Freshening Process Research Articles

Hydrogeochemical modelling of groundwater in a fractured Carboniferous sandstone aquifer

Deep groundwater monitoring in tight bedrock is not commonly performed and rarely documented. For a case study in Bochum, NRW, deep monitoring wells allow groundwater investigation in the outcropping Carboniferous down to a depth of 186 m. Groundwater monitoring was carried out by combining sampling of the monitoring wells and local springs at the site. Inverse models were developed using the hydrogeochemical code PHREEQC to understand the natural geochemical processes of deep groundwater in the coal-bearing sandstone. The identified hydrogeochemical processes are pyrite oxidation in shallow fracture systems and Na-Ca/Mg cation exchange in deep fracture systems. Core samples reveal fractures containing Fe-dolomite and calcite cements. A freshening process dominates the deep fractured rocks, where fresh water is flushing a saline aquifer. The data presented here support the planning of further monitoring wells and help to improve future modelling approaches.

Attribution of the Subsurface Temperature Change in the Southern Hemisphere

Abstract The Southern Hemisphere temperature has experienced obvious changes with great spatial differences over the past several decades. Most regions show extreme warming, especially those located at 35°–55°S. In contrast, subsurface cooling exists between 15° and 35°S in the Indian and Pacific basins. The subsurface temperature and salinity change can be divided into spiciness change and heave components. The results show the warming due to isopycnal movement being largely offset by significant spiciness cooling at middepth. Surface warming and subduction into the interior ocean account for subsurface spiciness cooling near 45°S, while surface freshening and penetration along isopycnals are more important to the subsurface spiciness cooling farther north. The isobaric temperature change is associated with pure warming and pure heaving, and the subsurface cooling observed in the Indian and Pacific subtropics is predominantly attributed to pure heaving. This study provides a quantitative estimate of the relative contribution of surface temperature, salinity change, and circulation adjustment in subsurface temperature change, highlighting the importance of circulation change in producing subsurface cooling. Further research is needed to understand why different processes dominate in different ocean sections. Significance Statement While the global ocean is warming, the subsurface temperature change exhibits a significant regional disparity. This paper attempts to explain the deep-reaching warming at 35°–55°S and cooling at 15°–35°S based on three historical observation datasets. We find that the cooling mostly occurs between 400 and 1000 m in the south Indian and Pacific subtropics (15°–35°S), which is attributed to pure heaving, indicating the importance of circulation change in these regions. The midlatitude warming (35°–55°S) is mainly caused by the pure warming process, which is related to heat uptake at the subpolar surface and northward and downward heat transport. The spiciness cooling near 45°S is mainly driven by the subduction of the surface warming signal while the freshening process has a stronger impact on spiciness cooling farther north.

Roles of Surface Forcing in the Southern Ocean Temperature and Salinity Changes under Increasing CO2: Perspectives from Model Perturbation Experiments and a Theoretical Framework

Abstract A rapid warming and freshening of the Southern Ocean have been observed over the past several decades and are attributed to anthropogenic climate change. In this study, ocean model perturbation experiments are conducted to separate roles of individual surface forcing in the Southern Ocean temperature and salinity changes. Model-based findings are compared with results from a theoretical framework including three idealized processes defined on the θ–S diagram. Under the future scenario of CO2 doubling, the heat flux forcing dominates the large-scale warming, deepening of isopycnals, and spiciness changes along isopycnals, which can be captured by an idealized pure warming process to represent the subduction of surface heat uptake. The poleward-intensifying westerly winds account for 24% of the enhanced warming between 35° and 50°S and would have comparable contribution as the heat flux forcing after removing the global ocean warming effect. In contrast, the widespread freshening in the Southern Ocean driven by increased surface freshwater input is largely compensated by the wind-driven saltening. The response to freshwater forcing could not be approximated as a similar pure freshening process as the induced cooling and freshening have comparable effects on density. The wind-driven changes are primarily through the local heave of isopycnals, thus resembling an idealized pure heave process, but contain considerable spiciness signals especially in the midlatitude Southern Ocean, resulting from anomalous northward transport and subduction of heat and salt that are largely density-compensating. These distinct signatures of individual surface forcing help us to better understand observed and projected changes in the Southern Ocean. Significance Statement Considerable changes including a rapid warming and freshening have been observed in the Southern Ocean as it absorbs most of the extra heat from the anthropogenic climate change, receives increased surface freshwater input, and experiences a poleward shift and intensification of the westerly winds. The purpose of this study is to distinguish different contributions from surface heat flux, freshwater flux, and wind forcing to the Southern Ocean temperature and salinity changes, based on ocean model experiments and three idealized processes from a theoretical framework. Our study reveals distinct signatures of individual surface forcing that help us to understand linkages between changes seen at the surface and in the interior Southern Ocean.

