Groundwater modelling to help diagnose contamination problems

Aquifer remediation for a contaminated site is complex, expensive, and long-term. Groundwater modelling is often used as a tool to evaluate remedial alternatives and to design a groundwater remediation system. Groundwater modelling can also be used as a useful process to identify aquifer characteristics and contaminant behaviour that are not realized prior to modelling, to help diagnose what happened and why it happened at contaminant sites. Three real-world modelling cases are presented to demonstrate how groundwater modelling is applied to help understand contamination problems and how valuable the improved understanding is to decision-making and/or to remedial design.

Analysis of travel time, sources of water and well protection zones with groundwater models

This study compares numerical models with analytical solutions in computing travel times and radius of protection zones for a pumping well located in an unconfined aquifer with uniform recharge and in a semi-confined aquifer. Numerical models were capable of delineating protection zones using particle tracking method in both cases. However, protection zones defined by travel time criterion can only protect small percent of source water to the well; large percent of source water is not protected which may pose a risk of pollution of source water to the well. The case study of Leggeloo well field in the Netherlands indicates that although a well field protection area was enforced in 1980 s, elevated nitrate concentration has been monitored in the abstracted water since 1990 s. The analysis of protection areas shows that the current protection area only protects 37.4% of recharge water to the well field. A large protection area must be adopted in order to safeguard the sustainable water supply for the local community.

Geochemical modeling of groundwater in southern plain area of Pengyang County,Ningxia,China

The purpose of this paper is to examine the evolution mechanisms of a hydrochemical field and to promote its benefits to the living standards of local people and to the local economy in the southern plain area of Pengyang County, in Ningxia, China. Based on understanding of the hydrogeological conditions in Pengyang County, the chemical evolution characteristics of groundwater in the plain area were analyzed. PHREEQC geochemical modeling software was used to perform hydrochemical modeling of water-rock interaction and to quantitatively analyze the evolution processes and the formation mechanisms of the local groundwater. Geochemical modeling was performed for two groundwater paths. The results showed that, along path 1, Na＋ adsorption played the leading role in the precipitation process and its amount was the largest, up to 6.08 mmol/L; cation exchange was significant along path 1, while along simulated path 2, albite accounted for the largest amount of dissolution, reaching 9.06 mmol/L, and the cation exchange was not significant. According to the modeling results, along the groundwater flow path, calcite and dolomite showed oversaturated status with a precipitation trend, while the fluorite and gypsum throughout the simulated path were not saturated and showed a dissolution trend. The total dissolved solids （TDS） increased and water quality worsened along the flow path. The dissolution reactions of albite, CO2, and halite, the exchange adsorption reaction of Na＋, and the precipitation of sodium montmorillonite and calcite were the primary hydrogeochemical reactions, resulting in changes of hydrochemical ingredients.

Modeling Groundwater Levels on the Calera Aquifer Region in Central Mexico Using ModFIow

A conceptual model for the Calera Aquifer has been created to represent the aquifer system beneath the Calera Aquifer Region （CAR） in the State of Zacatecas, Mexico. The CAR area was uniformly partitioned into a 500 x 500 m grid generating a high resolution model that represented the natural boundaries of the aquifer. A computer model was calibrated and validated to verify output from the model corresponding to situations that matched the historical aquifer performance. Predicted groundwater levels were compared with measured data collected from nine observation wells between 1954 and 2004 to evaluate model performance. The main objective of this study was to develop and evaluate a groundwater modeling system using ModFlow-2000 for the CAR. Performance statistics indicated that the model performed well in simulating historic groundwater levels in the central part of the CAR where irrigated agriculture was concentrated. Results evaluation yielded average coefficients of determination of 0.81 and 0.67 and root mean square error values lower than 25.1 m and 25.9 m for the calibration and validation processes, respectively. These results are indicative of a good agreement between predicted and observed groundwater levels. However, further improvements in the conceptual model may be needed to improve predictions in other parts of the CAR for evaluating alternative groundwater management strategies.

Modeling for Inter-Basin Groundwater Transfer Identification: The Case of Upper Rift Valley Lakes and Awash River Basins of Ethiopia

Groundwater movement beneath watershed divide is one component of the hydrological cycle that is typically ignored due to difficulty in analysis. Numerical ground-water models, like TAGSAC, have been used extensively for predicting aquifer responses to external stresses. In this paper TAGSAC code was developed to identify the inter-basin groundwater transfer (IBGWT) between upper Awash River basin (UARB) and upper rift valley lakes basin (URVLB) of Ethiopia. For the identification three steady state groundwater models (for UARB, URVLB and for the two combined basins) were first created and calibrated for the 926 inventoried wells. The first two models are conceptualized by considering the watershed divide between the two basins as no-flow. The third model avoids the surface water divide which justifies IBGWT. The calibration of these three models was made by changing the recharge and hydrogeologic parameters of the basins. The goodness of fit indicators (GoFIs) obtained was better for the combined model than the model that describes the URVLB. Furthermore, the hydraulic head distribution obtained from the combined model clearly indicates that there is a groundwater flow that doesn’t respect the surface water divide. The most obvious effect of IBGWT observed in these two basins is that it diminishes surface water discharge from URVLB, and enhances discharge in the UARB. Moreover, the result of this study indicates potential for internal and cross contamination of the two adjacent groundwater.

