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1.
This article analyses the hydrogeochemical processes, linked to the freshwater–seawater mixing zone, which can be caused by continuous pumping from a detrital coastal aquifer. It was necessary to construct an experimental plot, drilling three boreholes along a line perpendicular to the coast. A complete physico-chemical analysis was done of all water samples taken. The percentage of seawater, calculated from Chloride and 18Oxygen concentrations, varied between 55 and 90 %. The ionic deltas (Δ) calculated, and the saturation indices (SI) of mineral phases susceptible to precipitation or dissolution, allowed a series of hydrogeochemical processes to be identified that occur as a consequence of the advance of marine intrusion into the coastal band, and of aquifer flushing. Based on the major elements, the fraction of exchange (βI) was calculated for samples ranging from seawater to freshwater, and this revealed that differences in βI could explain the hydrochemistry of the mixing zone. The main processes recognised include precipitation of dolomite, dissolution of gypsum, fixation of sulphur salts and cation exchange. Most of the ion exchange took place between Na and Calcium + Magnesium ions. The process of fixation or liberation of these ions is probably determined by the advance or recession of the saline wedge, and/or by recharge during rainy periods. The behaviour of Magnesium is more sensitive to small variations in salinity, whilst Calcium behaves more homogeneously. The high percentage of seawater in the samples studied favours the speed and magnitude of processes such as ion exchange, and the intervention of magnesium is also a key.  相似文献   

2.
Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na+ and Cl? near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na–Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca–HCO3 and Ca–Mg–HCO3 groundwater types. Further along flow paths, Ca2+ and Na+ ion exchange causes groundwater evolution to Na–HCO3 type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system.  相似文献   

3.
The Panama coastal aquifer system is an important water resource in the southeast coast of Sri Lanka that provides adequate supplies of water for agriculture and domestic uses. One of the biggest threats to these fragile aquifers is the sea water intrusion. In this study, recharging mechanism and geochemical evaluation of groundwater in the coastal sandy aquifer of Panama were evaluated using chemical and stable isotope techniques. Thirty groundwater samples were collected and analyzed for their major ion concentrations and stable isotope ratios of oxygen (18O/16O) and hydrogen (D/H). All studied samples showed a ranking of major anions in the order Cl? > HCO 3 ?  > SO 4 2?  > N-NO3 ? while cations showed a decreasing order of abundance with Na+ > Ca2+ > Mg2+ > K+. Dominant groundwater hydrogeochemical types were Na–Cl and mixed Ca–Mg–Cl. Results of saturation index calculations indicate that the investigated groundwater body was mostly saturated with respect to calcite, dolomite and gypsum. In addition, stable isotope and geochemical data suggest that fresh groundwater in the aquifer is recharged mainly by local precipitation with slight modification from evaporation and saline water intrusions. Isotope data suggest that mixing of salt water with freshwater occurs in aquifers which are located towards the lagoon. Since the communities in the study area depend entirely on groundwater, an understanding of the hydrogeochemical characteristics of the aquifer system is extremely important for the better water resource management in the region.  相似文献   

4.
Uttarakhand geothermal area, located in the central belt of the Himalayan geothermal province, is one of the important high temperature geothermal fields in India. In this study, the chemical characteristics of the thermal waters are investigated to identify the main geochemical processes affecting the composition of thermal waters during its ascent toward the surface as well as to determine the subsurface temperature of the feeding reservoir. The thermal waters are mainly Ca–Mg–HCO3 type with moderate silica and TDS concentrations. Mineral saturation states calculated from PHREEQC geochemical code indicate that thermal waters are supersaturated with respect to calcite, dolomite, aragonite, chalcedony, quartz (SI > 0), and undersaturated with respect to gypsum, anhydrite, and amorphous silica (SI < 0). XRD study of the spring deposit samples fairly corroborates the predicted mineral saturation state of the thermal waters. Stable isotopes (δ18O, δ2H) data confirm the meteoric origin of the thermal waters with no oxygen-18 shift. The mixing phenomenon between thermal water with shallow ground water is substantiated using tritium (3H) and chemical data. The extent of dilution is quantified using tritium content of thermal springs and non-thermal waters. Classical geothermometers, mixing model, and multicomponent fluid geothermometry modeling (GeoT) have been applied to estimate the subsurface reservoir temperature. Among different classical geothermometers, only quartz geothermometer provide somewhat reliable estimation (96–140 °C) of the reservoir temperature. GeoT modeling results suggest that thermal waters have attained simultaneous equilibrium with respect to minerals like calcite, quartz, chalcedony, brucite, tridymite, cristobalite, talc, at the temperature 130 ± 5 °C which is in good agreement with the result obtained from the mixing model.  相似文献   