Hydrochemical evolution processes of multiple-water quality interfaces (fresh/saline water, saline water/brine) on muddy coast under pumping conditions.

A thorough understanding of the evolution of coastal brine-bearing groundwater systems under the influence of human activities contributes to the sustainable use of coastal groundwater resources. Therefore, this study comprehensively investigated the processes associated with hydrochemical changes in groundwater during brine exploitation based on long-term monitoring data. The dataset comprised 102 samples (including groundwater, seawater, rain and river samples) collected from 1966 to 2021 to capture the hydrochemical variability. Significant changes in the brine water table and concentration between the pre-overexploitation period (1965-2000) and the exploitation period (2000-2021) are observed. From the relationship between stable isotope (δ18O and δ2H) values and Cl- contents, shallow saline water (SSW) near the drawdown cone (with δ18O and δ2H values of -4.66 ‰ ~ -3.57 ‰ and - 42.1 ‰ ~ -32.8 ‰, respectively) is similar geochemically to the brine inside the drawdown cone (with values of -4.30 ‰ ~ -3.10 ‰ and - 39.0 ‰ ~ -32.3 ‰, respectively), indicating that the SSW has a recharge effect on the underground brine. The delta values of major cations were calculated to analyze the hydrochemical processes at different water quality interfaces (saline/freshwater interface, salinewater/brine interface). The results demonstrated that the SSW and deep saline water (DSW) at the offshore brine/saline water interface undergo seawater intrusion, where SSW undergoes a salinization process with cation exchange (Na+ is exchanged for Ca2+ and Mg2+ in clay), while DSW undergoes a freshening process. The SSW at the saline/freshwater interface is affected by freshwater pumping, and the DSW undergoes a salinization process influenced by brine intrusion presenting the common salinization process with cation exchange. The hydrochemical model shows that the brine concentration recovered during the restricted exploitation stage due to the dissolution of gypsum and halite. If no measures are taken to restrict exploitation, then brine will eventually be replaced by saline water.

Groundwater Salinization and Freshening Processes in the Luy River Coastal Aquifer, Vietnam

The Vietnamese Binh Thuan province located along the Southern Central part of the coast is one of the driest in the country. The population is relying largely on groundwater for irrigation, and the groundwater resources are threatened by climate change and saltwater intrusion in river estuaries. Recent studies, however, reveal that the extension of saltwater intrusions was larger than expected, raising some doubts on the actual origin of the intrusions. In this study, we use the geochemical characterization of groundwater samples collected in both the dry and rainy seasons in the Luy river coastal area to identify hydrochemical processes responsible for the salinization and variations taking place in the shallow aquifers. The distribution of chemical components of groundwater and the indicators presenting the freshening and salinization processes, such as cation exchange code and chloride conservative element, the ionic delta, the HFE-diagram, and geochemical modelling, were studied. The results show that 65% of the samples exceed the WHO limit for drinking water and 100% of them have a degree of restriction on use from slight–moderate to severe in potential irrigation problems according to FAO. In contrast to previous expectations, freshening is the dominant process in the aquifers, and it is more advanced in the rainy season. Due to a lack of recharge and aquifer exploitation, salinization is more severe in the deeper aquifer and during the dry season. Saltwater intrusion is not limited to the zone close to the river but extends further inland and at depth, and salinity can vary quickly over short distances. Based on these new insights, we develop a new conceptual model for the evolution of salinization in the Luy river catchment involving a natural freshening process of connate water combined with anthropic influence. The conceptual model will form the basis for the development of a groundwater model of the study area and eventually lead to sustainable management scenarios for this coastal region, preventing further deterioration of the groundwater resources.

