heavy metals in soil and plants martha rosemeyer ies january 27, 2003
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Heavy Metals in Soil and Plants
Martha Rosemeyer
IES
January 27, 2003
Major pointsHeavy metals are forever! We are concentrating them and spreading them in
the environment increasing human and animal contact
There are methods to manage them as long as there is vigilance for the next millenia
There is some remediationSystems thinking is critical to solving
environmental problems
Outline
What are heavy metals? How are they bound to the soilFood chain
Plant uptake of heavy metals Animal uptake
Reducing heavy metal contamination
What is a heavy metal?
Loose definition- specific density > 4-7 g/cm3
Usually associated with toxicity in plants (but some micronutrients produce toxicity symptoms as well) or animals
“trace metal” metals in ppm concentrations in earth’s crust
Periodic table of the elements
Heavy metals in the environmentMost in deposits in earth where safeGenerally low levels in soilBy mining and concentrating metals they have
become more toxic to the biosphereHave spread where humans come into contact with
them“The toxicity of inorganic contaminants released into the
environment every year is now estimated to exceed that from radioactive and organic sources combined. A fair share goes to contaminating soil. ”--Brady and Weil 1999, p740
Contamination vs. pollution
“Contamination” is above the background“Pollution” means concentration above some
level which is deemed safePollution levels are not agreed upon and depend
on who (child or adult), where (soil, water, air), over what time (8 hrs or chronic), workplace vs. public
Variability in action levels, recommended exposure limits
Some heavy metals and their environmental and physiological effects
Brady and Weil, 1999
* *
**
Canadian HM in soil stds also consider others Co, Mo, not Cr!
Reduction of HM in sewage sludge 1976 to 1990
Brady and Weil, 1999
Limits in sewage sludge and metal additions for WA
Brady and Weil, 1999 and Labno,2001
.089 Wa
2.2Wa
0.33 Wa
.022 Wa
Brady and Weil, 1999
Forms of heavy metals in soils
Brady and Weil, 1999
As- sources and background levelsOccurs naturally in soil and water (may be toxic in water, e.g.
Bangladesh and India)High As in upper end of Cascade Valleys in WA, may be high in
volcanic soils and hot springsPresent in coal burning and dusts from cement manufactureSmelter- within one mileSprayed in WA state as insecticide on apples for codling moth
until 1950, forest thinning to 1960sMay be near Chromated Copper Aresenate treated timber- get
$25 test kit from EWG if can’t get tested through classCauses cancer
As- regulationPermissible level in water (Bush changed to WHO levels of .01
ppm)- 0.01 ppmPermissible level in soil in out-of-print WSU extension bulletin
states: 25 mg/kg (ppm) is ‘probably not affecting plant growth’Residential soil cleanup - 250 mg/kgChildren should be < 37 ppm, adults with occasional exposure
to 175 mg/kg acceptableChronic exposure is of concern, e.g in gardeningSymptoms in humans depend on individual susceptibility, form
of As in soil, difficult to predict
As behavior in soil and plantsBackground level in soil
~6 ppm U.S. agricultural soils, but 7 Washington state Vashon-Maury Soil samples 2.3 - 460ppm (<2mm sieved)
More soluable and mobile in soil than Pb, so may have leached, increases in flooded, wet soils
Redistributed through tillage, but usually only in subsoil if soil is sandy
If high phosphate in soil may displace As to leachAs in soil can be 10-1000x higher in soil than plantCan be high enough to stunt plants and reduce yield-- binds to
energy exchange apparatus
Lead (Pb)
Sources of lead in soil include: former roadways <100ft., PbAs pesticide, smelter, within 20 ft of buildings, < 1 mile for smelter or fossil suel electrical power plants or cement manufacturing
Background level in soil 11 U.S. agricultural soils 17 Washington state, Vashon-Maury Island 5.3-1300ppm
Lead in soil usually not high enough to affect the plant growth because highly bound to the soil unless pH is low (acid)
Lead- regulated levels- can be contradictoryOSHA
blood (adults)- 40-50 g/dL air- 30-50 g/m3 over 8 hours
CDC blood (children)- 10 g/dL
EPA soil- 400 mg/kg, sewage sludge accumulated- 300 kg/ha water (drinking)- 15 g/L air- 1.5 g/m3 ambient
Cadmium
Background level 0.2 ppm U.S. agricultural soils 1 ppm Washington state 0-15ppm Vashon-Maury soil samples
Cadmium in wheat grain related to soil salinity, esp Chlorides, uptake as CdCl
Cadmium- health effects and regsHealth effects: carcinogen (respiratory and
testicular, pancreatic cancer), reproductive toxin: reduced birthrate, premature
birth, stillbirth and spontaneous abortionBehavioral and learning disabilitiesRegulation: air 0.05 g/day (1/10th that of Pb)Soil maximum: 4 ppm Cd (Wa and Canada,
FRA 1997)
Cd- Sources of exposureFood- major source of non-occupational exposure, esp.
wheat and potatoesIncineration- 71% Pb and 88% Cd due to plastics (vinyl
and other) in waste streamCd in fertilizer and food is regulated much more strictly
by Canadians and EuropeansNow same as Canada 4 kg/ha max acceptable cumulative
addition, 0.089 kg/ha max annual addition (Fert. Reg Act 1997)
Canada and Aust. have fertilizer “truth in labeling”
Cd has increased in soils due to P fertilizer use
In Columbia basin and around the world where high Cd P is applied-- even where low Cd P is applied
10% in exchangeable pool in Canadian prairie vs. 1% in Brady and Weil!
