammonium • Field parameters: pH, ORP, conductivity, DO, temp • Literature review • Geochemical modeling, statistical analysis

5. Arsenic Occurrence • Arsenic in rock and sediment at 1 to 100s mg/kg • Crustal average is 1.8 mg/kg • At 1.8 mg/kg, solubilization of <0.1% yields 10 ug/L arsenic in water • Certain geochemical conditions leach arsenic into ground water • Previous study proposed link between Des Moines lobe till and elevated arsenic

7. Database Compilation • State drinking water agencies/departments • USGS • State geological surveys

9. Understanding the Geology Northwest provenance late Wisconsin-aged till • Large fraction of fine-grained material • Entrained organics • Active anaerobic biological activity • Reduced conditions

10. Understanding the Geochemistry • Arsenate (As+5 H2AsO4-, HAsO4-2) • Oxidized form • Adsorbs to metal oxides • Arsenite (As+3 H3AsO3) • Reduced form; more toxic inorganic form • Adsorbs to iron oxides • Organic Arsenic (many forms) • In foods; highest in seafood • Uncommon in groundwater

11. Understanding the Geochemistry • Arsenic Release Mechanisms • Reductive Desorption • Reductive Dissolution • Anion Competition • Mineral Oxidation (often pyrite)

12. Understanding the Geochemistry • Northwest provenance sediment has 2 to 26 mg/kg arsenic • Northeast provenance sediment has 1 to 17 mg/kg arsenic • In 9 domestic and monitoring wells in Minnesota, As in sediment is not correlated to As in water

13. Understanding the Geochemistry

14. Understanding the Geochemistry

15. Understanding the Geochemistry

16. Understanding the Geochemistry

17. Understanding the Geochemistry • At 1.8 mg/kg arsenic, solubilization of <0.1% yields 10 ug/L arsenic in water • Total sediment arsenic concentration less important than the availability of arsenic • Measured 0.4 - 0.8 mg/kg arsenic adsorbed to/ coprecipitated with metal oxides • Adsorbed/coprecipitated arsenic labile

21. Statistics Results

22. Summary • Inside the footprint: • Glacial wells are deeper • A higher proportion of wells exceed 10 ug/l As • Glacial and bedrock wells 30 – 90 m deep most affected • Arsenic weakly correlated to iron • Arsenic not correlated to competing anions • Arsenic in ground water is dissolved • Arsenic in ground water is predominantly As(III)

23. Summary • Elevated arsenic concentrations spatially correlated • Total northwest provenance sediment arsenic not particularly high compared with other regional sediment • Sediment and ground water arsenic not correlated • 0.4 – 0.8 mg/kg labile arsenic present in aquifer sediment • Aquifers are moderately reduced

24. Conclusions • Late Wisconsin-aged till causes the upper Midwest’s widespread area of elevated arsenic in ground water • Reductive dissolution • Reductive desorption • High-arsenic sediment is not necessary to cause arsenic-impacted ground water • Large but imperfect data sets allow inexpensive observation and characterization of regional environmental problems

25. Well Characteristics

26. Till Sand Well characteristics

27. Well Characteristics

28. Well Characteristics

29. Summary and Conclusions • PWS and domestic wells have distinctly different well construction characteristics • PWS well construction coincidentally yields lower arsenic • Coarser aquifers • Larger aquifers • Longer screens • Reductive arsenic mobilization mechanisms active at the till-aquifer interface • Changing routine domestic well drilling practices may yield fewer high-arsenic domestic wells

30. Goals and Results • Characterize Upper Midwest arsenic occurrence • Northwest provenance late Wisconsin-aged sediment • Bedrock and glacial wells 30-90 m deep • Test potential geochemical mechanisms and geological controls • Reductive mobilization mechanisms • Labile arsenic is present • As(III) predominates • Fe correlation, no competing anion correlation • Well characteristics

31. Next steps • Micro-to-macro scale look at arsenic occurrence in conjunction with other metals • Assess new well As results • Build and test As prediction model

32. Questions? Mindy L. Erickson US Geological Survey merickso@usgs.gov 763-783-3231