Study of geothermal water intrusion due to groundwater exploitation in the Puebla Valley aquifer system, Mexico

Significant intrusion of geothermal water into fresh groundwater takes place in the Puebla Valley aquifer system, Mexico. The decline in the potentiometric surface due to the overexploitation of the groundwater induces this intrusion. This hydrological system comprises three aquifers located in Plio-Quaternary volcanic sediments and Mesozoic calcareous rocks. The hydraulic balance of the aquifer shows that the annual output exceeds the natural inputs by 12 million m³. Between 1973 and 2002, a drop in the potentiometric surface, with an 80 m cone of depression, was identified in a 5-km-wide area located southwest of the city of Puebla. Chemical analyses performed on water samples since 1990 have shown an increase in total dissolved solids (TDS) of more than 500 mg/L, coinciding with the region showing a cone of depression in the potentiometric surface. A three-dimensional flow and transport model, based on the hydrogeological and geophysical studies, was computed by using the MODFLOW and MT3D software. This model reproduces the evolution of the aquifer system during the last 30 years and predicts for 2010 an additional drawdown in the potentiometric surface of 15 m, and an increase in the geothermal water intrusion.     

Groundwater optimization model for sustainable management of the Valley of Puebla aquifer, Mexico

The Valley of Puebla aquifer (VPA), at the central region of Mexico, is subject to intensive exploitation to satisfy the urban and industrial demand in the region. As a result of this increased exploitation, a number of state and federal agencies in charge of water management are concerned about the problems associated with the aquifer (decline of groundwater table, deterioration in water quality, poor well productivity and increased pumping and water treatment costs). This study presents a groundwater management model that combines “MODFLOW” simulation with optimization tools “MODRSP”. This simulation–optimization model for groundwater evaluates a complex range of management options to identify the strategies that best fit the objectives for allocating resources in the VPA. Four hypothetical scenarios were defined to analyze the response of the hydrogeological system for future pumping schemes. Based on the simulation of flow with the MODFLOW program, promising results for the implementation of the optimization of water quantity were found in scenarios 3 and 4. However, upon comparison and analysis of the feasibility of recovery of the piezometric level (considering the policy of gradual reductions of pumping), scenario 4 was selected for optimization purposes. The response functions of scenario 4 were then obtained and optimized, establishing an extraction rate of 204.92 millions of m³/year (Mm³/year). The reduction in groundwater extraction will be possible by substituting the volume removed by 35 wells (that should be discontinued) by the same volume of water from another source.     

Hydrochemistry and salt-water intrusion in the Van aquifer,east Turkey

Groundwater in the Van coastal aquifer is one of the main sources of potable, industrial and irrigational water in Van City, because of its semi-arid climate. Groundwater extraction has been in excess of replenishment owing to increased agricultural and economic activities and a growing population during the last 20 years. A hydrochemical survey of the Van aquifer provided data on the groundwater chemistry patterns and the main mineralization processes. The main processes influencing the groundwater chemistry are salinisation from salt-water intrusion, silicate mineral dissolution, cation exchange and human activity. Deterioration in water quality has resulted from intrusion of the salt water of Lake Van along the coastal regions into the Van plain. At present, the mixing rate of salt water in the Van aquifer is between 1 and 5.5% and salt water has already invaded about 5 km inland in the iskele and the airport region.     

Hydrochemistry of waters from five cenotes and evaluation of their suitability for drinking-water supplies, northeastern Yucatan, Mexico

 Waters from five cenotes that are currently being used for aquatic recreational activities and that lie along the Cancun–Tulum touristic corridor, Mexico, were evaluated hydrochemically to determine whether the cenotes may be considered as potential drinking-water sources. Several parameters exceed the Mexican Drinking Water Standards (MDWS), but since they do not pose a significant health threat, four of the five cenotes may be used as drinking-water sources. The common contaminants in the Yucatan Peninsula, fecal coliforms and nitrate, are in most cases below the MDWS (0–460 MPN/100 ml and 0.31–1.18 mg/L, respectively). Although these four cenotes meet the MDWS, a careful groundwater management policy needs to be developed to avoid contamination (fecal and nitrates) and salt-water intrusion. Received, October 1996 Revised, June 1997; March 1998 Accepted, July 1997     

Naturally acidic surface and ground waters draining porphyry-related mineralized areas of the Southern Rocky Mountains,Colorado and New Mexico

