Regional Groundwater Flow Model Research Articles

Regional Groundwater Flow Modeling Using Improved Isogeometric Analysis: Application and Implications in Unconfined Aquifer Systems

Regional Groundwater Flow Modeling Using Improved Isogeometric Analysis: Application and Implications in Unconfined Aquifer Systems

Numerical modeling of groundwater flow and nitrate transport in karst and wastewater irrigated agricultural and forest landscapes, PA, USA

AbstractUnderstanding the influence of treated wastewater reuse for irrigation on nitrate groundwater contamination in karstic landscape is critical for developing long‐term management plans and assessing the sustainability of these practices. The Pennsylvania State University has been operating a secondary treated wastewater effluent irrigation site—the “Living Filter”—since the 1960s. The objective of this study is to develop a regional groundwater flow and nitrate transport model to assess the impact of treated wastewater and disposal of large volumes of wastewater for irrigation of agricultural and forested lands on nitrate groundwater contamination in a karst landscape in the Spring Creek Watershed in central Pennsylvania. The modular three‐dimensional finite‐difference ground‐water flow model (MODFLOW) was used to construct the groundwater flow model of the local aquifer and simulate three‐dimensional regional changes in the water table over time. The simulation of nitrate fate and transport in the aquifer was conducted by developing a groundwater mass transport model using MODFLOW and a modular three‐dimensional transport multi‐species model (MT3DMS). The calibrated groundwater model yielded a root mean square error of 5.41 ft for the area of irrigation and 24.67 ft for the entire modeled area. Results of the calibrated model show the extension of a nitrate plume over most of the study area with concentrations below 10 mg·L−1 under the current 2015 setting and the long‐term 2035 scenario of increased irrigation rates. The nitrate migration was dominated by the presence of two high conductivity groundwater troughs adjacent to the study area resulting in a near steady‐state total mass since the 1990s.

Estimating hydraulic conductivity correlation lengths of an aquitard by inverse geostatistical modelling of a pumping test

Aquitards are common hydrogeological features in the subsurface. Typically, pumping tests are used to parameterize the hydraulic conductivity of heterogeneous aquitards. However, they do not take spatial variability and uncertainty into account. Alternatively, core-scale measurements of hydraulic conductivity are used in geostatistical upscaling methods, for which their correlation lengths are needed, but this information is extremely difficult to obtain. This study investigates whether a pumping test can be used to obtain the correlation lengths needed for geostatistical upscaling and account for the uncertainty about heterogeneous aquitard conductivity. Random realizations are generated from core-scale data with varying correlation lengths and inserted into a groundwater flow model which simulates the outcome of an actual pumping test. The realizations yielded a better fit to the pumping test data than the traditional pumping test result, assuming homogeneous layers are selected. Ranges of horizontal and vertical correlation lengths that fit the pumping-test well are found. However, considerable uncertainty regarding the correlation lengths remains, which should be considered when parameterizing a regional groundwater flow model.

DETERMINATION OF PARAMETERS OF WATER EXCHANGE AND MODELLING OF GROUNDWATER FLOW PROCESS FOR URANIUM MILL TAILINGS “ZAHIDNE” OF THE PRYDNIPROVSKY CHEMICAL PLANT

Results are presented of field hydrogeological studies aimed at determining the parameters of water exchange and modelling predictions of groundwater flow processes in the zone of influence of the “Zahidne” uranium mill tailing situated at the Prydniprovsky Chemical Plant, Kamianske. Hydrogeological observations allowed to estimate the infiltration recharge rate to groundwater at the study site within the first terrace (200 mm/year) and within the second terrace (150 mm/year) of the Dnipro River. According to tracer test the real groundwater flow velocity in the unconfined aquifer in sandy alluvial deposits in the zone of influence of the tailing is estimated at 0.26–0.52 m/day. Determined water exchange parameters and groundwater level monitoring data were used to develop using Visual Modflow software and calibrate the groundwater flow model for the area between the “Zahidne” uranium mill tailing and Konoplyanka River. Groundwater travel time in the unconfined aquifer from uranium mill tailing to Konoplyanka River estimated using calibrated model is 13 years, which is significantly less than the previous estimates on regional groundwater flow model (20 years). Several remediation scenarios were modelled, which allowed to develop recommendations on improving the protection of groundwater and surface water from radioactive contamination

