Freshwater Head Research Articles

A coupled stochastic inverse/sharp interface seawater intrusion approach for coastal aquifers under groundwater parameter uncertainty

This paper presents an alternative approach to deal with seawater intrusion problems, that overcomes some of the limitations of previous works, by coupling the well-known SWI2 package for MODFLOW with a stochastic inverse model named GC method. On the one hand, the SWI2 allows a vertically integrated variable-density groundwater flow and seawater intrusion in coastal multi-aquifer systems, and a reduction in number of required model cells and the elimination of the need to solve the advective-dispersive transport equation, which leads to substantial model run-time savings. On the other hand, the GC method allows dealing with groundwater parameter uncertainty by constraining stochastic simulations to flow and mass transport data (i.e., hydraulic conductivity, freshwater heads, saltwater concentrations and travel times) and also to secondary information obtained from expert judgment or geophysical surveys, thus reducing uncertainty and increasing reliability in meeting the environmental standards. The methodology has been successfully applied to a transient movement of the freshwater-seawater interface in response to changing freshwater inflow in a two-aquifer coastal aquifer system, where an uncertainty assessment has been carried out by means of Monte Carlo simulation techniques. The approach also allows partially overcoming the neglected diffusion and dispersion processes after the conditioning process since the uncertainty is reduced and results are closer to available data.

Estimation of the position of the sharp interface in littoral aquifers with hydraulic head differences within the fresh-water zone

In littoral zones where fresh groundwater discharges into the sea, a vertical difference in hydraulic head always exists within the fresh-water zone, i.e., the Dupuit assumption is not satisfied. The hydraulic head difference may affect the sharp interface and needs to be taken into account when position of the interface is determined. Examination of the equilibrium status of a point on the interface reveals that the location of the interface in a homogeneous and isotropic coastal unconfined aquifer can be determined based on the fresh-water head and the salt-water head at one point on the sharp interface. Considering a linear increase or decrease in the hydraulic head within the fresh-water zone, a method for estimating the position of the sharp interface is described on the basis of the fresh hydraulic heads at two arbitrary points in the fresh-water zone and the salt hydraulic head at one arbitrary point in the salt-water zone in the same vertical straight line in the littoral zone. The method using corresponding pressures at the points is also given. The Ghyben-Herzberg model is a specific case of the method. This method requires three nearest observation wells, two revealing the fresh-water zone and one tapping the deeper salt-water zone, or three transducers installed in one well, two in the fresh-water zone and one in the deeper salt-water zone. The Observations of piezometric heads in the northwest Beihai coast in China are used to illustrate the application of the method to determine the position of the sharp interface, which ranges from 32.56 to 49.15 m under the mean sea level during the specific observation period.

Export of dissolved inorganic nutrients to the northern Indian Ocean from the Indian monsoonal rivers during discharge period

Coastal regions are highly productive due to the nutrients largely supplied by rivers. To examine the contribution of dissolved inorganic nutrients (DIN) by Indian rivers to coastal waters, data were collected near the freshwater heads of 27 monsoonal rivers of peninsular India during three weeks in late July to mid-August, the middle of the principal runoff period of the southwest monsoon of 2011. Twelve researchers in four groups, equipped with car and portable laboratory equipment, sampled mid-stream of each estuary using mechanized boat, and filtered and partly analyzed the water in the evening. The estimated exports were 0.22±0.05, 0.11±0.03, and 1.03±0.26Tgyr−1 for dissolved inorganic nitrogen, phosphorus and silicate, respectively. Higher amounts of DIN reach the Bay of Bengal than the Arabian Sea due to the higher volume (∼76%) of discharge to the former. In contrast, the export of dissolved inorganic nitrogen is almost same to the Bay of Bengal (0.12±0.03Tgyr−1) and Arabian Sea (0.10±0.02Tgyr−1) principally due to the polluted Narmada and Tapti rivers in the northwest. Including input from the glacial rivers, Ganges, Brahmaputra and Indus, it is estimated that the northern Indian Ocean receives ∼1.84±0.46, 0.28±0.07 and 3.58±0.89Tgyr−1 of nitrate, phosphate and silicate, respectively, which are significantly lower than the earlier estimates of DIN export from the Indian rivers based on DIN measured in the mid or upstream rivers. Such low fluxes in this study were attributed to efficient retention/elimination of DIN (∼91%) before reaching the coastal ocean. Hence, this study suggests that the importance of sampling locations for estimating nutrient fluxes to the coastal ocean. Riverine DIN export of 1.84±0.46Tgyr−1 would support 12.2±3.1TgCyr−1 of new production in coastal waters of the northern Indian Ocean that results in a removal of 12.2±3.1Tg atmospheric CO2yr−1.