Acidic, metal-rich waters produced by the oxidative weathering and resulting leaching of major and trace elements from pyritic rocks can adversely affect water quality in receiving streams and riparian ecosystems. Five study areas in the southern Rocky Mountains with naturally acidic waters associated with porphyry mineralization were studied to document variations in water chemistry and processes that control the chemical variations. Study areas include the Upper Animas River watershed, East Alpine Gulch, Mount Emmons, and Handcart Gulch in Colorado and the Red River in New Mexico. Although host-rock lithologies in all these areas range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, the mineralization is Tertiary in age and associated with intermediate to felsic composition, porphyritic plutons. Pyrite is ubiquitous, ranging from ∼1 to >5 vol.%. Springs and headwater streams have pH values as low as 2.6, SO₄ up to 3700 mg/L and high dissolved metal concentrations (for example: Fe up to 400 mg/L; Cu up to 3.5 mg/L; and Zn up to 14.4 mg/L). Intensity of hydrothermal alteration and presence of sulfides are the primary controls of water chemistry of these naturally acidic waters. Subbasins underlain by intensely hydrothermally altered lithologies are poorly vegetated and quite susceptible to storm-induced surface runoff. Within the Red River study area, results from a storm runoff study documented downstream changes in river chemistry: pH decreased from 7.80 to 4.83, alkalinity decreased from 49.4 to <1 mg/L, SO₄ increased from 162 to 314 mg/L, dissolved Fe increased from to 0.011 to 0.596 mg/L, and dissolved Zn increased from 0.056 to 0.607 mg/L. Compared to mine drainage in the same study areas, the chemistry of naturally acidic waters tends to overlap but not reach the extreme concentrations of metals and acidity as some mine waters. The chemistry of waters draining these mineralized but unmined areas can be used to estimate premining conditions at sites with similar geologic and hydrologic conditions. For example, the US Geological Survey was asked to estimate premining ground-water chemistry at the Questa Mo mine, and the proximal analog approach was used because a mineralized but unmined area was located adjacent to the mine property. By comparing and contrasting water chemistry from different porphyry mineralized areas, this study not only documents the range in concentrations of constituents of interest but also provides insight into the primary controls of water chemistry.     

Groundwater-flow modeling in the Yucatan karstic aquifer, Mexico

The current conceptual model of the unconfined karstic aquifer in the Yucatan Peninsula, Mexico, is that a fresh-water lens floats above denser saline water that penetrates more than 40 km inland. The transmissivity of the aquifer is very high so the hydraulic gradient is very low, ranging from 7–10 mm/km through most of the northern part of the peninsula. The computer modeling program AQUIFER was used to investigate the regional groundwater flow in the aquifer. The karstified zone was modeled using the assumption that it acts hydraulically similar to a granular, porous medium. As part of the calibration, the following hypotheses were tested: (1) karstic features play an important role in the groundwater-flow system; (2) a ring or belt of sinkholes in the area is a manifestation of a zone of high transmissivity that facilitates the channeling of groundwater toward the Gulf of Mexico; and (3) the geologic features in the southern part of Yucatan influence the groundwater-flow system. The model shows that the Sierrita de Ticul fault, in the southwestern part of the study area, acts as a flow barrier and head values decline toward the northeast. The modeling also shows that the regional flow-system dynamics have not been altered despite the large number of pumping wells because the volume of water pumped is small compared with the volume of recharge, and the well-developed karst system of the region has a very high hydraulic conductivity. Electronic Publication     

A reappraisal of the metamorphic history of the Tehuitzingo chromitite,Puebla state,Mexico

