Phytoremediation of Lead Contaminated Soil at a

Brownfield Site in Ramford County

Methods

The practical application of phytoremediation was recently demonstrated at a Brownfields site in Ramford County. The site is located in a residential/commercial area of the city and is adjacent to a school, churches and private residences. A 4800 sq. ft area on the site was selected for the study. The site is part of property occupied by Ramalives Battery Recycling Company, Inc. The soil at the site is contaminated with lead.

A preliminary site investigation was conducted in October of 1995 to determine the distribution of lead in the soil and collect surface (0-15 cm depth) samples for a laboratory treatability study. The soil samples were sieved to 4 mm and a subsample was submitted to the Ramford University Soil Testing Laboratory for a standard soil fertility analysis. An additional sample was analyzed at Phytotech for total metals by EPA Method 3050 (Table 1) and also extracted sequentially to assess metal associations with operationally defined soil fractions (exchangeable, carbonates, oxides, organic matter, and residual) (Table 2). The remaining soil was fertilized and potted in 3.5 inch diameter pots and planted with Brassica juncea. The soil was treated with Phytotech's proprietary amendments to induce metal uptake by the plants. After 4 weeks growth, the plants were harvested and analyzed for metal content.

Based on the results of the treatability study, a field trial was planned and conducted at the site in Ramford. An initial sampling of the site to obtain baseline soil data was conducted by sampling every 5 ft. at three depths (0-15, 15-30, and 30-45 cm). Contour maps describing the surface contamination were plotted using SurferTM (Surface Mapping System, 1994, Golden Software, Inc.). The site was tilled and fertilized according to the soil fertility test results and seeded with B. juncea. Soil solution lysimeters and tensiometers were installed at 4 depths (6", 12", 24" and 36") to collect soil solution samples and monitor soil watercontent. Irrigation was conducted according to a hydrogeological model developed specifically for the Ramalives site. Soil amendments were added based on the results obtained in the treatability study to optimize plant growth and metal uptake. The crop of B. juncea was harvested after 6 weeks and the plot was replanted within one week of the harvest. The harvested biomass was dried and removed from the plot. A total of three crops were grown and harvested during 1996.

Soil samples were collected again at three depths after the third crop to determine metal removal efficiency. The soil samples were air dried and analyzed for total metal content using EPA Method 3050. Contour maps of the contaminated areas were again plotted in the same manner as the initial sampling. The areas corresponding to specific levels of metal concentration were calculated using SurferTM.

Results and Discussion

The soil at the Ramalives site had significant variation in pH (5.1 to 7.1)and lead contamination (200 to 1800 mg/kg) (Figure 1). Soil characteristics fromthe sample used for the treatability study are presented in Tables 1 and 2. The soil lead was predominantly associated with the organic fraction (33% of the total lead) with the residual and oxide fractions containing 28% and 22%, respectively (Table 2). (Periodic Table)

Table 1. Soil characteristics and total metal content of a surface soil sample collected at the Ramalives site.

pH Texture % Organic Matter Cd Cr Cu Ni Pb Zn

------------- mg/kg -------------

 

Table 1. Soil characteristics and total metal content of a surface soil sample collected at the Ramalives site.  

pH Texture % Organic Matter Cd Cr Cu Ni Pb Zn
     

------------- mg/kg -------------

7.1 Loamy Sand 11.8 8 22 92 22 927 138

Table 2. Fractionation of metal contaminants based on sequential extraction of the surface soil (O to 15 cm depth) collected from the Ramalives site.

Fraction Cr Cu Ni Pb Zn
 

----------------- mg/kg---------------

Exchangeable 0 1 1 33 14
Carbonates 0 4 0 92 9
Oxide 1 14 2 152 23
Organic 6 79 6 232 62
Residual 14 45 17 196 81
Sum of Fractions 22 142 26 704 190

The greenhouse treatability study indicated that B. juncea plants were capable of removing significant quantities of lead from the soil. Shoot lead concentrations of 3900 mg/kg were achieved through the use of proprietary amendments in the greenhouse scale system. This data coupled with the soil analysis indicating that less than 200 mg/kg of the soil lead remained in the residual fraction after the sequential extraction, encouraged the application of phytoremediation as a means to reduce the surface soil lead concentrations to less than 400 mg/kg.

The implementation of phytoremediation technology at the field site was successful in reducing the area of lead contaminated soil. At the time of the initial sampling, 40% of the selected area exceeded the regulatory limit of 400 mg lead/kg and approximately 7% of the selected area exceeded 1000 mg/kg (Table 3). After three phytoremediation crops, the area that exceeded 400 mg/kg decreased from 40% to 28% of the treated area. In addition, none of the treated area exceeded 1000 mg/kg at the end of the third crop.

Table 3.
Reduction in surface area of soil contaminated with lead. Values given are the percent of the treated area that exceed the given soil concentration.

Soil Lead Concentration (mg/kg) Initial % of Treated Area after 3rd Harvest
>400 40% 28%
>500 30% 15%
>600 22% 6%
>800 12% 0%
>1000 7% 0%

Figure 2 presents the soil lead surface contour map showing the initial soil lead concentrations (top) and the soil lead distribution after two phytoremediation crops (bottom). Phytoremediation also reduced the surface area of lead contamination at the 400, 500, 600, 800, and 1 000 mg/kg levels (Table 3).

A further analysis of water soluble lead in the soil showed that the phytoremediation process did not increase the amount of water soluble lead in the soil at the 15-30 cm and 30-45 cm depths. Total subsurface soil lead concentrations also did not measurably increase as a result of remediation.

 

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Last Update:  06/30/04 09:44:16 PM