Numerical study of groundwater flow cycling controlled by seawater/freshwater interaction in a coastal karst aquifer through conduit network using CFPv2

In this study, a groundwater flow cycling in a karst springshed and an interaction between two springs, Spring Creek Springs and Wakulla Springs, through a subground conduit network are numerically simulated using CFPv2, the latest research version of MODFLOW-CFP (Conduit Flow Process). The Spring Creek Springs and Wakulla Springs, located in a marine estuary and 11 miles inland, respectively, are two major groundwater discharge spots in the Woodville Karst Plain (WKP), North Florida, USA. A three-phase conceptual model of groundwater flow cycling between the two springs and surface water recharge from a major surface creek (Lost Creek) was proposed in various rainfall conditions. A high permeable subground karst conduit network connecting the two springs was found by tracer tests and cave diving. Flow rate of discharge, salinity, sea level and tide height at Spring Creek Springs could significantly affect groundwater discharge and water stage at Wakulla Springs simultaneously.Based on the conceptual model, a numerical hybrid discrete-continuum groundwater flow model is developed using CFPv2 and calibrated by field measurements. Non-laminar flows in conduits and flow exchange between conduits and porous medium are implemented in the hybrid coupling numerical model. Time-variable salinity and equivalent freshwater head boundary conditions at the submarine spring as well as changing recharges have significant impacts on seawater/freshwater interaction and springs’ discharges. The developed numerical model is used to simulate the dynamic hydrological process and quantitatively represent the three-phase conceptual model from June 2007 to June 2010. Simulated results of two springs’ discharges match reasonably well to measurements with correlation coefficients 0.891 and 0.866 at Spring Creeks Springs and Wakulla Springs, respectively. The impacts of sea level rise on regional groundwater flow field and relationship between the inland springs and submarine springs are evaluated as well in this study.

Uncertainty and sensitivity analyses of the simulated seawater-freshwater mixing zones in steady-state coastal aquifers

The uncertainty and sensitivity of predicted positions and thicknesses of seawater-freshwater mixing zones with respect to uncertainties of saturated hydraulic conductivity, porosity, molecular diffusivity, longitudinal and transverse dispersivities were investigated in both head-control and flux-control inland boundary systems. It shows that uncertainties and sensitivities of predicted results vary in different boundary systems. With the same designed matrix of uncertain factors in simulation experiments, the variance of predicted positions and thickness in the flux-control system is much larger than that predicted in the head-control system. In a head-control system, the most sensitive factors for the predicted position of the mixing zone are inland freshwater head and transverse dispersivity. However, the predicted position of the mixing zone is more sensitive to saturated hydraulic conductivity in a flux-control system. In a head-control system, the most sensitive factors for the predicted thickness of the mixing zone include transverse dispersivity, molecular diffusivity, porosity, and longitudinal dispersivity, but the predicted thickness is more sensitive to the saturated hydraulic conductivity in a flux-control system. These findings improve our understandings for the development of seawater-freshwater mixing zone during seawater intrusion processes, and give technical support for groundwater resource management in coastal aquifers.

Performance of different assessment methods to evaluate contaminant sources and fate in a coastal aquifer.