5.
It is important to understand how phosphate sorption dynamics of coastal carbonate aquifers are affected by seawater intrusion, because many coastal aquifers are composed of carbonate rocks and subject to an increase in saltwater intrusion during relative sea-level rise. Twelve carbonate rock and unconsolidated sediment specimens were acquired from a test corehole spanning the full thickness of the Biscayne aquifer in southeastern Florida. All 12 samples exhibit low phosphorus content but variable contents of iron. Column leaching experiments were conducted with two carbonate aquifer samples, alternating between freshwater and saltwater flow. With the first influx of saltwater, phosphate concentration in leachate increased rapidly from a freshwater value of approximately 0.2 μM to peaks of between 0.8 and 1.6 μM. The phosphate concentration began to diminish as saltwater continued to flow, but sustained desorption continued for over 2 h. Overall, seawater drove sorption behavior much more than chemical composition for the aquifer rocks and sediment from the seven rock samples for which we did isotherm sorption experiments. Our results indicate that an immediate and intense pulse of phosphate desorption from carbonate rock and sediment with low phosphorus content occurs in response to an influx of seawater and that the duration of desorption will vary by layer within a single aquifer.  相似文献   

6.
The hydrogeological unit of Aguadulce (Campo de Dalías aquifers, SE Spain) has a complex geometry. This fact, together with a continuous rise in water demand due to intensive agriculture and tourism create problems for groundwater quantity and quality. In this paper classic geochemical tools managed by means of GIS software and geochemical simulations are combined to delineate, identify and locate the possible physicochemical processes acting in the Aguadulce groundwater. Two main aquifers can be distinguished: the carbonate or lower aquifer of Triassic age, and the calcodetritic or upper aquifer of Plio-Quaternary age. Groundwaters from the latter are more saline and, assuming all chlorinity originates from seawater intrusion, the seawater contribution to their composition would be up to 7%. Nevertheless the carbonate aquifer appears not to be homogeneous: it is compartmentalised into 4 zones where different processes explain the different groundwaters compositions. Zone 4 samples (E margin of the carbonate aquifer) resemble those of the Plio-Quaternary aquifer, where calcite precipitation, dolomite and gypsum dissolution and some cation exchange (water–rock interaction) together with seawater–freshwater mixing occur. In contrast, water–rock interaction predominates in zones 1 and 3 of the carbonate aquifer. Moreover, zone 2 samples, located between zones 1 and 3, are explained by water–rock interaction in addition to mixing with Plio-Quaternary aquifer waters. The combination of geochemical simulations with GIS and hydrogeochemical analyses has proven to be effective in identifying and locating the different physicochemical processes in the aquifer areas, thus improving understanding of hydrogeochemistry in complex aquifers.  相似文献   

7.
Salinization in coastal aquifers is usually related to both seawater intrusion and water–rock interaction. The results of chemical and isotopic methods were combined to identify the origin and processes of groundwater salinization in Daguansha area of Beihai, southern China. The concentrations of the major ions that dominate in seawater (Cl?, Na+, Ca2+, Mg2+ and SO 4 2– ), as well as the isotopic content and ratios (2H, 18O, 87Sr/86Sr and 13C), suggest that the salinization occurring in the aquifer of the coastal plain is related to seawater and that the prevailing hydrochemical processes are evaporation, mixing, dissolution and ion exchange. For the unconfined aquifer, groundwater salinization has occurred in an area that is significantly influenced by land-based sea farming. The integrated impacts of seawater intrusion from the Beibuwan Gulf and infiltration of seawater from the culture ponds are identified in the shallowest confined aquifer (I) in the middle of the area (site BBW2). Leakage from this polluted confined aquifer causes the salinization of groundwater in the underlying confined aquifer (II). At the coastal monitoring site (BBW3), confined aquifer I and lower confined aquifer II are heavily contaminated by seawater intrusion. The weak connectivity between the upper aquifers, and the seaward movement of freshwater, prevents saltwater from encroaching the deepest confined aquifer (III). A conceptual model is presented. Above all, understanding of the origin and processes of groundwater salinization will provide essential information for the planning and sustainable management of groundwater resources in this region.  相似文献   