Estimation of unrecorded groundwater abstractions in Jordan through regional groundwater modelling

Jordan suffers from water scarcity and groundwater covers the majority of Jordan’s water supply. Therefore, there is an urgent need to manage this resource conscientiously. A regional numerical groundwater flow model, developed as part of a decision support system for the country of Jordan, allows for quantification of the overexploitation of groundwater resources and enables determination of the extent of unrecorded agricultural groundwater abstraction. Groundwater in Jordan is abstracted from three main aquifers partly separated by aquitards. With updated geological, structural, and hydrogeological data available in the country, a regional numerical groundwater flow model for the whole of Jordan and the southernmost part of Syria was developed using MODFLOW. It was first calibrated for a steady-state condition using data from the 1960s, when groundwater abstraction was negligible. After transient calibration using groundwater level measurements from all aquifers, model results reproduce the large groundwater-level declines experienced in the last decades, which have led to the drying out of numerous springs. They show a reversal of groundwater flow directions in some regions, due to over-abstraction, and demonstrate that documented abstractions are not sufficient to cause the observed groundwater-level decline. Only after considering irrigation water demand derived from remote sensing data, the model is able to simulate these declines. Illegal abstractions can be quantified and predictive scenarios show the potential impact of different management strategies on future groundwater resources.

Regional Groundwater Flow Modelling of the Asturian Central Coal Basin (Spain)

Regional Groundwater Flow Modelling of the Asturian Central Coal Basin (Spain)

The Lake Chad transboundary aquifer. Estimation of groundwater fluxes through international borders from regional numerical modeling

Study RegionAfrica. The Lake Chad transboundary aquifer Study FocusTo understand transboundary groundwater hydrodynamics and estimate quantitative groundwater fluxes values between aquifer-sharing countries. To enable estimations, we developed an updated 3D ‘quasi steady-state’ regional groundwater flow model of the Chad Formation based on integrating extensive available information and reflecting the best current conceptual understanding to date. The conceptual model was tentatively assessed by a steady-state numerical model based on MODFLOW. New Hydrological Insights for the RegionThis model simulates lateral groundwater flows between neighboring countries, and also provides insights into the large-scale flow pattern and flows among hydroestratigraphic units. Modeling indicated that groundwater fluxes through international borders exist between Basin-sharing countries, except between Central African Republic and Cameroon, where a buffer area was considered for modeling purposes, leading to more uncertain results. From 14ºN parallel to further north, data are scanty and outcomes should be carefully considered. Forecasting transboundary impacts indicated that changes in recharge rates were more sensitive that changes in groundwater abstraction. To date, abstraction represents a small part of the water balance, but if it increases, it can become a driving factor in the future. Land use change and water use in the source areas (southern area) will have the strongest impact on transboundary groundwater flows due to changes in recharge, which will lead to quantitative changes in groundwater levels, artesian conditions, or even water quality.

Comparative Assessment of Methods for Coupling Regional and Local Groundwater Flow Models: A Case Study in the Beijing Plain, China

A coupled regional and local model is required when groundwater flow and solute transport are to be simulated in local areas of interest with a finer grid while regional aquifer boundary and major stresses should be retained with a coarser grid. The coupled model should also maintain interactions between the regional and local flow systems. In the Beijing Plain (China), assessment of managed aquifer recharge (MAR), groundwater pollution caused by rivers, capture zone of well fields, and land subsidence at the cone of depression requires a coupled regional and local model. This study evaluates three methods for coupling regional and local flow models for simulating MAR in the Chaobai River catchment in the Beijing Plain. These methods are the conventional grid refinement (CGR) method, the local grid refinement (LGR) method and the unstructured grid (USG) method. The assessment included the comparison of the complexity of the coupled model construction, the goodness of fit of the computed and observed groundwater heads, the consistency of regional and local groundwater budgets, and the capture zone of a well filed influenced by the MAR site. The results indicated that the CGR method based on MODFLOW-2005 is the easiest to implement the coupled model, capable of reproducing regional and local groundwater heads and budget, and already coupled with density and viscosity dependent model codes for transport simulation. However, the CGR method inherits shortcomings of finite difference grids to create multiple local models with inefficient computing efforts. The USG method based on MODFLOW-USG has the advantage of creating multi-scale models and is flexible to simulate rivers, wells, irregular boundaries, heterogeneities and the MAR site. However, it is more difficult to construct the coupled models with the unstructured grids, therefore, a good graphic user interface is necessary for efficient model construction. The LGR method based on MODFLOW-LGR can be used to create multiple local models in uniform aquifer systems. So far, little effort has been devoted to upgrade the LGR method for complex aquifer structures and develop the coupled transport models.