The present study deals with the application of different monitoring techniques and numerical models to characterize coastal aquifers affected by multiple sources of contamination. Specifically, equivalent freshwater heads in 243 monitoring wells were used to reconstruct the piezometric map of the studied aquifer; flow meter tests were carried out to infer vertical groundwater fluxes at selected wells; deuterium and oxygen isotopes were used to identify the groundwater origin, and tritium was analyzed to estimate the residence time; compound-specific isotope analyses and microbial analyses were employed to track different sources of contamination and their degradation; numerical modelling was used to estimate and verify groundwater flow direction and magnitude throughout the aquifer. The comparison of the information level for each technique allowed determining which of the applied approaches showed the best results to locate the possible sources and better understanding of the fate of the contaminants. This study reports a detailed site characterization process and outcomes for a coastal industrial site, where a comprehensive conceptual model of pollution and seawater intrusion has been built using different assessment methods. Information and results from this study encourages combining different methods for the design and implementation of the monitoring activities in real-life coastal contaminated sites in order to develop an appropriate strategy for control and remediation of the contamination.

Lipid biomarkers and organic matter carbon isotopes in estuarine sediments as proxies for evaluating seawater intrusion

Coastal wetlands and estuarine sediments are characteristics of tide-dominated environments and retain a record of seawater intrusion and possibly sea-level changes. A variety of methods including the C/N ratio and δ13C of bulk organic matter in the sediment have been employed in estuarine studies, but they are generally non-specific indicators. Here we report using lipid biomarkers to evaluate the extent of seawater intrusion based on an estuary in eastern China, Xinyanggang. Along the river from the estuary mouth upstream to the freshwater head, the vegetation shifts quickly from salt-tolerant Spartina (C4) to the less tolerant reed Phragmites (C3), both of which have distinguished δ13C values and biomarker distribution. The δ13C values of particulate organic matter (POM) and surface sediment decreased from the estuary mouth upstream, indicating the reduced contributions from Spartina and marine phytoplankton to the POM and surface sediment and increased inputs from Phragmites. The C32/C30 alkanol and cholesterol/sitosterol ratio decreased in the surface sediments, faithfully recording the variations in the contributions from Spartina and Phragmites. The combination of biomarker distribution and organic matter δ13C in the sediments can be used as indicators for sea water intrusion into the estuary/river.

Assessment of Density‐Induced Tracer Movement in Groundwater Velocity Measurements with Point Velocity Probes (PVPs)

AbstractThe concept of equivalent freshwater head was adapted to predict the conditions under which density‐driven flow would adversely impact measured groundwater velocities using point velocity probes (PVPs). Theoretically, vertical flow will result from any density contrast between the PVP tracer and the groundwater. However, laboratory testing of tracers with salinities ranging from 0 to 2000 mg NaCl/L showed that horizontal velocities could be determined with good accuracy with up to 60% of the total flow being vertical due to density effects in a gravel medium. The available data suggest that density effects are less likely to be pronounced in sandy sediments. The relative amount of vertical flow due to tracer density can be estimated from vertical and horizontal velocities measured with PVPs, or from the ratio of vertical to horizontal hydraulic gradients. The equivalent freshwater gradient produced from a given tracer salinity at 10 °C (a typical groundwater temperature at moderate latitudes) can be estimated from 7.80 × 10−7 × (MNaCl), where MNaCl is the mass of NaCl added, in mg, to 1 L of site groundwater in the mixing of the tracer. Equations for other temperatures were also determined.

Experimental evidence for seismically initiated gas bubble nucleation and growth in groundwater as a mechanism for coseismic borehole water level rise and remotely triggered seismicity

AbstractChanges in borehole water levels and remotely triggered seismicity occur in response to near and distant earthquakes at locations around the globe, but the mechanisms for these phenomena are not well understood. Experiments were conducted to show that seismically initiated gas bubble growth in groundwater can trigger a sustained increase in pore fluid pressure consistent in magnitude with observed coseismic borehole water level rise, constituting a physically plausible mechanism for remote triggering of secondary earthquakes through the reduction of effective stress in critically loaded geologic faults. A portion of the CO2 degassing from the Earth's crust dissolves in groundwater where seismic Rayleigh and P waves cause dilational strain, which can reduce pore fluid pressure to or below the bubble pressure, triggering CO2 gas bubble growth in the saturated zone, indicated by a spontaneous buildup of pore fluid pressure. Excess pore fluid pressure was measured in response to the application of 0.1–1.0 MPa, 0.01–0.30 Hz confining stress oscillations to a Berea sandstone core flooded with initially subsaturated aqueous CO2, under conditions representative of a confined aquifer. Confining stress oscillations equivalent to the dynamic stress of the 28 June 1992 Mw 7.3 Landers, California, earthquake Rayleigh wave as it traveled through the Long Valley caldera, and Parkfield, California, increased the pore fluid pressure in the Berea core by an average of 36 ± 15 cm and 23 ± 15 cm of equivalent freshwater head, respectively, in agreement with 41.8 cm and 34 cm rises recorded in wells at those locations.