8.
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 Ca2+ from freshwater displaced the marine cations Na+ and Mg2+ from the exchanger complex. The resulting loss of Ca2+ 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 Ca2+ was expelled from the exchanger by seawater Na+ and Mg2+. The released Ca2+ is being flushed from the aquifer by groundwater flow, and this export of Ca2+ 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.  相似文献   

9.
广东硇洲岛地下水化学演化及成因机理   总被引：4，自引：0，他引：4  
地下水一直是广东硇洲岛唯一的水资源,但近年来许多地区地下水咸化趋势加剧,严峻威胁着岛上居民的用水安全,研究地下水化学演化及成因机理,对预防和减缓水质变咸意义重大。本文结合区域水文地质条件、地形地貌条件及水化学资料分析了整个岛屿地下水水质状况,在此基础上采用Piper三线图研究了水化学特征,并利用PHREEQC软件对水文地球化学演化规律进行模拟研究,结果表明:(1)海水入侵是造成地下水咸化的主要原因,咸化对水质的影响主要表现为Cl-的增加,Ca2+在区分该地区不同水体时反应灵敏,是良好的识别指标之一;(2)海水入侵的过程中,过渡带前缘不存在经受长期古海水演化而成的卤水与地下淡水的混合作用,地下水中Mg2+、Ca2+与Na+之间存在着强烈的离子交换,石膏、白云石及方解石处于不饱和状态;(3)浅层地下水与深层地下水之间无明显的水力联系,属于相对独立的地下水流系统。在全球变暖,海平面上升的总趋势下,海岛地下水开发必须合理规划、严格管理,以防引发大规模海水入侵灾害的发生。  相似文献   

10.
This research aims to improve the current knowledge of groundwater salinisation processes in coastal aquifers using combined hydrochemical and isotopic parameters and inverse hydrochemical modelling. Field investigations were conducted in Laizhou Bay, which is the area most seriously affected by seawater intrusion in north China. During three sampling campaigns along a vertical transect in the Changyi-Liutuan area, 95 ground- and surface-water samples were collected for major ion and isotope analysis (2H/18O, 3H, 14C, 34S). The groundwater changes along the general flowpath towards the coast from fresh (<1 g/L), brackish (1-10 g/L), saline (10-100 g/L) to brine water (>100 g/L). Molar Cl/Br ratios are close to those of seawater in almost all groundwater samples, indicating that brines and deep seawater evolved from different events of palaeo-seawater intrusion. Depleted isotopic signatures of brines and deep saline water point to a former, initially depleted seawater reservoir due to runoff dilution. Tritium and 14C activities in deep saline water below confining units indicate isolation from modern precipitation and significant residence times. Brine water shows a wide range of 3H and 14C ages due to the complex conditions of mixing without isolation from modern groundwater. Sulphur-34 isotope ratios support seawater intrusion as a possible salt origin, although this parameter does not exclude gypsum dissolution. The combined use of Cl and 18O yields four different end-members of groundwater, and three different mixing scenarios were identified explaining the hydrochemical composition of groundwater samples with intermediate salinity in the different areas. To improve understanding of the various water types and their related processes in a spatial context, a conceptual model was developed integrating the results derived from the presented data in a vertical cross-section. Results of three inverse modelling simulations using PHREEQC-2 show that all hypothetical mixing scenarios derived from conservative components are thermodynamically feasible. In all scenarios, mixing, ion exchange, dissolution of dolomite and precipitation of gypsum and calcite account for the hydrochemical changes.  相似文献   