Characterization of a multi-layer karst aquifer through analysis of tidal fluctuation

Interaction between ocean tides and coastal aquifers is an important process influencing the quality and quantity of groundwater resources. In this study, high frequency groundwater fluctuation along two transects from the Gulf of Mexico to the Upper Floridan Aquifer (UFA) were analyzed to characterize the hydrogeologic structure of the multi-layer karst aquifer. Tidal efficiency and time lag of harmonic constants were calculated from groundwater and seawater level and aquifer diffusivity were estimated using the Jacob-Ferris equation. Measurable tidal signals were observed at a maximum inland distance of approximately 7–8 km due to the high conductivity of the unconfined karst aquifer. The largest tidal fluctuation was observed in the deeper unit of the aquifer (146–155 m below land surface), which indicate the hydraulic contact between seawater and the high permeability zone of Avon Park Formation. The estimated values of hydraulic diffusivities agreed well with the regional groundwater flow model despite the highly heterogeneous nature of karst aquifer. Our analysis suggested that tidal springs are important sea water/groundwater interfaces and the conduit networks of tidal creeks, caves, and fractures are preferential pathways of tidal propagation.

Evaluation of Groundwater Potential and Safe Yield of Heterogeneous Unconsolidated Aquifers in Chiang Mai Basin, Northern Thailand

Chiang Mai basin has an escalating population growth resulting in high demand for water consumption. Lack of surface water supply in most parts of the basin gives rise to the increasing use of groundwater which leads to a continuous decline in groundwater level in the past decades. This study is the first long-term groundwater monitoring and modeling study that aims at developing a transient, regional groundwater flow model of heterogeneous unconsolidated aquifers based on the MODFLOW program. Long-term groundwater monitoring data from 49 piezometers were used in model calibration and validation. The pilot points technique was used to account for the spatial variability of hydrogeologic parameters of heterogeneous aquifers. The simulation results and statistics showed that most sensitive and significant model parameters were spatially variable hydraulic conductivities and recharge rates. The Chiang Mai basin’s unconsolidated aquifers do not have high potential. The water table and/or potentiometric surface in the southeast and southwest areas of Chiang Mai city were continuously decreasing with no sign of recovery indicating critical groundwater condition and careful management must be considered. Safe yield calculation, based on a 2-m average drawdown threshold, suggested that unconsolidated aquifers of the Chiang Mai basin can sustain overall abstraction rates up to 51.2 Mm3/y or approximately 214% of the current extraction rates.

Physics-inspired integrated space–time artificial neural networks for regional groundwater flow modeling

Abstract. An integrated space–time artificial neural network (ANN) model inspired by the governing groundwater flow equation was developed to test whether a single ANN is capable of modeling regional groundwater flow systems. Model-independent entropy measures and random forest (RF)-based feature selection procedures were used to identify suitable inputs for ANNs. L2 regularization, five-fold cross-validation, and an adaptive stochastic gradient descent (ADAM) algorithm led to a parsimonious ANN model for a 30 691 km2 agriculturally intensive area in the Ogallala Aquifer of Texas. The model testing at 38 independent wells during the 1956–2008 calibration period showed no overfitting issues and highlighted the model's ability to capture both the observed spatial dependence and temporal variability. The forecasting period (2009–2015) was marked by extreme climate variability in the region and served to evaluate the extrapolation capabilities of the model. While ANN models are universal interpolators, the model was able to capture the general trends and provide groundwater level estimates that were better than using historical means. Model sensitivity analysis indicated that pumping was the most sensitive process. Incorporation of spatial variability was more critical than capturing temporal persistence. The use of the standardized precipitation–evapotranspiration index (SPEI) as a surrogate for pumping was generally adequate but was unable to capture the heterogeneous groundwater extraction preferences of farmers under extreme climate conditions.

