1. Heavy Metal Contamination in Agricultural Soils
Global Issues Seminar
Colorado Mountain College
Maggie Gaddis, adjunct faculty

2. Outline What is a heavy metal? Remediation Techniques
What causes heavy metal pollution?
International Examples- Zimbabwe, China
Remediation Techniques
Historic technique
Modern techniques (pros and cons)
Phytoremediation (natural v. chemically enhanced)
Case Study- Hayden Ranch

3. What is a heavy metal?
metal of relatively high density (specific gravity greater than about 5) or of high relative atomic weight (especially one that is poisonous ...
heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight, and some on chemical properties or toxicity.[1] The term heavy metal has been called a "misinterpretation" in an IUPAC technical report due to the contradictory definitions and its lack of a "coherent scientific basis".[1] There is an alternative term toxic metal, for which no consensus of exact definition exists either. As discussed below, depending on context, heavy metal can include elements lighter than carbon and can exclude some of the heaviest metals. Heavy metals occur naturally in the ecosystem with large variations in concentration. In modern times, anthropogenic sources of heavy metals, i.e. pollution, have been introduced to the ecosystem. Waste-derived fuels are especially prone to contain heavy metals so they should be a central concern in a consideration of their use.

4. What causes heavy metal pollution?
Anthropogenic causes
Heavy metal- released to the river/ocean from numerous sources.
Typical sources-
1. municipal wastewater-treatment plants
2. manufacturing industries,
3. mining,
4. transportation
5. rural agricultural cultivation & fertilization.
transported as either dissolved species in water/ integral part of suspended sediments.
may be volatilized to the atmosphere or stored in riverbed sediments.
Toxic heavy metal- taken up by organisms; the metals dissolved in water have the greatest potential of causing the most deleterious effects.

5. What is soil pollution? Another way of looking at it….
Soil is a resource
Increasing use of agrochemicals (herbicides, pesticides), Green Revolution
Soil pollution comprises the pollution of soils with materials, mostly chemicals, that are out of place or are present at concentrations higher than normal which may have adverse effects on humans or other organisms. It is difficult to define soil pollution exactly because different opinions exist on how to characterize a pollutant; while some consider the use of pesticides acceptable if their effect does not exceed the intended result, others do not consider any use of pesticides or even chemical fertilizers acceptable. However, soil pollution is also caused by means other than the direct addition of xenobiotic (man-made) chemicals such as agricultural runoff waters, industrial waste materials, acidic precipitates, and radioactive fallout.
Both organic (those that contain carbon) and inorganic (those that don't) contaminants are important in soil. The most prominent chemical groups of organic contaminants are fuel hydrocarbons, polynuclear aromatic hydrocarbons ( PAHs ), polychlorinated biphenyls ( PCBs ), chlorinated aromatic compounds, detergents, and pesticides. Inorganic species include nitrates, phosphates, and heavy metals such as cadmium, chromium and lead; inorganic acids; and radionuclides (radioactive substances). Among the sources of these contaminants are agricultural runoffs, acidic precipitates, industrial waste materials, and radioactive fallout.

6. Metals have unique chemical properties
1. Do not decay like organics
2. Necessary and beneficial to plants
3. Always present at background levels from parent rock weathering
4. Often occur as cations, which are actively exchanged in plant cell processes
In soil science, cation exchange capacity (CEC) is the capacity of a soil for ion exchange of cations between the soil and the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination. Cations can also be easier to understand by just adding the group number.
The quantity of positively charged ions (cations) that a clay mineral or similar material can accommodate on its negatively charged surface is expressed as milli-ion equivalent per 100 g, or more commonly as milliequivalent (meq) per 100 g or cmol/kg. Clays are aluminosilicates in which some of the aluminium and silicon ions have been replaced by elements with different valence, or charge. For example, aluminium (Al3+) may be replaced by iron (Fe2+) or magnesium (Mg2+), leading to a net negative charge. This charge attracts cations when the clay is immersed in an electrolyte such as salty water and causes an electrical double layer. The cation-exchange capacity is often expressed in terms of its contribution per unit pore volume, Qv.