11.
The Wadi Watir delta, in the arid Sinai Peninsula, Egypt, contains an alluvial aquifer underlain by impermeable Precambrian basement rock. The scarcity of rainfall during the last decade, combined with high pumping rates, resulted in degradation of water quality in the main supply wells along the mountain front, which has resulted in reduced groundwater pumping. Additionally, seawater intrusion along the coast has increased salinity in some wells. A three-dimensional (3D) groundwater flow model (MODFLOW) was calibrated using groundwater-level changes and pumping rates from 1982 to 2009; the groundwater recharge rate was estimated to be 1.58?×?106 m3/year. A variable-density flow model (SEAWAT) was used to evaluate seawater intrusion for different pumping rates and well-field locations. Water chemistry and stable isotope data were used to calculate seawater mixing with groundwater along the coast. Geochemical modeling (NETPATH) determined the sources and mixing of different groundwaters from the mountainous recharge areas and within the delta aquifers; results showed that the groundwater salinity is controlled by dissolution of minerals and salts in the aquifers along flow paths and mixing of chemically different waters, including upwelling of saline groundwater and seawater intrusion. Future groundwater pumping must be closely monitored to limit these effects.  相似文献   

12.
The study of brine aquifers in southern Taiwan is highly complicated by hybrid geochemical reactions, which obscure important geochemical information. Using multivariate analysis on major and minor ion compositions normalized by Cl− content, chemical constituents were combined into two principal components representing brine mixing and mineral precipitation. Comparing to multivariate analysis on the original data, this procedure reveals more geochemical information. It demonstrates that the brine groundwater of the region is primarily composed of highly evaporated seawater. The evaporation ratio is >70%; a point at which calcite, dolomite and gypsum precipitate. Oxygen and hydrogen isotopic compositions confirm this inference; and further, geochemical modeling quantitatively determined the evaporation ratio to be about 85%. Natural boron contamination is a consequence of brine groundwater. Two evolutionary trends in the plotting of the Cl/B ratio versus Cl− can be identified: (1) Cl/B ratio decreases with boron being released from clay minerals when brine aquifers are flushed with freshwater; and (2) Cl/B ratio increases when seawater of a high Cl/B ratio infiltrates coastal aquifers.  相似文献   

13.
The Rhône delta, South of France (Camargue, 750 km2) is a coastal saline wetland located along the Mediterranean Sea. The confined aquifer of this delta shows high values of electrical conductivity rising from the north (4 mS/cm) to the shoreline (58 mS/cm). This work aims to identify the origin of groundwater salinity and the geochemical processes occurring in this coastal confined aquifer according to the degree of salinity. A natural tracing approach is considered using monthly sampling in 8 piezometers for chemical and isotopic analyses (18O, 2H, 13CTDIC). Ionic and isotopic ratios demonstrate that strong salinities are due to a simple mixing between Mediterranean seawater and freshwater; seawater contribution reaches up to 98% at 8 km from the shoreline. Seawater intrusion induces a particular groundwater chemistry which varies with the degree of seawater contribution: (1) In the less saline part of the aquifer (seawater contribution <20%), the intrusion induces an increase of Na+ in groundwater leading to Ca2+/Na+ exchange processes. The δ13CTDIC analyses show that matrix exchange processes most likely occur for the less saline samples. (2) In the saline part of the aquifer (seawater contribution >20%), the intrusion induces SO4 reduction which is confirmed by highly depleted δ13CTDIC values (up to −19‰). The δ13CTDIC also reveals that methanogenesis processes may occur in the most reductive part of the aquifer. Due to SO4 reduction, the intrusion induces a shift in carbonate equilibrium leading to supersaturation with respect to dolomite and/or magnesian calcite. Thus carbonate precipitation may occur in the area strongly influenced by seawater.  相似文献   

