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)

.