7. International Example- Zimbabwe

8. International Example- China
Pathways- irrigation, atmospheric deposition
Inorganic fertilizer use was 3.9 kg/ha in 1949 (our green revolution)
was 379 kg/ha in 1995…2.5 times the world average (FAO)
Second largest producer of pesticides

9. Remediation Techniques
Traditional/historical approach: land fills, leave it on site, let it wash away
Microbial remediation
Site excavation and soil replacement
Soil washing, vapour extraction
Phytoremediation
gardening on contaminated sites
Site mitigation: excavation & soil replacement (+/-geotextiles)- very expensive, fast
soil washing – expensive
soil vapour – very expensive
extraction
microbial remediation - low cost; <1 year
phytoremediation (+/- chelating agents) - low cost; 2–5+ years

10. Phytoremediation Phyto- plant
Remediation- The act or process of correcting a fault or deficiency
million dollar industry in USA (2005)
Two types
Natural hyperaccumulators, metals accumulate in roots and shoots. Typically these plants have a high tolerance for metals. BUT are slow growing, and produce low biomass. With plant materials available, remediation could take years. Thlaspi

11. Hyperaccumulators
thlaspi caerulescens- Roots of Thlaspi caerulescens foraging for Zn patches in soil.
Metallophytes
The responses of metal tolerant and metal hyperaccumulator plants to heavy metals have been the subject of studies dating back to the early 20th century. However, there is still a long way to go before these processes will be completely understood. Our group is interested in many of these processes in metallophytes (Zn, Ni, Cd, Co, Mn, Hg, Cu and As), from their mineral nutrition and ability to scavenge metals from soils, through the coordination of the metals with ligands in the roots and xylem, to the storage of metals in the shoots. We also have an interest in the growth responses of metallophytes to other environmental stresses, such as drought and salinity. Recent studies include:
"There are some really cool plants out there that will not only tolerate very toxic soils but will accumulate some of those metals to very high levels in the shoot," says Leon Kochian, director of the U.S. Plant, Soil and Nutrition Laboratory located on Cornell's campus and a Cornell professor of plant biology and crop and soil sciences. Thlaspi caerulescens, a member of the cabbage family known to some as Alpine pennycress, for example, "accumulates metals to astounding levels," says Kochian.
Delicate yellow flowers unique to the Greek island of Lesbos have provided vital clues that could lead to the development of plants that will clean up contaminated soil. Andrew Smith and Ute Krämer from the Department of Plant Sciences at the University of Oxford and Janet Cotter-Howells of the University of Manchester (now at the University of Aberdeen) have discovered how the plant, Alyssum lesbiacum (Figure 1), mops up nickel. Armed with this knowledge, they hope to design plants that draw metals out of soil much more cheaply and cleanly than is possible with existing techniques.

12. Case Study- Hayden Ranch

13. Case Study- Hayden Ranch
California gulch

14. Case Study- Hayden Ranch

15. Citations
Catlett, K.M. et al Soil chemical properties controlling zinc activity in 18 Colorado soils. Soil Science Society of America Journal 66:
Ellis, R.J. et al Cultivation-dependent and –independent approaches for determining bacterial diversity in heavy-metal contaminated soil. Applied and Environmental Microbiology. 69(6):
Facchinelli, A. et al Multivariate statistical and GIS-based approach to identify heavy metal sources in soil. Environmental Pollution 114:
Fleming, S.W. Assessment of potential health effects of ingestion of garden produce containing arsenic and cobalt grown in Cobalt, Ontario. Risk Assessment unit, Hazardous Contaminants Branch.
Lombi, E. et al Phytoremediation of heavy-metal contaminated soils: natural hyperaccumulation versus chemical enhanced phytoextraction. Journal of Environmental Quality 30:
Muchaweti, M. et al. Heavy metal content of vegetables irrigated with mixtures of wastewater and sewage sludge in Zimbabwe: implications for human health. Dept. of Biochemistry, Univ. of Zimbabwe.
Sandhi, M.A. Heavy metal in the environment and effect on plant physiology. Powerpoint presentation accessed with Slideshare on
Wong, S.C. et al Heavy metals in agricultural soils of the Pearl River Delta, South China. Environmental Pollution 119:

16. Questions?