The Design of a Three-Dimensional Physical Modeling System for Real-Time Groundwater Flows

In the past decades,physical modeling has been widely used in hydrogeology for teaching,studying and exhibition purposes.Most of these models are used to illustrate hydrogeological profiles,but few can depict three-dimensional groundwater flows,making it impossible to validate groundwater flows simulated by numerical methods with physical modeling.

Groundwater flow through fractured rocks and seepage control in geotechnical engineering: Theories and practices

Groundwater flow through fractured rocks has been recognized as an important issue in many geotechnical engineering practices.Several key aspects of fundamental mechanisms,numerical modeling and engineering applications of flow in fractured rocks are discussed.First,the microscopic mechanisms of fluid flow in fractured rocks,especially under the complex conditions of non-Darcian flow,multiphase flow,rock dissolution,and particle transport,have been revealed through a com-bined effort of visualized experiments and theoretical analysis.Then,laboratory and field methods of characterizing hydraulic properties(e.g.intrinsic permeability,inertial permeability,and unsaturated flow parameters)of fractured rocks in different flow regimes have been proposed.Subsequently,high-performance numerical simulation approaches for large-scale modeling of groundwater flow in frac-tured rocks and aquifers have been developed.Numerical procedures for optimization design of seepage control systems in various settings have also been proposed.Mechanisms of coupled hydro-mechanical processes and control of flow-induced deformation have been discussed.Finally,three case studies are presented to illustrate the applications of the improved theoretical understanding,characterization methods,modeling approaches,and seepage and deformation control strategies to geotechnical engi-neering projects.

Evaluation of groundwater resource potential by using water balance model:A case of Upper Gilgel Gibe Watershed,Ethiopia

Groundwater resource potential is the nation’s primary freshwater reserve and accounts for a large portion of potential future water supply.This study focused on quantifying the groundwater resource potential of the Upper Gilgel Gibe watershed using the water balance method.This study began by defining the project area’s boundary,reviewing previous works,and collecting valuable primary and secondary data.The analysis and interpretation of data were supported by the application of different software like ArcGIS 10.4.1.Soil water characteristics of SPAW(Soil-plant-air-water)computer model,base flow index(BFI+3.0),and the water balance model.Estimation of the areal depth of precipitation and actual evapotranspiration was carried out through the use of the isohyetal method and the water balance model and found to be 1664.5 mm/a and 911.6 mm/a,respectively.A total water volume of 875829800 m^(3)/a is estimated to recharge the aquifer system.The present annual groundwater abstraction is estimated as 10150000 m^(3)/a.The estimated specific yield,exploitable groundwater reserve,and safe yield of the catchment are 5.9%,520557000 m^(3)/a,and 522768349 m^(3)/a respectively.The total groundwater abstraction is much less than the recharge and the safe yield of the aquifer.The results show that there is a sufficient amount of groundwater in the study area,and the groundwater resources of the area are considered underdeveloped.

Groundwater Management at West El-Minia Desert Area, Egypt Using Numerical Modeling

Water shortage is the main problem facing any development in Egypt especially in the desert lands. River Nile is considered the main source of water in Egypt but its water covers only the area of flood plain where its tributaries do not reach to the desert. The desert fringes, west of El-Minia governorate, Egypt, are areas of natural expansion for agricultural, industrial, and civil activities. This implies an increasing demand for groundwater. A numerical groundwater model is one of the main tools used for assessment of the resource potential and prediction of future impact under different circumstances and stresses. In this paper, a transient groundwater flow model in the desert district west of El-Minia, Egypt, was developed. The conceptual model was built by analyzing the hydrogeological data and previous work. Steady state model of year 1990 was used to investigate and calibrate the parameters such as hydraulic conductivities, recharge and conductance of the surface water streams. The storage coefficients are calibrated by the transient model based on the available data observed from 1990 to 2013, which provides insights to understand the behavior of groundwater system in Quaternary Aquifer and to predict spatial-temporal distributions of groundwater levels and groundwater flow in responding to extraction of water. The calibrated transient model will be used to predict the impacts of desert development schemes and water resources management schemes on groundwater in the study area.

