Alternate sources of water for capture by a well near a stream. A, Stream in an aquifer that overlies a regional aquifer with potentially different hydraulic properties. B, Stream in an aquifer system where pumping may interact with plants and change evapotranspiration losses from the system .

The importance of monitoring networks for resource-management decisions is becoming more recognized, in both theory and application. Quantitative computer models provide a science-based framework to evaluate the efficacy and efficiency of existing and possible future monitoring networks. In the study described herein, two suites of tools were used to evaluate the worth of new data for specific predictions, which in turn can support efficient use of resources needed to construct a monitoring network. The approach evaluates the uncertainty of a model prediction and, by using linear propagation of uncertainty, estimates how much uncertainty could be reduced if the model were calibrated with addition information (increased a priori knowledge of parameter values or new observations). The theoretical underpinnings of the two suites of tools addressing this technique are compared, and their application to a hypothetical model based on a local model inset into the Great Lakes Water Availability Pilot model are described. Results show that meaningful guidance for monitoring network design can be obtained by using the methods explored. The validity of this guidance depends substantially on the parameterization as well; hence, parameterization must be considered not only when designing the parameter-estimation paradigm but also-importantly-when designing the prediction-uncertainty paradigm.

This paper presents an analysis of several methods for approximately solving the equations governing multicomponent mass transport with homogeneous and heterogeneous chemical reactions. A derivation of the governing equations allowing a fairly general description of the chemistry is given. It is shown that a number of different formulations are possible depending on the choice of primary variables. Discretization of the governing equations reduces the problem to solving a set of nonlinear algebraic equations each time step. It is shown that the choice of method to solve the algebraic equations should be based on the nature of the chemical reactions. An analysis of the equations yields recommendations in some cases and direction for numerical experimentation in others.

To model ground water flow in a shallow aquifer underlying a salt marsh, the impact of total stress changes due to tidal loading of the marsh surface on the piezometric head within the aquifer must be considered. If the deformation of the porous medium is negligible, a ground water flow model may be modified to account for the changing total stress. The incorporation of these modifications into the U.S. Geological Survey (USGS) numerical model SUTRA is discussed, and example problems for which analytic solutions are available are presented to demonstrate the behavior and validity of the modified model. In addition, the form of the specific pressure storativity term used in the SUTRA model is discussed and modifications of this form are suggested.

All groundwater pumped is balanced by removal of water somewhere, initially from storage in the aquifer and later from capture in the form of increase in recharge and decrease in discharge. Capture that results in a loss of water in streams, rivers, and wetlands now is a concern in many parts of the United States. Hydrologists commonly use analytical and numerical approaches to study temporal variations in sources of water to wells for select points of interest. Much can be learned about coupled surface/groundwater systems, however, by looking at the spatial distribution of theoretical capture for select times of interest. Development of maps of capture requires (1) a reasonably well-constructed transient or steady state model of an aquifer with head-dependent flow boundaries representing surface water features or evapotranspiration and (2) an automated procedure to run the model repeatedly and extract results, each time with a well in a different location. This paper presents new methods for simulating and mapping capture using three-dimensional groundwater flow models and presents examples from Arizona, Oregon, and Michigan.