David A. Chin

An investigation of the validity of first-order stochastic dispersion theories in isotropie porous media

Monte Carlo simulations are used to (1) investigate the accuracy of approximations that are implicit in first-order stochastic dispersion theories and (2) identify the accuracy limits of first-order dispersion theories in isotropic porous media. The Fickian theory of Gelhar and Axness (1983), as well as the Fickian and non-Fickian theories of Dagan (1984) and Neuman and Zhang (1990) are investigated. All Monte Carlo simulations are in three dimensions. Confidence limits of ensemble-averaged Monte Carlo results in isotropic porous media are established for 0.1 ≤ σY ≤ 1.5. These results showed that first-order theoretical estimates of the Eulerian velocity covariance function are quite accurate for σY < 1; theoretical estimates of the non-Fickian longitudinal dispersivity do not deviate significantly from theory for at least σY ≤ 1.5; theoretical estimation of the transverse dispersivity is limited to σY < 1; and, the Fickian longitudinal dispersivity is overestimated by the theory of Gelhar and Axness (1983). Of all first-order dispersion theories, the theory of Dagan (1984) is most robust in estimating the dispersivity tensor.

An assessment of first-order stochastic dispersion theories in porous media

Abstract Random realizations of three-dimensional exponentially correlated hydraulic conductivity fields are used in a finite-difference numerical flow model to calculate the mean and covariance of the corresponding Lagrangian-velocity fields. The dispersivity of the porous medium is then determined from the Lagrangian-velocity statistics using the Taylor definition. This estimation procedure is exact, except for numerical errors, and the results are used to assess the accuracy of various first-order dispersion theories in both isotropic and anisotropic porous media. The results show that the Dagan theory is by far the most robust in both isotropic and anisotropic media, producing accurate values of the principal dispersivity components for σ y as high as 1.0, In the case of anisotropic media where the flow is at an angle to the principal axis of hydraulic conductivity, it is shown that the dispersivity tensor is rotated away from the flow direction in the non-Fickian phase, but eventually coincides with the flow direction in the Fickian phase.

Watershed-Scale Fate and Transport of Bacteria

The added dimensionality provided by using multiple models to predict the fate and transport of bacteria at the watershed scale were investigated. Both HSPF and SWAT were applied to the 15.6 km2 catchment K of the Little River Experimental Watershed (LREW) in Georgia. Over the seven-year period from 1996 to 2002, SWAT provided a more accurate description of fecal coliform concentrations, with an NSE of 0.73 compared to 0.33 for HSPF. For this particular watershed, the SWAT process equations are more representative of the watershed-scale fate and transport of bacteria than the HSPF process equations. Based on this comparative analysis, it can be inferred that elevated levels of fecal coliform in the receiving stream are primarily due to in-stream sources. This source characterization could not be achieved by using only the HSPF model, which indicates a much greater contribution from groundwater and terrestrial nonpoint sources. A model-averaging approach in which a weighted average of the HSPF and SWAT predictions are used to predict bacteria concentrations in the receiving stream demonstrates that model weights can be determined such that the NSE of the combined models will be greater than either of the models taken individually. However, in the present case, the marginal improvements in NSE obtained through this integration were small.

Comparative assessment of municipal wastewater disposal methods in Southeast Florida

A comparative assessment of the risks of three effluent disposal alternatives currently available to wastewater utilities in Southeast Florida is presented in this paper. The alternatives are: deep well injection and ocean outfalls following secondary treatment, and surface water (canal) discharges following secondary wastewater treatment, filtration and nutrient removal. Water quality data, relative to disposal of wastewater treatment plant effluent were gathered, along with water quality data on the receiving waters, from utilities. Comparisons and conclusions regarding potential health concerns associated with the three disposal alternatives are presented. The results indicated that health risks associated with deep wells were generally lower than those of the other two alternatives. The proximity of injection wells to aquifer storage and recovery wells was a determining factor relative to injection well risk. Urban ecological risks were also indicated to be lower, though impacts of urban water use/reuse to the Everglades were not studied. Additional data collection and analysis were recommended to understand the effects of wastewater management on the cycling of water, nutrients and other constituents on southeast Florida. In particular, it was recommended that monitoring of effluents for nitrosamines and pharmaceutically active substances be implemented on a broad scale.

