Hydrogeochemical Tools Research Articles

Trace elements (Li, B, Mn and Ba) as sensitive indicators for salinization and freshening events in coastal aquifers

The current global intrusion of seawater into coastal aquifers causes salinization of groundwater and thus significant degradation of its quality. This study quantified the effect of seawater intrusion and freshening events in coastal aquifers on trace elements (Li, B, Mn and Ba) across the fresh-saline water interface (FSI) and their possible use as indicators for these events. This was done by combining field data and column experiments simulating these events. The experiments enabled quantification of the processes affecting the trace element composition and examination of whether salinization and freshening events are geochemically reversible, which has been seldom investigated.The dominant process affecting trace element composition during salinization and freshening is ion exchange. The results of the experiments show that the concentrations of major cations and Li+ were reversible during salinization and freshening, whereas B, Mn2+ and Ba2+ were not. During salinization, Li+ and B were depleted due to sorption by 10 and 100μmol·L−1, respectively, to about half of their expected conservative concentrations. The relative depletion of Li+ increased with distance from the shore, representing the propagation of salinization. Ba2+ and Mn2+ were desorbed from the sediment during salinization and enriched by tenfold in the aqueous phase compared to their concentration in seawater (~0.1 μeq·L−1). During freshening both were depleted by almost tenfold compared to their concentration in fresh groundwater (~0.7 μeq·L−1). The depletion of Mn2+ is a sensitive marker for freshening because Mn2+ has a strong affinity to the solid phase. Moreover, this study shows that both Mn2+ and Ba2+ can be used as sensitive hydrogeochemical tools to distinguish between salinization and freshening events in the FSI zone in coastal aquifers.

Variation of hydrogeochemical characteristics of water in surface flows, shallow wells, and boreholes in the coastal city of Douala (Cameroon)

ABSTRACTGroundwater is used by 3 million inhabitants in the coastal urban city of Douala, Cameroon, but comprehensive data are too sparse for it to be managed in a sustainable manner. Hence this study aimed to (1) assess the potability of the groundwater; (2) evaluate the spatial variation of groundwater composition; and (3) assess the interaction and recharge mechanisms of different water bodies. Hydrogeochemical tools and methods revealed the following results in the Wouri and Nkappa formations of the Douala basin, which is beneath Douala city: 30% of water samples from hand-dug wells in the shallow Pleistocene alluvium aquifer were saline and highly mineralized. However, water from boreholes in the deeper (49–92 m depth) Palaeocene aquifer was saline-free, less mineralized and potable. Water in the shallow aquifer (0.5–22 m depth) was of Na+-K+-Cl−-NO3− type and not potable due to point source pollution, whereas Ca+-HCO3− unpolluted water dominates in the deeper aquifer. Water in the deep and shallow aquifers indicates the results of preferential flow pass and evaporative recharge, respectively. Possible hydrogeochemical processes include point source pollution, reverse ion exchange, remote recharge areas and mixing of waters with different chemical signatures.EDITOR D. Koutsoyiannis ASSOCIATE EDITOR M.D. Fidelibus

Geophysical, isotopic, and hydrogeochemical tools to identify potential impacts on coastal groundwater resources from Urmia hypersaline Lake, NW Iran.