A Correction on Coastal Heads for Groundwater Flow Models

We introduce a simple correction to coastal heads for constant-density groundwater flow models that contain a coastal boundary, based on previous analytical solutions for interface flow. The results demonstrate that accurate discharge to the sea in confined aquifers can be obtained by direct application of Darcy's law (for constant-density flow) if the coastal heads are corrected to ((α + 1)/α)hs - B/2α, in which hs is the mean sea level above the aquifer base, B is the aquifer thickness, and α is the density factor. For unconfined aquifers, the coastal head should be assigned the value hs1+α/α. The accuracy of using these corrections is demonstrated by consistency between constant-density Darcy's solution and variable-density flow numerical simulations. The errors introduced by adopting two previous approaches (i.e., no correction and using the equivalent fresh water head at the middle position of the aquifer to represent the hydraulic head at the coastal boundary) are evaluated. Sensitivity analysis shows that errors in discharge to the sea could be larger than 100% for typical coastal aquifer parameter ranges. The location of observation wells relative to the toe is a key factor controlling the estimation error, as it determines the relative aquifer length of constant-density flow relative to variable-density flow. The coastal head correction method introduced in this study facilitates the rapid and accurate estimation of the fresh water flux from a given hydraulic head measurement and allows for an improved representation of the coastal boundary condition in regional constant-density groundwater flow models.

Groundwater Flow Cycling Between a Submarine Spring and an Inland Fresh Water Spring

Spring Creek Springs and Wakulla Springs are large first magnitude springs that derive water from the Upper Floridan Aquifer. The submarine Spring Creek Springs are located in a marine estuary and Wakulla Springs are located 18 km inland. Wakulla Springs has had a consistent increase in flow from the 1930s to the present. This increase is probably due to the rising sea level, which puts additional pressure head on the submarine Spring Creek Springs, reducing its fresh water flow and increasing flows in Wakulla Springs. To improve understanding of the complex relations between these springs, flow and salinity data were collected from June 25, 2007 to June 30, 2010. The flow in Spring Creek Springs was most sensitive to rainfall and salt water intrusion, and the flow in Wakulla Springs was most sensitive to rainfall and the flow in Spring Creek Springs. Flows from the springs were found to be connected, and composed of three repeating phases in a karst spring flow cycle: Phase 1 occurred during low rainfall periods and was characterized by salt water backflow into the Spring Creek Springs caves. The higher density salt water blocked fresh water flow and resulted in a higher equivalent fresh water head in Spring Creek Springs than in Wakulla Springs. The blocked fresh water was diverted to Wakulla Springs, approximately doubling its flow. Phase 2 occurred when heavy rainfall resulted in temporarily high creek flows to nearby sinkholes that purged the salt water from the Spring Creek Springs caves. Phase 3 occurred after streams returned to base flow. The Spring Creek Springs caves retained a lower equivalent fresh water head than Wakulla Springs, causing them to flow large amounts of fresh water while Wakulla Springs flow was reduced by about half.

