Saltwater Upconing Research Articles

Determination of Exploitable Coefficient of Coral Island Freshwater Lens Considering the Integrated Effects of Lens Growth and Contraction

Groundwater on small coral islands (so-called freshwater lens) is an important water resource for residents and local ecosystems. However, an overexploitation of it may induce a contamination by saltwater. In this paper, we strive to determine the exploitable coefficient of the freshwater lens considering the integrated effects of lens growth and contraction and examine the impacts of well layout schemes on the evolution of the freshwater lens. For this purpose, a numerical model is setup to simulate the saltwater upconing and recovery process under pumping conditions during different evolution stages. Our results show that long-term and higher intensity pumping activities are suggested to be conducted at the latter stage of the lens evolution. Meanwhile, the seasonal contraction of the freshwater lens caused by the seasonal variation in rainfall is characterized by a quicker response of center thickness than maximum thickness of the lens, which further impacts the pumping intensity. The results also indicate that the exploitable coefficient (ρ) of the freshwater lens in small coral island is generally smaller than that in inland areas, ranging from 0.09 to 0.37 under different well layout schemes. Additionally, it is also affected by the uncertainty of hydrogeological parameters. Finally, a safe exploitable coefficient is proposed under the most unfavorable parameter combination for the studied island. The study has important implications for the protection and sustainable exploitation of subsurface freshwater resources on island.

Analyzing spatial and temporal evolution of groundwater salinization through Multivariate Statistical Analysis and Hydrogeochemical Facies Evolution-Diagram

The growing groundwater withdrawal rates in coastal aquifers in arid/semi-arid regions exacerbate seawater intrusion and saltwater upconing by causing groundwater salinization and potential adverse and cascading effects to related groundwater-depending systems. This study aims to highlight the dynamics of groundwater salinization in time and space by comparing the efficacy of statistical (hierarchical cluster and factor analyses) and hydrogeochemical (hydrogeochemical facies evolution) methods. Multi-temporal groundwater samples collected from the monitoring well network in the study area (Salento Aquifer, Puglia region, Southern Italy) have been considered to recognize such dynamics. By comparing the spatial and temporal evolution of water clusters, factor scores, and hydrogeochemical facies, the proposed methodological approach enables the identification of zones characterized by low dynamics of freshening and intrusion processes (with invariant features during the investigated period), which correspond to groundwater recharge areas and zones subject to groundwater salinization respectively. On the contrary, a high spatial and temporal variability of salinization dynamics typifies the zones subject to alternation of groundwater characteristics. These results allow outlining a preliminary hazard map related to groundwater salinization processes, which might be a useful tool for policymakers and stakeholders involved in groundwater management of coastal aquifers. Results suggest that generally, a thoughtful understanding of limitations concerning the aquifer heterogeneity and anisotropy, distribution and density of control points, and depth of sampling is crucial for handling the study outcomes, especially for the aims of management.

Assessing the protective effect of cutoff walls on groundwater pumping against saltwater upconing in coastal aquifers

Subsurface physical barriers are amongst the most effective methods to mitigate seawater intrusion in coastal aquifers. The main objective of this study was to examine the impact of cutoff walls on saltwater upconing using laboratory and numerical modelling experiments. Physical experiments were first completed to reproduce the saltwater upconing process in a laboratory-scale coastal aquifer model incorporating an impermeable cutoff wall. Numerical modelling was used for validation purposes and to perform additional simulations to explore the protective effect of cutoff walls against saltwater upconing. The results suggest that the cutoff wall did not substantially delay the saltwater upconing mechanism in the investigated configurations. Laboratory and numerical observations showed the existence of some residual saline water, which remained on the upper part of the aquifer on the seaward side of the wall following the retreat of the saltwater. The protective effect of cutoff walls was noticeably sensitive to the design parameters. Specifically, cutoff walls installed close to the pumping well enabled the implementation of higher pumping rates, therefore a more optimal use of the freshwater, especially for deeper wells. The results highlighted that the penetration depth of the cutoff walls may not necessarily need to exceed the depth of the pumping well to ensure effectiveness, which is of great importance from construction and economic perspectives.

