Heavy Metal Problems in Soil

Heavy Metals

• According to the North Carolina Department of Agriculture and Consumer Services, heavy metals are defined as toxic metals with a specific gravity higher than 5. Some of the naturally occurring heavy metals found in the soil include copper, zinc and nickel. Industrial runoff and sewage often deposit metals such as lead, mercury, arsenic, selenium, cadmium and chromium. These metals are typically present in the soil in small percentages and do not become harmful unless they become more concentrated. Some plants and animals are sensitive to the presence of certain heavy metals in the soil and will show the symptoms of metal toxicity before they reach a concentration strong enough to harm humans.

Soil Concentrations

• The levels of heavy metals in the soil are measured according to their concentration in parts per million (ppm) or milligrams of metal per kilogram of soil (mg/kg). Naturally occurring metals like aluminum or iron have relatively high average concentrations, reaching 71,000 and 38,000 mg/kg, respectively. Highly toxic metals such as mercury or lead are naturally present in much smaller amounts, making the soil more sensitive to pollution from these metals. On average the soil contains only 10 and 0.03 mg/kg of lead and mercury. Heavy metals including arsenic, selenium, cadmium and chromium are usually only present in levels below 100 mg/kg.

Issues

• Heavy metal contamination causes a wide variety of problems in the soil. Soils that are contaminated with these metals often have a substantially reduced pH level that reduces the ability of plants to absorb nutrients from the soil. This change in pH is also detrimental for the microbes living in the soil that plants rely on to compost organic matter into nutrients. Many heavy metals in the soil are transportable through the soil and into the groundwater, posing a threat to drinking water supplies and aquatic life.

Remediation

• There are several methods for removing toxic concentrations of heavy metals from the soil. Physically removing the contaminated soil to a secure location is a cost-intensive process that is environmentally disruptive and requires permanent storage for the contaminated soil. A more cost-effective alternative is to chemically neutralize the heavy metals, preventing them from interacting with organic material in the soil. Some other methods of neutralizing heavy metals in the soil use microbes to neutralize the metals or plants to draw the metals out of the soil. These methods are cost-effective compared to removal but are much slower.