Yanhui Dong

A Parallel PCG Solver for MODFLOW

In order to simulate large-scale ground water flow problems more efficiently with MODFLOW, the OpenMP programming paradigm was used to parallelize the preconditioned conjugate-gradient (PCG) solver with in this study. Incremental parallelization, the significant advantage supported by OpenMP on a shared-memory computer, made the solver transit to a parallel program smoothly one block of code at a time. The parallel PCG solver, suitable for both MODFLOW-2000 and MODFLOW-2005, is verified using an 8-processor computer. Both the impact of compilers and different model domain sizes were considered in the numerical experiments. Based on the timing results, execution times using the parallel PCG solver are typically about 1.40 to 5.31 times faster than those using the serial one. In addition, the simulation results are the exact same as the original PCG solver, because the majority of serial codes were not changed. It is worth noting that this parallelizing approach reduces cost in terms of software maintenance because only a single source PCG solver code needs to be maintained in the MODFLOW source tree.

Influence of South to North Water Transfer on groundwater dynamic change in Beijing plain

Beijing is a city of severe water shortage. The groundwater plays a key role in the water supply. However, the groundwater level has been gradually descending due to extensive pumping in consecutive drought years. How to satisfy the water demand and recover the groundwater level is an urgent work. With the implementation of the South to North Water Transfer Project, an opportunity has been provided for restoration of groundwater under over exploitation. On the basis of hydrogeology conditions of the Beijing plain, as well as the high-performance parallel computing platforms, a groundwater flow numerical model was established. And dynamic monitoring data of groundwater levels were used to calibrate the numerical model. The calculation results fit well with the measured data in the calibrated model. Therefore, the calibrated model can be used to predict the dynamic change of groundwater levels in the Beijing plain. The results show that several obvious depression cones of groundwater have been formed because of the rapid decline of groundwater levels in the Beijing plain in recent years. After the implementation of the South to North Water Transfer project and due to the restrictions on groundwater exploitation, the area of cone of depression will be reduced to different degrees, the central water level of depression cone will increase, and some cones of depression around wellhead will disappear. It is a benefit to relieve water shortage and control the development of land subsidence and the deterioration of the ecological environment.

Using hydrochemical and isotopic data to determine sources of recharge and groundwater evolution in an arid region: a case study in the upper-middle reaches of the Shule River basin, northwestern China

Understanding sources of recharge and mechanisms for hydrogeochemical evolution of groundwater in the Shule River basin, an arid inland river basin in northwestern China, is essential for successful water resource management. Sources of water and associated groundwater recharge processes were investigated using hydrogeological, hydrogeochemical, and isotopic methods. The study area was divided into three parts: Changma, the Shule valley, and the Beishan area. The hydrogeochemical and isotopic analysis results show that groundwater in Changma mainly originated from precipitation in the Qilian Mountains. Lateral flow from Changma is the primary recharge mechanism, while direct recharge by infiltration of precipitation can be neglected in the Shule valley. Minerals within the aquifer material (e.g., halite, calcite, dolomite, and gypsum) dissolve into water that recharges the groundwater system. Therefore, strong linear relationships were found between Na+ and Cl− and between Ca2++Mg2+ and SO42−+HCO3−, with stoichiometry ratios of approximately 1:1 in both cases. Groundwater quality in the region is generally good, with low salinity and hardness. Local precipitation is the only source of recharge for groundwater in the Beishan area. Low groundwater velocity and dry climatic conditions suggest that dissolution of minerals from the aquifer material controls hydrogeochemical evolution. Water rarely travels from the Beishan area to the Shule valley; therefore, almost all the groundwater in the Shule valley originates from Changma. Global climate change and loss of glaciers in the Qilian Mountains may lead to a reduction of recharge; therefore, the groundwater resources of the Shule valley must be managed to sustain water supply.

Short note: An areal recharge and discharge simulating method for MODFLOW

As a widely used groundwater flow model, MODFLOW offers a set of packages to simulate hydrologic stresses, inflows and outflows, to a groundwater system. Speci@?cally, MODFLOW lacks a general method to process areally distributed recharge and discharge to groundwater. One solution would be to create a new package for MODFLOW. Alternatively, it is also possible to make the best use of existing code to the same effect. In this note, a simple, yet effective method to simulate areal recharge and discharge is proposed based on the recharge (RCH) package of MODFLOW, allowing multiple instances of the RCH package to be used in one model. The method has been implemented in MODFLOW2000/2005 and has been successfully applied to a regional groundwater flow model to simulate areally distributed precipitation recharge, agricultural discharge and irrigation infiltration recharge in a simple approach.

