Salty Dust: Increasing the accessibility and mobility of toxic metals
Dr. James King
Acknowledgements
Richard Reynolds, Harland Goldstein, Jim Yount, George Breit, Suzette Morman
George Nikolich, Jack Gillies, Vic Etyemezian
Why salty dust?Aral Sea dust stormApril 18 2003
NASA MODISPhoto by W. Cox, GBUAPCD
• Evaporite-mineral dust contain elevated As, Cr, Cu, Ni, Pb, Th, U, Se
• No current limits on inhalation of toxins
• Bioaccessibility of toxic metals• Spatial variability of metal
content in dust vs. groundwater chemistry
• Influences from climatic variability
Dust from Owens (dry) Lake
Types of Playas
WET
DRY
(Stone, 1956; Neal, 1965; Rosen, 1994)
Ground water is at or near the surface (< 4 m)
Ground water is far below the surface (> 4 m) or cannot interact with surface
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
Playa Surface Characteristics
• Relatively stable with time
• Typically very hard
• Variable & Dynamic• Soft – in areas of
fluffy & puffy sediment
• Hard – in areas of crust
Wet playa
Dry playaHard, compact surfaces
Playa Sediment TypesWet 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 carbonateDry Playa• Typically compact clastic sediment
(commonly mud cracked)
• Evaporite minerals deposited originally in lake beds
Dust Emission from Playas
Wet playa
Dry playa
Conditions may promote dust emission. Efflorescent salts in near-surface sediments produce mineral fluff & soft surfaces
Low levels of dust emission when sediment supply is limited and surface is undisturbed
Hard, compact surfaces
Franklin Playa April 2005
Wet playa
Dry playa
Field study & Monitoring siteFranklin Lake Playa, USA
Mojave Desert
Franklin Lake
Amargosa River
Carson Slough
Ash Meadows
Quickbird satellite images
0.6-m resolutionApril 2006Czarnecki, J.B., 1997. USGS Water Supply Paper, 2377.
Ash Meadows Carson
Slough
Ash Meadows: 0.7
1.5
16
90
Specific Conductivity
(mS cm-1)
Spring Discharge: a60,000 m3
day-1
Evaporation:b22,800 m3
day-1
Precipitation: 100mm yr -1 Pan Evap. 2500 mm yr -1
aDudley & Larson, 1976; bCzarnecki & Stannard, 1997)
Groundwater Ion Content Trends
Franklin Playa
Carson Slough
Ash Meado
ws
Groundwater Metal Trends 85
180
190
83
93
As (ppm
)
As (ppm) predicted in anhydrous salts (Cl) by mass
balance from evaporation
Trace Metal and Ion Content with Depth
Sulfate (wt. %)
0 10 20 30
Chloride (wt. %)
0 10 20 30
Dep
th (c
m)
0
30
60
90
120
150
180
Arsenic (ppm)
0 200 400 600
Dep
th (c
m)
0
30
60
90
120
150
180
Uranium (ppm)
0 10 20 30
As U Cl SO4
Franklin Playa Auger Sediments
Evaporation Front:
Surface
Evaporation front
Water table Groundwater
vapor generated
Evaporation
Metals move with water
Water vapor rises with few metals
Metals accumulate in residual water
Chloride concentrated
Sulfates precipitated, few metals
Thick evaporation
zone
evaporation zone
Thin evaporation
zone
Evap. frontWater tableGroundwater
Evaporation
Metals move with waterSulfates, chlorides precipitated with metals
Surface and Dust sediment collection
Bulk dust collection
Dust
Wind-tunnel Tests
Assess the potential vulnerability of surfaces to wind erosion
Simulated winds to ~ 20 m/s to measure PM10 dust flux
Salt Crust Arsenic Spatial Trends
sulfate / chloride (wt/wt)
0 20 40 60 80 100
Arsenic (ppm)
0 100 200 300 400
AsSO4 : Cl
Ratio in ground water
Mobility of Sulfates
ground water
crust
dust
ground water
dust
dust
dust
ground water
ground water
crust
crust
crust
Fractionation increases sulfate in crust and dustSulfates are mobile
0 1 2
SO4 & Cl increase in groundwater
sulfate / chloride (wt. ratio)
0 20 40 60 80 100 120 140
Ash Meadows
Clay Dunes
East Transect
Discovery
Bioaccessibility of Toxic MetalsExtraction pH Temp (C) Time Mixing control methodGastric 1.5 37 I hr Shaker in
Enviro ChamberIntestinal 5.5 37 I
hr Shaker inEnviro Chamber 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.
Micrograms leached / gram solid
0 2 4 6 8 10
Ash Meads
Carson Slough
Clay dunes
Coppice
Discovery Disturb
Discovery Undist
Uranium
Micrograms leached / gram solid
0 50 100 150 200 250
Ash Meads
Carson Slough
Clay dunes
Coppice
Discovery Disturb
Discovery Undist
Arsenic
IntestinalGastricLung
North
South
Extractions from dust in simulated biofluids
Extractions from dust in simulated biofluids
Micrograms leached / gram solid
0 50 100 150 200 250
Ash Meads
Carson Slough
Clay dunes
Coppice
Discovery Disturb
Discovery Undist
Intestinal
GastricLung
85
180
190
83
93
As (ppm
)
As (ppm) predicted from Cl
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
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
PI-SWERL Portable In-Situ Wind ERosion Laboratory
m/s mph1000 4.24 9.48 0.2312000 7.17 16.03 0.3953000 10.13 22.66 0.5414000 12.71 28.44 0.6775000 14.92 33.38 0.805
Windspeed Shear Velocity RPM
0.0001 0.01 1 1000.0001
0.001
0.01
0.1
1
10
100Series1
Non Gravel Surfaces
Gravel Sur-faces
Saltation Above 100 mg/ms
All Data: R=0.76, b=1.0, a=0.02
PI-SWERL PM10 Emissions (mg.m-2s-1)
Win
d Tu
nnel
PM
10
Emiss
ions
(mg.
m-2
s-1)
.