(54a) Determining the Rate of Biodegradation in Fuel Impacted Karst Aquifers

Hydrologic and geologic characteristics of fractured rock aquifers have been described as not being suited for natural bioremediation because of small microbial populations. It is also widely perceived that karst groundwater often has insufficient residence time for biodegradation to occur. It is perhaps due to these perceptions that less research has been conducted for quantitative modeling of karst as compared to consolidated aquifers.  Modeling biodegradation in karst is in the domain of non-ideal chemical reaction kinetics.  For all non-ideal flow reactors, the concentration response to an input of tracer must lie between the limiting cases of completely mixed flow and purely plug flow. The residence time distribution function (RTD) for tracer molecules in a single karst conduit or a complex system of conduits is a probability density function which can be interpreted to define the probability that contaminant molecules present at the influent at time equals zero will arrive at the effluent after a particular amount of time.   In this work, the biodegradation rate of a contaminant (toluene) in raw karst groundwater from a BTEX impacted site in central Kentucky was quantitatively measured in microcosm studies and the extent of biodegradation of toluene in the same groundwater was measured for a complex flow system.  Data from conservative tracer studies and for toluene biodegradation were used in conjunction with the advection dispersion equation to investigate the biodegradation rate. Results indicated that the biodegradation of toluene in karst groundwater is a volumetric reaction which can be described by pseudo first order reaction kinetics.  The values of the rate constant (k') obtained from the dispersion-like model ranged from 0.017 (hr)^-1 to 0.0210 (hr)^-1 compared to 0.0186 (hr)^-1 for the microcosm experiments.  This corresponds to a half life of less than two days for toluene and has major implications to issues regarding natural attenuation of fuel impacted sites.