Estimation of the depth to the fresh-water/salt-water interface from vertical head gradients in wells in coastal and island aquifers (original) (raw)
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A Simple Method for Locating the Fresh Water–Salt Water Interface Using Pressure Data
Ground Water, 2007
Salt water intrusion is a key issue in dealing with exploitation, restoration, and management of fresh ground water in coastal aquifers. Constant monitoring of the fresh water-salt water interface is necessary for proper management of ground water resources. This study presents a simple method to estimate the depth of the fresh water-salt water interface in coastal aquifers using two sets of pressure data obtained from the fresh and saline zones within a single borehole. This method uses the density difference between fresh water and saline water and can practically be used at coastal aquifers that have a relatively sharp fresh water-salt water interface with a thin transition zone. The proposed method was applied to data collected from a coastal aquifer on Jeju Island, Korea, to estimate the variations in the depth of the interface. The interface varied with daily tidal fluctuations and heavy rainfall in the rainy season. The estimated depth of the interface showed a good agreement with the measured electrical conductivity profile.
Biased Monitoring of Fresh Water-Salt Water Mixing Zone in Coastal Aquifers
Ground Water, 2009
In coastal aquifers, significant vertical hydraulic gradients are formed where fresh water and underlying salt water discharge together upward to the seafloor. Monitoring boreholes may act as ''short circuits'' along these vertical gradients, connecting between the higher and the lower hydraulic head zones. When a sea tide is introduced, the fluctuations of both the water table and the depth of the mixing zone are also biased due to this effect. This problem is intensified in places of long-screen monitoring boreholes, which are common in many places in the world. For example, all approximately 500 boreholes of the fresh water-salt water mixing zone in the coastal aquifer of Israel are installed with 10 to 50 m long screens. We present field measurements of these fluctuations, along with a three-dimensional numerical model. We find that the in-well fluctuation magnitude of the mixing zone is an order of magnitude larger than that in the porous media of the actual aquifer. The primary parameters that affect the magnitude of this bias are the anisotropy of the aquifer conductivity and the borehole hydraulic parameters. With no sea tide, borehole interference is higher for the anisotropic case because the vertical hydraulic gradients are high. When tides are introduced, the amplitude of the mixing zone fluctuation is higher for the isotropic case because the overall effective hydraulic conductivity is greater than the conductivity in the anisotropic case. In the aquifer, the fresh water-salt water mixing zone fluctuations are dampened, and tens of meters inland from the shoreline, the fluctuations are on the order of few centimeters.
EFFECT OF DIFFERENT PATTERNS OF INJECTION WELL SYSTEMS ON SALTWATER INTRUSION IN COASTAL AQUIFERS
This research investigates and compares three various patterns of external injection well systems on the final position of the interface toe. These patterns are: 1) One row of injection wells, 2) Double rows of injection wells with different locations, and 3) Staggered rows of injection wells. This paper also studied the effect of different rates of injection and variable distances from the coast on seawater interface movement. The results were compared with the results for other researchers. Finally this research studied the effect of the rainfall percolation on the behavior of the interface toe. This research deals with hypothetical coastal confined and unconfined aquifers under steady state conditions. The finite element method was applied for the groundwater flow equation under assumption of steady sharp interface in homogenous aquifer. A computer program named NUMERICAL with FORTRAN (77) language was used after several modifications to solve the mathematical concepts of the problem. Analytical method was used to verify the computer program. The Image theory and the superposition principle were the main tools used through the analytical procedures.
The construction of desalination plants along the Mediterranean coast that are supplied with seawater via pumped boreholes in coastal aquifers has given rise to novel hydrogeological situations. At the experimental site on the Andarax delta (SE Spain), a monitoring system has been set up, consisting of piezometer clusters. Piezometric level and electrical conductivity are monitored continuously at various depths in the aquifer. The data obtained allow the response of the aquifer to the intensive saltwater extraction to be assessed. Under a natural regime, the situation is highly stable and only the influence of the tides is detected. Under a regime of seawater extraction, the response becomes very dynamic, with pronounced falls in water level in the deepest piezometers and a marked descent in the position of the interface (25 m). This leads to a gradual decline in electrical conductivity in the slotted piezometers situated at the interface as a result of ingress of fresh water via slotted portions of the production boreholes. To cite this article: S. Jorreto et al., C. R. Geoscience 341 (2009). # 2009 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Acque Sotterranee - Italian Journal of Groundwater
Saltwater intrusion (SWI) is a widespread environmental problem that poses a threat to coastal aquifers. To address this issue, this research employs both numerical and experimental methods to study saltwater intrusion under the impact of sea level rise and varying freshwater boundary conditions in two homogeneous aquifers. The study compares transient numerical groundwater heads and salt concentrations to experimental results under receding-front and advancing front conditions. In the low permeability aquifer, the root mean square error is 0.33 cm and the R2 is greater than 0.9817. Similarly, in the high permeability aquifer, the root mean square error is 0.92 cm and the R2 is greater than 0.9335. The study also compares the results of ten experimental tests for steady-state saltwater intrusion wedge and toe length with seven different analytical solutions. The experimental results are then compared to these analytical solutions to find the most suitable equation. The Rumer and Har...
