Water quality performance of wetlands receiving nonpoint-source nitrogen loads: Nitrate and total nitrogen removal efficiency and controlling factors

Nonpoint-source nitrogen (N) loads in the U.S. Corn Belt are a major concern both for local impacts on receiving waters and for contributing to hypoxia in the Gulf of Mexico. Nonpoint-source nutrient loads can be ameliorated by a combination of in-field and offsite practices, and wetland restoration is a particularly promising approach for reducing N loads from agricultural drainage. However, there is considerable variability among wetlands, and adequate performance data are available for relatively few systems receiving unregulated nonpoint-source loads. We measured N mass balances of 26 restored wetlands receiving a wide range of unregulated, naturally varying hydraulic and nutrient loads to evaluate the N removal performance of these systems and the effects of major factors controlling their performance. Nitrogen loads were primarily in the form of nitrate, and all of the wetlands were effective in reducing both nitrate and total N loads. Nitrate N and total N removal rates averaged 1,500 and 1,440 kg N ha^-1  yr^-1 , respectively, with the slightly lower total N removal rates reflecting a small net export of reduced N (averaging 66 kg N ha^-1  yr^-1 ). Average nitrate and total N removal rates were substantially higher than typically reported for Corn Belt wetlands but comparable with highly loaded systems elsewhere. Nitrate removal efficiency ranged from 9 to 92% and was strongly related to hydraulic loading rate and temperature. Results demonstrate the substantial capacity of wetlands to reduce unregulated and highly variable nonpoint-source N loads over a broad range of weather and loading conditions and provide a reasonable basis for predicting average wetland performance based on hydraulic loading rate, temperature, and nitrate concentration.

Assessment of intrinsic bioremediation of a coal-tar-affected aquifer using Two-dimensional reactive transport and Biogeochemical mass balance approaches

Expedited site characterization and groundwater monitoring using direct-push technology and conventional monitoring wells were conducted at a former manufactured gas plant site. Biogeochemical data and heterotrophic plate counts support the presence of microbially mediated remediation. By superimposing solutions of a two-dimensional reactive transport analytical model, first-order degradation rate coefficients ((day-1) ) of various compounds for the dissolved-phase plume were estimated (i.e., benzene [0.0084], naphthalene [0.0058], and acenaphthene [0.0011]). The total mass transformed by aerobic respiration, nitrate reduction, and sulfate reduction around the free-phase coal-tar dense-nonaqueous-phase-liquid region and in the plume was estimated to be approximately 4.5 kg/y using a biogeochemical mass-balance approach. The total mass transformed using the degradation rate coefficients was estimated to be approximately 3.6 kg/y. Results showed that a simple two-dimensional analytical model and a biochemical mass balance with geochemical data from expedited site characterization can be useful for rapid estimation of mass-transformation rates.

Impact of transverse and longitudinal dispersion on first-order degradation rate constant estimation

A two-dimensional analytical model is employed for estimating the first-order degradation rate constant of hydrophobic organic compounds (HOCs) in contaminated groundwater under steady-state conditions. The model may utilize all aqueous concentration data collected downgradient of a source area, but does not require that any data be collected along the plume centerline. Using a least squares fit of the model to aqueous concentrations measured in monitoring wells, degradation rate constants were estimated at a former manufactured gas plant (FMGP) site in the Midwest U.S. The estimated degradation rate constants are 0.0014, 0.0034, 0.0031, 0.0019, and 0.0053 day(-1) for acenaphthene, naphthalene, benzene, ethylbenzene, and toluene, respectively. These estimated rate constants were as low as one-half those estimated with the one-dimensional (centerline) approach of Buscheck and Alcantar [Buscheck, T.E., Alcantar, C.M., 1995. Regression techniques and analytical solutions to demonstrate intrinsic bioremediation. In: Hinchee, R.E., Wilson, J.T., Downey, D.C. (Eds.), Intrinsic Bioremediation, Battelle Press, Columbus, OH, pp. 109-116] which does not account for transverse dispersivity. Varying the transverse and longitudinal dispersivity values over one order of magnitude for toluene data obtained from the FMGP site resulted in nearly a threefold variation in the estimated degradation rate constant-highlighting the importance of reliable estimates of the dispersion coefficients for obtaining reasonable estimates of the degradation rate constants. These results have significant implications for decision making and site management where overestimation of a degradation rate may result in remediation times and bioconversion factors that exceed expectations. For a complex source area or non-steady-state plume, a superposition of analytical models that incorporate longitudinal and transverse dispersion and time may be used at sites where the centerline method would not be applicable.