Understanding the effects of periodic freshening and salinization on coastal water quality and aquifer sediments through laboratory-based column experiments

The coastal aquifers experience periodic freshening and salinization events due to frequent changes in hydrological conditions. Such frequent changes have vital effects on the coastal groundwater quality and aquifer properties. This forms the basis of the present study. Laboratory-based column experiments have been conducted to investigate the long-term impact of such periodic events on coastal groundwater quality and aquifer sediment characteristics. Conditions similar to the natural coastal aquifer systems were generated in sediment-filled columns by injecting freshwater and seawater periodically. Changes in pore water chemistry and sediment properties were monitored over a period of one year. It has been observed that periodic injections of freshwater (freshening) and seawater (salinization) caused significant changes in the pore water chemistry, mainly due to sediment-water interactions. In addition to seawater-freshwater mixing, geochemical processes such as ion-exchange, mineral precipitation/dissolution, and adsorption/desorption of elements have been identified to control the pore water chemistry. Trace elements, including barium, boron, bromide, lithium, and strontium, have been identified as effective tracers for monitoring the saltwater intrusion. Further, the changes in pore water chemistry are found to be interlinked with the changes in aquifer properties. It is observed that with the increase in the number of salinization cycles, the effectiveness of the freshening process to bring back the original aquifer pore water chemistry diminishes, and the impact of salinization lasts longer. At the end of the experiment, changes in the porosity and permeability of the sediments have been observed. Such changes in the aquifer properties may affect the specific yield and storativity of the aquifer.

Hydrochemical and Isotopic Characterization of the Waters of the Manglaralto River Basin (Ecuador) to Contribute to the Management of the Coastal Aquifer

Coastal aquifers are strategic and fundamental in the development of touristic areas. The coastal aquifer within the Manglaralto River Basin in Ecuador is essential, as it is the only source of water supply for a large part of the northern part of the Santa Elena province. It is a semi-arid region where high volumes of water are pumped from the aquifer, causing a significant drawdown of groundwater levels, thus affecting the water quality. This work aims to characterize the characteristics of groundwater in the coastal aquifer using hydrochemistry and stable isotopes to propose a hydrogeological conceptual model. The methodology for determining the chemical and isotopic characteristics of groundwater follows the following scheme: (i) studies of ionic concentrations using the Piper diagram, (ii) assessment of the origin of salinity through the Cl/Br ratio, the presence of seawater intrusion through the Hydrochemical Facies Evolution Diagram HFE-D, (iii) characterization of precipitation events using stable isotopes (18O and 2H), and, (iv) development of a hydrogeological conceptual model of the study area. The results indicate that in the basin there are mixing processes of the existing water in the aquifer with recharge water, direct cation exchange processes in the freshening process during recharge, and evaporation in the unsaturated zone. A conceptual model of the flow system in the basin is built, based on the mentioned processes. The main conclusions are: seawater intrusion is present in the areas of the wells located closest to the coast, urban activity through septic tanks is affecting the quality of the aquifer, and rainfall is highly relevant in the different hydrochemical and isotopic processes that operate in the basin.

Groundwater and soil salinization and geochemical evolution of Femenin-Ghahavand plain, Iran.

