Distribution of Microorganisms and their Activities in Capillary Barriers: Implications for Modeling of Hydrologic Transport through Capillary Barriers

Abstract

The capping of buried waste with surface barriers is a remediation approach designed to prevent the infiltration of water through the buried waste to minimize migration of waste constituents from the burial ground. The hydraulic performance of surface barriers and their long-term effectiveness have been modeled based on soil physical and chemical characteristics, neglecting the potential contribution of soil microorganisms. We hypothesized that soil microorganisms may affect the long-term performance of surface barriers by altering soil structure, soil wettability, or soil pore water surface tensions. Two field-scale barrier prototypes were studied: the “thick soil” design and “capillary barrier” design. Two conceptual models for microbial distribution in the barriers were postulated: (i) due to excavation, mixing, and emplacement, soil microbial numbers and activity would be uniformly distributed throughout the barrier profile; and (ii) in capillary barriers, the presence of the coarse–fine interface would locally enhance microbial growth and create local effects on barrier properties. Our initial studies involved field sampling of thick and capillary barrier prototypes at two different locations, examination of the distributions of microorganisms and their activities in vertical transects through the barriers, and correlation of the biological measures with barrier hydraulic properties. We found relatively uniform distributions of microorganisms and activities across the barriers (both designs), consistent with the first conceptual model. The presence of a capillary barrier layer was not associated with a clear increase in microbial activities; however, finer resolution sampling may be required to evaluate the second conceptual model. Our observations of uniform (or increasing) microbial activities with depth in the barriers contrast with commonly observed decreases in soil microbial numbers and activities with depth at undisturbed sites. The indigenous soil microorganisms did not affect soil wettability or soil pore water interfacial tensions in these prototype barriers of <10 yr of age. However, on the time scales for which barriers are expected to be effective (100s to 1000s of years), microbially produced surface-active substances may alter barrier hydraulic performance. We propose laboratory studies to evaluate long-term consequences of microbially produced surface-active substance on barrier integrity and indicate how these effects can be incorporated into models predicting long-term barrier performance.