14.
Groundwater is a crucial resource on the Manukan Island as it is the only source of freshwater available on the island. The aquifer has deteriorated to a high degree, during the last decade. Nine domestic wells were sam-pled from March 2006 to January 2007 to probe the hydrochemical components that influence the water quality. Geochemical data on dissolved major constituents in groundwater samples from the Manukan Island revealed the main processes responsible for their geochemical evolution. The results using statistical analyses, graphical method and numerical model output (PHREEQC) showed that the groundwater was chemically highly enriched in Na and Cl, indicative of seawater intrusion into the aquifer as also supported from the Na-Cl signature on the Piper diagram. From the PHREEQC simulation model, calcite, dolomite and aragonite solubility showed positive values of the saturation indices (SI), indicating supersaturation which led to mineral precipitation condition of water by these min-erals.  相似文献   

15.
Karstic aquifers are considered as the main sources of groundwater in the northeast of Rudbar, Iran. The present study was conducted to evaluate the hydrogeological properties of karstic springs in this region. For this purpose, saturation indices (SI values) were calculated using the geochemical PHREEQC model for a number of minerals in the groundwater in the karstic aquifer. Moreover, AqQA-RockWare software packages were used to prepare hydrogeochemical plots for the aquifer, using which the sources of the ions in the water were identified. The origin of bicarbonate, calcium, and magnesium ions in water was determined using chloro-alkaline indices. Moreover, through plotting a Piper diagram for spring water samples, it was discovered that water type of all springs is the Ca-HCO3 type, confirming the karstic characteristic of springs in the area. A Durov diagram also suggests that the water composition of the springs is of the bicarbonate type with the dominant Ca cation, suggesting the calcareous effects of the region on the quality of groundwater and exhibiting a single source for the springs. The calculated saturation indices show that most of the water samples are undersaturated with respect to calcite, dolomite, and CO2. The stable isotopes (δ18O and δ2H) and deuterium excess values were used to get information about transport pathways in groundwater, atmospheric moisture, and the degree of interaction between these reservoirs. The degree of karstification of the recharge area of the karst aquifer was determined to be 5.5 from an analysis of the hydrograph Sefidab Spring.  相似文献   

16.
广西北海市海城区西段含水层海水入侵地球化学过程研究   总被引：2，自引：0，他引：2  
姚锦梅  周训  谢朝海 《地质学报》2011,85(1):136-144
广西北海市海城区西段地下含水层出现过海水入侵.海水入侵过程中可能发生哪些地球化学作用是人们关注的问题.本文运用绘制Piper图、理论混合线(TML)、计算离子delta值和饱和指数SI以及水文地球化学模拟等方法研究了该区含水层在海水入侵后发生的地球化学作用.通常海水入侵后地下水含水层容易发生白云岩化作用,要通过模拟计算...  相似文献   

17.
Groundwater salinization in the Azores archipelago (Portugal)   总被引：1，自引：1，他引：0  
Groundwater salinization in coastal regions causes severe constraints to water supply and economic losses to society worldwide. In the Azores archipelago, groundwater abstraction in wells drilled in coastal aquifers is very important for water supply, and quality problems have been reported. Therefore, a groundwater chemistry dataset from wells was compiled to study groundwater salinization in these aquifers. Waters are mainly of the Na–Cl type, presenting a slightly acidic to slightly alkaline character, with a pH between 5.63 and 8.50 (median 7.40). Electrical conductivity measurements range from 127 to 9,670 μS/cm (median 862), suggesting highly variable mineralization, with higher values observed on Santa Maria, São Miguel, Pico, Graciosa, and São Jorge islands. The major-ion composition reflects the contribution of seawater to the groundwater compositional evolution, which is essentially explained by seawater intrusion into wells. In many samples, exchange reactions of Na+ + K+ for Ca2+ + Mg2+ are associated with salinization. The seawater fraction in groundwater composition reaches a maximum of 22.5%. These results are extremely challenging to water managers in the Azores because failure to comply with national water quality regulations and with European Union groundwater directive requirements often occurs.  相似文献   