Wellhead protection area delineation using multiple methods: a case study in Beijing

Determining a wellhead protection area (WHPA) is a basic and important step in protecting groundwater from contamination. Based on availability of data and complexity of hydrogeological conditions, different delineation methods can be adopted. This study’s objective was to examine the relation and difference of the results calculated using six WHPA delineation methods, which ranged from simplified shapes to stochastic models with superior sampling means. All the methods were applied to a well field in Beijing, China. The WHPA probability distribution from a stochastic simulation was used as reference for comparison with other methods. WHPAs calculated using simplified and analytical approaches turned out to be smaller than reference WHPA. Those calculated using the numerical model provided better results but still cannot include a considerable area of reference WHPA. The stochastic approach based on efficient orthogonal Latin hypercube sampling provided an estimation of the probability distribution of WHPA delineation with different degrees of uncertainty, which played an important role in the WHPA delineation of the well field in Beijing. To attain a realistic WHPA delineation in a complex aquifer system, various delineation approaches can be adopted and all results that validate each other must be considered.

Distributed Parallel Computing in Stochastic Modeling of Groundwater Systems

Stochastic modeling is a rapidly evolving, popular approach to the study of the uncertainty and heterogeneity of groundwater systems. However, the use of Monte Carlo-type simulations to solve practical groundwater problems often encounters computational bottlenecks that hinder the acquisition of meaningful results. To improve the computational efficiency, a system that combines stochastic model generation with MODFLOW-related programs and distributed parallel processing is investigated. The distributed computing framework, called the Java Parallel Processing Framework, is integrated into the system to allow the batch processing of stochastic models in distributed and parallel systems. As an example, the system is applied to the stochastic delineation of well capture zones in the Pinggu Basin in Beijing. Through the use of 50 processing threads on a cluster with 10 multicore nodes, the execution times of 500 realizations are reduced to 3% compared with those of a serial execution. Through this application, the system demonstrates its potential in solving difficult computational problems in practical stochastic modeling.

Assessment of uncertainty in discrete fracture network modeling using probabilistic distribution method

There have been widespread concerns about solute transport problems in fractured media, e.g. the disposal of high-level radioactive waste in geological fractured rocks. Numerical simulation of particle tracking is gradually being employed to address these issues. Traditional predictions of radioactive waste transport using discrete fracture network (DFN) models often consider one particular realization of the fracture distribution based on fracture statistic features. This significantly underestimates the uncertainty of the risk of radioactive waste deposit evaluation. To adequately assess the uncertainty during the DFN modeling in a potential site for the disposal of high-level radioactive waste, this paper utilized the probabilistic distribution method (PDM). The method was applied to evaluate the risk of nuclear waste deposit in Beishan, China. Moreover, the impact of the number of realizations on the simulation results was analyzed. In particular, the differences between the modeling results of one realization and multiple realizations were demonstrated. Probabilistic distributions of 20 realizations at different times were also obtained. The results showed that the employed PDM can be used to describe the ranges of the contaminant particle transport. The high-possibility contaminated areas near the release point were more concentrated than the farther areas after 5E6 days, which was 25,400 m2.

Efficient natural attenuation of acidic contaminants in a confined aquifer

As the demand for energy grows steadily and fossil fuels are depleted gradually, nuclear power will continue to play an important role in meeting energy needs in the future. Uranium, as the fuel for nuclear power generation, has experienced the increase in production in recent years. Acid in situ leach uranium mining is an effective and popular technique to extract uranium without exposure of workers to radioactive ore deposits. Despite this advantage, this technique causes extremely high concentrations of contaminants in confined aquifers after mining is completed. These contaminants will undergo natural attenuation while migrating downgradient with regional groundwater. Mounting concerns have been raised regarding widespread groundwater contamination due to concentrations of contaminants and lack of published studies for comparison. In this study, the fate of various contaminants was examined to infer the extent of natural attenuation in a confined aquifer in northwest China. Results indicate the efficiency of natural attenuation in the confined aquifer. In addition, the contaminant plume migrated much slower than regional groundwater. The dual-domain theory has been invoked to aid in the interpretation of the decoupling of the plume migration with the regional groundwater flow. This study also implies that widespread contamination in confined aquifers is more likely to be caused by external factors (e.g., mechanical failure, human errors) than post-mining spreading of contaminants. To be always safe, strict monitoring schemes must still be established and operated.