Timescales of seawater intrusion and retreat

Quantifying the timescales associated with moving freshwater–seawater interfaces is critical for effective management of coastal groundwater resources. In this study, timescales of interface movement in response to both inland and coastal water level variations are investigated. We first assume that seawater intrusion (SWI) and retreat (SWR) are driven by an instantaneous freshwater-level variation at the inland boundary. Numerical modelling results reveal that logarithmic timescales of SWI (lnTi) and SWR (lnTr) can be described respectively by various simple linear equations. For example, SWI timescales are described by lnTi=a+blnh′f–s, where a and b are linear regression coefficients and h′f–s is the boundary head difference after an instantaneous drop of inland freshwater head. For SWR cases with the same initial conditions, but with different increases in freshwater head, lnTr=c+dΔXT, where c and d are regression coefficients and ΔXT is the distance of toe response that can be estimated by a steady-state, sharp-interface analytical solution. For SWR cases with the same freshwater head increase, but with different initial conditions, in contrast, lnTr=e+flnΔXT, where e and f are regression coefficients. The timescale of toe response caused by an instantaneous variation of sea level is almost equivalent to that induced by an instantaneous inland head variation with the same magnitude of water level change, but opposite in direction. Accordingly, the empirical equations of this study are also applicable for sea-level variations in head-controlled systems or for simultaneous variations of both inland and coastal water levels. Despite the idealised conceptual models adopted in this study, the results imply that for a particular coastal aquifer, SWI timescales are controlled by the boundary water levels after variations, whereas SWR timescales are dominated by the distance of toe response.

Modeling flow and reactive transport to explain mineral zoning in the Atacama salt flat aquifer, Chile

Geochemical processes are typically neglected when modeling groundwater flow in highly saline systems. In the Atacama salt flat, northern Chile, field data indicate that the main aquifer presents a mineralogical zoning consisting of carbonates, sulfates and halite, which is a result of precipitation and/or dissolution reactions. When precipitation and/or dissolution of minerals are active processes in the aquifer, physical properties that control groundwater flow, such as porosity and permeability, would also change. Until now, groundwater flow in the Atacama salt flat has been simulated with conventional constant- or variable-density flow models. This study, for the first time, investigated the importance of incorporating geochemical processes in modeling the Atacama salt flat aquifer. In particular, the impact of mineral precipitation/dissolution on aquifer’s physical properties and flow patterns was studied. To this end, the SHEMAT code was utilized to develop a two-dimensional numerical groundwater flow model in the eastern border of the Atacama salt flat. The SHEMAT code solves the classic groundwater flow equation coupled to the reactive transport of ions and mineral precipitation, with variable density, viscosity, porosity and permeability. When geochemical processes were included in the simulations, the model reproduced reasonably well the mineral zoning evidenced by field observations. Mineral precipitation was responsible of a strong permeability reduction in areas of the aquifer associated with halite and calcite formation. Permeability alterations were responsible of changes in the equivalent freshwater head that resulted in significant flow pattern variations compared to variable-density flow models. These results highlight the importance of considering geochemical processes that modify the aquifer’s physical properties when modeling groundwater flow in highly saline aquifers such as the Atacama salt flat aquifer.

Incorporating the concept of equivalent freshwater head in successive horizontal simulations of seawater intrusion in the Nile Delta aquifer, Egypt

Summary A new approach to study seawater intrusion problems in coastal aquifers is presented. The approach is demonstrated for the case of the Nile Delta aquifer in Egypt. FEFLOW, a 3D finite element variable density model, is employed, however, because of the lack of 3D data, and to demonstrate the proposed approach, the simulations are performed in 2D horizontal views. The concept of equivalent freshwater head (usually implemented in 2D vertical simulations) is adapted in the horizontal (areal) simulations. After calibration against field observations, the simulations are conducted at four horizontal sections located at different levels (100, 200, 300 and 400 m) below the mean seawater level. The depth of the horizontal section is identified through assigning an appropriate pressure “equivalent freshwater” head at the boundaries. The study domain is modified for the horizontal sections at 300 and 400 m, respectively, to account for the aquifer geometry at these depths. The effect of freshwater recharge from the Nile River on the seawater intrusion is observed in the upper layer around its two main branches. The results of the horizontal simulations clearly demonstrate the variation of water concentration in the vertical direction. As the depth increases, the transition zone (in which the concentration varies from the seawater to the freshwater concentration) is shifted toward the landside and become more extensive. At the lower levels of the Nile Delta aquifer, the seawater migrates much further inland as compared to the shallower levels. The concept of horizontal simulations at different levels is further developed to produce meaningful concentration distributions in the vertical sections. This approach allows for a better realization of seawater intrusion in coastal aquifers.

