Coupled Groundwater Flow Research Articles

Development of a simulation-optimization model for sustainable operation of groundwater heat pump system

Groundwater heat pump (GWHP) system directly uses a stable temperature of groundwater for cooling and heating of buildings or districts. In this study, a simulation-optimization model for the sustainable operation of GWHP system was developed. The simulation model computed the coupled groundwater flow and heat transport with considering thermal recycling. As an optimization technique, genetic algorithm was linked with the simulation model. The developed model can determine the optimal extraction and injection rates of GWHP system to maximize system performance and efficiency, to maintain stable operation, and to minimize costs and environmental impacts. Assuming a 2D hypothetical aquifer, it was demonstrated how the optimal operation of GWHP system obtained from the model can be changed according to several operational and hydrogeological conditions, including well distance, regulation on injection water temperature, hydraulic conductivity, and regional hydraulic gradient. The results highlighted a necessity to determine an operational strategy of GWHP system through the optimization able to consider site-specific condition. In addition, the performance and capability of the developed model were demonstrated by obtaining the optimal strategy for the cooling operation of a research-oriented GWHP facility installed in Korea.

Development of a groundwater flow and reactive solute transport model in the Yongding River alluvial fan, China

The Yongding River in the western suburbs of Beijing has been recharged with reclaimed water since 2010 for the purpose of ecological restoration. Where the reclaimed water is not well treated, it poses a danger to the aquifer underneath the river. To provide a reliable tool which could be used in future research to quantify the influence of reclaimed water in the Yongding River on the local groundwater environment, a transient groundwater flow and reactive solute transport model was developed using FEFLOW™ in the middle-upper part of the Yongding River Alluvium Fan. The numerical model was calibrated against the observed groundwater levels and the concentrations of typical solutes from June 2009 to May 2010 and validated from June 2010 to December 2010. The average RMSE and R2 of groundwater level at four observation wells are 0.48 m and 0.61, respectively. The reasonable agreement between observed and simulated results demonstrates that the developed model is reliable and capable of predicting the behavior of groundwater flow and typical contaminant transport with reactions. Water budget analysis indicates that the water storage in this aquifer had decreased by 43.76×106m3 from June 2009 to December 2010. The concentration distributions of typical solutes suggest that the middle and southern parts of the unconfined aquifer have been polluted by previous discharge of industrial and domestic sewage. The results underscore the necessity of predicting the groundwater response to reclaimed water being discharged into the Yongding River. The study established a coupled groundwater flow and reactive solute transport model in the middle-upper part of the Yongding River Alluvium Fan, one of the drinking water supply sites in Beijing city. The model would be used for risk assessment when reclaimed water was recharged into Yongding River.

Alternating direction implicit scheme as Finite-Difference method to solve coupled groundwater flow and contaminant transport model in the coastal aquifer

The simulation of coupled groundwater flow-contaminant transport equation can be conducted numerically. The Finite-Difference Method (FDM) through Alternating Direction Implicit (ADI) Scheme will be applied in this research. The ADI scheme is chosen because the FDM in two-dimensional form. The ADI scheme recognized to solve two-dimensional PDE (Partial Differential Equation) due the Crank-Nicholson has matrix dimension problem and other problem from another scheme of FDM. This research will use ADI scheme to transform the groundwater flow and contaminant transport equation become the discrete form. The model algorithm translated into VBA (Visual Basic Algorithm) code and the physical parameter determined hypothetically. The result of model simulation describes that the ADI scheme can be applied to solve coupled groundwater flow-contaminant transport equation. The result can be stated as stable because it reached the steady condition after 60 days of simulation.

Optimal remediation design and simulation of groundwater flow coupled to contaminant transport using genetic algorithm and radial point collocation method (RPCM)

The simulation-optimisation models of groundwater and contaminant transport can be a powerful tool in the management of groundwater resources and remediation design. In this study, using Multiquadratic Radial Basis Function (MRBF) a coupled groundwater flow and reactive transport of contaminant and oxidant was developed in the framework of the Meshfree method. The parameter analysis has determined the optimum shape parameter (0.97), and the results of the model were compared with a physical sandbox model which were in good agreement. The genetic algorithm approach was used to find the optimum design of the remediation using permanganate as an oxidant. To find the optimum design we considered two objectives and two constraints. The results revealed that the breakthrough of contaminant to the downstream area of interest and the concentration of the contaminant in this area is reduced significantly with optimisation.