Impact of a Low-Permeability Layer on the Pumping Efficiency Under Threats of Saltwater Up-Coning

Previous studies on saltwater up-coning are mostly based on the assumption of a homogeneous aquifer and neglect the effect of heterogeneity. A numerical study was carried out to explore the impact of a low-permeability (k) layer on the pumping efficiency under threats of saltwater up-coning. It is found that the presence of the low-k layer could impede up-coning of saltwater, thereby improving significantly pumping efficiency, as expected. For the cases adopted, when the permeability of the 2 m thick low-k layer is three orders of magnitude less than that of the corresponding homogeneous aquifer, the pumping duration (i.e., the time before the salinity of pumped water reaches 2%) can be up to about 10 times longer than that of the homogeneous aquifer case. Additionally, a sound linear relationship could be developed between the normalized pumping duration of the layered case (E) (defined as the difference of the pumping duration between layered and homogeneous cases relative to the pumping duration in the homogeneous case) and the permeability of the homogeneous aquifer relative to that of the low-k layer (β). However, a short low-k layer may lead to the failure of this linear relationship. Moreover, a thicker low-k layer leads to a larger E when all other conditions are unchanged. When varying the location of the low-k layer, the large E occurs for the low-k layer located below and near the well bottom. The results obtained in the current study offer significant implications for preventing saltwater up-coning and improving the pumping efficiency through natural and artificial barriers.

Laboratory and Numerical Study of Saltwater Upconing in Fractured Coastal Aquifers

This study investigated the saltwater upconing mechanism in fractured coastal aquifers. Head-induced saline intrusion was initiated into three narrow sandbox aquifers containing individual horizontal discontinuities placed on different positions. Subsequently, using a peristaltic pump, freshwater was abstracted from the aquifers’ center, triggering saltwater upconing. Progressively larger pumping rates were applied until critical conditions, resulting in the wells’ salinization, were achieved. Advanced image analysis algorithms were utilized to recreate the saltwater concentration fields and quantify the extent of the saline wedges with a high accuracy. A numerical model was successfully employed to simulate the laboratory results and conduct a comprehensive sensitivity analysis, further expanding the findings of this investigation. The impact of the fractures’ length, permeability and position on the upconing mechanism was identified. It was established that the presence of high permeability discontinuities significantly affected aquifer hydrodynamics. The conclusions of this study could constitute a contribution towards the successful management of real-world fractured coastal aquifers.

Experimental and modeling investigation of pumping from a fresh groundwater lens in an idealized strip island

Groundwater lens floating on the seawater in surficial coastal aquifers is an important freshwater supply for small island inhabitants. This study addresses an unanswered, simple but important question for groundwater resources management in small islands: how much freshwater can be sustainably withdrawn from a groundwater lens in small islands. We derive an approximate analytical solution to describe the freshwater-seawater sharp interface profile in an idealized strip island subject to a pumping well located at the domain center to determine the critical or maximum allowable pumping rate that prevents saltwater upconing. A simple formula of the critical pumping rate is provided as a function of hydrogeologic parameters, including the recharge rate, densities, hydraulic conductivity and well penetrating depth. It shows that the critical pumping rate increases linearly with the recharge rate and decreases linearly with the hydraulic conductivity and the squared well penetrating depth. Laboratory visualization experiments and numerical simulations were conducted to validate the derived analytical solution. Results indicate that the maximum pumping rate can be as high as 50% of the total recharge for a single well located at the top of the domain center. Our solutions and results provide useful insights for sustainable groundwater management in small islands and can potentially render significant social and economic impact for inhabitants on small islands.