ABSTRACT

The Tehuitzingo ultramafic body (Acatlán Complex, southern Mexico) is a strongly serpentinized harburgitic body that encloses chromitite bodies and blocks of eclogitic rocks. Hydrous retrograde metamorphism on chromitite bodies resulted in the formation of: i) partly altered chromite characterized by unaltered cores surrounded by Fe²⁺-rich and Al-depleted porous chromite containing chlorite; ii) porous chromite corresponding to a chromite that was entirely transformed to Fe²⁺-rich and Al-depleted porous chromite; and iii) zoned chromite formed by modified cores surrounded by Fe³⁺-rich non-porous chromite and magnetite rims. The content of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the cores of partly altered chromite preserves the magmatic fingerprint of back-arc-bearing chromitites, while the cores of zoned chromite are enriched in Zn, Co and Mn but depleted in Ga, Ti, Ni and Sc, and display a metamorphic signature. P-T pseudosections performed in the system CrMFASH allow us to constrain the temperature of formation of Fe²⁺-rich chromite by reaction of magmatic chromite with olivine between 584 and 449 °C; while the new thermodynamic calculations performed in the FMASH system shows that the Fe³⁺-rich non-porous chromite and magnetite rims in zoned chromites were formed at lower temperatures (i.e. 270–340 ºC and < 20 kbar). Temperatures of alteration obtained using our new thermodynamic model are in agreement with the qualitative estimation that use minor and trace elements in Fe²⁺-rich chromite (i.e. 560–700 ºC). These temperatures are more restricted than those estimated for chlorite hosted in partly altered (193–481 ºC) and porous chromite (158–255 ºC), and those reported in host serpentinite and associated eclogitic rocks (210–399 ºC). A comparison of P-T conditions estimated by our new thermodynamic models and the distribution of minor and trace elements in the different zones of chromites allow us to infer that the alteration took place during the hydrous retrograde metamorphism, and that the high-pressure metamorphism did not modify the composition of chromites. Thus, the microstructural zoning displayed by chromite grains was formed during the exhumation of a segment of a back-arc-related oceanic lithosphere, at similar temperature conditions than the Tehuitzingo serpentinite.     

Hydrochemistry and hydrogen sulfide generating processes in the Malm aquifer,Bavarian Molasse Basin,Germany

Knowledge about the hydrochemical conditions of deep groundwater is crucial for the design and operation of geothermal facilities. In this study, the hydrochemical heterogeneity of the groundwaters in the Malm aquifer, Germany, is assessed, and reasons for the extraordinarily high H₂S concentrations in the central part of the Bavarian Molasse Basin are proposed. Samples were taken at 16 sites, for a total of 37 individual wells, to analyze cations, anions, gas loading and composition. The hydrochemical characteristics of the Malm groundwater in the center of the Molasse Basin are rather heterogeneous. Although the groundwater in the central basin is dominated by meteoric waters, there is a significant infiltration of saline water from higher strata. Care has to be taken in the interpretation of data from geothermal sites, as effects of chemical stimulation of the boreholes may not be fully removed before the final analyses. The distribution of H₂S in the gas phase is correlated to the gas loading of the water which increases in the central basin. Temperatures, isotopic data and the sulfur mass balance indicate that H₂S in the central basin is related to thermochemical sulfate reduction (south of Munich) and bacterial sulfate reduction (north of Munich).     

Hydrochemistry and gas geochemistry of the northeastern Algerian geothermal waters

This study focuses on the water and gas chemistry of the northeastern Algerian thermal waters. The helium gas was used to detect the origin of the geothermal fluid. In the Guelma Basin, the heat flow map shows an anomaly of 120 ± 20 mW/m² linked to the highly conductive Triassic extrusion. The chemical database reveals the existence of three water types, Ca-SO₄/Na-Cl, which are related to evaporites and rich in halite and gypsum minerals. The third type is Ca (Na)-HCO₃, which mostly characterizes the carbonated Tellian sector. The origin of thermal waters using a gas-mixing model indicates a meteoric origin, except for the El Biban hot spring (W10), which shows a He/Ar ratio of 0.213, thus suggesting the presence of batholith. The helium distribution map indicates a lower ³He/⁴He ratio between 0 Ra and 0.04 Ra in the W10 and W15 samples, which is compatible with the crustal ratio. Reservoir temperatures estimated by silica geothermometers give temperatures less than 133 °C. The geothermal conceptual model suggests that a geothermal system was developed by the deep penetration of infiltrated cold waters to a depth of 2.5 km and then heated by a conductive heat source (batholith for El Biban case). The thermal waters rise up to the surface through the deep-seated fractures. During their ascension, they are mixed with shallow cold groundwater, which increase the Mg content and cause the immature classification of the water samples.     