The hydrogeological and hydrogeochemical data of 75 groundwater samples and Landsat 8 OLI satellite data were used to identify the source and mechanism of groundwater and soil salinization in Femenin-Ghahavand aquifer (FGA) in Hamadan area, Iran. Generally, the EC values range from 350 to 7556μS/cm with a mean value of 1785μS/cm. The salinity value increases in the groundwater along the flow direction from southeast to northwest. The water samples are categorized into two water types of HCO3-Ca and Ca2+-Mg2+-Cl-. The water samples are super-saturated with calcite and dolomite due to dissolution of carbonate rocks through the flow path from recharge to discharge zones and under-saturated with respect to gypsum and halite. The most probable source of salinity is dissolution of interlayer of halite and gypsum in Miocene Marl and Qom formation in the bedrock of FGA. Also, the ion exchange is another major process that affects the deep groundwater quality in FGA. The water samples of FGA were divided into two groups based on the HFE diagram. About 71% of the samples are plotted below the mixing line, indicating intrusion process with reverse ion exchange and the rest (29%) is under freshening process with direct ion exchange. Consequently, the source of salinity in FGA groundwater is dissolution of halite and gypsum in deep saline groundwater which is likely upconing and mixing with fresh groundwater during overexploitation. The result of soil investigation using NDS index based on Landsat 8 OLI satellite data shows severe increase in salinization from 2013 to 2019. The areas with saline groundwater have saline soil too, indicating same source of groundwater and soil salinization. However, the main cause of soil salinization would be land use change and irrigation by saline groundwater and high evaporation in the region.

Hydrochemical Facies and Ionic Exchange in Coastal Aquifers of Puducherry Region, India: Implications for Seawater Intrusion

Salinization of groundwater by seawater intrusion is a major concern for the coastal aquifers worldwide. Seawater intrusion occurs mainly due to overpumping of freshwater and sea-level rise which causes lateral and vertical movements of seawater into the coastal aquifers. There are several methods to identify and assess the extent of seawater ingress towards land. In the present study, a hydrochemical approach is adopted to understand the status of intrusion. The ions such as Na+, K+, Ca2+, Mg2+, Cl−, SO4 2−, HCO 3 2− , and in-situ parameters namely TDS, pH, and EC were determined for 174 water samples collected both in pre- and post-monsoon in Puducherry region, India. Ionic ratios such as HCO3 2−/Cl−, Na+/Cl−, Ca2+/Cl−, Ca2+/Na+, Mg2+/Cl−, K+/Cl−, SO4 2−/Cl−, and Cl−/Br− were calculated and correlated with total dissolved solids (TDS) to evaluate seawater intrusion. The ionic ratios such as HCO3 2−/Cl−, Ca2+/Cl−, Mg2+/Cl−, K+/Cl−, and SO4 2−/Cl− vs. TDS shows negative correlation indicating salinization of groundwater. Cl−/Br− ratio is used to distinguish the causes of salinity in groundwater. The Hydrochemical Facies Evolution diagram (HFE-diagram) and heat maps generated out of it have been very well used to understand evolution of seawater intrusion and freshening process in the coastal aquifer to time. The majority of samples in pre-monsoon fall under the facies Na–HCO3/SO4, followed by Na–mixHCO3/mix SO4, Mix Na–HCO3/Mix SO4, and MixNa–MixHCO3/mix SO4 facies indicating direct cations exchange process, whereas, in post-monsoon, Na–Cl, Mix Na–Cl, and Mix Ca–Cl facies are dominant indicating reverse ion exchange process. In the study area, five locations, viz. Ariyankuppam, Kariambattur, Kalapet, Mutialpet, and Parikalpet, fall under Na–Cl and Ca–Cl facies in pre- and post-monsoon which indicates consistent seawater intrusion. The hydrochemical changes that take place during seawater freshwater interaction along coastal aquifer are determined by ionic exchange. About 24.2% of samples in pre-monsoon and 13.5% of samples in post-monsoon show mixing of seawater. The highly negative ionic exchange values of sodium during pre-monsoon indicate increased amount of seawater fraction in groundwater.

