Quantifying biological organic carbon removal in groundwater recharge systems

This paper presents a novel approach to study and predict the removal of organic carbon in groundwater recharge systems by combining microbial community description with advanced methods for organic carbon characterization. Soil microbial biomass was characterized using three methods: dehydrogenase activity (general heterotrophic activity), substrate induced respiration (rapid mineralization potential), and phospholipid extraction (total viable biomass). These methods proved to be sensitive, robust, and relatively simple in their application for soils of various groundwater recharge sites. Findings indicated that microbial biomass was positively correlated to the organic carbon removal capacity of different laboratory-scale test systems. Organic carbon seems to be a limiting factor for biomass growth in recharge systems. Organic carbon removal rates are increased by higher initial organic carbon concentrations. The removal of three organic carbon fractions (natural organic matter, effluent organic matter, and glucose and glutamic acid) in soil column studies followed a first-order kinetic with distinctly different rate constants and correlated positively with respective total viable biomass in the column systems. These results supported the assumption that during groundwater recharge organic carbon is preferably removed by biological processes. The transformation of organic carbon fractions during travel through the subsurface became apparent in size-exclusion chromatograms indicating a shift to smaller molecular weight. The presented approach showed promise to reveal new insights into removal mechanisms of organic carbon and to serve as a tool to predict organic carbon removal in groundwater recharge systems to improve design and operation of vadose zone treatment.