Laboratory Physical Experiments on the Saltwater Upconing and Recovery of Island Freshwater Lenses: Case Study of a Coral Island, China

The research of saltwater upconing is crucial for the development and utilization of island freshwater resources. In this paper, a laboratory physical experiment device was developed, and the saltwater upconing and recovery of island freshwater lenses were investigated using rainfall simulations and single-well and multi-well pumping tests with various pumping intensities. The results of the experiment revealed that: (1) The thickness of the freshwater lens increased continuously and linearly during the early stages of rainfall. As the rainfall continued, the upward trend slowed and eventually leveled off. (2) Under the same pumping duration, when the pumping intensity was less than the critical pumping intensity, the increasing height rate of the saltwater upconing increased linearly at a small gradient, and when the pumping intensity was greater than the critical value, the rising height rate of the saltwater upconing increased linearly at a large gradient. (3) Under the same pumping intensity conditions, the pumping duration of the multi-well was longer than that of the single-well pumping, and the pumping volume of the multi-well was also greater than that of the single-well pumping. The experiment results can provide support for the development and utilization of island freshwater lens.

Influence of aquifer heterogeneity on sea level rise-induced seawater intrusion: A probabilistic approach

Seawater intrusion (SWI) is influenced by a variety of coastal phenomena, such as sea level rise (SLR), inundation of low-lying coastal regions, coastal storms, recharge rate variations, and pumping-induced saltwater upconing. Quantification of the influence of heterogeneity in the hydraulic conductivity field on SWI combined with SLR, land-surface inundation, and recharge rate variations in an unconfined aquifer is the main objective of the present study. The principal SWI indicators used in this study are length of the SWI wedge, seawater volume, and weighted average transition zone width. Characterized by the hydraulic conductivity field variance (σlnk2), the longitudinal correlation length (λx), the type of SLR (gradual or instantaneous SLR), the land-surface inundation consideration, and the recharge rate variations, 72 scenarios have been introduced, and for each of them, 50 sets of heterogeneous hydraulic conductivity fields have been generated. Based on two approaches, namely ensemble Monte-Carlo and a Bayesian framework, it is demonstrated that: (1) the land-surface inundation consideration increases the SWI wedge length and the seawater volume regardless of the type of SLR, while it decreases the weighted average transition zone width in gradual SLR scenarios; (2) λx has a more significant impact on SWI characteristics compared to σlnk2; (3) increasing the degree of aquifer heterogeneity results in larger effective dispersion values; (4) Numerical bootstrapping suggests that the introduced Bayesian framework could be adopted as an alternative to computationally demanding methods such as bootstrapping for stochastic analysis of SWI; (5) Reliability analysis indicates the general belief that considering the heterogeneity decreases the SWI wedge length and the seawater volume, while increases the transition zone width compared to the homogeneous modeling is associated with huge amounts of uncertainty proportional to the aquifer heterogeneity itself; and (6) the results show that the impact of heterogeneity on the SWI indicators is similar under different recharge rates.

Impact of layered heterogeneity on transient saltwater upconing in coastal aquifers

This research investigated the effect of layered heterogeneity on transient saltwater upconing in a laboratory-scale coastal aquifer. The experiments were conducted in a 2D-laboratory flow tank, and the response of the saltwater wedge to pumping was analysed in a heterogeneous aquifer system, where a low permeability layer was constructed in the middle of the aquifer. The SEAWAT code was used for validation and to perform additional simulations to explore the sensitivity of the critical pumping rate and the critical time to the main parameters characterising the low-permeability layer, which included its permeability, thickness and position. The experimental results showed that the presence of layered heterogeneity noticeably altered the shape and the intrusion length of the upconing wedge without inducing a change in the abstraction rate “triggering” saltwater upconing mechanism compared to the homogeneous case. The numerical results of the layered aquifer provided excellent agreement with the experimental data for both the transient toe length and the shape of the steady-state saltwater wedges. The sensitivity analysis revealed that the critical pumping rate and the critical time was found to decrease considerably with decreasing hydraulic conductivity and thickness of the middle layer, which evidences the higher vulnerability of such layered aquifer systems to the saltwater upconing, in comparison to idealised homogeneous systems. The results nonetheless showed that varying the position of the interlayer induced very little change on the critical pumping rate, but the critical time would tend to decrease as the low permeability layer was moved deeper away from the pumping well, particularly for smaller middle layer thickness.