Numerical simulations of shallow groundwater flow and heat transport in continuous permafrost setting under impact of climate warming

Numerical simulations of coupled groundwater flow and heat transport are used to address how hydrogeological conditions can affect permafrost dynamics. The simulations are based on a two-dimensional vertical-plane conceptual model of a study site at the Iqaluit Airport, Nunavut, Canada, which includes a 50 m deep permafrost terrain with a shallow active layer, overlain by a paved taxiway with winter snow-covered embankments. Coupled groundwater flow and advective–conductive heat transport with freeze–thaw dynamics, temperature-dependent pore-water freezing functions, and latent heat are included in the model. The simulation results show that a smooth (low-slope) freezing function with a higher residual unfrozen moisture content produced a deeper thaw front compared to that using a steeper freezing function, generating a maximum increase in the depth to permafrost of 17.5 m after 268 years. Permafrost thaw rates in high-permeability zones within a heterogeneous system were also relatively higher compared to an otherwise equivalent homogeneous soil, resulting in a maximum increase of 2.6 m in the depth to permafrost after 238 years. As recharge water cools while flowing along the upgradient permafrost table, advectively driven heat transport is paradoxically shown to temporarily increase the height of the permafrost table in downgradient areas.

Application of geochemical and groundwater data to predict sinkhole formation in a gypsum formation in Manitoba, Canada

Numerical modelling approaches were used to investigate coupled groundwater flow and reactive transport processes in gypsum karst sub-terrain. A regional equipotential map and steady-state flow model were created using scarce data to gain insights into flow patterns and identify potential areas at risk for cavity and sinkhole development in a shallow gypsum formation. Coupled flow and reactive transport modelling was used to simulate the dissolution of gypsum between a sinkhole in a man-made drainage ditch and a quarry, where freshwater enters the drainage ditch and flows toward the quarry. Field data from a tracer test were used to characterize flow in the study area. The resulting regional equipotential map was valuable in identifying potential areas of sinkhole development; sinkholes occurred in areas underlain by thick gypsum formations with high flow gradients and radial flow. The reactive transport model was valuable in identifying the growth of the cavity and the timeline for the potential risk to road infrastructure. The reactive transport model indicated that cavity growth could be slowed by removing the inflow of freshwater into the drainage ditch. Groundwater equipotential maps, flow models and reactive transport models are valuable tools for the investigation of sub-terrain karst development including cavity development and sinkhole formation in evaporite minerals.

Numerical simulation of groundwater age distributions in the hierarchical subsurface flow system: a case study of Jinan in northern China

Understanding groundwater age is essential for the aquifer management. In this study, a typical hydrogeological cross section in Jinan is selected to set up a coupled groundwater flow and age transport model. The simulation results show that hierarchical groundwater flow systems, composed by the nested local, intermediate and regional subsystems, are developed from the northern piedmont of Mount Taishan to Qiguang Fault. The scales of these flow subsystems are primarily controlled by the topography. Aquifer lithology variations also influence the groundwater flow system. Groundwater age distributions are closely related to the flow system. Groundwater age becomes older along the flow paths and increases abruptly near the internal stagnation point, which can be used as a characteristic boundary between different groundwater flow hierarchies. The results of the present research will not only help understand the groundwater circulation regimes in Jinan, promote the rational development and utilization of groundwater resources, but also provide references for the practical applications of groundwater flow system theory.

Controls on geothermal heat recovery from a hot sedimentary aquifer in Guardbridge, Scotland: Field measurements, modelling and long term sustainability

Geothermal heat from Hot Sedimentary Aquifers represents a promising intermediate (30°C) resource for district heating systems. A key control on the geothermal productivity of these aquifers is the architecture of faults, which can significantly enhance or reduce the natural permeability of these systems. We present the first three-dimensional coupled groundwater flow and heat transport numerical model, combining multiple data from field mapping and fracture surveys, of two intersecting major fault systems in Central-East Scotland. This includes fault-zone fracture permeability modelling, depth-dependent permeability modelling, geo-mechanical facies assessment and heat productivity estimates for single well and multiple well extraction scenarios. Simulations indicate that with careful location of extraction wells within permeable fault systems, production is sustainable for over 50 years for multiple-well extraction scenarios in this region.