Hydrology and Earth System Sciences Discussions, 2019
Saltwater intrusion is a worldwide problem increasingly affecting coastal aquifers, due to climate changes and growing demand of freshwater for irrigation and human consumption. Therefore, research efforts on this topic have been intensified, aiming to achieve better predictions of the saltwater wedge evolution and design suitable countermeasures to limit the saltwater intrusion. Both physical and numerical models are essential for these purposes. This work presents a laboratory facility designed and built to simulate saltwater intrusion in coastal aquifers, with the overall goal of providing benchmarks for numerical models by means of different measurement techniques. The laboratory facility has been specifically designed to limit errors and provide redundant evaluation in the measurement of hydraulic heads and discharged flow rates. Moreover, the size of the facility allows us to monitor the saltwater wedge evolution by electrical resistivity tomography (ERT). A specifically designed ERT monitoring system was developed and verified by comparison with photos of the saltwater wedge collected at regular intervals during an experiment in a homogeneous porous medium. The experiment consisted of two phases: for the initial 24 h, the saltwater wedge evolved without any external forcing, while in the following 12 h, freshwater was pumped out through a channel drain, to simulate aquifer exploitation. The SUTRA code was adopted to reproduce the experimental results, by calibrating only the longitudinal and transversal dispersivities. Overall, the agreement between observed data, numerical simulations, and ERT results, albeit preliminary, demonstrates that the proposed laboratory facility can provide valuable benchmarks for future studies of seawater intrusion, even in more complex settings. 1 Introduction Seawater intrusion in coastal aquifers is a worldwide problem caused, among other factors, by aquifer overexploitation related to human activities, such as irrigation and drinking water supply, and climate changes, whose main effect is the reduction of natural groundwater recharge rather than sea level rise, as recently demonstrated by Ketabchi et al. (2016). To prevent or limit the deterioration of both surface water and groundwater quality due to saltwater contamination, research studies have been developed to fully comprehend the problem and identify its fundamental parameters, as well as to evaluate possible counter
HYDROGEOPHYSICAL APPLICATIONS IN COASTAL AQUIFERS
NATO Science Series, 2006
TDEM (time domain electromagnetic) traverses in the Dead Sea (DS) coastal aquifer help to delineate the configuration of the interrelated fresh-water and brine bodies and the interface in between. A good linear correlation exists between the logarithm of TDEM resistivity and the chloride concentration of groundwater, mostly in the higher salinity range, close to that of the DS brine. In this range, salinity is the most important factor controlling resistivity. The configuration of the fresh-saline water interface is dictated by the hydraulic gradient, which is controlled by a number of hydrological factors.
Integrating Surface and Borehole Geophysics in the Characterization of Salinity in a Coastal Aquifer
2003
A combination of geologic descriptions of cores, borehole geophysics, surfacegeophysical soundings and water-sample analyses was used to characterize flow in the surficial aquifer in the vicinity of Big Cypress National Preserve, Collier County, south Florida. The geophysical analysis augmented by aquifer tests and piezometer measurements showed that the surficial aquifer is under unconfined conditions, is in hydraulic connection with surface water bodies, and is underlain by a series of confining units and confined aquifers. Seawater intrusion appears as a simple wedge of saline water underneath the coastal region. The pattern of salinity in the underlying aquifers is variable and shows no relation to distance from the coast or from the surficial wedge of seawater intrusion. Borehole flow-log data document the existence of a strong vertical hydraulic-head gradient throughout the study area. The subsurface distribution of salinity in the study area apparently is determined by variations in the amount of seepage of saline water from underlying aquifers and not by lateral seawater intrusion except near the wedge of sea water imaged by the surface geophysics.
Journal of African Earth Sciences, 2017
Several factors can affect the quantity and the quality of groundwater resources, but in coastal aquifers seawater intrusion is often the most significant issue regarding freshwater supply. Further, saltwater intrusion is a worldwide issue because about seventy percent of the world's population lives in coastal regions. Generally, fresh groundwater not affected by saltwater intrusion is characterized by low salinity and therefore low electrical conductivity (EC) values. Consequently, high values of EC in groundwater along the coastline are usually associated to seawater intrusion. This effect is amplified if the coastal aquifer is overexploited with a subsequent gradual displacement of the freshwater-saltwater interface towards the continent. Delineation of marine intrusion in coastal aquifers has traditionally relied upon observation wells and collection of water samples. This approach may miss important hydrologic features related to saltwater intrusion in areas where access is difficult and where wells are widely spaced. Consequently, the scarcity of sampling points and sometimes their total absence makes the number of data available limited and most of the time not representative for mapping the spatial and temporal variability of groundwater salinity. In this study, we use a series of geophysical methods for characterizing the aquifer geometry and the extension of saltwater intrusion in the Martil-Alila coastal region (Morocco) as a complement to geological and hydrogeochemical data. For this reason, we carried out three geophysical surveys: Gravity, Electrical Resistivity and Frequency Domain Electromagnetic. The geometry of the basin has been determined from the interpretation of a detailed gravity survey. Electrical resistivity models derived from vertical electrical soundings allowed to characterize the vertical and the lateral extensions of aquifer formations. Finally, frequency domain electromagnetic methods allowed delineating the extension of the saltwater intrusion. Highlights The geometry of the basin has been determined from the interpretation of a detailed gravity survey. Electrical resistivity allowed to characterize the vertical and the lateral extensions of aquifer formations. Frequency domain electromagnetic methods allowed delineating the extension of the saltwater intrusion and the saltwater freshwater interface position. The freshwater / saltwater interface reaches in some areas a distance about 4 km from the coastline.