NUTRIENT LIMITATIONS AND THE PERFORMANCE OF PERMEABLE REACTIVE BARRIERS

In situ treatment systems have been applied increasingly over the past decade. Permeable reactive barriers containing organic carbon have been used to treat acidic drainage at mine sites and nitrate derived from wastewater disposal systems. The design of permeable reactive barrier systems typically involves an evaluation of the relative performance of candidate reactive materials. The most reactive materials are typically selected through this procedure. Screening studies conducted for the assessment of two treatment systems evaluated the reactivity of different candidate materials. The materials identified to be most reactive contained greater masses of labile organic carbon and higher concentrations of N and P, suggesting that these nutrients provide a limitation on the performance of the reactive material. Field-scale test plot installations show results that the rate of metal removal is significantly greater in the test plot constructed using the nutrient rich reactive material. The nutrient-rich material supported greater numbers of sulfate reducing bacteria and showed higher levels of enzyme activity. Improved performance was indicated by more rapid and extensive removal of SO₄, Fe, Zn and other metals, and greater generation of carbonate alkalinity. Carbon isotope ratios show greater utilization of organic carbon in the nutrient rich test plot. These results suggest that nutrient availability is an important consideration in the design of in situ treatment systems.