Geothermal potential assessment of the Nevado del Ruiz volcano based on rock thermal conductivity measurements and numerical modeling of heat transfer

This work presents an estimation of the geothermal potential of the Nevado del Ruiz (NDR) volcano, bridging the knowledge gap to develop geothermal energy in Colombia and improve resource estimates in South America. Field work, laboratory measurements, geological interpretations, 2D numerical modeling, and uncertainty analysis were conducted to the northwest of the NDR to assess temperature at depth and define thermal energy content. About 60 rock samples were collected at outcrops to measure thermal conductivity with a needle probe. A 2D numerical model, built from an inferred geological cross-section, was developed with the software OpenGeoSys to simulate the underground temperature distribution and then estimate the geothermal potential of a 1 km2 area with sufficient temperature, assuming a recovery factor equal to 2.4% and a 30 years exploitation time. Coupled groundwater flow and heat transfer were simulated in steady-state considering two different thermal conductivity scenarios. Results show that the average estimated potential is 1.5 × 10−2 MWt m−1 of the reservoir thickness, considering temperatures greater than 150 °C located at a depth of approximately 2 km, in a selected area situated outside of the Los Nevados National Natural Park (NNP), to avoid any direct intervention on this protected area. According to a Monte Carlo analysis considering pessimist and optimist scenarios of thermal conductivity, the estimated geothermal power was 1.54 × 10−2 MW m−1 (σ = 2.91 × 10−3 MW m−1) and 1.88 × 10−2 MW/m (σ = 2.91 × 10−3 MW m−1) for the two modeling scenario considered.

Direct and Inverse Modeling of Seawater Intrusion: A Perspective

ABSTRACT Sea Water Intrusion (SWI) is one of the major hydrological problem in coastal aquifers. This global issue is aggravated by increasing demands for freshwater in coastal regions. In this paper, different approaches of modeling SWI and the parameters affecting the process of SWI are introduced. This is followed by the discussion on the numerical models to solve the complex, three dimensional (3D) groundwater and solute transport problems in coastal aquifer. The importance of considering aquifer characteristics while modeling the groundwater system for flow and solute transport is emphasized. A brief discussion on previous methodology, novelty and limitations on direct simulation of SWI are tabulated. The significant aspects to be considered while direct modeling of coastal aquifers are discussed and the recent focus of research in this area of interest are stated. In direct modeling, information on aquifer parameters are often unknown, therefore, an inverse approach is explained briefly. The previous studies relating to determistic inverse modeling for coupled groundwater flow and solute transport problems reported in literature are summarized. The insights about prior information, estimated parameter sensitivities, variances and correlations are reported. The paper identifies some of the existing gaps in the modeling of SWI based on the previous work and provides comprehensive understanding on direct and deterministic inverse SWI modeling.

Geothermal assessment of the Pisa plain, Italy: Coupled thermal and hydraulic modeling

This paper explores the possibility of a development project with a geothermal well doublet in the Pisa plain, Italy. The performance of the system has been evaluated with a 3-dimensional field-scale numerical model that simulates the evolution of temperature and pressure conditions in the aquifer, under different exploitation scenarios. Coupled groundwater flow and thermal transport processes in the reservoir are considered together with non-Darcy fluid flow in the wellbores, and heat exchange between boreholes and surrounding rock formations. Calculations are performed with a parallelized version of the wellbore-reservoir simulator T2Well. This code allows for the efficient modeling of coupled hydraulic-thermal processes over a domain about 40 km2 wide and 1.5 km thick. Simulation results indicate that the energy of the reservoir is sufficient for the designed extraction rate (between 80 and 150 m3/h), but also suitable for much larger rates, up to 250 m3/h. Although aimed at assessing the long-term performance of a specific system, this modeling approach could be profitably applied for the design of similar projects elsewhere.

Reactive transport modelling of the hydro-geochemical behaviour of partially oxidized acid-generating mine tailings with a monolayer cover

The efficiency of a monolayer cover to prevent acid mine drainage (AMD) generation from two pre-oxidized tailings impoundments was assessed using the MIN3P code, a finite volume model for coupled groundwater flow, oxygen diffusion and multi-component reactive transport. Numerical simulations were validated using large column tests set up in the laboratory and monitored during 19 wetting and drying cycles, over approximately two years. Results indicate that a monolayer cover made of either non acid-generating tailings or a till was not able to prevent sulfide oxidation in the underlying reactive tailings, both for the conditions applied in the laboratory and under conditions more representative of field observations. The efficiency of the simulated monolayer cover was highly dependent on the position of the water table. Despite improved water quality, the reactive tailings nonetheless continued to oxidize and generate AMD, even when the water table was close to the surface of the tailings (1 m depth). The results from this study indicate that the efficiency criteria for cover systems on fresh unoxidized tailings may not be directly applicable for pre-oxidized tailings. The paper also presents a few modifications to the MIN3P code that were successfully applied to simulate the hydrogeological and geochemical response of pre-oxidized, already acidic and highly contaminated tailings.