Diagenetic and other highly mineralized waters in the Polish Carpathians

Highly mineralized waters of different chemical types and origin occur in the flysch formations and their bedrocks in the western part of the Polish Carpathians. The marine sedimentation water of the flysch formations is not preserved, as the most mineralized and the heaviest isotopic values of flysch waters are characterized by δ¹⁸O and δ²H values in the ranges of 5–7‰ and −(20–30)‰, respectively. Their origin is related to the dehydration of clay minerals during burial diagenesis, with molecules of marine water completely removed by molecules of released bound water. They are relatively enriched in Na⁺ in respect to the marine water, supposedly due to the release of Na⁺ during the illitization of smectites and preferable incorporation of other cations from the primary brine into newly formed minerals. In some parts of younger formations, i.e. in the Badenian sediments, brines occur with isotopic composition close to SMOW and Cl⁻ contents greatly exceeding the typical marine value of about 19.6 g/L, supposedly due to ultrafiltration. Most probably, the marine water of the flysch formations was similarly enriched chemically in its initial burial stages. Final Cl⁻ contents in diagenetic waters depend on different Cl⁻ contents in the primary brines and on relationships between diagenetic and further ultrafiltration processes. In some areas, diagenetic waters migrate to the surface along fault zones and mix with young local meteoric waters becoming diluted, with the isotope composition scattering along typical mixing lines. In areas with independent CO₂ flow from great depths, they form chloride CO₂-rich waters. Common CO₂-rich waters are formed in areas without near-surface occurrences of diagenetic waters. They change from the HCO₃–Ca type for modern waters to HCO₃–Mg–Ca, HCO₃–Na–Ca and other types with elevated TDS, Mg²⁺ and/or Na⁻ contents for old waters reaching even those of glacial age. Bedrocks of the flysch are represented by Mesozoic and Paleozoic mudstones, sandstones and carbonates, and in some areas by Badenian sediments. Brines of the Mesozoic and Paleozoic bedrocks are usually significantly enriched in Ca²⁺ and Mg²⁺ in comparison with the Badenian brines. By analogy to the deepest brines in the adjacent Upper Silesian Coal Basin, they are supposed to originate from paleometeoric waters of a hot climate.     

Thermal sensitivity analysis of emplacement of the magma chamber in Los Humeros caldera,Puebla, Mexico

We present results of our simulation study of the effects of the depth (top of the magma chamber at 5–10 km) and volume (1000–1400 km³) of the primary heat source beneath the Los Humeros caldera. The thermal gradient in the vicinity of the magma chamber calculated from the temperature excess (difference between the simulated and the initial temperatures prior to emplacement of the magma) is more sensitive to its depth of intrusion than to its volume. This relationship was quantified from multiple linear regression equations. The temperature excess at 2–3 km depth due to the emplacement of magma and its conductive cooling is also more dependent on the chamber depth than on its volume. Therefore, in the study of calderas, volcanoes, and geothermal fields, constraining the chamber depth is more important than its volume. Similarly, comparison of the thermal regime inferred along vertical and horizontal profiles shows the importance of solving the thermal transport equations in three dimensions instead of one or two dimensions.     

Hydrochemistry of surface water and groundwater from a fractured carbonate aquifer in the Helwan area,Egypt

Groundwater is an important water resource in the Helwan area, not only for drinking and agricultural purposes, but also because several famous mineral springs have their origin in the fractured carbonate aquifer of the region. The area is heavily populated with a high density of industrial activities which may pose a risk for groundwater and surface water resources. The groundwater and surface water quality was investigated as a basis for more future investigations. The results revealed highly variable water hydrochemistry. High values of chloride, sulphate, hardness and significant mineralization were detected under the industrial and high-density urban areas. High nitrate contents in the groundwater recorded in the southern part of the study area are probably due to irrigation and sewage infiltrations from the sewage treatment station. The presence of shale and marl intercalation within the fissured and cavernous limestone aquifer promotes the exchange reactions and dissolution processes. The groundwater type is sodium, sulphate, chloride reflecting more mineralized than surface water. The results also showed that water in the study area (except the Nile water) is unsuitable for drinking purposes, but it can be used for irrigation and industrial purposes with some restrictions.     

Hydrochemistry in coastal aquifer of southwest Bangladesh: origin of salinity

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.     

Hydrochemistry of an Albian sandstone aquifer in a semi arid region,Ain oussera,Algeria

The studied area is in the south of Algeria. Chemical data are used to determine the status of water quality in the Albian sandstone aquifer of Ain oussera, as well as to clarify the hydrological regime in the study area, to identify spatial and temporal variations of water quality and sources of contamination (natural and anthropogenic). Waters in sandstone are dominated by a magnesium chloride type and sodium chloride type. Interpretation of chemical data with thermodynamic calculation suggests that the chemical evolution of groundwater is primarily controlled by water–rock interactions. Piezometric map suggests that water is moving from the south toward north. The agricultural irrigation effluent and domestic effluents have largely contributed to contamination of groundwater.     