Application of numerical models to assess multi-source saltwater intrusion under natural and pumping conditions in the Great Maputo aquifer, Mozambique

Regional three-dimensional groundwater-flow and saltwater transport models were built to analyse saltwater intrusion in the Great Maputo area, southern Mozambique. Increased water demand has led to many private groundwater abstractions, as the local public water supply network has already reached maximum capacity. Pushing for new strategies to tackle the water-supply shortages exposes the aquifer system to saltwater intrusion from entrapped fossil saline groundwater and seawater. Previous attempts at modelling have been frustrated by data limitations. This study compiled all the available data to build the models, which were subsequently calibrated with observed heads, discharges and salt concentrations. The transport models were used to test hypotheses of potential sources of saltwater resulting in the current salinity distribution. Furthermore, scenarios were simulated to assess the impacts of sea-level rise and projected groundwater abstractions. Results show that saline groundwater is widely distributed in the aquifer’s western sector, where it is a limiting factor for groundwater development, and seawater intrusion is a risk along the coastline. Newly constructed wells (46) along the Infulene River can be operated with some impacts of saltwater upconing and must be closely monitored. Although current groundwater abstractions (60,340 m3/day) are still small compared with groundwater recharge (980,823 m3/day), larger volumes of abstraction are feasible only when using a high number of production wells further away from the city with relatively low yields to avoid saltwater upconing. Capture of fresh groundwater upstream of discharge areas by wells for water supply is possible while maintaining groundwater discharges for groundwater dependent ecosystems.

Study on groundwater abstraction by horizontal drains for minimization of saltwater upconing from below

