Saturated Unsaturated Transport Research Articles

Composite Modeling for Evaluation of Groundwater and Soil Salinization on the Multiple Reclaimed Land Due to Sea-Level Rise

In this study, the applicability of the composite model for assessing seawater intrusion and soil salinization in coastal aquifers due to climate change was investigated. In this approach, flow in the saturated zone of a coastal aquifer is simulated using a three-dimensional saturated–unsaturated transport model and flow in the unsaturated zone between the surface and groundwater level is simulated using a one-dimensional model in the vertical direction. Long-term sea-level predictions obtained using the representative concentration pathway (RCP) 4.5 and 8.5 scenarios were applied for computing the sea-level rise for 91 country-managed reclaimed areas in the Republic of Korea. Composite results were obtained and analyzed for seawater intrusion and soil salinization due to sea-level rise. In the results of groundwater and soil salinity in all 91 reclaimed land, the increasing rate of groundwater and soil salinity in the RCP 4.5 scenario was 13.5% and 10.4%, respectively. In the RCP 8.5 scenario, the increasing rate of groundwater and soil salinity was 14.1% and 11.1%, respectively. The groundwater level increased to 0.41 m in the RCP 4.5 scenario and 0.51 m in the RCP 8.5 scenario. The results for two representative reclaimed land areas in the Heungwang and Deokchon districts were examined in detail. The composite analysis revealed that widespread damage could be caused by sea-level rise in the reclaimed land and that seawater intrusion in many regions will accelerate groundwater salinization over time. Moreover, the reclaimed land areas were characterized in terms of watershed size, presence of ponds, water levels of the ponds, and pond locations. In reclaimed land located in small watersheds, the groundwater recharge area was smaller than in land located in larger watershed areas. Consequently, the seawater in small watersheds penetrated further inland. Ponds with water levels higher than the sea level effectively prevented seawater intrusion into groundwater. If the water level of a pond is similar to or lower than the sea level, it indicates that seawater has already penetrated a large part of the aquifer. The composite model developed in this study seems to be one of the simulation methods that can be applied when simulating saturated and unsaturated zone to a large number of sites. Also, the study results could be used to establish and implement a long-term comprehensive plan for water resources at the national level, considering seawater intrusion due to climate change and providing a basis for establishing countermeasures against future seawater intrusion.

A Modeling Approach for Assessing Groundwater Resources of a Large Coral Island under Future Climate and Population Conditions: Gan Island, Maldives

This study assesses the future groundwater supply of a large coral island, Gan Island, Republic of Maldives, under influences of rainfall patterns, sea level rise, and population growth. The method described in this paper can be used to estimate the future groundwater supply of other coral islands. Gan is the largest inhabited island (598 ha) of the Republic of Maldives with a population of approximately 4500. An accurate estimate of groundwater supply in the coming decades is important for island water security measures. To quantify future groundwater volumes in Gan, a three-dimensional, density-dependent groundwater and solute transport model was created using the SUTRA (Saturated Unsaturated Transport) modeling code. The Gan model was tested against observed groundwater salinity concentrations and then run for the 2012–2050 period to compare scenarios of future rainfall (from General Circulation Models), varying rates of population growth (i.e., groundwater pumping), and sea level rise. Results indicate that the total fresh groundwater volume increases approximately 20% if only future rainfall patterns are considered. If moderate pumping is included (2% annual population growth rate), the volume increases only by 13%; with aggressive pumping (9% annual population growth rate), the volume decreases by 24%. Sea level rise and associated shoreline recession leads to an additional 15–20% decrease in lens thickness and lens volume. Results can be used to make decisions about water resource management on Gan and other large coral islands in the Indian and Pacific Oceans. Methods used herein can be applied to any coral island to explore future groundwater security.