18.
The Agadir-Essaouira area in the occidental High Atlas Mountains of Morocco is characterized by a semi-arid climate. The scarcity and quality of water resources, exacerbated by long drought periods, constitute a major problem for a sustainable development of this region. Groundwater resources of carbonate units within Jurassic and Cretaceous aquifers are requested for drinking and irrigation purposes. In this study, we collected 84 samples from wells, boreholes, springs, and rivers. Hydrochemical and isotopic data were used to examine the mineralization and origin of water, which control groundwater quality. The chemical composition of water seems to be controlled by water-rock interactions, such as dissolution of carbonates (calcite and dolomite), weathering of gypsum, as well as ion exchange processes, which explain the observed variability. Stable isotopes results show that groundwater from the mainly marly Cretaceous aquifer are submitted to an evaporation effect, while samples from the chiefly calcareous Jurassic aquifer indicate a meteoric origin, due to a rapid infiltration of recharge runoff through the karstic outcrops. The low values of δ18O and δ2H suggest a local recharge from areas with elevations ranging from 400 to 1200 m for the Cretaceous aquifer and from 800 to 1500 m for the Jurassic units.  相似文献   

19.
In the coastal region of Bangladesh, groundwater is mainly used for domestic and agricultural purposes, but salinization of many groundwater resources limits its suitability for human consumption and practical application. This paper reports the results of a study that has mapped the salinity distribution in different aquifer layers up to a depth of 300 m in a region bordering the Bay of Bengal based on the main hydrochemistry and has investigated the origin of the salinity using Cl/Br ratios of the samples. The subsurface consists of a sequence of deltaic sediments with an alternation of more sandy and clayey sections in which several aquifer layers can be recognized. The main hydrochemistry shows different main water types in the different aquifers, indicating varying stages of freshening or salinization processes. The most freshwater, soft NaHCO3-type water with Cl concentrations mostly below 100 mg/l, is found in the deepest aquifer at 200–300 m below ground level (b.g.l.), in which the fresh/saltwater interface is pushed far to the south. Salinity is a main problem in the shallow aquifer systems, where Cl concentrations rise to nearly 8000 mg/l and the groundwater is mostly brackish NaCl water. Investigation of the Cl/Br ratios has shown that the source of the salinity in the deep aquifer is mixing with old connate seawater and that the saline waters in the more shallow aquifers do not originate from old connate water or direct seawater intrusion, but are derived from the dissolution of evaporite salts. These must have been formed in a tidal flat under influence of a strong seasonal precipitation pattern. Long dry seasons with high evaporation rates have evaporated seawater from inundated gullies and depressions, leading to salt precipitation, while subsequent heavy monsoon rains have dissolved the formed salts, and the solution has infiltrated in the subsoil, recharging groundwater.  相似文献   

20.
This study is an attempt to quantify the geochemical processes and the timescale of seawater intrusion into a coastal aquifer from changes in the major ionic composition of the water and the natural distribution of the cosmogenic isotopes 14C and 3H. For that purpose, we sampled saline and brackish groundwaters from the Israeli coastal aquifer. A multilayer sampler (MLS) was used to obtain very high resolution (10 cm) profiles across the fresh-saline water interface (FSI).The chemical and stable isotope data revealed three distinct water types (end members) that are located in different zones on the route to the coastal aquifer: (1) slightly modified Mediterranean seawater (SWS); (2) slightly diluted (with up to 20% fresh groundwater) saline groundwater (SDS); and (3) fresh groundwater (FGW).The SWS samples generally show an excess of total alkalinity and total dissolved inorganic carbon (DIC), and a depletion of 13CDIC and 14CDIC with respect to normal seawater indicating that anaerobic oxidation of organic matter is the first diagenetic reaction that affects seawater during its penetration into the bottom sediments. SDS waters appear when SWS is slightly diluted, gain Ca2+ and Sr2+, and is depleted in K+, suggesting that the main processes that transform SWS into SDS are slight dilution with fresh groundwater and cation exchange. At the fresh-saline water interface, SDS generally shows conservative mixing with FGW.Inspection of chemical data from coastal aquifers around the world indicates that intensive ion exchange in slightly diluted saline groundwater is a globally important phenomenon of seawater intrusion. Most of our saline groundwater samples contain substantial amounts of 3H suggesting that penetration of Mediterranean seawater and its inland travel to a distance of 50-100 m onshore occurred 15-30 yr ago. This is supported by the 14CDIC mass balance that explains the relatively low 14CDIC activities in the SDS as influenced by diagenesis and not by simple radioactive decay.  相似文献   

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