In many places in Vietnam central region, the fresh coastal sand dune aquifers have an interface with salty water below, so upon groundwater (GW) abstraction, there is a risk of salty water upconing to the abstraction facilities. With the same abstraction rate, the horizontal drains shall significantly reduce the groundwater level (GWL) drawdown, which significantly reduces the development of saltwater upconing. The results analysis for the design of groundwater abstraction facilities in Thach Tri commune, Thach Ha district, Ha Tinh province have shown the effectiveness of the solution. For infinite distributed aquifers, the GWL drawdown in the facility area with the abstraction rate of 100m3/day by eight 4m- and 8m-long horizontal drains is from 11.7% to 16.8% lower in comparison to a vertical well, respectively, (GWL drawdown in vertical well is 0.733m, GWL drawdown in center of 4m-long horizontal drains is 0.646m and in 8m-long horizontal drains is 0.603m). For 1km x 1km aquifer bounded by no-flow boundaries, GWL drawdown in the vertical well and in the center of eight 4m-and 8m-long horizontal drains are 2.313m, 1.293m and 1.255m, respectively. This is corresponding to GWL drawdown decrease of 44.1% and 45.7% in comparison to the vertical well. The significantly reduced GWL drawdown in abstraction facilities of horizontal drains will significantly reduce the level of salty water upconing and therefore increase the time of abstraction of fresh water.ReferencesCPEO, 2010. Updated 2010 tech tree list of summary cleanup technology descriptions: Horizontal Wells. http://www.cpeo.org/techtree/ttdescript/horzwel.htmDagan, G. and Bear, J., 1968. Solving the problem of local interface upconing in a coastal aquifer by the method of small perturbations. J. Hydr. Res., 6, 15-44.Diersch,H.-J. Prochnow D. and Thiele M., 1984. Finite-element analysis of dispersion-affected saltwater upconing below a pumping well. Appl. Math. Modelling, 8, 305-312.Falkovich G., 2011. Fluid Mechanics. Cambridge University Press, 167p.Fletcher G. Driscoll, 1987. Groundwater and Wells. Published by Johnson Division, St. Paul, Minnesota 55112, USA, 1089p.Kushtanova G.G., 2015. Well test analysis. Kazan University, 31p. https://doi.org/10.15625/0866-7187/38/2/8603.Marta Faneca Sanchez, Khairul Bashar, Gijs Janssen, Gualbert Dude Essink, Marjolein Vogels, Jan Snel, Yangxiao Zhou and Roelof Stuurman, 2015. SWIBANGLA: Managing salt water intrusion impacts in coastal groundwater systems of Bangladesh. Deltares, The Netherlands UNESCO-IHE, The Netherlands Jahangirnagar University, Bangladesh, 153p. https://doi.org/10.15625/0866-7187/39/1/9231.Nguyen Ngoc Quynh, 2012. Determination of required geometric sizes of sand dunes for their stability in the Central Plains from Quang Ngai province to Binh Thuan province. Journal of Water Resources Science and Technology, 9, 2-6.Nguyen Tien Hai, Tran Nghi, Nguyen Van Bach, 2004. Formation characteristics and evolution of coastal strips of sands in Quang Binh. Journal of Geology, ISSN 0866-7381. Series A, 281, 30-40.Nguyen Van Dan, Vo Cong Nghiep, Dang Huu On, 1996. Groundwater of the North Trung Bo coastal plains. Geological Survey of Vietnam, 86p.Nguyen Xuan Truong, Nguyen The Tiep, 1998. Sea terrace in Vietnam with corresponding geological formations. Journal of Science 1. Hanoi University, 69-76.Phan Van Truong, 2015. Hydrogeochemistry characteristics and salinity of groundwater in Quaternary sediments in the coastal zone of Ha Tinh province. J. Sci. of the Earth, 37(1), 70-78. https://doi.org/10.15625/0866-7187/37/1/6557.Pieter S. Pauw Sjoerd E.A.T.M. van der Zee Anton Leijnse Gualbert H.P. Oude Essink, 2015. Saltwater Upconing Due to Cyclic Pumping by Horizontal Wells in Freshwater Lenses. J. Ground Water, 54(4), 521-531. https://doi.org/10.1111/gwat.12382.Soylu M. E., Istanbulluoglu E., Lenters J. D. and T. Wang, 2011. Quantifying the impact of groundwater depth on evapotranspiration in a semi-arid grassland region. Hydrol. Earth Syst. Sci., 15, 787-806,. www.hydrol-earth-syst-sci.net/15/787/2011/doi:10.5194/hess-15-787-2011.Truong Minh Duc, 2015. Climate change and environment in the Central coastal plain. Journal Vietnam Social Sciences, 89, 34-45.

Assessment of Salt Water Upconing to above Groundwater Abstraction Facilities

Abstract: A methodology for identifying salt water upconing to groundwater abstraction wells has been based on Dagan and Bear method for an abstraction point. The results of the analysis for different design cases of well field and screen length and positions for coastal sand dune aquifer in Thach Ha district, Ha Tinh province has showed that the height of salt water upconing is inversely proportional to the distance from the well screen bottom to the salt and fresh water interface. At the same time, for abstraction wells with the same depth, the height of salt water upconing as well as the time of salt water upconing to the abstraction wells are inversely proportional to the length of the well screen (i.e. the longer the well screen the less salt water upconing). The use of several abstraction wells to reduce the abstraction rate of each well (the total abstraction rate remains unchanged) has the effect of significantly reducing the salt water upconing process. The proposed methodology for salt water upconing assessment has been shown an effective role in supporting the design of groundwater abstraction facilities that are at risk of salinization from the lower part or lower aquifers for sustainable groundwater exploitation on the viewpoint of ensuring the quality of abstracted water and of protecting groundwater resources by limiting (even preventing) salt water intrusion.