Concentrated on clays and organic matter
Phosphate fertilizers as source of Cd, Pb, As: Western states to 340 mg/kg Cd
J.R. Simplot’s phosphate mine near ID/WY border
International Pb and Cd limits in foods-- no established US limits
Labno 2001
Heavy metals in the food chain
Brady and Weil, 1999
Message
Exact relationship between soil and plant depends on soil type, climate, management, chemical form, plant species and variety
It is complicated and data is lackingOther countries have been able to regulate despite
this-- why not US?
Uptake of HM by corn from sewage sludge
Brady and Weil, 1999
*
* not true for wheat
Species dependent where it accumulates
Greater accumulation of Pb and Cd in stover than wheat grain
Corn from previous table concerning corn grown with sewage sludge grain greater than stover
Cd and Pb uptake by wheat and potato WSU studies, Labno and Kuo 2001 International Cd stds: grain 0.1 mg Cd/kg, tuber 0.05
mg Cd/kg (US mean 0.03 (.06 p 20)) International stds for Pb: grain 0.35 mg Pb/kg, tuber
1.5 mg Pb/kgRates of application 1x, 2x, 8x for 2 yearsUsed 2 sources of DAP, TSP and RP, one Zn fertilizerUsed a low Cd waste-derived Zn fert. (application
0.1kg/ha/yr), Pb (5.6 kg/ha/yr at highest level), but results not reported for Cd
At the yr 1, 1x rate all grain and tubers except ID TSP were below international stds for Cd, 8x rate above for ID DAP and ID TSP (150ppm Cd)
At 1x and 8x application rate the levels in the grain and tuber were under Pb stds
Later study looked at Ironite and flue dust (676 mg Cd/kg, 180000 mg Pb/kg) but plant uptake was not performed. Prev study on uptake was performed with materials that were lower in Cd and Pb
Cd from TSP in grainNote in yr 2 all levels above stds
Labno 2001
Cd in tuber yr 2 with TSP
Labno, 2001
Close relationship between soil and plant levels of Cd (DTPA extractant)
Where applied Cd goes
Labno 2001
Message to legislators“It was estimated that 1% of total soil Cd is in the
wheat, indicating that most remains in the soil” Transfer coefficient = .01
Transfer coefficient (Ratio of uptake over applied) was 0.005 for Pb indicating a low potential for plant uptake.
Transfer coefficient for As was small 0.012 indicating a low potential for plant uptake.
Report from Dept Ag based on Kuo’s student’s work, Dec 2001
Plant uptake
Element Crop Uptake
As Root crops Roots
Cd Leafy veges Roots, tuberGrains, tuber leaves
Lead Fruits, grains Surface or in tuber
Arsenic and human healthFood and water major source of exposure for
US citizens (Yorktimes)High concentrations: Internal bleeding and
deathKnown to cause cancer: lung, skin, liver,
kidney; Reproductive damageCauses arsenic keratosis
of skin
Arsenic: Source
Has been used for centuries (China 900 AD)Many different forms of arsenate (200),e.g
CaAsForm influences mobility and toxicityMined with other minerals esp. Au, Cu, Sn and
mined, from mine waste or tailingsNatural or mine waste in waterFound in hydrothemal deposits
Arsenic in drinking water in US
Arsenic: Soil/plant relationshipExact relationship between soil and plant depends on soil
type, climate, management, chemical form, plant species and variety
Plant levels tends to increase until some level where plateausSignificant on alkaline soils where have >10000 ppm
(Bowell and Parshley)As stunts or kills plants by acting as P and binding with
energy transport mechanisms, green beans and legumes most sensitive --
--Peryea, 1999
Concentration of Pb and As in plantsRoots > leaves> fruits and seedsRoot skin is higher than inner flesh--Roots absorb but do not transport PbApples and apricots contain low Pb and AsHaven’t found any regulations on As in foodOrganic As may be less toxic than inorganic compounds
of As; Organic As may be predominant in fruits and vegetables, although inorganic As more common in grain
HM in earthworms after application of sewage sludge
Brady and Weil, 1999
Cd uptake in snailsNew evidence from France, Renaud
Scheifler of University of Franche-ComteSnails took up 12% of Cd from supposedly
bound fraction of smelter soil with high Pb and As
Animal uptake of soil-- not via plant!Up to 30% of diet is soil for sheep, goatsUp to 18% for cattleDepends on management how much the animals
get soilDirect ingestion
of soil particles
may increase
uptake of HM
The browse line
Above the browse line!
How can we manage Pb and As contaminated soil soils?Add organic matter (test to make sure low in
Pb and As)Keep pH high with lime (check to make sure
not contaminated with Pb and As or others)Add phosphate to bind with lead (TSP
lowest), but may increase plant uptake of As. Rock P may have Cd.
Biological remediation
Organic matter binds heavy metals (make sure not contaminated) --the case of Cr
Brady and Weil, 1999
Add lime (make sure source not contain heavy metals)
Brady and Weil, 1999
Plant “hyperaccumulation”
Hyperaccumulator
Plant tissue concentrations of 4% Zn can be used as ore-- Thalpsi
Brake fern (Pteris vittata) can accumulate AsPb- add chelators, solubilize lead and plant can
take it upGenetic engineering
moving genes into canola (rapeseed) and Indian mustards to accumulate heavy metals!!
Is this systems thinking?
Bioremediation by fungi
Fungi can accumulate from mine tailings and contaminated soil, but then what!?
References
Peryea, F. 1999. Gardening on Lead- and Arsenic- Contaminated soils. WSU Ext. Pub # EB 1884
Lab tips
Use non-metal soil samplers (WSU)May want to sample to 15 cm in ag soils
(WSU)Sieve soil less that 2 mm (Vashon-Maury)Dry plant material at 45-60°C for at least 24
hrs (WSU)
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