Application of machine learning models in groundwater quality assessment and prediction:progress and challenges

Groundwater quality assessment and prediction(GQAP)is vital for protecting groundwater resources.Traditional GQAP methods can not adequately capture the complex relationships among attributes and have the disadvantage of being computationally demanding.Recently,the application of machine learning(ML)in GAQP(GQAPxML)has been widely studied due to ML’s reliability and efficiency.While many GQAPxML publications exist,a thorough review is missing.This review provides a comprehensive summary of the development of ML applications in the field of GQAP.First,the workflow of ML modeling is briefly introduced,as are data preparation,model development,model evaluation,and model application.Second,299 publications related to the topic are filtered,mainly through ML modeling.Subsequently,many aspects of GQAPxML,such as publication trends,the spatial distribution of study areas,the size of data sets,and ML algorithms,are discussed from a bibliometric perspective.In addition,we review in detail the well-established applications and recent findings for several subtopics,including groundwater quality assessment,groundwater quality modeling using groundwater quality parameters,groundwater quality spatial mapping,probability estimation of exceeding the groundwater quality threshold,groundwater quality temporal prediction,and the hybrid use of ML and physics-based models.Finally,the development of GQAPxML is explored from three perspectives:data collection and preprocessing,model building and evaluation,and the broadening of model applications.This review provides a reference for environmental scientists to better understand GQAPxML and promotes the development of innovative methods and improvements in modeling quality.

Groundwater regime and calculation of yield response in North China Plain: a case study of Luancheng County in Hebei Province

The groundwater table has been declining at a rate of 0.65 m/yr in Luancheng County since large scale groundwater extraction carried out in the 1960s. The drop of precipitation, substantial increase in agricultural output, variations of crop planting structure and construction of water conservancy projects in the headwater area all tie up with the decline of the groundwater table. On the basis of analyzing the hydrogeological conditions and the water resources utilization of Luancheng County, a three-dimensional groundwater flow model was developed to simulate the county抯 groundwater flow through finite-difference method using Visual Modflow software. We divide the research field into four parts after analyzing the hydrogeological condition. Based on parameter calibration and adjustment using measured data, the hydraulic conductivity and specific yield were simulated. Using the calibrated model, we analyze the agricultural water saving potentiality and its influence on the groundwater. The results are as follows: (1) if we decrease the amount of water extracted by 0.14×108 m3, the average groundwater table of the five observation wells in December will rise by 0.33 m; (2) if we decrease the water by 0.29×108 m3, the average groundwater table of the five observation wells in December will rise by 0.64 m; and (3) if we increase the water by 0.29×108 m3, the average groundwater table of the five observation wells in December will decline by 0.45 m. So we can draw a conclusion that controlling the agricultural water use is an important way to prevent the decline of groundwater table.

Numerical simulation of groundwater and early warnings from the simulated dynamic evolution trend in the plain area of Shenyang,Liaoning Province(P.R. China)

Groundwater level is the most direct factor reflecting whether groundwater is in a virtuous cycle. It is the most important benchmark for deciding whether a balance can be struck between groundwater discharge and recharge and whether groundwater exploitation will trigger problems pertinent to environment, ecology and environmental geology. According to the borehole and long-term monitoring wells data in the plain area of Shenyang, a numerical groundwater model is established and used to identify and verify the hydrogeological parameters and balanced items of groundwater. Then the concept of red line levels, the control levels of groundwater is proposed, the dynamic evolution trend of groundwater under different scenarios is analyzed and predicted and groundwater alerts are given when groundwater tables are not between the lower limit and the upper limit. Results indicated:(1) The results of identification and verification period fitted well, and the calculation accuracy of balanced items was high;(2) with the implementation of shutting wells, groundwater levels in urban areas of Shenyang would exceed the upper limit water level after 2020 and incur some secondary disasters;(3) under the recommended scenario of water resources allocation, early-warnings for groundwater tables outside the range would occur in the year of 2020, 2023, 2025 respectively for successive wet, normal and dry years. It was imperative to reopen some groundwater sources and enhance real-time supervision and early-warning to prevent the occurrence of potential problems.

Impact of longwall mining on groundwater above the longwall panel in shallow coal seams

Since longwall mining causes subsidence through the overlying strata to the ground surface, the surface water and groundwater above the longwall panels may be affected and drained into the lower levels.Therefore, loss or interruption of streams and overburden aquifers is a common concern in coal industry.This paper analyzed the potential effects of longwall mining on subsurface water system in shallow coal seam. In order to monitor different water level fluctuations throughout the mining period, three water wells were drilled down to the proposed deformation zone above the longwall panel. A GGU-SS-FLOW3 D model was used to predict water table contours for the periods of pre- and post-mining conditions. The field data from the three water wells were utilized to calibrate the model. The field test and numerical model can help to better understand the dewatering of shallow aquifers and surface waters related to ground subsidence from longwall mining in shallow coal seam.