SummaryIncreasing availability of technologies, such as CD-ROMs and the WWW, in schools means that more teachers will have the potential to implement student-centered, inquirybased approaches to learning. Assessing what each student knows in a broad subject area, such as science, is difficult. Assessing students' understanding in circumstances where each student may pursue different topics of study, where there is no way to predict in advance what those topics of study will be, and where the possible topics of study include natural phenomena which are only beginning to be studied by professional scientists is more difficult. The authors recently faced such a challenge. To meet the challenge, the authors chose to assess studentlearning using an open-ended concept map activity combined with a rubric which extracts quantitative information about the quality of understanding from each map. This article describes the method the authors developed, including tests of reliability and validity.

Abstract. Land disposal of sewage effluent resulted in contamination of a sand and gravel aquifer (Cape Cod, Massachusetts) with zinc (Zn). The distribution of Zn was controlled by pH-dependent adsorption; the Zn extended 15 rn into the 30-m-thick sewage plume within approximately 100 rn of the source but only 2-4 rn into the plume between 100 and 400 rn downgradient. A two-dimensional vertical cross section model coupling groundwater flow with solute transport and equilibrium adsorption is 'used to simulate the influence of pH on Zn transport. Adsorption is described using semiempirical surface complexation models (SCM) by writing chemical reactions between dissolved Zn and mineral surface sites. SCM parameters were determined in independent laboratory experiments. A 59-year simulation with a one-site SCM describes the influence of pH on Zn transport well, with greater mobility at the low pH values near the upper sewage plume boundary than at the higher pH values deeper in the sewage-contaminated zone. Simulation with a two-site SCM describes both the sharpness and approximate location of the leading edge of the Zn-contaminated region. Temporal variations in pH of incoming groundwater can result in large increases in Zn concentration and mobility. The influence of spatial and temporal variability in pH on adsorption and transport of Zn was accomplished much more easily with the semiempirical SCM approach than could be achieved with distribution coefficients or adsorption isotherms.

Abstract:In the paper that is the foundation for this study, VanderKwaak and Loague (2001 The results from two stages of model calibration are presented. The uncertainty in initial soil-water content estimates for event-based simulation is shown to be a major limitation for physics-based models. The performance of InHM, relative to past event-based simulation efforts with a quasi-physically based rainfall-runoff model, is better for both peak stormflow and the time to peak stormflow, but worse for stormflow depth. The InHM simulations reported here set the stage for continuous simulation of near-surface response for the R-5 catchment with InHM.

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Abstract. The work reported here is the first part of a larger effort focused on efficient numerical simulation of redox zone development in contaminated aquifers. The sequential use of various electron acceptors, which is governed by the energy yield of each reaction, gives rise to redox zones. The large difference in energy yields between the various redox reactions leads to systems of equations that are extremely ill-conditioned. These equations are very difficult to solve, especially in the context of coupled fluid flow, solute transport, and geochemical simulations. We have developed a general, rational method to õolve such systems where we focus on the dominant reactions, compartmentalizing them in a manner that is analogous to the redox zones that are often observed in the field. The compartmentalized approach allows us to easily solve a complex geochemical system as a function of time and energy yield, laying the foundation for our ongoing work in which we couple the reaction network, for the development of redox zones, to a model of subsurface fluid flow and solute transport. Our method (1) solves the numerical system without evoking a redox parameter, (2) improves the numerical stability of redox systems by choosing which compartment and thus which reaction network to use based upon the concentration ratios of key constituents, (3) simulates the development of redox zones as a function of time without the use of inhibition factors or switching functions, and (4) can reduce the number of transport equations that need to be solved in space and time. We show through the use of various model performance evaluation statistics that the appropriate compartment choice under different geochemical conditions leads to numerical solutions without significant error. The compartmentalized approach described here facilitates the next phase of this effort where we couple the redox zone reaction network to models of fluid flow and solute transport.