Assessment of groundwater salinity and quality in Gaza coastal aquifer, Gaza Strip, Palestine: An integrated statistical, geostatistical and hydrogeochemical approaches study

A comprehensive study was conducted to identify the salinization origins and the major hydrogeochemical processes controlling the salinization and deterioration of the Gaza coastal aquifer system through a combination approaches of statistical and geostatistical techniques, and detailed hydrogeochemical assessments. These analyses were applied on ten physicochemical variables for 219 wells using STATA/SE12 and Surfer softwares. Geostatistical analysis of the groundwater salinity showed that seawater intrusion along the coastline, and saltwater up-coning inland highly influenced the groundwater salinity of the study area. The hierarchical cluster analysis (HCA) technique yielded seven distinct hydrogeochemical signature clusters; (C1&C2: Eocene brackish water invasion, C3 saltwater up-coning, C4 human inputs, C5 seawater intrusion, C6 & C7 rainfall and mixing inputs). Box plot shows a wide variation of most of the ions while Chadha's plot elucidates the predominance of Na-Cl (71.6%) and Ca/Mg-Cl (25%) water types. It is found that, the highest and the lowest levels of salinization and the highest level of nitrate pollution were recorded in the northern area. This result reflects the sensitivity of this area to the human activities and/or natural actions. Around 90.4% of the wells are nitrate polluted. The main source of nitrate pollution is the sewage inputs while the farming inputs are very limited and restricted mostly in the sensitive northern area. Among the hydrogeochemical processes, ion exchange process was the most effective process all over the study area. Carbonate dissolution was common in the study area with the highest level in clusters 6, 7, 4 and 2 in the north while Gypsum dissolution was significant only in cluster 1 in the south and limited in the other clusters. This integrated multi-techniques research should be of benefit for effective utilization and management of the Gaza coastal aquifer system as well as for future work in other similar aquifers systems.

Saltwater Upconing Due to Cyclic Pumping by Horizontal Wells in Freshwater Lenses.

This article deals with the quantification of saltwater upconing below horizontal wells in freshwater lenses using analytical solutions as a computationally fast alternative to numerical simulations. Comparisons between analytical calculations and numerical simulations are presented regarding three aspects: (1) cyclic pumping; (2) dispersion; and (3) finite horizontal wells in a finite domain (a freshwater lens). Various hydrogeological conditions and pumping regimes within a dry half year are considered. The results show that the influence of elastic and phreatic storage (which are not taken into account in the analytical solutions) on the upconing of the interface is minimal. Furthermore, the analytical calculations based on the interface approach compare well with numerical simulations as long as the dimensionless interface upconing is below 1/3, which is in line with previous studies on steady pumping. Superimposing an analytical solution for mixing by dispersion below the well over an analytical solution based on the interface approach is appropriate in case the vertical flow velocity around the interface is nearly constant but should not be used for estimating the salinity of the pumped groundwater. The analytical calculations of interface upconing below a finite horizontal well compare well with the numerical simulations in case the distance between the horizontal well and the initial interface does not vary significantly along the well and in case the natural fluctuation of the freshwater lens is small. In order to maintain a low level of salinity in the well during a dry half year, the dimensionless analytically calculated interface upconing should stay below 0.25.

Saltwater Upconing Below a Pumping Well in an Inland Aquifer: a Theoretical Modeling Study on Testing Different Scenarios of Deep Saline-Groundwater Pathways

To date, studies on the geological conditions in inland aquifers leading to pathways for upwelling deep saline groundwater due to pumping have not been published yet. Therefore, this paper conducted a theoretical modeling study to raise two hypotheses about deep saline-groundwater pathways leading to saltwater upconing below a pumping well in an inland aquifer based on the field situation at the Beelitzhof waterworks in southwestern Berlin (Germany), defined as follows: (1) there are windows in the Rupelian clay caused by glacial erosion, where their locations are uncertain, and (2) there are no windows in the clay, but the clay is partially thinned out but not completely removed by glacial erosion, so salt can merely come through the clay upward by diffusion and eventually accumulate on its top. These hypotheses were tested to demonstrate the impact of the lateral distance between windows in the clay and the well, as well as salt diffusion through the clay depending on its thickness on saltwater intrusion in the pumping well, respectively, using a density-dependent groundwater flow and solute transport model. Hypothesis 1 was validated with four scenarios that windows could occur in the clay at the site, and their locations under some conditions could significantly cause saltwater intrusion, while hypothesis 2 could be excluded, because salt diffusion through the clay with thickness greater than 1 m at the site was not able to cause saltwater intrusion.