Hydrochemical assessment of freshening saline groundwater using multiple end-members mixing modeling: A study of Red River delta aquifer, Vietnam

In this study, we evaluated the water quality status (especially, salinity problems) and hydrogeochemical processes of an alluvial aquifer in a floodplain of the Red River delta, Vietnam, based on the hydrochemical and isotopic data of groundwater samples (n=23) from the Kien Xuong district of the Thai Binh province. Following the historical inundation by paleo-seawater during coastal progradation, the aquifer has been undergone progressive freshening and land reclamation to enable settlements and farming. The hydrochemical data of water samples showed a broad hydrochemical change, from Na-Cl through Na-HCO3 to Ca-HCO3 types, suggesting that groundwater was overall evolved through the freshening process accompanying cation exchange. The principal component analysis (PCA) of the hydrochemical data indicates the occurrence of three major hydrogeochemical processes occurring in an aquifer, namely: 1) progressive freshening of remaining paleo-seawater, 2) water-rock interaction (i.e., dissolution of silicates), and 3) redox process including sulfate reduction, as indicated by heavy sulfur and oxygen isotope compositions of sulfate. To quantitatively assess the hydrogeochemical processes, the end-member mixing analysis (EMMA) and the forward mixing modeling using PHREEQC code were conducted. The EMMA results show that the hydrochemical model with the two-dimensional mixing space composed of PC 1 and PC 2 best explains the mixing in the study area; therefore, we consider that the groundwater chemistry mainly evolved by mixing among three end-members (i.e., paleo-seawater, infiltrating rain, and the K-rich groundwater). The distinct depletion of sulfate in groundwater, likely due to bacterial sulfate reduction, can also be explained by EMMA. The evaluation of mass balances using geochemical modeling supports the explanation that the freshening process accompanying direct cation exchange occurs through mixing among three end-members involving the K-rich groundwater. This study shows that the multiple end-members mixing model is useful to more successfully assess complex hydrogeochemical processes occurring in a salinized aquifer under freshening, as compared to the conventional interpretation using the theoretical mixing line based on only two end-members (i.e., seawater and rainwater).

Natural and anthropogenic factors affecting freshwater lenses in coastal dunes of the Adriatic coast

This study characterizes the near-shore portion of the shallow coastal aquifer included in the Ravenna area (Northern Italy) with special attention to the roles of coastal dunes as freshwater reservoirs and their buffer on groundwater salinity. The paper focuses on the presence and evolution of freshwater lenses below coastal dunes and highlights the existing differences between preserved natural dunes and dunes strongly affected by human intervention. The influence that multiple natural and anthropogenic factors, such as land cover, local drainage network, and beach erosion have on the presence, size and evolution of the freshwater lenses in the aquifer is quantified and discussed. The methodology includes multiple seasonal monitoring and sampling campaigns of physical (water level, salinity, and temperature) and chemical (major cations and anions) groundwater parameters. Results indicate that freshwater lenses, where existing, are limited in thickness (about 1–2m). Proximity to drainage ditches as well as limited dune elevation and size do not allow the formation and permanent storage of large freshwater lenses in the aquifer below the dunes. The pine forest land cover, that replaced the typical bush or sand cover, intensifies evapotranspiration reducing net infiltration and freshwater storage. The cation species distribution in the water shows that a freshening process is ongoing in preserved natural sites with stable or advancing beaches, whereas a salinization process is ongoing in anthropogenic-impacted areas with strongly-fragmented dune systems. Currently, the thin freshwater lenses in the shallow Ravenna coastal aquifer are limited in space and have no relevance for irrigation or any other human activity. The dune-beach system, however, is the recharge zone of the coastal aquifer and its protection is important to reduce water and soil salinization, which in turn control the health of the whole coastal ecosystem.

