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)