Ecological restoration strategies for an abandoned sulfuric acid recycling factory soil with heavy metal contaminants
Concentrated sulfuric acid used to clarify kerosene at oil refineries in Titusville, PA in the 1800s was recycled at a nearby factory. The factory, which operated between ca. 1870 and 1917, contributed multiple heavy metals and acid to the soil from lead-lined tanks and distillation pans. Today, large portions of the site remain only partially vegetated in an otherwise densely-forested area, and vegetation is limited to isolated “islands” of wind-blown plant litter that serve as buffers from the underlying contaminated soil. In this study, we assessed the relationship between contaminant distribution at the site, soil microbial activity, and the ability of specific amendments/treatments to ameliorate factors that limit natural site restoration. Surface soil samples were taken in a grid pattern over the study site and analyzed for pH, total and bioaccessible metals, soil enzyme activity and Biolog ECO plates. Existing plant species were inventoried along transects that paralleled contaminant concentration gradients. Three specific low-cost restoration strategies, including the amendment of soil with rock phosphate, limestone and biochar, were tested ex-situ on the most contaminated soil from the site. Potted studies were also conducted using several configurations of hardwood biochar amendment (surface-applied, mixed and layered).
The soil at the site was highly contaminated with lead (up to 43,000 mg/kg; 4 to 55% bioavailable), antimony, and arsenic. The distribution of metals was consistent with the location of acid distillation pans and storage tanks and was spatially correlated with enzyme activity and microbial diversity. Plants native to northwestern Pennsylvania were identified at the site and evaluated as candidates for phytoremediation. Vegetative species richness (including trees, shrubs, grasses, forbs, and bryophytes) varied significant among transects and generally increased as soil contamination decreased. None of the native candidates germinated in the contaminated soils, so a known metal-tolerant grass, Canada wildrye (Elymus canadensis), was used in a potted study. All soil amendments enhanced plant growth in contaminated soil by added alkalinity, reduced metal toxicity, and improvement in soil properties. These results suggest that: 1) natural revegetation of this site has, in part, been limited by the extreme level of soil contamination as well as limited natural buffer “islands” and metal-tolerant plants and 2) remediation of the site using E. canadensis, biochar, limestone and rock-phosphate is feasible.