Groundwater flow and chemistry of the oases of Al Wahat, NE Libya

The quality and geochemistry of groundwater are significantly affected by the depositional environment of aquifer sediments. Miocene sediments in Al Wahat area (Jalu, Awjilah and Shakherah Oases) in the Libyan Desert at the north-east of the country have been deposited in fluvial marginal marine and marine environments. The purposes of this paper are to describe the areal distribution of the dominant water quality constituents, to identify the major hydro-geochemical processes that affect the quality of water and to evaluate the relations of sedimentary depositional environments and groundwater flow to the quality and geochemistry of water in aquifer sediments of Post-Eocene. This study is the first investigation in Al Wahat Oasis and also in the whole Sahara, which introduces the importance of considering the end members and the synsedimentary influence for the interpretation of the aquifer hydrochemistry. The area involved in this study is within the boundaries 28°N–30°N and 21°E–23°E. Eighteen wells are selected in the area, including eight piezometers, and ten samples were analysed from wells used for domestic and agricultural purposes. Results show high and significant increase in total dissolved solids, especially Na+, Cl−, SO4 2− and NO3 − compared with the previous years. The chemical results for the groundwater samples in Al Wahat are classified according to the Stuyfzand groundwater classification system; the water type is mostly brackish and brackish-saline NaCl in the downstream direction and fresh-brackish NaHCO3 upstream. These water types indicate that groundwater chemistry is changed by cation exchange reactions during flushing of the diluted saline aquifer by freshwater from the south. The different stages of cation exchange produce a chromatographic sequence of groundwater types. These cation exchange reactions during the freshening process occur mainly in the intercalated clay, resulting in a Na+ increase, and peaks of K+ and Mg2+ in the aquifer. In the north, the synsedimentary marine influence on the groundwater is stronger and the abstraction for irrigation is higher. Upconing of deep saline water and anthropogenic pollution may contribute significantly to the aquifer water quality. Calcite equilibrium and gypsum dissolution are also important hydrochemical processes in the aquifer.

Formation Mechanism of Barrier Layer in the Subtropical Pacific

AbstractSeasonal and interannual variations of the barrier layer (BL) and its formation mechanism in the subtropical North and South Pacific were investigated by using raw and gridded Argo profiling float data and various surface flux data in 2003–12 and hydrographic section data from the World Ocean Circulation Experiment Hydrographic Programme. BLs detected by raw Argo profiles, which existed within the sea surface salinity (SSS) front located on the equator side of SSS maxima, were thickest and most frequent in winter and had a temporal scale shorter than 10 days, indicating their transient nature. Surface and subsurface processes for the BL formation suggested by previous studies were evaluated. Poleward Ekman advection of fresher water was dominant as the surface freshening process but cannot explain the observed seasonal variations of the BL. Subsurface equatorward intrusion of high-salinity tropical water was too deep to produce salinity stratification within isothermal layers. These results strongly suggest that BLs in the subtropical Pacific are formed mainly through tilting of the SSS front due to the poleward Ekman flow near the sea surface and the equatorward geostrophic flow in the subsurface. This idea is supported by the dominant contribution of the meridional SSS gradient to the meridional sea surface density gradient within the SSS front and the correspondence between the seasonal variations of the BL and isothermal layer depth. On an interannual time scale, the winter BL thickness in the North and South Pacific was related to the Pacific decadal oscillation and the El Niño–Southern Oscillation, respectively, through the intensity of trade winds controlling isothermal layer depth.

Origins and processes of groundwater salinization in the urban coastal aquifers of Recife (Pernambuco, Brazil): A multi-isotope approach