Measurements of major ions, trace elements, water-stable isotopes, and geophysical soundings were made to examine the interaction between Urmia Aquifer (UA) and Urmia Lake (UL), northwest Iran. The poor correlation between sampling depth and Cl(-) concentrations indicated that the position of freshwater-saltwater interface is not uniformly distributed in the study area, and this was attributed to aquifer heterogeneities. The targeted coastal wells showed B/Cl and Br/Cl molar ratios in the range of 0.0022-2.43 and 0.00032-0.28, respectively. The base-exchange index (BEI) and saturation index (SI) calculations showed that the salinization process followed by cation-exchange reactions mainly controls changes in the chemical composition of groundwater. All groundwater samples are depleted with respect to δ(18)O (-11.71 to -9.4 ‰) and δD (-66.26 to -48.41 ‰). The δ(18)O and δD isotope ratios for surface and groundwater had a similar range and showed high deuterium excess (d-excess) (21.11 to 31.16 ‰). The high d-excess in water samples is because of incoming vapors from the UL mixed with an evaporated moisture flux from the Urmia mainland and incoming vapors from the west (i.e., Mediterranean Sea). Some saline samples with low B/Cl and Br/Cl ratios had depleted δ(18)O and δD. In this case, due to freshwater flushing, the drilled wells in the coastal playas and salty sediments could have more depleted isotopes, more Cl(-), and consequently smaller B/Cl and Br/Cl ratios. Moreover, the results of hydrochemical facies evolution (HFE) diagram showed that because of the existence fine-grained sediments saturated with high density saltwater in the coastal areas that act as a natural barrier, increasing the groundwater exploitation leads to movement of freshwaters from recharge zones in the western mountains not saltwater from UL. The highly permeable sediments at the junction of the rivers to the lake are characterized by low hydraulic gradient and high hydraulic conductivity. These properties enhance the salinization of groundwater observed in the study area. The main factors influencing the salinity are base-exchange reactions, invasion of highly diluted saltwater, dissolution of salty pans, and water chemistry evolution along flow paths.

Hydrogeochemical characterization of oilfield waters from southeast Maracaibo Basin (Venezuela): Diagenetic effects on chemical and isotopic composition

Free water and water in emulsion with oil from hydrocarbon wells onshore–offshore Lake Maracaibo were extracted and analyzed. The use of classical and new chemical diagrams evidenced that Na–HCO3 and Na–Cl are the main chemical facies, whereas Na–SO4 facies was rare and revealed only in faulted areas in which shallow groundwater was involved in deep circulation. Cretaceous Ca–Cl waters were not revealed, but their imprint on some samples was traced and evidenced by Langelier–Ludwig, Brine-Differentiation and Caexcess vs. Nadeficit diagrams. These square plots, allowing a discrimination between the water contributions from the different source rocks, should be more exploited as hydrogeochemical tools in hydrocarbon exploration. Original data on monocarboxylic anions showed acetate concentration higher than ten times that of formate at sampling conditions, but speciation under deep conditions indicated a reversed activity dominance. Calculation of saturation indexes and kinetic modeling showed an active precipitation of chalcedony, kaolinite and clays, especially where the temperature was lower (T < 100 °C). Formation waters and lake water showed quite similar chloride concentration and hydrogen isotope composition. However, thermodynamic calculations at the oilfield P-T conditions revealed chlorite and illite, produced during previous diagenetic stages of Misoa sandstones, recrystallization to smectite and vermiculite in present-day stage. The diagenetic water produced in these reactions could have enriched the δ18O isotopic composition of approximately +2‰. This isotope diagenetic effect was more evident in Na–HCO3 waters, which also showed a decrease in chloride at constant boron concentration as a consequence of clay dewatering. In this regard, a new boron geothermometer useful for formation waters is also proposed:T°C=[5.1364×ln(1/B)mg/1]+5.728(±0.180)−0.164(±0.012)

Hydrogeochemical tools applied to the study of carbonate aquifers: examples from some karst systems of Southern Spain

Karst aquifers present hydrogeological characteristics that distinguish them from others such as fractured and granular aquifers. In particular, they are characterized by highly heterogeneous functioning (large temporal and spatial variability), which makes difficult the application of traditional research methods to karst hydrogeology. For this reason, during the last decades, hydrogeochemical tools have been developed for carbonate aquifers characterization by means of spatial and temporal hydrochemical variability, mixing processes (recharge vs. stored water in the system), groundwater origin (saturated/unsaturated zones, epikarst, etc.), karstification degree and, in general, the hydrogeological functioning of karst aquifers. In the present work, some of the most used techniques to study this type of media are applied to pilot sites from South Spain, in order to show hydrogeochemical processes related to karst aquifers functioning: groundwater origin, distribution of its chemical composition, aquifer behavior, water residence time, substratum influence and geochemical modelling. The results display that the combination of different hydrogeochemical tools provides a comprehensive understanding of the functioning of karst systems, which is useful for groundwater management and protection as well as for environmental applications (groundwater ecodependent systems, wetlands, coastal aquifers, etc).