Groundwater flow modelling for an assessment of abstraction and climate change impacts on groundwater resources—an example from the Wadi Araba Basin (Jordan)

The Wadi Araba Basin is a desert area in which finding water represents a real challenge. One potential source for the abstraction of groundwater is a sandstone aquifer which is partly covered by alluvial sediments distributed along the Wadi Araba Valley. The study analyses the impacts of abstraction and climate changes on groundwater quantity. It also aims to define a safe yield for the groundwater resources and to locate new wellfields in hydrogeological areas that will minimize high drawdown. Numerical modelling (MODFLOW NWT version) was applied in order to develop a regional groundwater flow model for the Wadi Araba Basin. The modelling area covers 6268 km2 (the Wadi Araba Basin on the Jordanian side). The cell sizes in horizontal directions are 500 m, and there are four layers in a vertical direction represented by the main aquifers and aquitards of the hydrogeological system (alluvial aquifer, A7/B2 carbonate aquifer, A1-A6 marly aquitard and Kb/Ram sandstone aquifer). Results of the modelling show that renewable groundwater resources stand at around 20 million cubic meters per year (MCM/year). According to the most pessimistic predictive scenario, the water table might decrease by 60 m by 2050, with the most optimistic scenario putting that figure at 24 m. Five new wellfield locations are proposed (four in the central part and one in the northern part of the Wadi Araba Valley). According to the model, the opening of new wellfields in these optimal locations, and the closing of some old wells where relatively large drawdowns were recorded, revealed that a more sustainable abstraction of groundwater could be achieved.

Groundwater flow modeling - A tool for water resources management in the khondalite and sandstone formations

The groundwater levels in the non-command areas of East Godavari (E.G.) district of Andhra Pradesh State, India are gradually plummeting over the years. To estimate the impact of excessive use of groundwater and to suggest appropriate water management strategies, a regional groundwater flow model has been constructed for a watershed of 50 km2 namely ‘Kondakalava watershed’ (a non-command area) which is composed of khondalitic (in the northern part) and sandstones (in the southern part) formations. The groundwater flow model has revealed that the sandstone aquifer is experiencing huge aquifer stress due to heavy pumping rates, high well density, etc. If the present rate of pumping continued by 1% increase in every year, the sandstone aquifer is going to be dried in the future year 2036 by encountering the drawdown of 12 m whereas the khondalitic aquifer sustains with the minimum drawdown of 4 m without producing the dry cells up to the future year 2040. The higher recharge conditions and decreased pumping rates are helped for rising future groundwater levels by nearly 4 m in sandstone formation. Hence it is suggested that the runoff can be injected by deep tube wells in sandstone aquifer. The new ponds are to be constructed in order to improve the water table conditions of sandstone aquifer and the diversion of the water can be made from the newly proposed canal which is passing nearby watershed. In the watershed, two zones are identified to connect the existing ponds in order to improve the recharge conditions in the sandstone formation.

Assessing changes in nitrogen contamination in groundwater using water aging: Waikato River, New Zealand

Groundwater and surface water contamination by nitrogen can produce serious health issues particularly regarding the contraction of methemoglobinemia in infants; prevention of this disorder is a main reason for regulating drinking water quality in New Zealand. To assist the management of water quality in the Upper Waikato Catchment in the light of growing intensification of nitrogen producing land uses, a regional groundwater flow and contamination transport model was developed to assess probable future nitrogen concentrations. Calibration of the steady state model was achieved with a root mean square error of 4.7% for groundwater levels and 7.9% for gains and losses in river flow within the catchment. Model performance was assisted by relationships determined between SiO2 concentrations in groundwater and water age. Modelling coupled with water aging indicates that the mean residence time of contributing water in the Upper Waikato Catchment ranges from 5 to 101 years with an average of 50 years. Some 77% of surface water is sourced from groundwater which is likely to convey the predominant contribution to the total nitrogen load. Nitrogen concentrations are increasing but the long-term total load is difficult to estimate owing largely to lack of information about historical land use and the degree of attenuation of nitrogen. The model and results obtained may be employed to provide information on water travel time distributions, appropriate source location and surface water receiving reaches in rivers and streams all of which is important and necessary for effective water quality management in the Catchment.