In the coastal multilayer aquifer system of a highly urbanized southern city (Recife, Brazil), where groundwaters are affected by salinization, a multi-isotope approach (Sr, B, O, H) was used to investigate the sources and processes of salinization. The high diversity of the geological bodies, built since the Atlantic opening during the Cretaceous, highly constrains the heterogeneity of the groundwater chemistry, e.g. Sr isotope ratios, and needs to be integrated to explain the salinization processes and groundwater pathways. A paleoseawater intrusion, most probably the 120kyB.P. Pleistocene marine transgression, and cationic exchange are clearly evidenced in the most salinized parts of the Cabo and Beberibe aquifers. All 87Sr/86Sr values are above the past and present-day seawater signatures, meaning that the Sr isotopic signature is altered due to additional Sr inputs from dilution with different freshwaters, and water–rock interactions. Only the Cabo aquifer presents a well-delimitated area of Na-HCO3 water typical of a freshening process. The two deep aquifers also display a broad range of B concentrations and B isotope ratios with values among the highest known to date (63–68.5‰). This suggests multiple sources and processes affecting B behavior, among which mixing with saline water, B sorption on clays and mixing with wastewater. The highly fractionated B isotopic values were explained by infiltration of relatively salty water with B interacting with clays, pointing out the major role played by (palaeo)-channels for the deep Beberibe aquifer recharge. Based on an increase of salinity at the end of the dry season, a present-day seawater intrusion is identified in the surficial Boa Viagem aquifer. Our conceptual model presents a comprehensive understanding of the major groundwater salinization pathways and processes, and should be of benefit for other southern Atlantic coastal aquifers to better address groundwater management issues.

Research on the critical conditions for clay particle release during saline aquifer freshening process

Water sensitivity phenomenon occurs during saline aquifer freshening process in seawater intrusion area, and clay particles released in the phenomenon can damage the infiltration capacity of the aquifer. In order to find out the factors and mechanisms for clay particle release, laboratory column infiltration experiments simulating saline aquifer freshening process were designed to measure the critical conditions (critical flow velocity, critical salt concentration and critical ionic strength) and force analysis for clay particle according to DLVO electric double layer theory was employed to illustrate the mechanisms for particle release. The research results showed that critical flow velocity for clay particle release is influenced by salt concentration of injecting solution. When salt concentration of injecting solution is very high, clay particles are not released, indicating that there does not exist a critical flow velocity in this situation. As salt concentration of injecting solution decreases, particles start to be released. The critical salt concentration for clay particle release is 0.052 mol L−1 in our work, which was determined by a constant-flux experiment for stepwise displacement of high concentration NaCl solution. The critical ionic strength for clay particle release decreases as Ca2+ molar content percentage of the mixed solution of NaCl and CaCl2 increases following the first-order exponential decay equation y = 0.0391e−0.266x + 0.0015.

Multicomponent heterovalent chromatography in aquifers. Modelling salinization and freshening phenomena in field conditions

Analysis of hydrochemical data using diagrams and the geochemical transport model PHREEQC confirmed that for the Argos plain aquifer and for other heterogeneous confined alluvial coastal aquifers in Greece, the variation of water qualities along a flow path has basically a chromatographic origin due to seawater intrusion and freshening processes. Additionally, calcite precipitation, dolomite and gypsum dissolution take place. Intense fertilization and overexploitation of the area resulted in degradation of the groundwater quality by high amounts of nitrate ions and increased salinization. The main physical factor affecting groundwater quality in this area is the cation exchange capacity that is calculated to be 4.3 meq(100 gr) −1. Good adaptation of the transport model to the field data showed that the model can be used to predict the freshening process after an artificial recharge. Hence in a first stage for Cl −, there is a continuous decrease to the initial fresh water concentrations corresponding to about 1–1.5 pores volumes that should have been shifted through the aquifer, while for both cations Ca 2+ and Mg 2+ this decrease is even lower and leads to concentrations lower than that of the initial fresh water. Complete restoration of the aquifer requires a great amount of recharge water that should be transported through the aquifer and corresponds to 32 pores volumes.