Assessing the Effect of Saltwater Intrusion on Petroleum Hydrocarbons Plumes Via Numerical Modelling

A contamination by petroleum hydrocarbons was detected in a sandy aquifer below a petrochemical plant in Southern Italy. The site is located near the coastline and bordered by canals which, together with pumping wells, control submarine groundwater discharge toward the sea and seawater intrusion (SWI) inland. In this study, a three-dimensional flow and transport model was developed using SEAWAT-4.0 to simulate the density-dependent groundwater flow system. Equivalent freshwater heads from 246 piezometers were employed to calibrate the flow simulation, while salinity in 193 piezometers was used to calibrate the conservative transport. A second dissolved species, total petroleum hydrocarbons (TPH), was included in the numerical model to simulate the plumes originating from light non-aqueous-phase liquid. A detailed field investigation was performed in order to determine the fate of dissolved hydrocarbons. Fifteen depth profiles obtained from multilevel samplers (MLS) were used to improve the conceptual model, originally built using a standard monitoring technique with integrated depth sampling (IDS) of salinity and TPH concentrations. The calibrated simulation emphasises that density-dependent flow has a great influence on the migration pattern of the hydrocarbons plume. This study confirms that calibration of density-dependent models in sites affected by SWI can be successfully reached only with MLS data, while standard IDS data can lead to misleading results. Thus, it is recommended to include MLS in the characterization protocols of contaminated sites affected by SWI, in order to properly manage environmental pollution problems of coastal zones.

Electrical Conductivity as a Proxy for Groundwater Density in Coastal Aquifers

Groundwater density is an important parameter in the interpretation of flow patterns. This paper investigates the relationship between electrical conductivity (EC) and groundwater density in coastal aquifers and evaluates the suitability of the UNESCO 1980 equation of state, developed for the world's oceans, for determining the density of groundwater based on its EC. To achieve this aim, a dataset of groundwater samples from four different types of coastal aquifers was collected. It is found that the density of a sample can be estimated to a good approximation on the basis of its EC using the UNESCO 1980 equation of state. Deviations from the equation of state were found to be due to the changes in EC and the density caused by geochemical reactions, such as the dissolution of carbonates, degradation of organic carbon, cation exchange, and sulfate loss. Owing to these deviations, the UNESCO 1980 equation of state may underestimate the density by up to 1.5 kg/m(3). The effect of this uncertainty on the correction terms applied to the hydraulic head required for a proper interpretation of groundwater flow patterns and rates is quantified. It was found that the fresh water head may be wrong by centimeters to a few decimeters. From this it is concluded that, unless the purpose of a groundwater investigation requires great accuracy, the equation of state provides an efficient and inexpensive way to estimate density from EC.

Hydrogeological study for improved nature restoration in dune ecosystems–Kleyne Vlakte case study, Belgium

In dune slacks a close coupling exists between changes in the hydrology and changes in species composition and vegetation structure. Consequently, there is a need to underpin nature restoration projects not only with ecologically relevant knowledge but also with scientifically sound hydrogeological data. In this paper, this necessity is illustrated through a study of the Flemish Nature Reserve ‘The Zwindunes and Zwinpolders’ (Belgian coastal plain) as an example. The management plan for the nature reserve suggests rewetting part of it to enhance its ecological value. The groundwater aspect was studied by means of field observations and mathematical modelling. First, fresh water head observation showed a mean groundwater flow from the nature reserve to the adjacent polder. Secondly, groundwater quality was studied with borehole measurements and water samples, resulting in a map of the fresh-salt water distribution and of water types. All available information was then put together in a density dependent groundwater flow model. The aim of this model was the description of current flow and fresh-salt water distribution and to simulate the impact of three possible rewetting scenarios. Rewetting will be accomplished by the infiltration of water in a depression, different lay-outs for which are considered. A zoomed in flow model based on a regional model was used to incorporate both local scale, which is of importance to ecology, and the larger scale, which determines general groundwater flow and fresh-salt water distribution. This modelling indicated differences between scenarios and was used to decide on the best rewetting strategy.