Occurrence and hydrogeochemistry of fluoride in alluvial aquifer of Weihe River, China

Fluoride (F−) has significant impacts on human health. High fluoride groundwater (up to 1.90 mg/L) has been found in upper confined aquifer underlying the first terrace of Weihe River during a hydrogeological investigation for water supply in 2005. To reveal the occurrence and hydrogeochemistry of high F− groundwater, hydrogeochemical tools such as saturation index, ionic ratios and correlation analysis were used in this study. The study shows that the concentrations of most physiochemical parameters from phreatic water, influenced by intensive evaporation and anthropogenic activities such as unregulated sewage and excreta disposal and agricultural practices in the area, are higher than those of confined water. The F− concentration in phreatic water is within the acceptable limits set by China and the World Health Organization (WHO), while that of upper confined water shows a decreasing trend northwestward as the Weihe River approaches, with F− concentration in the first terrace beyond the national and the WHO standards. High F− groundwater is observed in alkaline environment associated with high Na+, pH, HCO3 − and low Ca2+ and Mg2+. The enrichment of F− is controlled by geologic and hydrogeological conditions, fluorine-bearing minerals presented in alluvial formations and their dissolution/precipitation under the alkaline environment along groundwater flow. Ion exchange, human activities and the mixing of different recharge waters may influence the enrichment of F− as well.

Groundwater–wetlands interaction in coastal lagoon of Almería (SE Spain)

Water resources management in coastal wetlands requires the degree of interdependence between groundwater and terrestrial ecosystems to be known. This is especially so in semiarid areas where surface inflows are restricted, marine influence is marked and the evaporation rate is high. Thus, chemistry of surface waters is very variable in the Cerrillos-Punta Entinas wetlands system. Using classical hydrogeochemical tools, the main processes that favor a diversity of water types were described, related to: presence of salt deposits on the lagoon beds, marine origin of the water, and local influence of groundwater. All these factors make it difficult to establish what reference conditions should be used to define “good” water quality of the surface waters—as required by the Water framework directive—and to understand the influence of groundwater on these coastal wetlands. Knowledge about the influence of the different interaction of these factors on the hydrogeochemical dynamics is required for the sustainable management of this protected natural site.

Assessment of seawater intrusion and nitrate contamination on the groundwater quality in the Korba coastal plain of Cap-Bon (North-east of Tunisia)

In recent years, seawater intrusion and nitrate contamination of groundwater have become a growing concern for people in rural areas in Tunisia where groundwater is always used as drinking water. The coastal plain of Korba (north-east of Tunisia) is a typical area where the contamination of the aquifer in the form of saltwater intrusion and high nitrate concentrations is very developed and represents the major consequence of human activities.The objective of this study is to evaluate groundwater resource level, to determine groundwater quality and to assess the risk of NO3- pollution in groundwater using hydrogeochemical tools. Groundwater were sampled and analyzed for physic-chemical parameters: Ca2+, Mg2+, Na+, K+, Cl−, SO42-, HCO3-, NO3-, Total Dissolved Solid and of the physical parameters (pH, electrical conductivity and the temperature). The interpretation of the analytical results is shown numerically and graphically through the ionic deviations, Piper Diagram, seawater fractions and binary diagrams. Moreover, electrical conductivity investigations have been used to identify the location of the major intrusion plumes in this coastal area and to obtain new information on the spatial scales and dynamics of the fresh water–seawater interface. Those processes can be used as indicators of seawater intrusion progression.First, the hydrogeochemical investigation of this aquifer reveals the major sources of contamination, represented by seawater intrusion. Thus, the intensive extraction of groundwater from aquifer reduces freshwater outflow to the sea, creates several drawdown cones and lowering of the water table to as much as 12m below mean sea level in the center part of the study area especially between Diarr El Hojjej and Tafelloun villages, causing seawater migration inland and rising toward the wells.Moreover, the results of this study revealed the presence of direct cation exchange linked to seawater intrusion and dissolution processes associated with cations exchange.Second, a common contaminant identified in groundwater is dissolved nitrogen in the form of nitrate. The average nitrate concentration of the aquifer is about 30.44mg/l, but contents as great as about 50mg/l occur in the central region where seawater has been identified. Nitrate survey reveals that nitrate concentration above the drinking water standard (50mg/l) covered an area of 122.64km2, which accounts for 28% of the whole area. Irrigation with the nitrogen fertilizers, domestic sewage, industrial wastewater and movement of contaminants in areas of high hydraulic gradients within the drawdown cones probably are responsible for localized peaks of the nitrate concentration.It is suggested that risk assessment of nitrate pollution is useful for a better management of groundwater resources, preventing soil salinisation and minimizing nitrate pollution in groundwater.

