Salty Dust: Increasing the accessibility and mobility of toxic metals

1. Salty Dust: Increasing the accessibility and mobility of toxic metals Dr. James King

2. Acknowledgements • Richard Reynolds, Harland Goldstein, • Jim Yount, George Breit, Suzette Morman • George Nikolich, Jack Gillies, Vic Etyemezian

3. Why salty dust? • Evaporite-mineral dust contain elevated As, Cr, Cu, Ni, Pb, Th, U, Se • No current limits on inhalation of toxins • Bioaccessibilityof toxic metals • Spatial variability of metal content in dust vs. groundwater chemistry • Influences from climatic variability Aral Sea dust storm April 18 2003 Dust from Owens (dry) Lake NASA MODIS Photo by W. Cox, GBUAPCD

4. Types of Playas WET Ground water is at or near the surface (< 4 m) Ground water is far below the surface (> 4 m) or cannot interact with surface DRY (Stone, 1956; Neal, 1965; Rosen, 1994)

5. Dust Emission Mechanics • Direct entrainment • Highly dependent on surface conditions • Generates relatively smaller amounts of dust • Sensitive to wind regime • F = Au*3 5 • Saltation Bombardment • Dependent on sand supply conditions • Fetch effects are important • q = Bu*3 • Surface Roughness • Vegetation, rocks, and crusts can modify the efficiency of dust emission mechanics

6. Playa Surface Characteristics Hard, compact surfaces Dry playa Wet playa • Relatively stable with time • Typically very hard • Variable & Dynamic • Soft – in areas of fluffy & puffy sediment • Hard – in areas of crust

7. Playa Sediment Types • Wet Playa • Fluffy sediment – very soft; abundant evaporite minerals produced continuously; high volume of pore space • Puffy sediment – soft, hummocky surface; fewer evaporite minerals. • Crusts – salts and carbonate • Dry Playa • Typically compact clastic sediment (commonly mud cracked) • Evaporite minerals deposited originally in lake beds

8. Dust Emission from Playas Hard, compact surfaces Dry playa Wet playa Low levels of dust emission when sediment supply is limited and surface is undisturbed Conditions may promote dust emission. Efflorescent salts in near-surface sediments produce mineral fluff & soft surfaces Wet playa Dry playa Franklin Playa April 2005

9. Field study & Monitoring siteFranklin Lake Playa, USA Amargosa River Ash Meadows Mojave Desert Carson Slough Franklin Lake

10. 0.7 Spring Discharge: a60,000 m3 day-1 Ash Meadows: Ash Meadows Carson Slough 1.5 Specific Conductivity (mS cm-1) Precipitation: 100mm yr -1 Pan Evap. 2500 mm yr -1 16 Evaporation: b22,800 m3 day-1 90 Quickbird satellite images 0.6-m resolution April 2006 aDudley & Larson, 1976; bCzarnecki & Stannard, 1997) Czarnecki, J.B., 1997. USGS Water Supply Paper, 2377.

11. Groundwater Ion Content Trends Ash Meadows Carson Slough Franklin Playa

12. Groundwater Metal Trends 85 As (ppm) 83 190 180 93 As (ppm) predicted in anhydrous salts (Cl) by mass balance from evaporation

13. Trace Metal and Ion Content with Depth As U Cl SO4 Franklin Playa Auger Sediments Evaporation Front:

14. Thick evaporation zone Thin evaporation zone Evaporation Evaporation Sulfates precipitated, few metals Sulfates, chlorides precipitated with metals Surface Evap. front Water vapor rises with few metals Metals move with water Water table Groundwater Evaporation front vapor generated evaporation zone Metals accumulate in residual water Chloride concentrated Metals move with water Water table Groundwater

15. Surface and Dust sediment collection Wind-tunnel Tests Simulated winds to ~ 20 m/s to measure PM10 dust flux Dust Assess the potential vulnerability of surfaces to wind erosion Bulk dust collection

16. Salt Crust Arsenic Spatial Trends SO4: Cl As Ratio in ground water

17. Mobility of Sulfates ground water crust dust ground water crust dust ground water SO4 & Cl increase in groundwater crust dust ground water crust dust Fractionation increases sulfate in crust and dust Sulfates are mobile

18. Bioaccessibility of Toxic Metals Extraction pH Temp (C) Time Mixing control method Gastric 1.537I hr Shaker in EnviroChamber Intestinal 5.5 37 I hr Shaker in EnviroChamber Lung 7.4 37 24 hr Incubator Physiologically based extractions in simulated biofluids to assess bioaccessibility of As, Cd, Cr, Pb, Mo, Sb, W Se, U, etc.

19. Extractions from dust in simulated biofluids North Arsenic Uranium South Lung Gastric Intestinal

20. Extractions from dust in simulated biofluids 85 As (ppm) 83 190 180 93 Lung Gastric As (ppm) predicted from Cl Intestinal

21. Summary on accessibility of toxic metals • Extractions from dust in simulated biofluids demonstrate that for both Ar and U, the potential for concentrations exceeding current ingestion limits could be reached • For these results there is no bias of the accessibility of Ar or U based on dust chemistry – this simplifies any prediction of other potential sources of toxic dust • Differences in the accessibility of Ar and U exists between the three tested biofluids, with the intestinal biofluid having the lowest ability to access the metals

22. Summary of mobility • Sulfate salts are the most mobile; easily precipitating from the groundwater and concentrating further when eroded • Toxic metals, in this case mainly As and U, are precipitated with the salts but mainly rely on the movement of chlorides to accumulate at the surface • The conceptual model proves that any history of a thin evaporation zone could lead to concentration of toxic metals near the surface if present in the groundwater and therefore groundwater chemistry alone is not a good predictor of the potential mobility and accessibility • Further work is currently under way to model wind erosion emissions based on local climate and surface conditions

24. PI-SWERL • Portable In-Situ Wind ERosion Laboratory