Coupled groundwater flow and contaminant transport simulation in a confined aquifer using meshfree radial point collocation method (RPCM)

In this study, a meshfree radial point collocation method is used to model the contaminant transport through confined aquifer. The discretization of the governing equations is done by a point collocation method and radial basis functions (RBF) are used as the interpolation function. For comparative study, two widely used radial basis functions namely multi-quadrics and exponential RBF are used. A local circular support domain is employed to construct the shape functions. In the model, no information on nodal inter-relationship is required for shape function construction except the nodal coordinates, unlike in finite-difference (FDM) or finite-element (FEM) based methods. The developed model is validated through benchmark problems in one and two dimensions. Further, application of the model for advective transport with high Peclet number has been studied and the model has been found to be effective in handling the instability of high Peclet problems. For the field problem considered, the results obtained from the model have been compared with the FEM solution and was found to be satisfactory. This method is relatively easy to implement and offers better accuracy with acceptable computational time. Considering the significant advantages offered by this method, it can serve as a good alternative to the conventional methods.

Analytical solution and computer program (FAST) to estimate fluid fluxes from subsurface temperature profiles

AbstractThis study details the derivation and application of a new analytical solution to the one‐dimensional, transient conduction‐advection equation that is applied to trace vertical subsurface fluid fluxes. The solution employs a flexible initial condition that allows for nonlinear temperature‐depth profiles, providing a key improvement over most previous solutions. The boundary condition is composed of any number of superimposed step changes in surface temperature, and thus it accommodates intermittent warming and cooling periods due to long‐term changes in climate or land cover. The solution is verified using an established numerical model of coupled groundwater flow and heat transport. A new computer program FAST (Flexible Analytical Solution using Temperature) is also presented to facilitate the inversion of this analytical solution to estimate vertical groundwater flow. The program requires surface temperature history (which can be estimated from historic climate data), subsurface thermal properties, a present‐day temperature‐depth profile, and reasonable initial conditions. FAST is written in the Python computing language and can be run using a free graphical user interface. Herein, we demonstrate the utility of the analytical solution and FAST using measured subsurface temperature and climate data from the Sendia Plain, Japan. Results from these illustrative examples highlight the influence of the chosen initial and boundary conditions on estimated vertical flow rates.

Numerical investigation of coupled density‐driven flow and hydrogeochemical processes below playas

AbstractNumerical modeling approaches with varying complexity were explored to investigate coupled groundwater flow and geochemical processes in saline basins. Long‐term model simulations of a playa system gain insights into the complex feedback mechanisms between density‐driven flow and the spatiotemporal patterns of precipitating evaporites and evolving brines. Using a reactive multicomponent transport model approach, the simulations reproduced, for the first time in a numerical study, the evaporite precipitation sequences frequently observed in saline basins (“bull's eyes”). Playa‐specific flow, evapoconcentration, and chemical divides were found to be the primary controls for the location of evaporites formed, and the resulting brine chemistry. Comparative simulations with the computationally far less demanding surrogate single‐species transport models showed that these were still able to replicate the major flow patterns obtained by the more complex reactive transport simulations. However, the simulated degree of salinization was clearly lower than in reactive multicomponent transport simulations. For example, in the late stages of the simulations, when the brine becomes halite‐saturated, the nonreactive simulation overestimated the solute mass by almost 20%. The simulations highlight the importance of the consideration of reactive transport processes for understanding and quantifying geochemical patterns, concentrations of individual dissolved solutes, and evaporite evolution.

Optimizing conjunctive use of surface water and groundwater for irrigation to address human-nature water conflicts: A surrogate modeling approach

In arid and semi-arid areas where agriculture competes keenly with ecosystem for water, integrated management of both surface water (SW) and groundwater (GW) resources at a basin scale is crucial, but often lacks scientific support. This study implemented physically-based, fully integrated SW–GW modeling in optimizing water management, and performed surrogate modeling to replace the computationally expensive model with simple response surfaces. Water use conflicts between agriculture and ecosystem in Heihe River Basin (HRB), the second largest inland river basin in China, were investigated. Based on the integrated model GSFLOW (Coupled Ground-Water and Surface-Water Flow Model), the conjunctive use of SW and GW for irrigation in the study area was optimized using a surrogate-based approach named DYCORS (DYnamic COordinate search using Response Surface models). Overall, the study demonstrated that, with the surrogate modeling approach, an expensive integrated model could be efficiently incorporated into an optimization analysis, and the integrated modeling would make feasible a physically based interpretation of the optimization results. In the HRB case study, the surrogate-based optimization suggested a very different time schedule of water diversion in opposite to the existing one, indicating the critical role of SW–GW interactions in the water cycle. With the temporal optimization, a basin-scale water saving could be achieved by reducing non-beneficial evapotranspiration. In addition, the current flow regulation in HRB may not be sustainable, because the ecosystem recovery in the lower HRB would be at the cost of the ecosystem degradation in the middle HRB.

