Kenneth L. Kipp

A geochemical transport model for redox‐controlled movement of mineral fronts in groundwater flow systems: A case of nitrate removal by oxidation of pyrite

A one-dimensional prototype geochemical transport model was developed in order to handle simultaneous precipitation-dissolution and oxidation-reduction reactions governed by chemical equilibria. Total aqueous component concentrations are the primary dependent variables, and a sequential iterative approach is used for the calculation. The model was verified by analytical and numerical comparisons and is able to simulate sharp mineral fronts. At a site in Denmark, denitrification has been observed by oxidation of pyrite. Simulation of nitrate movement at this site showed a redox front movement rate of 0.58 m yr−1, which agreed with calculations of others. It appears that the sequential iterative approach is the most practical for extension to multidimensional simulation and for handling large numbers of components and reactions. However, slow convergence may limit the size of redox systems that can be handled.

Approaches to the simulation of unconfined flow and perched groundwater flow in MODFLOW.

Various approaches have been proposed to manage the nonlinearities associated with the unconfined flow equation and to simulate perched groundwater conditions using the MODFLOW family of codes. The approaches comprise a variety of numerical techniques to prevent dry cells from becoming inactive and to achieve a stable solution focused on formulations of the unconfined, partially-saturated, groundwater flow equation. Keeping dry cells active avoids a discontinuous head solution which in turn improves the effectiveness of parameter estimation software that relies on continuous derivatives. Most approaches implement an upstream weighting of intercell conductance and Newton-Raphson linearization to obtain robust convergence. In this study, several published approaches were implemented in a stepwise manner into MODFLOW for comparative analysis. First, a comparative analysis of the methods is presented using synthetic examples that create convergence issues or difficulty in handling perched conditions with the more common dry-cell simulation capabilities of MODFLOW. Next, a field-scale three-dimensional simulation is presented to examine the stability and performance of the discussed approaches in larger, practical, simulation settings.

Parallel processing for PHAST: a three-dimensional reactive-transport simulator

A parallel algorithm for the reactive-transport simulator PHAST was developed for a Beowulf cluster of Linux PCs. The Local Area Multicomputer implementation of the Message Passing Interface standard was used for communication among processors. PHAST simulates reactive transport by operator splitting the calculation into a flow and transport step and a chemical reaction step. A load-balancing algorithm was developed using random mapping of cells to the processors for the reaction task and rebalancing after each time step. Testing with a demonstration field-scale example showed that the parallel algorithm was scalable over a range of processors and problem sizes. Load balancing is an important element of the parallel algorithm. Super-linear speedup indicated that the sequential program is not optimal in cache usage. Relative speedups on 16.8 effective processors ranged from 10 to 24 which can bring field-scale reactive-transport simulations into a reasonable timeframe.