Journal of Hydrologic Engineering

Mobile subsurface colloids can carry ground-water contaminants sorbed onto their surfaces to far greater distances than predicted using a conventional advective/dispersive transport equation with normal retardation values. In this study, we present a dimensional analysis of colloid-facilitated ground-water contaminant transport. The significance of the contaminant distribution coefficient with colloids and the colloid distribution coefficient with the solid matrix on the retardation coefficient is demonstrated. Figures plotted in terms of dimensionless parameters show that the smaller the sorption of contaminant with colloids, the smaller the effect of colloid presence on contaminant transport. The effect of retardation coefficient on the normalized contaminant concentration profile is illustrated in terms of dimensionless parameters. It has been demonstrated that a reduction in contaminant retardation coefficient in the presence of colloids is accompanied by a corresponding reduction in the modified hydrodynamic dispersion coefficient. Furthermore, the relation between the change in the retardation coefficient and modified hydrodynamic dispersion coefficient has been obtained for different values of the colloid distribution coefficient with the solid matrix. As the retardation coefficient of contaminant in the presence of colloids, ${\mathit{R}}_{\mathit{c}}^{*}$ decreases, the log-log relation between ${\mathit{R}}_{\mathit{c}}^{*}$ and the hydrodynamic dispersion of contaminant in the presence of colloids is a linear one.

A general methodology to develop dispersion models in three-dimensional heterogeneous aquifers under nonstationary conditions is presented. Under this procedure, fundamental hydrologic processes influencing chemical dispersion in porous media, such as recharge rate, ground-water flow boundary conditions, regional hydraulic gradients and their transient behavior, as well as nonstationary statistical properties of the flow and dispersion parameters (if known), may be included in the analysis. The method of decomposition, which is a general analytic technique not requiring many of the restricting assumptions of the current small perturbation methods, is used as a basic tool to solve the resulting stochastic partial differential equations. A special application that reproduces the enhanced dispersion of the Borden aquifer tests is presented. The results suggest that the longitudinal and transverse field dispersion coefficients do not exhibit asymptotic values, even in the absence of recharge, and rather grow as linear functions of time and the corresponding longitudinal and transverse velocity variances. If the regional recharge rate is important, the rate of growth of the dispersion coefficients is expected to increase, and the corresponding mean concentration plume would be nonsymmetric with respect to its center of mass. Finally, a comparison between perturbation and decomposition solutions to ground-water equations, for the case of large conductivity variance, is presented.

A methodology is presented for assessing the relative risk of a given chemical inventory to produce a ground-water contamination event in accordance with wellhead protection (WHP) planning. Modeled after the U.S. Environmental Protection Agency's hazardous ranking system, the approach is one based on a vulnerability assessment incorporating factors for chemical quantity, storage, toxicity, and fate and transport characteristics. The ground-water vulnerability scoring system (GVSS) tool provides a means for ranking the various chemical holders within a delineated WHP (capture) zone with respect to their likelihood to produce a ground-water contamination event. The information generated by GVSS may be useful to land-use planners and water quality regulators in the development of future planning of commercial/industrial activities within a delineated WHP area and is a valuable tool for educating the general public regarding the issues of long-term preservation of the quality of ground-water supplies. In highlighting the utility of the tool, an application is presented using select contaminant holdings located in Spokane County, Washington.

The method of characteristics is used to develop analytical expressions for the transient one-dimensional movement of fine particles at the local scale. These solutions are applicable when the porous medium is homogeneous with a uniform steady-state velocity field, and under a local capture/detachment probability law for the fine particles. These local-scale solutions are then used for field-scale averaging operations, which involve prediction of the time-space evolutions of the moments of particle concentrations. At this large scale, the porous medium is represented by vertical, noninteracting, parallel pathways. Spatial variability, in the pore-water velocities and the net deposition, is assumed to exist in the horizontal directions. Further analysis leads to the development of analytical expressions for the univariate moments (to any arbitrary order) of the concentration of fine particles as functions of space and time. The initial and boundary conditions are allowed to be fairly general. Some hypothetical examples are considered to illustrate the utility of the method.

Estimates of temporal variation of the sediment yield are required for studies of reservoir sedimentation, river morphology, and soil and water conservation. In this paper, a method is proposed for prediction of the variation of sediment yield with time. This method is based on the assumption that the time-area curve of a catchment can be used to obtain the catchment response to an input. The sediment delivery ratio in a catchment is considered to be the function of land slope, area, and extent of forest cover. Carefully collected data from the experimental catchments are used for calibration and verification of the proposed method. The present method is found to produce realistic estimates for the total sediment yield.

In the present technical note, using recorded rainfall data h _t of given duration t ≤ 60 min for four regions of South Italy (Basilicata, Calabria, Campania, and Puglia) published by the Italian Hydrographic Service, the applicability of a power-rainfall depth-duration relationship is tested. At first, for 15 Calabrian recording rain gauges, the independence of the ratio h _t,T /h_60,T from the return period T is tested by hypothesizing that h _t are distributed according to an Extreme Value Distribution Type 1 (EV1) law, using six values of the return period. Then, for each geographical region, the exponent of the proposed power-rainfall depth-duration relationship is estimated by the least squares method. The comparison among the regional rainfall depth-duration relationships of the four regions of South Italy showed that the short-duration rainfall is dependent on geographical factors.