Hydrochemistry of a complex volcano-sedimentary aquifer using major ions and environmental isotopes data: Dalha basalts aquifer, southwest of Republic of Djibouti

In the Republic of Djibouti (Horn of Africa), fractured volcanic aquifers are the main water resources. The country undergoes an arid climate. Alluvial aquifers exist in the wadis (intermittent streams) valleys and, in relation with volcanic aquifers, form complex volcano-sedimentary systems. Due to increasing water demands, groundwater resources are overexploited and require a rigorous management. This paper is focused on the Dalha basalts aquifer, located in the Dikhil area (Southwest of Djibouti). This aquifer is of vital importance for this area. Hydrochemical data and isotopic tracers (¹⁸O and ²H) were used to identify factors and phenomena governing the groundwater’s mineralization. The Piper diagram shows complex water types. Results from multivariate statistical analyses highlight three water families according to their locations: (1) groundwater characterized by low ionic concentrations located at the wadis zones; (2) groundwater characterized by moderate salinity and (3) highly mineralized waters mainly flowing in the eastern and central part of the study area, in volcanic aquifers. Results from scatter plots, especially Na versus Cl and Br versus Cl, suggest that the origin of more saline waters is not from dissolution of halite. The δ¹⁸O and δ²H data indicate that the groundwater flowing in the alluvial aquifer is of meteoric origin and fast percolation of rainwater occurs in the volcanic aquifers. These findings provide a preliminary understanding of the overall functioning of this complex volcano-sedimentary system. Additional investigations (pumping tests, numerical modeling) are in progress to achieve a more comprehensive understanding of this system.     

The Basin of Mexico aquifer system: regional groundwater level dynamics and database development

The aquifer system of the Basin of Mexico is the main source of water supply to the Mexico City Metropolitan Zone. Management of the Basin’s water resources requires improved understanding of regional groundwater flow patterns, for which large amounts of data are required. The current study analyses the regional dynamics of the potentiometric groundwater level using a new database called the Basin of Mexico Hydrogeological Database (BMHDB). To foster the development of a regional view of the aquifer system, data on climatological, borehole and runoff variables are part of the BMHDB. The structure and development of the BMHDB are briefly explained and then the database is used to analyze the consequences of groundwater extraction on the aquifer’s confinement conditions using lithology data. The regional analysis shows that the largest drawdown rates are located north of Mexico City, in Ecatepec (a region that has not yet received attention in hydrogeological studies), due to wells that were drilled as a temporary solution to Mexico City’s water-supply problem. It is evident that the aquifer has changed from a confined to an unconfined condition in some areas, a factor that is responsible for the large subsidence rates (40 cm/year) in some regions.     

Isotopic evidence of the origin of mineralized waters from the Central Carpathian Synclinorium,SE Poland

Stable isotopes of hydrogen and oxygen were determined in 45 samples of water (27 samples of oil-associated waters, 17 samples of mineral waters used by spas, 1 sample of surface river water) from the Central Carpathian Synclinorium, covering a stratigraphic range of flysch sediments from Upper Cretaceous to Oligocene. Moreover, oxygen isotope compositions of authigenic calcite (vein and cement) from core samples of four boreholes were made to evaluate isotopic equilibrium between waters and diagenetic carbonates as a function of temperature. The saline and brackish waters (TDS from1 g/l to 48.9 g/l) considered here, generally belong to four hydrogeochemical classes: Na-Cl, Cl-HCO₃-Na, HCO₃-Cl-Na and HCO₃-Na. Their isotopic composition causes them to fall to the right of Global Meteoric Water Line (GMWL) showing enrichment in ¹⁸O and ²H. On the other hand, relative to Standard Mean Ocean Water (SMOW) they are depleted in ²H and both depleted and enriched in ¹⁸O. The observed isotopic composition can be explained by the three-component mixing of surface water, diagenetically modified sea water (kind of connate water) and metamorphic water. The mixing is accompanied by an exchange of oxygen isotopes between water and carbonate cements causes ¹⁸O enrichment of interstitial waters. The contribution of isotopic exchange between water and clay minerals in shales was evaluated only theoretically basing of the literature.