Indication of deep groundwater flow through the crystalline rocks of southern Norway

We assess the major aspects of the subsurface thermal regime within crystalline rocks of southern Norway. Results of two-dimensional modeling of coupled groundwater flow and heat transfer demonstrate that advective cooling due to groundwater flow is an important factor for the reduction of subsurface temperatures in southwestern Norway (the Bergen and Stavanger areas) where the normal annual precipitation is high on the western (windward) side of the Scandes mountains. On the other hand, the influence of groundwater flow on subsurface temperatures is most likely relatively low in southeastern Norway (the Moss area), which represents the rain-shadow area with light precipitation and is characterized by smoothed landforms. Therefore, where the reduced subsurface temperatures are associated with high atmospheric precipitation and complex surrounding topography, deep groundwater flow is most likely present within crystalline rocks. In this case, the observed low values of the thermal gradient within crystalline rocks can be used as groundwater flow tracers.

A coupled geothermal model of the Alpokalja area, Hungary—Austria

A coupled groundwater flow and heat transport model was developed for a trans-boundary geothermal reservoir located in the Alpokalja area. The study area lies in the western part of the Pannonian Basin, at the border between Hungary and Austria. The study area contains several famous geothermal water utilizations on both sides of the border, which has an impact on natural groundwater conditions. The aim of the modeling study was to evaluate the natural-state and production-state groundwater conditions, and to make predictions on cross-boundary interferences. A three-dimensional finite element-type coupled geothermal model was constructed to provide a coherent quantitative representation of geothermal flow systems. The model described the hydraulic behavior of the flow system, the interaction between different reservoirs, and geothermal conditions.

Modeling surface water-groundwater interaction in arid and semi-arid regions with intensive agriculture

In semi-arid and arid areas with intensive agriculture, surface water-groundwater (SW-GW) interaction and agricultural water use are two critical and closely interrelated hydrological processes. However, the impact of agricultural water use on the hydrologic cycle has been rarely explored by integrated SW-GW modeling, especially in large basins. This study coupled the Storm Water Management Model (SWMM), which is able to simulate highly engineered flow systems, with the Coupled Ground-Water and Surface-Water Flow Model (GSFLOW). The new model was applied to study the hydrologic cycle of the Zhangye Basin, northwest China, a typical arid to semi-arid area with significant irrigation. After the successful calibration, the model produced a holistic view of the hydrological cycle impact by the agricultural water use, and generated insights into the spatial and temporal patterns of the SW-GW interaction in the study area. Different water resources management scenarios were also evaluated via the modeling. The results showed that if the irrigation demand continuous to increase, the current management strategy would lead to acceleration of the groundwater depletion, and therefore introduce ecological problems to this basin. Overall, this study demonstrated the applicability of the new model and its value to the water resources management in arid and semi-arid areas.

3D hydro-mechanically coupled groundwater flow modelling of Pleistocene glaciation effects

Pleistocene glaciation led to temporal and spatial variations of sub-surface pore fluid pressure. In basins covered by ice sheets, fluid flow and recharge rates are strongly elevated during glaciations as compared to inter-glacial periods. Present-day hydrogeological conditions across formerly glaciated areas are likely to still reflect the impact of glaciations that ended locally more than 10 thousand years before present.3D hydro-mechanical coupled modelling of glaciation can help to improve the management of groundwater resources in formerly glaciated basins.An open source numerical code for solving linear elasticity, which is based on the finite element method (FEM) in 3D, has been developed. By coupling this code with existing 3D flow codes it is possible to enable hydro-mechanical coupled modelling.Results of two benchmark simulations are compared to existing analytical solutions to demonstrate the performance of the newly developed code. While the result for a fluid–structure coupled case is in reasonable agreement with the analytical model, the result for a classical structure–fluid coupled benchmark showed that the analytical solution only matches the numerical result when the relevant coupling parameter (loading efficiency) is known in advance. This indicates that the applicability of widely applied approaches using an extra term in the groundwater flow equation for vertical stress to simulate hydro-mechanical coupling might have to be re-evaluated.A case study with the commercial groundwater simulator FEFLOW demonstrates the newly developed solution.