Sea Level Rise Impact on Atoll Islands: Implication to SDG 6

Low-lying atoll islands that depend heavily on fresh groundwater for survival are particularly vulnerable to sea level rise (SLR), which calls for appropriate climate action SDG 13. As the sea level rises, the associated increase in surface seawater inundation and subsurface saltwater intrusion will reduce the availability of fresh groundwater due to permanent salinization of groundwater with corresponding thinning of freshwater lens. This paper provides scientific insights on how freshwater lens in atoll islands respond to SLR. Simulations on saturated-unsaturated variable-density groundwater flow with salt transport are performed by the groundwater flow and solute transport model SUTRA (Saturated-Unsaturated Transport) developed by the U.S. Geological Survey. Model simulations and statistical analyses suggest that freshwater lens thickness depends mainly on groundwater recharge rate, island size and aquifer hydraulic conductivity. The impact of various geo-hydrologic parameters on fresh groundwater sustainability is then analyzed to explore feasibility of increasing groundwater recharge through rainwater harvesting, as a mitigation measure. The implication to the achievement of sustainable clean water and sanitation for all (SDG 6) is also discussed.

Assessing Impacts of Climate Change and Sea-Level Rise on Seawater Intrusion in a Coastal Aquifer

The objectives of this study were to assess the climate change impacts on sea-level rise (SLR) and freshwater recharge rates and to investigate these SLR and freshwater recharge rates on seawater intrusion in coastal groundwater systems through the Saturated-Unsaturated Transport (SUTRA) model. The Gunsan tide gauge station data were used to project SLR based on polynomial regressions. Freshwater recharge rates were assumed as 10% of the projected annual precipitation under climate change. The Byeonsan2 groundwater monitoring well for seawater intrusion was selected for the study. A total of 15 scenarios, including the baseline period (2005–2015), were made based on SLR projections and estimated freshwater recharge rates. The changes in salinity relative to the baseline at the monitoring well for each scenario were investigated through the SUTRA model. From the scenario of 0.57 m SLR with a freshwater recharge rate of 0.0058 kg s−1, the largest salinity increase (40.3%) was simulated. We concluded that this study may provide a better understanding of the climate change impacts on seawater intrusion by considering both SLR and freshwater recharge rates.

Control of saltwater intrusion by aquifer storage and recovery

This paper presents the results obtained from the application of aquifer storage and recovery (ASR) technique to control seawater intrusion (SWI) in coastal aquifers. The study is based on the numerical modelling experiments performed using the SUTRA (Saturated–Unsaturated TRAnsport) finite-element code on the Wadi Ham aquifer in the UAE. A three-dimensional numerical model of this aquifer is developed and calibrated based on the available hydrogeological data in real scale. A significant amount of SWI has been calculated for the year 2015 due to the high rates of pumping from the available local well fields. To study the future responses of the aquifer to different control actions, the transient responses of SWI are simulated over a 10-year planning horizon. The proposed management measure (ASR) is implemented in repeated cycles of artificial recharge, storage and recovery using an additional set of wells defined in the model. The results show that ASR is a reliable method in controlling SWI in coastal aquifer systems besides its conventional role in subsurface water banking.

Interaction between salinity intrusion and vegetation succession: A modeling approach

Predicting potential changes in groundwater salinity in low-lying coastal regions due to climate change is important, where coastal vegetation is abundant, succession competition between halophytes and glycophytes plays a significant role in the salinity budget. Sea level rise enhances salinity intrusion, contributing an additional dimension to vegetation competition. A new simulation model known as mangrove-hardwood hammock model coupled with saturated-unsaturated transport (MANTRA) has recently been developed by the authors to simulate groundwater salinity regimes in the presence of vegetation competition, subject to climate change. MANTRA is based upon linking two existing Unites States geological survey (USGS) simulation models known as mangrove-hardwood hammock model (MANHAM) and saturated-unsaturated transport (SUTRA). MANHAM simulates the evolution of vegetation succession subject to changing groundwater salinity. SUTRA simulates saturated and unsaturated transport of solutes and salinity in groundwater given sea salinity. MANTRA improves the simulation robustness to simultaneously simulate groundwater hydrology, salinity and coastal vegetation succession subject to sea level rise. Some simulation results will be presented to demonstrate the impact of sea level rise on coastal vegetation succession and groundwater salinity.