Natural saltwater upconing by preferential groundwater discharge through boils

Natural saltwater upconing caused by the preferential groundwater discharge of boils is a key proce ss in the salinization of Dutch deep polders. The factors controlling upconing by boil discharge and boil water salinities are poorly constrained and have not been previously documented. We addressed this knowl- edge gap by investigating upconing mechanisms using field measurements and numerical simulation s of simplified situations. Boils occur as conduits in the upper aquitard connecting the underlying aquifer to the surface and allowing groundwater to discharge at rates up to 100 m3 d?1 with Cl concentrations up to5gL?1. Boils are found as isolated features or clustered in small areas of 20–100m2. Field observations show that preferential flow through boils creates localized and narrow saltwate r upconing spikes, caus- ing the elevated boil water salinities. Modeling results indicate that boil water in Dutch polders com- prises mixtures of groundwater from a wide range of depths and salinities with larger contributions from shallower and less saline groundwater than from the deeper and more saline water. Similar to pre- vious numerical studies of pumping-in duced upconing, the numerical results show that the most impor- tant factors controlling the boil salinity in Dutch polders are boil discharge, the horizontal hydraulic conductivity of the aquifer, the depth of the transition zone and the salinit y (or density) contrast within the aquifer. When boils are clustered, natural saltwater upconing is a function of the total discharge of a boil cluster, whereas the boil-to-boil salinity variations within a cluster are determined by the discharge of individual boils and their position relative to neighboring boils. Regional lateral flow significantly mod- ifies flow patterns by dividing the groundwater flow system into a local boil system overlying the regio- nal flow system. Despite this, regional flow has only a minor effect on the relative contribu tions of saline and fresh groundwater to boil discharge and thus on boil salinity as well.

Estimation of Salt Water Upconing Using a Steady‐State Solution for Partial Completion of a Pumped Well

A new steady-state analytical solution to the two-dimensional radial-flow equation was developed for drawdown (head) conditions in an aquifer with constant transmissivity, no-flow conditions at the top and bottom, constant head conditions at a known radial distance, and a partially completed pumping well. The solution was evaluated for accuracy by comparison to numerical simulations using MODFLOW. The solution was then used to estimate the rise of the salt water-fresh water interface (upconing) that occurs under a pumping well, and to calculate the critical pumping rate at which the interface becomes unstable, allowing salt water to enter the pumping well. The analysis of salt water-fresh water interface rise assumed no significant effect on upconing by recharge; this assumption was tested and supported using results from a new steady-state analytical solution developed for recharge under two-dimensional radial-flow conditions. The upconing analysis results were evaluated for accuracy by comparison to those from numerical simulations using SEAWAT for salt water-fresh water interface positions under mild pumping conditions. The results from the equation were also compared with those of a published numerical sharp-interface model applied to a case on Cape Cod, Massachusetts. This comparison indicates that estimating the interface rise and maximum allowable pumping rate using the analytical method will likely be less conservative than the maximum allowable pumping rate and maximum stable interface rise from a numerical sharp-interface model.

Seawater intrusion processes, investigation and management: Recent advances and future challenges