New insights into the hydrogeology and groundwater flow in the Great Barrier Reef catchment, Australia, revealed through 3D modelling

Study regionThe Great Barrier Reef catchment is located adjacent to the world's largest coral reef system, the Great Barrier Reef, in eastern Queensland, Australia. Study focusThis study characterized the geologic and hydrogeologic settings and evaluated the influence of regional faults on groundwater flow. 3D geological models of six regions within the catchment were constructed using drill-log data from >49,000 wells, digital elevation models and surface geological maps. The 3D models were then integrated with potentiometric surface maps and faults data to conceptualize the hydraulic relationships of aquifer units and estimate groundwater development potentials. Potentiometric surfaces and fault orientations were used to conceptualize groundwater flow directions. New hydrological insights for the regionThe 3D geological and hydrogeological characterizations revealed previously unknown faults and aquifer units in the study area. The study found that the central regions consisted of fractured and porous-unconfined aquifers, while confined aquifers, which extend to the coast and likely beyond, were also found in the northern and southern most regions. The orientations of the faults trended in NW-SE directions and could form conduits for south-easterly groundwater flow as opposed to the predominate easterly flow in the porous-unconfined and confined aquifers. The 3D models, aquifer connectivities and geometries provided crucial information to determine groundwater development potentials and offer a first step in developing local and regional groundwater flow and contaminant transport models.

A regional-scale conceptual and numerical groundwater flow model in fluvio-glacial sediments for the Milan Metropolitan area (Northern Italy)

Study regionThe Milan metropolitan area lies on one of the most important aquifer in Italy, heavily exploited for public and industrial water supply. The area, covering 3135 km2 in the Po Plain (Northern Italy) with a continental climate, is bounded by the Po, the Adda and the Ticino rivers and by the prealpine foothills. Regional hydrology is characterised by a network of natural and man-made elements, and lowland springs. The sedimentary sequence, from bottom to top, is formed by meandering river plain deposits, the distal fringe of the glacial outwash plains and proximal braid-plain deposits. Study focusThis study proposes a general approach for aquifer geometry reconstruction and hydrodynamic parametrization of hydrofacies in fluvio-glacial deposits, and their implementation into a 3D regional groundwater flow model. This approach is based on sedimentologically-defined lithofacies/hydrofacies and their correlation in space to obtain nearly homogeneous subunits starting from available data (i.e. 8628 borehole logs, grain size distributions, well tests) and sedimentological knowledge. New hydrological insights for the regionThe calibrated 3D FEM groundwater model allows quantifying the main components of the hydrogeological budget at the regional scale, and the fluxes among the different hydro-stratigraphic units. A sensitivity analysis of groundwater levels to the main recharge components suggests importance of anthropogenic disturbances with respect to natural recharge, and that land-use change may impact water resources more than climate change.

Groundwater mean residence times of a subtropical barrier sand island

Abstract. Fresh groundwater on barrier islands is affected by changing sea levels and precipitation variability due to climate change and is also vulnerable to anthropogenic processes, such as contamination and groundwater over-abstraction. Constraining groundwater mean residence times (MRTs) and flow paths is essential for understanding and managing these resources. This study uses tritium (3H) and carbon-14 (14C) to determine the MRTs of groundwater along a transect across subtropical North Stradbroke Island, south-east Queensland, Australia. Hydraulic properties, major ion geochemistry and stable isotopes are used to validate residence times and to identify the processes responsible for their variability. 3H activities range from less than 0.01 to 1 TU (tritium units), which are values lower than those of local average rainfall (1.6–2.0 TU). 14C concentrations range from 62.5 to 111 pMC (percent modern carbon). Estimated MRTs determined using lumped parameter models and 3H activities range from 37 to more than 50 years. Recharge occurs over the entire island, and groundwater MRTs generally increase vertically and laterally towards the coastal discharge areas, although no systematic pattern is observed. MRTs estimated from 14C concentrations display similar spatial relationships but have a much greater range (from modern to approximately 5000 years). Water diversion and retention by lower-permeability units in the unsaturated parts of the dune systems are the most likely course for relatively long MRTs to date. The results indicate that the internal structures within the dune systems increase MRTs in the groundwater system and potentially divert flow paths. The structures produce perched aquifer systems that are wide-spread and have a significant influence on regional recharge. The geochemical composition of groundwater remains relatively consistent throughout the island, with the only irregularities attributed to old groundwater stored within coastal peat. The outcomes of this study enhance the understanding of groundwater flow, recharge diversion and inhibition for large coastal sand masses in general, especially for older sand masses that have developed structures from pedogenesis and dune movement. With respect to south-east Queensland, it allows the existing regional groundwater flow model to be refined by incorporating independent MRTs to test models' validity. The location of this large fresh groundwater reservoir, in dry and populous south-east Queensland, means that its potential to be used as a water source is always high. Background information on aquifer distribution and groundwater MRTs is crucial to better validate impact assessment for water abstraction.

