Mass Flux Analysis of Abiotic Tetrachloroethene Dense Nonaqueous Phase Liquid Pool Dissolution in a Heterogeneous Flow Environment

Porous aquifer materials are often characterized by layered heterogeneities that influence groundwater flow and present complexities in contaminant transport modeling. Such flow variations also have the potential to impact the dissolution flux from dense nonaqueous phase liquid (DNAPL) pools. This study examined how these heterogeneous flow conditions affected the dissolution of a tetrachloroethene (PCE) pool in a two-dimensional intermediate-scale flow cell containing coarse sand. A steady-state mass-balance approach was used to calculate the PCE dissolution rate at three different flow rates. As expected, aqueous PCE concentrations increased along the length of the PCE pool and higher flow rates decreased the aqueous PCE concentration in the effluent. Nonreactive tracer studies at two flow rates confirmed the presence of a vertical flow gradient, with the most rapid velocity located at the bottom of the tank. These results suggest that flow focusing occurred near the DNAPL pool. Effluent PCE concentrations and pool dissolution flux rates were compared to model predictions assuming local equilibrium (LE) conditions at the DNAPL pool/aqueous phase interface and a uniform distribution of flow. The LE model did not describe the data well, even over a wide range of PCE solubility and macroscopic transverse dispersivity values. Model predictions assuming nonequilibrium mass-transfer-limited conditions and accounting for vertical flow gradients, however, resulted in a better fit to the data. These results have important implications for evaluating DNAPL pool dissolution in the field where subsurface heterogeneities are likely to be present

Numerical modeling analysis of hydrodynamic and microbial controls on DNAPL pool dissolution and detoxification: Dehalorespirers in co-culture

Dissolution of dense non-aqueous phase liquid (DNAPL) contaminants like tetrachloroethene (PCE) can be bioenhanced via biodegradation, which increases the concentration gradient at the DNAPL-water interface. Model simulations were used to evaluate the impact of ecological interactions between different dehalorespiring strains and hydrodynamics on the bioenhancement effect and the extent of PCE dechlorination. Simulations were performed using a two-dimensional coupled flow-transport model, with a DNAPL pool source and two microbial species, Dehalococcoides mccartyi 195 and Desulfuromonas michiganensis, which compete for electron acceptors (e.g., PCE), but not for their electron donors. Under biostimulation, low vx conditions, D. michiganensis alone significantly enhanced dissolution by rapidly utilizing aqueous-phase PCE. In co-culture under these conditions, D. mccartyi 195 increased this bioenhancement modestly and greatly increased the extent of PCE transformation. Although D. michiganensis was the dominant population under low velocity conditions, D. mccartyi 195 dominated under high velocity conditions due to bioclogging effects

Effects of temperature on bacterial transport and destruction in bioretention media: Field and laboratory evaluations

Microbial activities are significantly influenced by temperature. This study investigated the effects of temperature on the capture and destruction of bacteria from urban stormwater runoff in bioretention media using 2-year field evaluations coupled with controlled laboratory column studies. Field data from two bioretention cells show that the concentration of indicator bacteria (fecal coliforms and Escherichia coli) was reduced during most storm events, and that the probability of meeting specific water quality criteria in the discharge was increased. Indicator bacteria concentration in the input flow typically increased with higher daily temperature. Although bacterial removal efficiency was independent of temperature in the field and laboratory, column tests showed that bacterial decay coefficients in conventional bioretention media (CBM) increase exponentially with elevated temperature. Increases in levels of protozoa and heterotrophic bacteria associated with increasing temperature appear to contribute to faster die-off of trapped E. coli in CBM via predation and competition

Long-term sustainability of escherichia coli removal in conventional bioretention media

Bioretention has significant potential for reduction of bacterial levels in urban storm-water discharge. The long-term performance of bacteria removal was evaluated using column studies over an 18-month period, during which synthetic urban storm-water runoff was loaded into conventional bioretention media (CBM) columns once every two weeks. CBM initially achieved a mean of 72% removal efficiency for Escherichia coli O157:H7 strain B6914. The removal efficiency improved over time, achieving 97% or higher efficiency after six months. The trapped B6914 cells died off rapidly between runoff application events. Mechanistic studies indicated that decreased porosity and increased hydrodynamic dispersion observed in mature CBM are favorable for improvement of physical straining of cells and for bacterial adhesion. The temporal change in surface charge on CBM may not be a key factor in the improved bacterial removal. Indigenous protozoa in the CBM grew logistically, and may play an important role in enhancement of bacterial capture and rapid decline in numbers of trapped bacteria via predation. Overall, the long-term bacterial removal process in CBM can be efficient and sustainable

The capture and destruction of Escherichia coli from simulated urban runoff using conventional bioretention media and iron oxide-coated sand

The performance, sustainability, and mechanisms of bacterial removal from stormwater runoff by bioretention systems are poorly understood. The potential for removal of microorganisms in bioretention systems was evaluated using column studies and simulated urban stormwater runoff. Conventional bioretention media (CBM) removed 82% of Escherichia coli O157:H7 strain B6914 cells; iron-oxide coated sand (IOCS) significantly enhanced capture, with 99% efficiency. This improvement possibly was because of the greater positive surface charge and roughness of the IOCS. Trapped strain B6914 cells decayed more rapidly in CBM, however, with more than 99.98% die-off within one week compared with the IOCS in which approximately 48% of trapped cells survived. Predation and competition from native microorganisms in CBM were verified to play a dominant role in rapid destruction of trapped strain B6914. In particular, protozoan grazing appeared to play an important role, with the die-off of trapped B6914 increasing with increasing concentrations of protozoa