Holocene dynamics of the salt–fresh groundwater interface under a sand island, Inhaca, Mozambique

The configuration of coastal groundwater systems in southeast Africa was strongly controlled by the Holocene sea-level changes, with an Early Holocene transgression ∼15 m (10,000–5000 cal BP), and two assumed high-stand events in the Middle and Late Holocene with levels higher than the present. The fluctuation of the salt–fresh groundwater interface under Inhaca Island in Mozambique during the Holocene has been studied using an adapted version of the numerical code SUTRA (Saturated-Unsaturated Transport). In this study, small-scale variations such as tidal effects have not been considered. A number of transient simulations were run with constant boundary conditions until the steady state condition was reached in order to study the sensitivity of response time, salt–fresh interface position, and thickness of the transition zone to different parameters such as hydraulic conductivity, porosity, recharge, and dispersivity. A 50% increase in horizontal hydraulic conductivity yields a rise in the location of the interface of >15 m, while an increase in recharge from 8% to 20% of mean annual precipitation (MAP) causes a downward shift in the interface position of >40 m. A full transient simulation of the Holocene dynamics of the salt–fresh groundwater interface showed a response time of several hundred years, with a duration sensitive to porosity, hydraulic conductivity and recharge and a position determined by the recharge rate and the hydraulic conductivity. Dispersivity controls the thickness of the transition zone in this non-tidal model. Physical processes, such as changes in recharge and/or the sea level, may cause rapid shifts in the interface position and affect the thickness of the transition zone.

Artificial neural network model as a potential alternative for groundwater salinity forecasting

The paper evaluates the prospect of artificial neural network (ANN) simulation over mathematical modeling in estimating safe pumping rate to maintain groundwater salinity in island aquifers. Feed-forward ANN model with quick propagation (QP) as training algorithm has been used to forecast the salinity under varied pumping rates. The accuracy, generalization ability and reliability of the model are verified by real-time field data. The model is trained with 2 years of real-time field data and prediction on water quality with varying pumping rate is made for a span of 5 years. The same is then compared with both real-time field data and the prediction based on SUTRA (Saturated–Unsaturated Transport) computations. The proposed ANN model has surfaced as a simpler and more accurate alternative to the numerical method techniques. The ANN methodology using minimal lag and number of hidden nodes, along with the optimal number of spatial and temporal variables consistently produced the best performing network based simulation models. The prediction accuracy of the ANN model has been extended for another 5 years to further define the limit of pumping at a permissible level of groundwater salinity.

Application of groundwater model in coastal aquifer: A case study of Veraval area of Gujarat

The development and management of fresh ground water resources in coastal aquifers are seriously constrained by the presence of seawater intrusion. Seawater intrusion is a process that occurs in virtually all coastal aquifers, where they are in hydraulic continuity with seawater. This process is caused by the higher density of sea water than freshwater. Over the years, many models have been developed to represent and study the problems related to seawater intrusion. Numerical models provide effective tool to handle ground water problems. This paper presents simulation of seawater intrusion in a section of the Veraval coast of Gujarat through Saturated and Unsaturated Transport model and examines the impact of increased pumping scenarios on extent of seawater intrusion. The density driven salt-water intrusion process was simulated with the use of commercially available finite element based software, SUTRA (Saturated–Unsaturated TRAnsport). Initial and boundary conditions determined from the aerial calibration were used to evaluate steady-state, hydraulic heads. Pumping and inflow of coastal area was estimated for three different time periods using the model.