Seawater intrusion (SI) is a global issue, exacerbated by increasing demands for freshwater in coastal zones and predisposed to the influences of rising sea levels and changing climates. This review presents the state of knowledge in SI research, compares classes of methods for assessing and managing SI, and suggests areas for future research. We subdivide SI research into categories relating to processes, measurement, prediction and management. Considerable research effort spanning more than 50years has provided an extensive array of field, laboratory and computer-based techniques for SI investigation. Despite this, knowledge gaps exist in SI process understanding, in particular associated with transient SI processes and timeframes, and the characterization and prediction of freshwater–saltwater interfaces over regional scales and in highly heterogeneous and dynamic settings. Multidisciplinary research is warranted to evaluate interactions between SI and submarine groundwater discharge, ecosystem health and unsaturated zone processes. Recent advances in numerical simulation, calibration and optimization techniques require rigorous field-scale application to contemporary issues of climate change, sea-level rise, and socioeconomic and ecological factors that are inseparable elements of SI management. The number of well-characterized examples of SI is small, and this has impeded understanding of field-scale processes, such as those controlling mixing zones, saltwater upconing, heterogeneity effects and other factors. Current SI process understanding is based mainly on numerical simulation and laboratory sand-tank experimentation to unravel the combined effects of tides, surface water–groundwater interaction, heterogeneity, pumping and density contrasts. The research effort would benefit from intensive measurement campaigns to delineate accurately interfaces and their movement in response to real-world coastal aquifer stresses, encompassing a range of geological and hydrological settings.

Tracer adsorption in sand-tank experiments of saltwater up-coning

Summary This study aims to substantiate otherwise unresolved double-peaked plumes produced in recent saltwater up-coning experiments (see Jakovovic et al. (2011) , Numerical modelling of saltwater up-coning: Comparison with experimental laboratory observations, Journal of Hydrology 402, 261–273) through additional laboratory testing and numerical modelling. Laboratory experimentation successfully reproduced the double-peaked plume demonstrating that this phenomenon was not an experimental nuance in previous experiments. Numerical modelling by Jakovovic et al. (2011) was extended by considering adsorption effects, which were needed to explain the observed up-coning double peaks of both previous and current laboratory experiments. A linear adsorption isotherm was applied in predicting dye tracer (Rhodamine WT) behaviour in the sand-tank experiments using adsorption parameters obtained experimentally. The same adsorption parameters were tested on all laboratory experiments and it was found that adsorption had insignificant effect on experiments with high pumping rates. However, low pumping rates produced pronounced spatial velocity variations within the dense salt plume beneath the pumping well, with velocities within the plume increasing from the centre of the plume towards the interface. The dye tracer was retarded relative to the salt and was transported preferentially along the higher-velocity paths (i.e. along the edges of the salt plume) towards the well forming double-peaked up-coning patterns. This illustrates the sensitive adsorptive nature of Rhodamine WT and that care should be taken when it is used in similar sand-tank experiments. Observations from this study offer insight into the separation of chemicals in natural systems due to different adsorption characteristics and under conditions of density-dependent flow.

Experimental observations of saltwater up-coning

Summary The proper management of coastal aquifers commonly requires an understanding of the behaviour of saltwater plumes underlying production wells. The current understanding of saltwater up-coning (i.e. the pumping-induced rise in the freshwater–saltwater interface) is based predominantly on theoretical constructs of salinity and flow responses to groundwater pumping, plus a limited number of field-scale and laboratory investigations. There is a need to produce direct observations of saltwater up-coning, given the considerable and inherent uncertainties associated with the prediction of density-dependent flow and dispersive solute transport behaviour. In this study, time-series observations of saltwater up-coning were made using controlled sand-tank experiments of a two-dimensional aquifer cross-section. A range of mixed convection ratios was tested by invoking different groundwater abstraction rates and saltwater densities. The temporal development of up-coning was characterised in terms of the height of the interface apex, the width of the salt plume, and the salinity of the pumping well. Experimental results were compared to an existing sharp-interface analytical solution and close agreement was obtained for two of the cases. Higher freshwater–saltwater density differences and lower pumping appeared to induce the largest deviations from theoretical up-coning. Dispersive transport processes dominated during phases of near-well up-coning, causing gradual increases in pumped water salinity, although plumes became non-dispersive after the interface made contact with the bore. An interesting departure from the classical up-coning shape was obtained for the situation of highest saltwater density and lowest pumping, in which up-coning occurred as an almost horizontal interface for the majority of the experiment, eventually producing a double-apex shape as the interface neared the well. The experimental results could form a useful basis for the testing of density-dependent groundwater flow and transport models, since they offer controlled laboratory analogues of saltwater up-coning.