Geochemical and biological processes in surface water and groundwater systems: Field observation and modeling studies

This dissertation includes four self-contained papers describing the results of field and modeling studies aimed at understanding and quantifying geochemical and biological reactions in surface water and groundwater systems. The first paper describes the results of a field study of a CaCO₃ precipitating stream in Huanglong Natural Scenic District in Sichuan, China. CO₂ out-gassing and CaCO₃ precipitation are shown to increase with flow velocities based on a mass balance model. Glass slide precipitation experiments indicate that pool development is a consequence of spatial variations in pH resulting from varying hydrodynamics, and reveal that vaterite, a rare polymorph of CaCO₃, coprecipitates with calcite in milky opalescent water in the pool groups. SEM micrographs show the signs of etching on the natural surfaces of diatom-adhered calcite, suggesting that calcite dissolution may be aided by diatoms. The second paper presents the theoretical background and field application of a biologically reactive transport model for simulating the fate and transport of BTEX compounds (benzene, toluene, ethyl benzene and xylene) through a multispecies sequential pathway at a site in the Hill Air Force Base (AFB), Utah. The first-order biodegradation rate coefficients, estimated from the model Base (AFB), Utah. The first-order biodegradation rate coefficients, estimated from the model calibration, are 0.051. 0.031, 0.005, 0.004, and 0.002 day−1 for aerobic respiration, denitrification, Fe(III) and sulfate reductions, and methanogenesis, respectively. The third paper discusses the results of a Monte Carlo analysis designed to quantify the influence of heterogeneity on the fate and transport of BTEX at the Hill AFB site. Simulations show that the total dissolved BTEX masses calculated for all hydraulic conductivity realizations follow a lognormal distribution with the mean approximately 10% larger than that of the calibrated base case. The fourth paper presents a preliminary analysis of a field natural gradient experiment on natural attenuation of BTEX compounds at the Columbus AFB, Mississippi. An electron-donor model and a sequential degradation model are developed and calibrated for multi-species plume distributions 35 and 100 days after the source release. The transport modeling, conducted in conjunction with a back propagation neural network, yields estimates of the spatially varying degradation rate coefficients that show an increasing trend with hydraulic conductivity. The degradation rate coefficients from both models appear to decrease with time for the simulations.