Geoquímica de los sistemas de flujo de agua subterránea en rocas sedimentarias y rocas volcanogénicas de Loreto, BCS, México

The chemical composition of groundwater is the result of continuous water-rock interaction between rain water (and hail or snow) that filters into the ground and the minerals in the lithology it flows through. The objective of this study is to determine the origin of the chemical composition of groundwater in the Loreto area, Baja California Sur. Hydrogeochemical mass balance and water mixture models as well as other hydrogeochemical tools were used. All related processes are marked by an increase in the concentrations of Cl - , Br - , I - , As total , B - , Li and Sr. We identified seven groups of water corresponding to shallow springs, wells, thermal springs and submarine springs. The hydrogeochemical patterns of the underground water flows (defined numerically and by graphic expressions of water chemistry) are related to the prevailing lithology and are typical of albite, andesine and oligoclase dissolution. The remaining patterns result from the mixture of different groundwater flows, and show influence of ionic exchange processes. Thus, the chemical composition of groundwater circulating through the felsic volcanic rocks and their derived sediments has its origin in the weathering of those materials. Processes of water-rock interaction and water mixture take place among local flows and others of intermediate or regional hierarchy. Furthermore, the influences of the arid climate and intensive agriculture development also play an important role in how groundwater in the area acquires its chemical composition.

Groundwater flow regime, recharge and regional-scale solute transport in the semi-arid Kalahari of Botswana derived from isotope hydrology and hydrochemistry

Summary A synthesis of isotope hydrological (2H, 3H, 18O, 13C, 14C, 3He, 4He) and hydrogeochemical tools was used to analyze flow regimes and their time scales, to determine recharge conditions, and to describe the regional-scale hydrogeochemical evolution of groundwater from the Triassic Ntane Sandstone Aquifer between Serowe and Orapa in the Kalahari of Botswana. This location is taken as an example of a complex hydrodynamic setting relating to semi-arid climatic conditions, low recharge amounts, thick unsaturated zones (up to 100 m) and long groundwater residence time scales. Capabilities and limits of the individual methods are discussed, and relations to solute transport are demonstrated for nitrate. Chemical classification allows the distinction of six different groundwater types. This characterization is reflected in the isotopic composition and in part can be attributed to different age ranges (up to greater than 20 ka). We show that the regional-scale groundwater flow regime in the aquifer is only slow, and that groundwater is recharged in Serowe and Orapa. However in both investigated regions, mixing between over- and underlying aquifers is evident which limits a clear derivation of one regional flow regime. The challenges imposed on the identification of the regional groundwater flow regime by resulting mixing processes and for groundwater dating are discussed and evaluated.

Assessment of the Salinization Process at the Coastal Area with Hydrogeochemical Tools and Geographical Information Systems (GIS): Selçuk Plain, Izmir, Turkey

The salinization process was investigated with hydrogeochemical tools to evaluate the origin of salinity in the Selcuk Plain. Na/Cl and Cl/Br molar ratios of coastal zone that covers an alluvial aquifer, karstic discharges, and a wetland are similar to the local seawater ratio. According to mixing ratios, seawater addition can reach 9–18% in both seasons at the coastal zone especially in karstic springs that are a kind of seawater–freshwater mixing points. A thematic map of the Salinization zone was constructed for the Selcuk Plain using geographical information system tools with different parameters such as major ion ratios, EC values, mixing ratios, and sodium adsorption ratio values. High correlation between Cl and Br, B, Li and Sr explains the salinization process that comes from marine intrusion. The δ18O and δD composition of water samples varies between −6.7‰ and −2.9‰; −37‰ and −20‰, respectively. The coastal zone waters are plotted on the mixing line due to the effects of the salinization process in the plain.