SIMULATION OF SALTWATER INTRUSION IN THE PAVANJE-GURPUR BASINS OF KARNATAKA

ABSTRACT The development and management of fresh groundwater resources in coastal aquifers are seriously constrained by the saltwater intrusion. The Dakshina Kannada district is one of the rapidly developing districts of Karnataka. Due to the increase in population, urbanization and industrialization, the district is facing severe shortage of freshwater resources during the summer period. Also, the rivers of the district are being seasonal and tidal in nature, saltwater intrusion into the adjoining aquifers during the non-monsoon period is greatly felt up to several kilometers inland along and around the river courses. Hence the present work is aimed at simulation of saltwater intrusion in the Gurpur and Pavanje river basin aquifers using Saturated-Unsaturated Transport (SUTRA) model and comparing it with the field observations. The model is also used to predict the impact of increasing stress on freshwater aquifers due to the developmental activities. The results from the investigation indicate that the model prediction agrees well with the observed salinity levels in the study area. As such, the saltwater intrusion is not a severe problem in the region except a few pockets. The study also includes the evaluation of the basin characteristics through field tests and water quality/level observations through a network of wells. The coastal aquifers are predominantly shallow unconfined aquifers with transmissivity and specific yield values ranging from 49.2–461.4 m2/day and 0.00058–0.2805 respectively. The aquifers along the river courses are getting contaminated during the summer months only and region in the proximity of the sea is affected throughout the year. The optimal pumping rates have also been worked out for the region with potential groundwater development to avoid the problem of saltwater intrusion. The results from the study would be useful in the sustainable groundwater development in the region.

지하댐 운영시 발생하는 염수침입 저감기법에 관한 연구

최근 들어 도서지역이나 해안지역의 제한된 수자원을 보다 효과적으로 사용하기 위하여 지하댐을 이용한 지하수자원 개발 방안이 제시되고 있다. 그러나 해안선에 인접한 지하댐의 경우 과도한 지하수 양수는 대수층의 지하수위를 저하시켜 결국 염수침입이라는 심각한 문제를 발생시키게 된다. 염수침입은 대수층의 지하수위 하강에 기인하므로 본 연구에서는 지하댐 하류에 함양정(Recharging well)을 설치하여 대수층의 지하수위를 상승시킴에 따른 염수침입 저감효과를 분석하였다. 이 방안을 쌍천유역에 적용하였으며, 이를 위해 염분의 이송ㆍ확산을 분석할 수 있는 SUTRA(Saturated-Unsaturated Transport) 수치모형을 적용하였다. 그 결과 지하댐 하류지역에 대한 함양기법이 양수정 염분 저하에 매우 효율적인 방법임이 증명되었고, 염해저감을 위한 함양정 운영시 차수벽으로부터 40∼60m거리에 함양정을 설치하고 함양률을 총 양수량의 6∼7％정도로 하여 운영을 할 때 가장 효율적으로 염해를 저감시킬 수 있다는 결론을 얻을 수 있었다. Recently the ground-water development using the ground-water dam was proposed for the efficient use of the limited water resources especially for islands or seaside area. But in operating the ground-water dam adjacent to seaside an excessive pumping causes the sea-water intrusion which is caused by the drawdown of ground-water level. In this study, the effect of the recharging well method to reduce the sea-water intrusion was evaluated, and was applied to the downstream of the Ssangcheon ground-water dam site. The SUTRA model was used to simulate the salinity transport in the unsaturated and saturated zone. As the results, the effect of recharging method on the downstream of the ground-water dam was proven to be very efficient to reduce the salinity in the pumping well, and especially the best result was shown at the case that the recharging well is located at 40∼60m from the cutoff wall and the recharging rate is up to 6∼7％.