Geochemical processes and solute transport at the seawater/freshwater interface of a sandy aquifer

Geochemical processes occurring at a seawater/freshwater interface were studied in a shallow coastal siliclastic aquifer containing minor amounts of calcite. Data were collected from 106 piezometers in a 120-m transect from the coastline and landward. In the first 40 m from the coastline, a wedge of saltwater is intruding below the freshwater aquifer. The aquifer is strongly reduced with mineralization of organic matter by methanogenesis in the freshwater aquifer, and sulfate reduction dominating in the most seaward part of the saline aquifer. The spatial separation of cations in the aquifer indicated a slow freshening process where Ca 2+ from freshwater displaced the marine cations Na + and Mg 2+ from the exchanger complex. The resulting loss of Ca 2+ from solution decreases the saturation state for calcite and possibly causes calcite dissolution. A storm-flooding event was recorded where pulses of dense seawater sank through the fresh aquifer. As a result, the terminal electron accepting process switched from methanogenesis to sulfate reduction. The pulses of sinking seawater also triggered cation exchange reactions where Ca 2+ was expelled from the exchanger by seawater Na + and Mg 2+. The released Ca 2+ is being flushed from the aquifer by groundwater flow, and this export of Ca 2+ will, in the long term, cause decalcification of the sediment. The water composition in the aquifer is in a transient state as the result of various processes that operate on different timescales. Oxidation of organic matter occurs continuously but at a rate decreasing on a geological time scale. The freshening of the aquifer operates on the timescale of a few years. The episodic flooding and sinking of seawater through the aquifer proceeds in the course of days to weeks, but occurs irregularly with years in between.

Reaction of subsurface coastal aquifers to climate and land use changes in Greece: modelling of groundwater refreshening patterns under natural recharge conditions

This paper studies the multicomponent ion exchange process and freshening time under natural recharge conditions for three coastal aquifers in Greece. Due to over-pumping and the dry years of 1980–1990 decline in groundwater quality has been observed in most of the Greek coastal aquifers. This decline is caused by a lack of reliable water resource management, water abstraction from great depths, and seawater intrusion resulting in a rise of the fresh/salt water interface (salinisation process) due to a negative water balance. The reverse phenomenon, which should lead to groundwater freshening, is a long process. The freshening process shows chromatographic patterns that are due to chemical reactions such as calcite dissolution and cation exchange, and simultaneously occurring transport and dispersion processes. Using the geochemical simulation codes PHREEQE and PHREEQM (Parkhurst et al., US Geol. Surv. Water Resour. Invest., 80–96 (1980) 210; Appelo and Postma, Geochemistry, Groundwater and Pollution (1994)), these patterns were analysed and the above-mentioned processes were simulated for carefully selected aquifers in Peloponnesus and Crete (Greece). Aquifers of the Quaternary basin of Glafkos in Peloponnesus, the Neogene formations in Gouves, Crete, and the carbonate aquifer of Malia, Crete, were examined as representative examples of Greek coastal aquifer salinisation. The results show that when pumping was discontinued, the time required for freshening under natural conditions of the former two aquifers is long and varies between 8000 and 10,000 years. The Malia aquifer on the other hand, has a freshening time of 15 years. Freshening time was shown to depend mainly on cation exchange capacities and the recharge rate of the aquifers.

湾内の淡水化に関する水流数値シミュレーションと塩分濃度の変動特性の分析

湾内の淡水化に関する水流数値シミュレーションと塩分濃度の変動特性の分析

ANALYSIS ON THE FRESHENING PROCESS OF FRESHENING RESERVOIR BY DESALT FRESH WATER

This paper is associated with the computer simulation and the field observations of salinity to analyze the freshening process in the Yongam lake, which is a standard freshening reservoir. The freshening process was predicted by salinity equivalent model based on three kinds of hydraulic mechanisms comprising the convection, the vertical diffusion and the mixing of salinity. The model was applied to the conditions of two different times, which were decided by the introducing time of desalt fresh water and the operation of desalinization facilities. As the results, the observed salinity was greatly reduced with the lapse of time and varied according to the season, river discharge, and tidal phenomena. The spring layer of salinity related to hydraulic stability was found 15-16meters below the water surface. In the desalinization process, the computed salinity was 1, 000-1, 700ppm and 2, 000-4, 000ppm separately, in case of the introduction and without of desalt fresh water.