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Hanshaw, B.B.; Back, W. |
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Title |
Major geochemical processes in the evolution of carbonate—Aquifer systems |
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Journal Article |
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Year |
1979 |
Publication |
Journal of Hydrology |
Abbreviated Journal |
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43 |
Issue |
1 |
Pages |
287-312 |
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Abstract |
As a result of recent advances by carbonate petrologists and geochemists, hydrologists are provided with new insights into the origin and explanation of many aquifer characteristics and hydrologic phenomena. Some major advances include the recognition that: (1) most carbonate sediments are of biological origin; (2) they have a strong bimodal size-distribution; and (3) they originate in warm shallow seas. Although near-surface ocean water is oversaturated with respect to calcite, aragonite, dolomite and magnesite, the magnesium-hydration barrier effectively prevents either the organic or inorganic formation of dolomite and magnesite. Therefore, calcareous plants and animals produce only calcite and aragonite in hard parts of their bodies. Most carbonate aquifers that are composed of sand-size material have a high initial porosity; the sand grains that formed these aquifers originated primarily as small shells, broken shell fragments of larger invertebrates, or as chemically precipitated oolites. Carbonate rocks that originated as fine-grained muds were initially composed primarily of aragonite needles precipitated by algae and have extremely low permeability that requires fracturing and dissolution to develop into aquifers. Upon first emergence, most sand beds and reefs are good aquifers; on the other hand, the clay-sized carbonate material initially has high porosity but low permeability, a poor aquifer property. Without early fracture development in response to influences of tectonic activity these calcilutites would not begin to develop into aquifers. As a result of selective dissolution, inversion of the metastable aragonite to calcite, and recrystallization, the porosity is collected into larger void spaces, which may not change the overall porosity, but greatly increases permeability. Another major process which redistributes porosity and permeability in carbonates is dolomitization, which occurs in a variety of environments. These environments include back-reefs, where reflux dolomites may form, highly alkaline, on-shore and continental lakes, and sabkha flats; these dolomites are typically associated with evaporite minerals. However, these processes cannot account for most of the regionally extensive dolomites in the geologic record. A major environment of regional dolomitization is in the mixing zone (zone of dispersion) where profound changes in mineralogy and redistribution of porosity and permeability occur from the time of early emergence and continuing through the time when the rocks are well-developed aquifers. The reactions and processes, in response to mixing waters of differing chemical composition, include dissolution and precipitation of carbonate minerals in addition to dolomitization. An important control on permeability distribution in a mature aquifer system is the solution of dolomite with concomitant precipitation of calcite in response to gypsum dissolution (dedolomitization). Predictive models developed by mass-transfer calculations demonstrate the controlling reactions in aquifer systems through the constraints of mass balance and chemical equilibrium. An understanding of the origin, chemistry, mineralogy and environments of deposition and accumulation of carbonate minerals together with a comprehension of diagenetic processes that convert the sediments to rocks and geochemical, tectonic and hydrologic phenomena that create voids are important to hydrologists. With this knowledge, hydrologists are better able to predict porosity and permeability distribution in order to manage efficiently a carbonate—aquifer system. |
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0022-1694 |
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THL @ christoph.kuells @ Hanshaw1979 |
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26 |
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Author |
Hanshaw, B.B.; Back, W. |
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Title |
Deciphering hydrological systems by means of geochemical processes |
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Journal Article |
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Year |
1985 |
Publication |
Hydrological Sciences Journal |
Abbreviated Journal |
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30 |
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2 |
Pages |
257-271 |
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The distribution of permeability and chemical character of groundwater in carbonate aquifers is significantly influenced by the many diagenetic processes
and reactions that occur in the early development of these rocks. Many of these diagenetic processes occur in the transition zone formed as the carbonate sediments emerge from the marine environment and become fresh-water aquifers. Analyses of trace elements and isotopes
indicate that calcite cements and dolomites are formed in this groundwater mixing zone. Reverse reactions such as mineral dissolution and dedolomitization occur in carbonate aquifer systems. The geochemical reactivity of the fresh-water/salt-water mixing zone results from the nonlinearity of geochemical parameters as a function of ionic strength and causes extensive dissolution in coastal carbonate rocks. Interpretation of geochemical reactions and isotopic composition of groundwater provides a method to determine hydrological parameters
such as porosity, hydraulic conductivity, and groundwater flow rates. This geochemical method is largely independent of the more conventional approach of determining these parameters by an evaluation of physical properties of aquifer systems. |
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0262-6667 |
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THL @ christoph.kuells @ Hanshaw1985 |
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25 |
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Han, D.; Post, V.E.A.; Song, X. |
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Title |
Groundwater salinization processes and reversibility of seawater intrusion in coastal carbonate aquifers |
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2015 |
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Journal of Hydrology |
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531 |
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1067-1080 |
