B A R E N C O

Preliminary Findings – Assessment of Soils and Crops in the Zacatecas Area

– Mexico – July 2002

Mercury Task Force Meeting and Public Workshop – Zacatecas, MexicoCommission for Environmental

Cooperation

Ron Pearson, M.Sc.BARENCO INC

Ontario, Canada

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Scope of Work To undertake a literature review and scoping investigation

related to the environmental impact of historical silver and gold mining operations in the Zacatecas area on agricultural activities as well as on ecological and human health.

As part of the scoping investigation, to undertake a preliminary sampling program involving the collection of a limited number of soil and vegetation samples for exploratory analysis of mercury and other environmental parameters.

Based on the results of the soil and vegetation sampling

program and on information gathered pertaining to the potential receptors and exposure pathways, to develop a more comprehensive sampling program for consideration by the CEC.

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Sampling Objectives

o to obtain soil and crop tissues from a field located as close to the edge of the lagoon as possible to represent a maximum potential for contamination in this initial area of study;

o to sample as many different species of crops as possible;o to collect samples of crop tissues that would represent both human and

animal consumption;o to ensure that when crop tissues were sampled, a corresponding sample of

soil from the 0-15 cm plough layer (rooting depth) was sampled;o to obtain soil and crop samples from the southeastern area of the lagoon

where water rises and falls depending on seasonal rainfall and eventually seeps into the neighbouring fields;

o to obtain surficial soil (0-5 cm depth) from an area in La Zacatecana where children would be exposed to the greatest extent;

o to obtain soil from an unpaved dirt road in the village of La Zacatecana which, during dry times of the year, would be a source of wind-blown dust exposure; and,

o to obtain a surficial soil sample from an area remote from La Zacatecana to gain insight on the natural background concentrations of chemical parameters selected for analysis.

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Site SelectionSite Number Site Description Soil Depth Sampled

(cm)Vegetation Type

1  Farm Field – West of La Zacacetana

0-15 cm Corn leaves

Bean seeds 

2 School Playground – La Zacacetana

0-5 cm No vegetation

3 Unpaved Dirt Road – La Zacacetana

0-5 cm No vegetation

4 Farm Field – NW edge of lagoon

0-15 cm Corn leaves

Corn kernels

5 Farm Field – SE edge of lagoon

0-15 cm No vegetation

6 Farm Field – SE edge of lagoon

0-15 cm Green Bean pods

7 Mountain parkland – N of Zacatecas

0-5 cm No vegetation

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Site 1

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Site 2

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Site 3

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Site 4

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Site 5

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Site 6

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Site 7

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Sampling of Soil and Vegetation

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Summary of Findings

Chemical

Ontario Criterion

Agr. & Res/Park

Coarse/Fine Texture(ug/g)

Site 1

Farm Fieldug/g

Site 2

Schoolug/g

Site 3

Roadug/g

Site 4

Farm Fieldug/g

Site 5

Farm Fieldug/g

Site 6

Farm Fieldug/g

Site 7

Park Area

ug/g

Hg 10 1.3 13 12 28 2.5 1.1 2.0

As 20/25 21 16 20 82 12 7.8 18

Cd 3/4 ND 1.5 0.97 8.8 0.34 ND 2.1

Pb 200 44 170 140 550 36 17 460

Zn 600/800 82 220 180 1000 48 25 440

E.C.(uS/cm)

700 230 670 1300 1200 190 110 250

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Chemical Parameters Analyzed

Soil pH, Hg, Methyl Hg, As, Sb, Ba, Be, Cd, Cr, Cr

VI, Co, Cu, Pb, Mn, Mo, Se, Ag, Tl, V, Zn E.C., CN, Nitrite, Nitrate, Sulphate, SAR, B, Fe TCLP Inorganics

Vegetation Hg, Methyl Hg, Al, Sb, As, Ba, Be, Bi, B, Cd, Cr,

Co, Cu, Fe, Pb, Mn, Mo, Ni, Se, ag, Sr, Tl, Sn, Ti, U, V, Zn, Sulphur

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Summary of Findings

Soil Soil exceeded ecological or human health risk-based criteria at

four of the seven sites; Chemical parameters found to exceed Ontario’s site

remediation criteria included:• Arsenic• Cadmium• Lead• Mercury• Zinc• Electrical Conductivity (EC)

Contamination was greatest at Site 4 (Corn field -NW side of lagoon);

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Summary of Findings (cont’d)

Soil Soil from farm fields SE of the lagoon did not exceed

any criteria;

Soil from the school playground and unpaved road exceeded only the mercury criterion which is based on vegetation protection;

Soil from the control site north of Zacatecas (Paseo de la Bufa) exceeded the soil criterion for lead.

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Summary of Findings

Soil – Hazardous Waste Classification

None of the TCLP leachate analyses (including Hg, As, Pb and Ag) exceeded Ontario’s drinking water-based criteria

Based on these findings, the soil from the sampled sites would be classified as non-hazardous

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Summary of Findings

Vegetation Concentrations of mercury and other related contaminants

in vegetation from the field where soil quality criteria were exceeded were elevated and warrant further investigation in terms of the risk posed from consumption;

Methyl mercury, the most toxic form of mercury in terms of human health, was found in very low concentrations in all vegetation samples, with the highest concentrations being 4 orders of magnitude (30,000-40,000 times) lower than levels that were recorded in contaminated flour that resulted in severe health problems for people exposed to methyl mercury in Iraq

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Preliminary Conclusions

mercury and other related contaminants are present in soils of some farm fields in the La Zacatecana area at concentrations in excess of risk-based human and ecological criteria;

Concentrations of mercury and other related contaminants were not detected in concentrations exceeding criteria which are used in Ontario for the remediation of sites to levels considered acceptable for residential/parkland use;

Concentrations of methyl mercury in vegetation were very low in comparison to dietary exposure levels reported in a study of health impacts in another country;

Concentrations of mercury and other related contaminants in vegetation from the field where soil quality criteria were exceeded were elevated and warrant further investigation in terms of the risk posed from consumption

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Preliminary Conclusions

Based on the foregoing findings, a more comprehensive assessment of the degree and extent of mercury and related chemical contamination throughout the entire zone where mine tailings are known to have accumulated over the past 500 years appears warranted to provide input data for assessment of potential human and ecological health impacts.

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The CEC and Mexican Team