A Compartmental–Spatial System Dynamics Approach to Ground Water Modeling

High-resolution, spatially distributed ground water flow models can prove unsuitable for the rapid, interactive analysis that is increasingly demanded to support a participatory decision environment. To address this shortcoming, we extend the idea of multiple cell (Bear 1979) and compartmental (Campana and Simpson 1984) ground water models developed within the context of spatial system dynamics (Ahmad and Simonovic 2004) for rapid scenario analysis. We term this approach compartmental–spatial system dynamics (CSSD). The goal is to balance spatial aggregation necessary to achieve a real-time integrative and interactive decision environment while maintaining sufficient model complexity to yield a meaningful representation of the regional ground water system. As a test case, a 51-compartment CSSD model was built and calibrated from a 100,000+ cell MODFLOW (McDonald and Harbaugh 1988) model of the Albuquerque Basin in central New Mexico (McAda and Barroll 2002). Seventy-seven percent of historical drawdowns predicted by the MODFLOW model were within 1 m of the corresponding CSSD estimates, and in 80% of the historical model run years the CSSD model estimates of river leakage, reservoir leakage, ground water flow to agricultural drains, and riparian evapotranspiration were within 30% of the corresponding estimates from McAda and Barroll (2002), with improved model agreement during the scenario period. Comparisons of model results demonstrate both advantages and limitations of the CCSD model approach.     

A Practical Approach to Evaluating Natural Attenuation of Contaminants in Ground Water

The extent of natural attenuation is an important consideration in determining the most appropriate corrective action at sites where ground water quality has been impacted by releases of petroleum hydrocarbons or other chemicals. The objective of this study was to develop a practical approach that would evaluate natural attenuation based on easily obtained field data and field tested indicators of natural attenuation. The primary indicators that can he used to evaluate natural attenuation include plume characteristics and dissolved oxygen levels in ground water. Case studies of actual field sites show that plumes migrate more slowly than expected, reach a steady state, and decrease in extent and concentration when natural attenuation is occurring. Background dissolved oxygen levels greater than 1 to 2 mg/L and an inverse correlation between dissolved oxygen and contaminant levels have been identified through laboratory and field studies as key indicators of aerobic biodegradation. an important attenuation mechanism. Secondary indicators such as geochemical data, and more intensive methods such as contaminant mass balances, laboratory microcosm studies, and detailed ground water modeling can demonstrate natural attenuation as well. The recommended approach for evaluating natural attenuation is to design site assessment activities so that required data such as dissolved oxygen levels and historical plume flow path concentrations are obtained. With the necessary data, the primary indicators should be applied to evaluate natural attenuation. II the initial evaluation suggests that natural attenuation is a viable corrective action alternative, then a monitoring plan should be implemented to verify the extent of natural attenuation.     

Ground Water Quality Monitaring: The Arkansas Approach

Monitoring ground water quality on a statewide basis is a challenge being faced by a number of state and federal agencies involved with water quality. Many of these agencies have come forth with publications that are of some use to those who are engaged in developing monitoring networks. A review of this literature could save those involved much time and money by providing insight into what can be accomplished and/or avoided. Ultimately, each monitoring system has to be designed to meet the purposes and conditions for which it is created, no two situations ever being exactly alike. However, there are approaches and methods that can be borrowed and profitably utilized where both the problems confronted and the geology in which they are found are similar. It was the attempt to resolve these two conflicting tenets, situational uniqueness and methodological transferability, that impelled the state of Arkansas to develop the prototype approach that is to be described, along with some of the more important documents that were of use in the development of that approach.
Figure I highlights the main tenets of the prototype approach and Figure II locates each prototype within the state of Arkansas.     

Modeling Ground Water Quality Sampling Decisions

Questions such as what, where, when, and how often to sample play a central role in the development of monitoring strategies. Limited resources will not permit sampling for many contaminants at the same frequency at all well sites. Therefore, a resource allocation strategy is necessary to arrive at answers for the preceding types of questions. Such a strategy for a ground water quality monitoring program is formulated as an integer programming model (an optimization model). The model will be of use in the process of deciding what constituents to sample and where to sample them so as to maximize a given objective, subject to a set of budget, sampling, and regulatory constraints. The maximization objective in the model is defined as a weighted function of population exposure to a scaled measure of observed chemical concentrations. The sampling constraints are based on the observed variability of contaminants in the aquifer, needed precision in estimates, a chosen level of significance, the available budget for implementing the program, and selected regulatory constraints. The model is tested with field data obtained for 10 selected constituents from more than 650 wells in the Cambrian-Ordovician aquifer in Iowa. Results from two alternative formulations of the model are compared, analyzed, and discussed. Further avenues for research are briefly outlined.     

Modeling Management Practice Effects on Pesticide Movement to Ground Water

The assessment of agricultural impacts on water quality are now being redirected to include both ground water and surface water. Mathematical models have enhanced the ability of scientists'to evaluate these impacts. A variety of public domain models are available that can aid in evaluating the effects of managerial activities on pesticide movement to ground water. However, the ideal non-point source (NPS) pollution management model does not exist. Current models fail to adequately describe the transport of chemicals to ground water and, simultaneously, the effect of managerial practices on transport mechanisms. Much more work is necessary to develop a model that can describe water quality impacts of agricultural practices in a holistic framework that includes ground water and surface water concerns.     

Modeling a Ground Water Circulation Well Alternative

Ground water circulation wells (GCWs) provide an appealing alternative to typical pump-and-treat ground water remediation systems because of the inherent resource-conservative nature of the GCW systems. GCW performance prediction is challenging because the consideration of extraction and recharge in a single well is unusual for most practitioners, the technology is relatively new, and a meaningful body of literature has not been published. A three-part evaluation process using state-of-the-practice numerical ground water flow and mass transport models was developed for application during GCW pilot studies at the former Nebraska Ordnance Plant site. A small-scale ground water flow model was developed during the pilot study planning process to predict the system performance and to locate performance-measuring monitoring wells. Key predictions included the capture zone predicted to develop upgradient of the GCW, the downgradierit GCW recharge zone, and the circulation zone centered on the GCW. The flow model was subsequently verified using ground water elevation data and contaminant concentration data collected during pilot study operation. Aquifer parameters were reestimated as a result of the verification process. Those parameter values were used as input to a larger scale model, which was used to develop a remedial alternative consisting of multiple GCW systems.     

A Gas Chromatographic/Chemical Indicator Approach to Assessing Ground Water Contamination by Petroleum Products

The water-soluble fractions of unleaded gasoline, kerosene and diesel fuel were evaluated by U.S. EPA Methods 602, 610, and 625.
Several chemical indicator compounds useful in assessing petroleum contamination of ground water, including benzene, substituted benzenes, n-alkanes, and polynuclear aromatic hydrocarbons, were identified. These were applied to the interpretation of data collected from monitoring wells at gasoline service stations that were undergoing ground water remediation. The chemical indicators are used to identify the likely type(s) of petroleum contamination. Certain hydrocarbons may be unique to specific fuel types.
Gas chromatograms of field sample extracts were compared with chromatograms of laboratory water-soluble fractions (WSFs) and neat fuels (unleaded gasoline, kerosene, and diesel). In some situations, field samples represented water-soluble fractions of the contaminating fuel. In others, a fuel-water agglomeration was indicated, with the chromatograms showing peaks that represented components of both the WSFs and the neat fuels.
The use of both gas chromatography pattern identification and chemical indicators appears to be a viable approach to assessing ground water contamination caused by petroleum products.