Numerical modelling of salt-water intrusion due to human activities and sea-level change in the Godavari Delta, India

The effects of human activities and sea-level changes on the spatial and temporal behaviour of the coupled mechanism of salt-water and freshwater flow through the Godavari Delta of India were analysed. The density driven salt-water intrusion process was simulated with the use of a SUTRA (Saturated-Unsaturated TRAnsport) model. Physical parameters, initial heads, and boundary conditions of the delta were defined on the basis of available field data, and an areal, steady-state groundwater model was constructed to calibrate the observed head values corresponding to the initial development phase of the aquifer. Initial and boundary conditions determined from the areal calibration were used to evaluate steady-state, hydraulic heads. Consequently, the initial position of the hydraulic head distribution was calibrated under steady-state conditions. The changes of initial hydraulic distribution, under discharge and recharge conditions, were calculated, and the present-day position of the interface was predicted. The present-day distribution of hydraulic head was estimated via a 20-year simulation. The results indicate that a considerable advance in seawater intrusion can be expected in the coastal aquifer if current rates of groundwater exploitation continue and an important part of the freshwater from the river is channelled from the reservoir for irrigation, industrial and domestic purposes.

SIMULATION OF SEA WATER INTRUSION AND TIDAL INFLUENCE

ABSTRACT Coastal zones contain some of the most densely populated areas in the world as they generally present the best conditions for productivity. However, these regions face many hydrological problems like flooding due to cyclones and wave surge and drinking fresh water scarcity due to salt water intrusion. This paper presents the simulation of sea water intrusion in Nauru Island through Saturated-Unsaturated TRAnsport (SUTRA) model and examine the effect of tidal forcing on the fresh water resources.

Mathematical modeling of leachate routing in a leachate recirculating landfill

Full-scale application of leachate recirculation has been hampered by an inability to uniformly apply leachate to the waste mass. Much of this problem is due to the varied nature of the disposed waste which promotes preferential routing and short-circuiting of leachate flows. The United States Geological Survey’s Saturated-Unsaturated Flow and Transport (SUTRA) model [Voss C. I. (1984) SUTRA, Saturated–Unsaturated Transport, A Finite-Element Simulation Model for Saturated–Unsaturated, Fluid-Density-Dependent Ground-Water Flow with Energy Transport or Chemically Reactive Single-Species Solute Transport, US Geological Survey, National Center, Reston, VA.] was used to model leachate application to homogeneous anisotropic and heterogeneous waste masses. The homogeneous anisotropic scenario was modeled using vertical hydraulic conductivities of 10 −3 and 10 −4 cm/s while the horizontal hydraulic conductivity was simulated as one order of magnitude higher than these values. The heterogeneous waste mass was simulated by applying statistical relationships to the local hydraulic conductivities. Normal, exponentially increasing, and exponentially decreasing probability density functions (PDFs) were used to model the frequency-hydraulic conductivity relationship. Median hydraulic conductivities of 10 −3 and 10 −4 cm/s with hydraulic conductivity ranges of 10 −1–10 −5 cm/s and 10 −2–10 −6 cm/s, respectively, were simulated for each probability distribution. Leachate was applied 8 h/day via a horizontal trench at a daily rate of 4 m 3/m of trench. Saturation iso-clines are used to illustrate the effect of the waste mass hydraulic parameters on leachate routing. The ability to develop predictions and evaluate a variety of different scenarios without the effort and expense of physical experimentation is a great strength of mathematical models. However, a model’s predictive capabilities must be verified by comparing the model results with field studies. In an attempt to verify model estimations, SUTRA was used to simulate the site recirculation system and operational procedures used at the Delaware Solid Waste Authority Test Cells and the Yolo County Leachate Recirculation Demonstration Project. Cumulative measured and simulated leachate generation were compared. Results from this verification effort indicate that channeled flow is the major leachate movement mechanism that is not well understood at this time.