On the Optimal Spatial Design for Groundwater Level Monitoring Networks

AbstractEffective groundwater monitoring networks are important, as systematic data collected at observation wells provide a crucial understanding of the dynamics of hydrogeological systems as well as the basis for many other applications. This study investigates the influence of six groundwater level monitoring network (GLMN) sampling designs (random, grid, spatial coverage, and geostatistical) with varying densities on the accuracy of spatially interpolated groundwater surfaces. To obtain spatially continuous prediction errors (in contrast to point cross‐validation errors), we used nine potentiometric groundwater surfaces from three regional MODFLOW groundwater flow models with different resolutions as a priori references. To assess the suitability of frequently‐used cross‐validation error statistics (MAE, RMSE, RMSSE, ASE, and NSE), we compared them with the actual prediction errors (APE). Additionally, we defined upper and lower thresholds for an appropriate spatial density of monitoring wells. Below the lower threshold, the observation density appears insufficient, and additional wells lead to a significant improvement of the results. Above the upper threshold, additional wells lead to only minor and inefficient improvements. According to the APE, systematic sampling lead to the best results but is often not suited for GLMN due to its nonprogressive characteristic. Geostatistical and spatial coverage sampling are considerable alternatives, which are in contrast progressive and allow evenly spaced and, in the case of spatial coverage sampling, yet reproducible coverage with accurate results. We found that the global cross‐validation error statistics are not suitable to compare the performance of different sampling designs, although they allow rough conclusions about the quality of the GLMN.

Trend prediction of regional groundwater level with GISFEFLOW model in Beijing Mihuaishun plain, China

A three dimensional finite element model (FEFLOW) has been used for regional ground water flow modeling of Mihuaishun plain located in the south of Beijing, China. The numerical groundwater flow model is developed considering recharge components (precipitation infiltration, river leakage, and irrigation return flow). GIS interface is created for each source of recharge. Hydraulic conductivities and storage coefficient have been calibrated by the steady state model using the recorded data from 2007 to 2009. The model results is useful to identify the aquifer characteristics and to analyze the groundwater dynamics. The groundwater level monitoring network will be improved by analyzing groundwater levels. The future development scenarios are proposed to predict the trend changes of groundwater levels.

Assessment of groundwater depletion caused by excessive extraction through groundwater flow modeling: the Celaya aquifer in central Mexico

A numerical groundwater flow model was developed for the Celaya aquifer in central Mexico to assess the potential impacts of excessive groundwater use in the region. The numerical simulations were made with the program FEFLOW, which allowed the use of an irregular mesh that was adjusted to boundaries with irregular geometries and refined in wells and streams. Considering mountain block hydrology, both diffuse and stream-focused recharges were simulated. Hydraulic properties were assigned according to the three-dimensional distribution of lithologic units, which were estimated using geological information from the region. The model was calibrated using static water level measurements in wells and qualitative information of some hydrological processes. Some of the trends observed in the residuals were related to time-constant boundary conditions which can only be solved by developing a regional groundwater flow model. The simulations were undertaken for predevelopment, present, and future scenarios, up to the year 2030. The future simulations considered two possible scenarios: (1) future extractions are equal to current extractions; and (2) future extractions increase gradually in time. The results show that current groundwater extractions are not sustainable in the long term and decreases in extraction rates are required to achieve sustainable groundwater use in the region.