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Seawater intrusion (SWI) has led to salinization of fresh groundwater reserves in coastal areas worldwide and has forced the closure of water supply wells. There is a paucity of well-documented studies that report on the reversal of SWI after the closure of a well field. This study presents data from the coastal carbonate aquifer in northeast China, where large-scale extraction has ceased since 2001 after salinization of the main well field. The physical flow and concomitant hydrogeochemical processes were investigated by analyzing water level and geochemical data, including major ion chemistry and stable water isotope data. Seasonal water table and salinity fluctuations, as well as changes of δ2H–δ18O values of groundwater between the wet and dry season, suggest local meteoric recharge with a pronounced seasonal regime. Historical monitoring testifies of the reversibility of SWI in the carbonate aquifer, as evidenced by a decrease of the Cl− concentrations in groundwater following restrictions on groundwater abstraction. This is attributed to the rapid flushing in this system where flow occurs preferentially along karst conduits, fractures and fault zones. The partially positive correlation between δ18O values and TDS concentrations of groundwater, as well as high NO3− concentrations (\textgreater39mg/L), suggest that irrigation return flow is a significant recharge component. Therefore, the present-day elevated salinities are more likely due to agricultural activities rather than SWI. Nevertheless, seawater mixing with fresh groundwater cannot be ruled out in particular where formerly intruded seawater may still reside in immobile zones of the carbonate aquifer. The massive expansion of fish farming in seawater ponds in the coastal zone poses a new risk of salinization. Cation exchange, carbonate dissolution, and fertilizer application are the dominant processes further modifying the groundwater composition, which is investigated quantitatively using hydrogeochemical models. |
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Elsevier |
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0022-1694 |
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THL @ christoph.kuells @ Han2015 |
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24 |
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Bouchaou, L.; Michelot, J.L.; Vengosh, A.; Hsissou, Y.; Qurtobi, M.; Gaye, C.B.; Bullen, T.D.; Zuppi, G.M. |
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Application of multiple isotopic and geochemical tracers for investigation of recharge, salinization, and residence time of water in the Souss–Massa aquifer, southwest of Morocco |
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Journal Article |
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2008 |
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Journal of Hydrology |
Abbreviated Journal |
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352 |
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3 |
Pages |
267-287 |
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Groundwater and surface water in Souss–Massa basin in the west-southern part of Morocco is characterized by a large variation in salinity, up to levels of 37gL−1. The high salinity coupled with groundwater level decline pose serious problems for current irrigation and domestic water supplies as well as future exploitation. A combined hydrogeologic and isotopic investigation using several chemical and isotopic tracers such as Br/Cl, δ18O, δ2H, 3H, 87Sr/86Sr, δ11B, and 14C was carried out in order to determine the sources of water recharge to the aquifer, the origin of salinity, and the residence time of water. Stable isotope, 3H and 14C data indicate that the high Atlas mountains in the northern margin of the Souss–Massa basin with high rainfall and low δ18O and δ2H values (−6 to −8‰ and −36 to −50‰) is currently constitute the major source of recharge to the Souss–Massa shallow aquifer, particularly along the eastern part of the basin. Localized stable isotope enrichments offset meteoric isotopic signature and are associated with high nitrate concentrations, which infer water recycling via water agricultural return flows. The 3H and 14C data suggest that the residence time of water in the western part of the basin is in the order of several thousands of years; hence old water is mined, particularly in the coastal areas. The multiple isotope analyses and chemical tracing of groundwater from the basin reveal that seawater intrusion is just one of multiple salinity sources that affect the quality of groundwater in the Souss–Massa aquifer. We differentiate between modern seawater intrusion, salinization by remnants of seawater entrapped in the middle Souss plains, recharge of nitrate-rich agricultural return flow, and dissolution of evaporate rocks (gypsum and halite minerals) along the outcrops of the high Atlas mountains. The data generated in this study provide the framework for a comprehensive management plan in which water exploitation should shift toward the eastern part of the basin where current recharge occurs with young and high quality groundwater. In contrast, we argued that the heavily exploited aquifer along the coastal areas is more vulnerable given the relatively longer residence time of the water and salinization processes in this part of the aquifer. |
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0022-1694 |
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THL @ christoph.kuells @ Bouchaou2008 |
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17 |
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Author |
Cardenal, J.; Benavente, J.; Cruz-Sanjulián, J.J. |
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Title |
Chemical evolution of groundwater in Triassic gypsum-bearing carbonate aquifers (Las Alpujarras, southern Spain) |
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Journal Article |
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1994 |
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Journal of Hydrology |
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161 |
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1 |
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3-30 |
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A hydrochemical study employing modelling techniques, was carried out using samples taken at 65 points (springs and wells) in Triassic carbonate aquifers (Lújar-Gádor Unit, Alpujárride Complex, Betic Cordillera). These aquifers are made up of limestones and dolomites with some gypsum scattered or interbedded. Though the area is semi-arid, recharge is relatively high because of their mountainous nature. The carbonate rocks contain dense microfissuration; the groundwater flow regime is predominantly diffuse. The karstic forms are in general poorly developed. Two main hydrochemical processes have been identified in these aquifers. One is incongruent dissolution of dolomite that determines the chemical composition of the less mineralised water. The other is dedolomitisation (dolomite dissolution together with calcite precipitation caused by dissolution of gypsum), which becomes predominant when the flow encounters interbedded gypsum. This reaction is also frequently associated with low temperature thermalism, and can play a part in more intense local karstification (cavities, sinkholes, high transmisivity in wells) observed in the sectors of these aquifers where gypsum is more abundant. A reaction path model has been used to simulate the geochemical processes through a hypothetical aquifer (with similar lithology to the Alpujárride carbonate aquifers). Successive stages of evolution through the carbonate sequence, represented by different saturation states with respect to calcite, dolomite gypsum and CO2, have been modelled and then compared with the field data. |
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0022-1694 |
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THL @ christoph.kuells @ Cardenal1994 |
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18 |
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