This demonstration entailed the extraction of lead contamination from soil using in-situ phytoremediation in a manner that was both practical and economically feasible. Phytoremediation is a process in which ionic, or non-particulate, metals are removed from the soil by plant roots. A chelating agent (EDTA) and soil acidifier (acetic acid) were used to convert the lead into a water-soluble state amenable to plant uptake. The field design of the project utilized thirty-six 15-foot square grids, and the 2-year field demonstration was carried out through 1998 and 1999 at the Twin Cities Army Ammunition Plant, Arden Hills, Minnesota. Preliminary soil samples collected in 1997 mapped out the extent of lead contamination, after which, the crops were grown in 1998 and 1999 to test the technology.
The performance objectives for this project were not achieved. Lead uptake for corn planted in 1998 was 0.65 percent (6,500 mg/kg) lead in the biomass. This value was near expected levels, although total biomass yields were less than expected because of low uptake for the white mustard planting. Adverse weather conditions and a slow EDTA application rate caused plant damage and negated expected lead uptake. In 1999, a deeper rooting corn variety was used and the EDTA application system was changed from hose applicator to drip delivery. Lead uptake was much less in 1999 due to poor weather conditions, bird damage to crops, excess rain that limited root growth, and variable crop growth. No planting occurred in 2000 because sampling prior to the planned demonstration indicated the presence of lead and EDTA in groundwater.
Phytoremediation is a technically and economically viable remediation alternative when properly designed and managed, used in combination with other remediation technologies, and applied where site conditions are best suited for phytoremediation. Based on specific conditions and uptake efficiency assumptions associated with this project, the cost per crop for phytoremediating one acre to a depth of one foot is approximately $42,145 or $26.13 per cubic yard per crop. These costs will be higher if conditions are not optimal or additional site preparation is required.
While phytoremediation is clearly a viable remedial alternative in some settings, it is best used in combination with other technologies at most project locations. There are many factors that play a role in determining the relative success of phytoremediation: (1) type and consistency of soil; (2) presence of debris in the soil; (3) length of the growing season; (4) initial lead concentration; (5) weather conditions during the growing season; and (6) application rate of chelate and soil acidifier. Time is also a phytoremediation consideration because several years may be required, even under the best conditions, to obtain acceptable remediation results. (Project Completed - 1999)