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Arsenic in Minnesota Groundwater

Arsenic in Minnesota Groundwater. Mindy Erickson, P.E. Water Resources Science University of Minnesota Funding provided by CURA, MDH, WRC and USGS. Why Arsenic? Why Now?. In 2001, MCL changed from 50 u g/L to 10 ug/L Public Water Suppliers (PWS) must comply by January 1, 2006

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Arsenic in Minnesota Groundwater

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  1. Arsenic in Minnesota Groundwater Mindy Erickson, P.E. Water Resources Science University of Minnesota Funding provided by CURA, MDH, WRC and USGS

  2. Why Arsenic? Why Now? • In 2001, MCL changed from 50 ug/L to 10 ug/L • Public Water Suppliers (PWS) must comply by January 1, 2006 • About 40 MN PWS affected • Treatment plants for arsenic are expensive

  3. Exposure Living with a smoker Arsenic at 50 ug/L Contaminant (EPA’s standard threshold) Deaths / 1,000 10 20 0.1 Why the Change?Arsenic and Health Arsenic is classified by EPA as a known human carcinogen. It can cause skin, lung, prostate, and bladder cancers; skin lesions; nerve damage; and other health effects.

  4. Arsenic Occurrence • Arsenic naturally present 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 groundwater • Arsenic in mid-western groundwater is widespread and naturally occurring

  5. Arsenic Chemistry • 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

  6. Arsenic Release Mechanisms • Reductive Desorption • Reductive Dissolution • Anion Competition • Mineral Oxidation (often pyrite)

  7. Project Goals • Characterize arsenic variability • Spatial • Temporal • Investigate geochemical connections and mechanisms • Provide information for developing regulations and guidelines

  8. Research Tasks • Database building • PWS Sampling • Sediment Sampling • Private Well Sampling • Statistical Analysis • Geochemical/hydrological modeling

  9. Database Building • Quaternary geology information • Water quality data • Sediment geochemical data • Well construction information

  10. Minnesota Arsenic Data • National Uranium Resource Evaluation • Public Water Supplies • Centers for Disease Control • MN Pollution Control Agency GWMAPS • Minnesota Arsenic Study (MARS) • Minnesota Geological Survey • Other

  11. Minnesota Arsenic and Geology

  12. Recent Glacial History

  13. Regional Arsenic and Geology

  14. Arsenic and the Des Moines Lobe Till • 10% of PWS within the Des Moines lobe footprint exceed 10 ug/L • 2% of PWS outside of Des Moines lobe footprint exceed 10 ug/L • 3,200 measurements

  15. Arsenic in Sediment • Sediment arsenic concentrations vary from <1 to 50 mg/Kg in the region • Average sediment arsenic concentrations are similar across the region • No direct link between sediment arsenic concentration and water arsenic concentration • Des Moines lobe till is not the source of arsenic in groundwater

  16. As and Well Characteristics Till Sand

  17. As and Well Characteristics

  18. Research Tasks • Database building • PWS Sampling • Sediment Sampling • Private Well Sampling • Statistical Analysis • Geochemical/hydrological modeling

  19. PWS Sampling Arsenic Variability • Local Private well sampling • Temporal sampling of city well(s) • Filtered/Unfiltered pairs (selected sites) • Water analyzed for metals, arsenic species, major anions, TOC, ammonium

  20. Participating Communities

  21. Nielsville SI Wells

  22. Nielsville SI Wells

  23. Long-Term Temporal Results

  24. Long-Term Temporal Results

  25. Short-Term Temporal Results

  26. PWS Results • No difference between raw water filtered and unfiltered arsenic concentrations • Significant difference between chlorinated filtered/unfiltered arsenic concentrations • No long-term trends • Some significant short-term variability • Low arsenic private wells identified near some high arsenic PWS wells

  27. PWS Results Implications • Most water arsenic is dissolved • Four quarterly samples from an old well is likely representative • Short-term temporal sampling is worthwhile for wells with As close to the MCL • Site investigation is worthwhile • Low cost • Quick implementation • Significant potential cost savings

  28. Research Tasks • Database building • PWS Sampling • Sediment Sampling • Private Well Sampling • Statistical Analysis • Geochemical/hydrological modeling

  29. Sediment Sampling • Sediment and water samples from 8 new private wells, mud-rotary drilling • Sediment and water samples from 2 new monitoring wells, rotasonic drilling • Sediment analyzed for metals, organic carbon, carbonate • Selected sediment samples sequentially extracted • Water analyzed for metals, major anions, arsenic species, ammonium, and TOC

  30. Mud-Rotary Water Results

  31. Mud-Rotary Results • No direct correlation between sediment arsenic concentration and water arsenic concentration • Arsenic concentration in till averaged 6 mg/Kg, Fe 2% • Arsenic in aquifer sediment averaged 1.5 mg/Kg, Fe 0.8% • High arsenic concentration increases over time in new wells

  32. Monitoring Wells

  33. Monitoring Wells

  34. Monitoring Wells

  35. Preliminary Rotasonic Results • Organic layer observed between the till and sand • 0.5 – 0.7 mg/Kg arsenic is adsorbed to the sediment grains • Broad sediment and water geochemistry yet to come…

  36. Research Tasks • Database building • PWS Sampling • Sediment Sampling • Private Well Sampling • Statistical Analysis • Geochemical/hydrological modeling

  37. Private Well Sampling • 60 private wells selected based on well characteristics and arsenic concentration • Water analyzed for metals, major anions, arsenic species, ammonium, and TOC • Filtered/unfiltered pairs at 20 wells

  38. Private Well Sampling Results

  39. Private Well As Speciation Results

  40. Preliminary Private Well Sampling Results • Arsenic concentration is not directly correlated to TOC or ammonium concentration • Most arsenic is As3+ • Most arsenic is dissolved, not particulate • Well water with a ‘rotten egg’ smell has relatively low arsenic: about 10 ug/L

  41. Research Tasks • Database building • PWS Sampling • Sediment Sampling • Private Well Sampling • Statistical Analysis • Geochemical/hydrological modeling

  42. Outcomes • Quantify arsenic phases • Support or refute preliminary observation of increasing arsenic concentration in new wells • Narrow potential geochemical mechanisms and geological/hydrogeological controls • Characterize arsenic variability • Provide MDH with info for developing guidelines and regulations

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