Predictive Uncertainty in Water-Quality Modeling

A general and integrated approach to parameter identification, model calibration, and estimation of predictive uncertainty in water-quality models is proposed and validated. The proposed approach determines the maximal conditional likelihood functions of each of the model parameters using a transformation that forces the model errors to be normally distributed, with predictive uncertainty characterized by random normally distributed and homoscedastic model errors in the transform space. The proposed approach is demonstrated using a watershed-scale model to predict the fecal coliform levels in a third-order stream within the Little River Experimental Watershed in Georgia. Maximal conditional likelihood functions were identified for all parameters in the log, square root, and no-transformation cases. The key results are: (1) the number of sensitive parameters and the optimal parameter values can depend on the transformation; (2) only in the case of the log-transformation are the errors normally distributed and consistent with the assumed Gaussian likelihood function; (3) the standard error in the model is least for the no-transform case and highest for the log-transform case; and (4) the observed model errors are most predictable using the log-transform and least predictable using the no-transform approach.

Quantifying Pathogen Sources in Streams by Hydrograph Separation

A new technique for quantifying pathogen sources to streams is proposed and demonstrated. Hydrograph separation is used to partition measured streamflow into surface runoff and base flow, and characteristic pathogen concentrations are assigned to each flow component along with a background source flux. The maximum-likelihood characteristic concentrations and background flux are determined from measured instream pathogen concentrations. This approach is shown to yield comparable to superior performance in predicting instream pathogen concentrations compared with much more complex terrestrial fate and transport models. Application of the proposed approach to six catchments yields Nash-Sutcliffe efficiencies of the log-transformed fecal-coliform concentrations in the range of 0.21 to 0.48. The characteristic fecal-coliform concentrations in surface runoff are in the range of 200–700  cfu/dL and the base-flow characteristic concentrations are in the range of 20–100  cfu/dL. It is shown that the frequency dist...

Designing Bioretention Areas for Stormwater Management

Governing equations for designing bioretention areas for both flood control and water-quality control are developed, and a design protocol for applying these equations is also presented. Factors taken into account include the flood-control return period, the local intensity-duration-frequency (IDF) function, the catchment volumetric runoff coefficient, and the depth and infiltration capacity of the bioretention-area bowl. It is shown that the IDF functions nested within the conventional Natural Resources Conservation Service (NRCS) rainfall distributions can be described by a common functional form, with different parameters for each of the four rainfall types. These extracted IDF functions are used to show that designing bioretention areas for flood control is more feasible in the western part of the United States that have Type I rainfall, compared to other parts of the county that have Types IA, II, and III rainfall. It is demonstrated that practical bioretention areas that are sized for water-quality control can also meet flood-control regulations in some areas. A design example is provided to demonstrate the typical sizing of bioretention areas for both flood control and water-quality control.

A scale model of multivariate rainfall time series

Abstract A multivariate time-series model that uses a factor-analytic approach is shown to provide an effective description of both monthly and annual rainfall in south Florida. In the case of monthly rainfall, the scale model shows that deviations from monthly means are caused primarily by large-scale phenomena that have temporal structure. These sort of phenomena are not accounted for by using conventional contemporaneous ARMA models. In the case of annual rainfall, the majority of variance is associated with random normally distributed large-scale phenomena that do not have temporal structure.

Hydraulic Resistance versus Flow Depth in Everglades Hardwood Halos

The variation of the Manning roughness with flow depth in two hardwood-dominated vegetation halos that surround water-delivery structures in Everglades National Park was investigated. The results show that the hydraulic resistance of the halos decreases approximately linearly with increasing flow depth. For flow depths less than 15–20 cm, the hydraulic resistance is similarly high in both the halo and the downstream marsh vegetation, however, as the flow depth increases the hydraulic resistance in the halo decreases to below that in the downstream marsh. As a consequence, for increased stages at the delivery structure, the halo vegetation will become less restrictive relative to the marsh vegetation in controlling water deliveries.

In-stream bacteria modeling as a function of the hydrologic state of a watershed

AbstractThis paper presents a new way of modeling in-stream bacteria concentrations by examining a watershed in terms of wet and dry hydrologic states. Flow-duration curves were developed for four catchments of the Little River Experimental Watershed in Tifton, Georgia, and HSPF and SWAT bacteria models were built for the catchments. The flow-duration curves were used to designate wet and dry states of the catchments based on flow conditions instead of calendar day, the bacteria data sets were divided into wet and dry groups accordingly, and the models were calibrated to the wet and dry states. Water-quality parameter sensitivities revealed that each model placed varying emphasis on the parameters in each state according to the model’s structure, and certain parameters were insensitive in both wet and dry states. A custom parameter added to the models to represent background in-stream and/or distributed loads was consistently sensitive across hydrologic states and improved model predictions in both comput...