Decreased precipitation exacerbates the effects of sea level on coastal dune ecosystems in open ocean islands

AbstractThe alteration of fresh and marine water cycling is likely to occur in coastal ecosystems as climate change causes the global redistribution of precipitation while simultaneously driving sea‐level rise at a rate of 2–3 mm yr−1. Here, we examined how precipitation alters the ecological effects of ocean water intrusion to coastal dunes on two oceanic carbonate islands in the Bahamas. The approach was to compare sites that receive high and low annual rainfall and are also characterized by seasonal distribution (wet and dry season) of precipitation. The spatial and temporal variations in precipitation serve as a proxy for conditions of altered precipitation which may occur via climate change. We used the natural abundances of stable isotopes to identify water sources (e.g., precipitation, groundwater and ocean water) in the soil–plant continuum and modeled the depth of plant water uptake. Results indicated that decreased rainfall caused the shallow freshwater table on the dune ecosystem to sink and contract towards the inland, the lower freshwater head allowed ocean water to penetrate into the deeper soils, while shallow soils became exceedingly dry. Plants at the drier site that lived nearest to the ocean responded by taking up water from the deeper and consistently moist soil layers where ocean water intruded. Towards the inland, decreased rainfall caused the water table to sink to a depth that precluded both recharge to the upper soil layers and access by plants. Consequently, plants captured water in more shallow soils recharged by infrequent rainfall events. The results demonstrate dune ecosystems on oceanic islands are more susceptible to ocean water intrusion when annual precipitation decreases. Periods of diminished precipitation caused drought conditions, increased exposure to saline marine water and altered water‐harvesting strategies. Quantifying species tolerances to ocean water intrusion and drought are necessary to determine a threshold of community sustainability.

南海地震前の井戸水の低下について―次の南海地震の予知に向けて―

The water level in wells along the Paci.c coast from the Kii peninsula to Shikoku showed remarkable decreases a few days before the 1946 Nankai earthquake. If pre-slip occurred on a deep portion of the earthquake fault, an uplift .eld would cover the area of the decreased well water. However, the expected uplift is only a few centimeters. Can the drastic changes of well water be explained by the slight uplift? By our field surveys, we confirmed that the wells were located in a small delta. The seawater permeates under the delta which faced the sea, and the fresh water .oats on the seawater due to the different density. The Ghyben-Herzberg’ law shows that the depth between the horizon and the sea-and fresh water boundary is balanced to 40 times of the height between the horizon and fresh water head which is called groundwater table. This law implies that a slight uplift of the fresh water in a delta induces a 40 times drop of the sea-and fresh water boundary. In this case, much fresh water will be required to suspend the slight uplift of fresh water. If the new fresh water does not be supplied from the outside of delta, the much fresh water flows from the upper level of delta to take the new balance. Then, the well water on the upper level of the delta must decrease or dry up. The decrease of well water before the great earthquake was also con.rmed at the time before the 1854 Nankai earthquake (M 8.4). The reproducibility of the decrease of well water level caused by the pre-slip of Nankai earthquakes is fairly high. These understandings are effective for the prediction of the next great Nankai earthquake.

Brief Review on Methods of Estimation of the Location of a Fresh Water–Salt Water Interface with Hydraulic Heads or Pressures in Coastal Zones

AbstractMethods of estimation of the location of a sharp fresh water–salt water interface with hydraulic heads or pressures are relatively simple and are widely used. Progress has been made in the recent decade toward the mathematical relations describing the position of the sharp interface using hydraulic heads or pressures in coastal zones. This paper reviews several methods for estimation of the location of fresh water–salt water interface in coastal aquifers, including the classical Ghyben–Herzberg relation. The location of the fresh water–salt water interface in a coastal homogeneous, isotropic unconfined aquifer can be estimated based on piezometric heads at two points in the same vertical line tapping, respectively, the salt water zone (including the interface) and the fresh water zone (from the water table to the interface) when the groundwater flow system is in a steady state and satisfies the Dupuit assumption. If pressures are measured at two points in the fresh water and salt water zones in the same vertical line in the coastal aquifer under the same assumption, then the position of the interface can still be estimated with the pressure data. If the Dupuit assumption is not met in coastal aquifers and the vertical fresh water head gradients can be approximated with a straight line, the position of the interface can roughly be estimated by using the water level data in a partially penetrating well during drilling of the well.