On a test case for density‐dependent groundwater flow and solute transport models: The Salt Lake Problem

Like any other computer model, density‐dependent mathematical groundwater models must be “verified” to ascertain that they accurately represent the physics implied by a governing set of equations. Very few test cases for density‐dependent groundwater numerical models exist. As such, there is still a need for new and more robust tests of these modeling codes. In this paper a numerical model of an idealized evaporating salt lake is produced using the two‐dimensional density‐dependent saturated‐unsaturated transport (SUTRA) model, the results of which are compared with an equivalent laboratory Hele‐Shaw cell system developed by Wooding et al. [1997a, b]. Evaporation results in dense brine overlying less dense fluid, which is hydrodynamically unstable and leads to downward convection of salt fingers or plumes. A comparison of experimental and numerical plume growth shows good spatial and temporal agreement. The numerically generated plume pattern is sensitive to changes in random noise level applied just below the evaporation surface that serves as a trigger for the growth of instabilities. Experimental plume patterns were best matched with a noise level corresponding to 1% of the total salinity difference between boundary layer and background fluid concentrations at saturation. In a second comparison the stream‐function‐based finite difference model described by Wooding et al., [1997a, b] which differs significantly in principle from SUTRA is shown, after revision, to give good spatial and temporal agreement with experimental results. This test for density‐dependent groundwater models appears to be the most comprehensive and detailed available to date.

Application of environmental models to different hydrological systems

In recent years global problems such as climatic change, acid rain, and water pollution in surface and subsurface environments dominate discussions of world environmental problems. In this paper, the roles of hydrologic processes and hydrogeochemical processes are investigated through development, modification, and application of mathematical models for addressing point and non-point source water quality modelling of receiving waters: surface water, subsurface water and lake water. The paper describes the use of models to simulate the movements of pollutants and water: subsurface water, surface water and lake sediments. A hydrological model was applied to Northeast Pond River watershed to understand climate change effects in the watershed. Four watershed acidification models were applied to compute hydrogen ion, alkalinity and sulphate concentrations from Turkey Lakes watershed, Canada. The computed hydrogen ion was used to estimate acidic events, magnitude of hydrogen ion, and duration using a stochastic model. There exist uncertainties in environmental models due to imperfect knowledge of processes controlling water quality parameters as well as errors in data. Monte Carlo, first order, and inverse method analyses were used to assess uncertainty in models. SUTRA (saturated–unsaturated transport) and SUTRA −1 models were applied to Lambton county, Ontario, Canada to locate groundwater discharge areas for St. Clair River, calculate discharge rates, and hydrogeologic parameters. A sediment contamination model was developed and applied to Great Lakes sediment data to estimate transport parameters by 210Pb data. It was then coupled with fatty acid data and results were compared with observed data. A contaminant transport model was developed and applied to two North American streams to compute stream water concentration. The computed data was compared with observed data using a simple statistical method. A hydrological model was coupled with water quality models and RAISON (regional analysis by intelligent systems on) expert system and applied to Canadian watersheds. Digital satellite data was used to locate groundwater discharge and recharge areas in the watershed. This data is useful as observed data for hydrological modelling and GIS (Geographical Information System) system.

Mixed convection processes below a saline disposal basin

Saline groundwater and irrigation drainage are commonly diverted and stored in both natural and artificial depressions throughout the Murray–Darling Basin of Australia. The disposal basin brines that are formed are often denser than ambient groundwater. Under certain conditions these dense brines may become unstable causing them to mix with groundwater over distances several orders of magnitude greater than due to diffusion alone. A model is developed to study the mixed convection processes below a saline disposal basin located between a recharge and discharge zone. Numerical simulations are performed in cross-section using the 2-D density dependent model SUTRA (saturated–unsaturated transport). It is shown that the salt front movement is related to both the ability of the dense saline brines to mix convectively with underlying groundwaters (Rayleigh convection) and the strength of the regional advective velocity. Both homogeneous and heterogeneous aquifer systems are studied and the effects of anisotropy are considered. Our numerical results suggest that the behaviour of a dense brine plume overlying less dense groundwater in a homogeneous porous medium depends on the magnitude of at least two non-dimensional numbers, a Rayleigh number and modified Peclet number, defined in terms of basin scale hydrogeologic parameters including dispersion. It is shown that the onset of gravitational instabilities and the formation of free convective cells begins when the magnitude of a non-dimensional parameter combining the Rayleigh and modified Peclet number exceeds a certain critical value. © 1997 Elsevier Science B.V. All rights reserved.