Low-salinity Groundwater Research Articles

Isotopic and morphological features of fracture calcite from granitic rocks of the Tono area, Japan: a promising palaeohydrogeological tool

This study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system, using fracture-filling and fracture-lining calcite. Fracture calcite in deep (to ca. 1000 m) granitic rocks of the Tono area, central Japan, was investigated by optical and electron microscopy, and chemical and isotopic analysis. Coupled with geological evidence, these new data imply 3 main origins for the waters that precipitated calcite: (1) relatively high-temperature hydrothermal solutions, precipitating calcite distinguished by δ 18O SMOW from −3 to ca. 10‰, and with δ 13C PDB from ca. −18 to −7‰; (2) seawater, probably partly of Miocene age, which precipitated calcite distinguished by δ 13C PDB of ca. 0‰ and δ 18O SMOW > ca. 20‰; (3) fresh water, with a variable δ 13C PDB composition, but which precipitated calcite distinguished by δ 13C PDB that was significantly < 0‰ and as low as ca. −29‰ and δ 18O SMOW > ca. 17‰. Data for 14C suggest that at least some of the fresh-water calcite formed within the last 50 ka. The present day hydrogeological regime in the Tono area is also dominated by fresh groundwater. However, the marine calcite of probable Miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved. Studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters. When this correlation is applied to the Tono observations, the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data. Thus, the present study suggests that even in presently low-salinity groundwater systems, calcite morphological variations may record the changing salinity of coexisting groundwaters. It is suggested that calcite morphological data, coupled with isotopic data, could provide a powerful palaeohydrogeological tool in such circumstances.

Review of Some Recent Developments in Sustainable Shrimp Farming Practices in Texas, Arizona, and Florida

The world shrimp-farming industry is currently experiencing major crop losses due to disease outbreaks, which are often associated with environmental degradation. Such losses can be minimized through the adaptation of technologies that enhance biosecurity and environmental control. Current technologies suggest that a shrimp yield as high as 10 kg/m2/crop can be achieved in indoor, super-intensive, closed-recirculation systems in which environmental parameters are controlled. Nevertheless, high construction and operating costs make the financial viability of these systems questionable. Production of shrimp with reduced water exchange in outdoor ponds is another promising method to minimize monetary losses and environmental degradation. Data from commercial shrimp farms in south Texas suggest that significant reduction in water exchange and nutrient release is feasible with no impact on production when an adequate level of aeration is provided. Researchers of the Texas Agricultural Experiment Station, Corpus Christi, Texas, are currently testing other potential management tools for intensification of outdoor pond productions. These trials demonstrated the feasibility of producing ayield of almost 0.9 kg/m2 of marketable size shrimp with no water exchange. Inland production of shrimp in low-salinity ground water can provide another potential solution to disease and environmental problems, as production is conducted in isolated areas away from other host species and where effluent water can be used for crop irrigation. Recent studies with this water showed that high-density nursery and grow-out of Pacific white shrimp, Litopenaeus vannamei are feasible, with excellent survival and yield.

Salt dissolution and salt-related deformation of the Forth Approaches Basin, UK North Sea

This paper addresses the potential role of regional-scale salt dissolution as a mechanism responsible for salt-related deformation. This analysis is based on an investigation of the Late Permian to Triassic succession of the Forth Approaches Basin, on the NW margin of the North Permian Basin and makes use of a regional 2D seismic grid tied to five key exploration wells. This basin is ideal for this study because it consists of a deformed central region with a NE–SW trending axis, and an undeformed periphery which provides a template and key datums for reconstruction of the adjacent zone of deformation. Structural styles in this zone include simple salt-cored anticlines, salt walls, salt diapirs, detachment faults and, unusually for the North Sea, intra-Zechstein salt-cored folds (pre-Triassic). Detailed mapping was carried out in UK Block 26/10, and this shows that the dominant structural trend is NE–SW, suggesting that basement tilting influenced structural development. The stratal geometry of the numerous Triassic mini-basins is used to constrain the way in which accommodation space in the mini-basins was created by depletion of mobile salt. The spatial pattern of salt depletion can be accounted for by two contrasting mechanisms (or some combination thereof): (1) deformation through downbuilding during a phase of regional subsidence, or (2) flow induced by regional salt dissolution. Volumetric arguments necessary to discriminate between these two mechanisms are inconclusive due to later deformation of the Top Zechstein datum. Selective preservation of Z3 and Z4 cycles of the Zechstein over large parts of the Block 26/10 study area suggests that much of the salt depletion process preferentially removed Z2 halite. This observation is compatible with a process of subjacent dissolution possibly resulting from low salinity groundwater circulating in the pre-Zechstein aquifers.

Fluid geochemistry of Ambon Island (Indonesia)

The hot spring waters discharging in the Tulehu peninsula of Ambon island Indonesia vary in composition from Na–Cl-type to Na–Cl–HCO3-type Most chemical characteristics of the Na–Cl-type waters are typical of liquids circulating in high-enthalpy geothermal reservoirs Mixing trends in the plots of alkali vs Cl suggest the existence of a parent geothermal liquid with Cl content of 14000 mg/kg The Na–K geothermometer indicates deep temperatures of 230–245°C The parent liquid cools through conduction and/or mixing with cold low-salinity groundwaters Sodium–Cl–HCO3 -type waters originate from dilution of the parent liquid accompanied by water–rock interaction under high PCO2 conditions.

Dynamics of Eucalyptus largiflorens growth and water use in response to modified watertable and flooding regimes on a saline floodplain

Reduced flooding and raised watertables have caused increased soil salinity and die-back of native forests on the floodplains of the lower River Murray of south Australia. Proposed management options include increasing flood frequency by regulating flows from upstream storages, and groundwater pumping to lower the watertable. This paper uses a soil-vegetation-atmosphere-transfer model (WAVES) to evaluate the impact of these proposals on soil salinisation processes and vegetation growth (black box, Eucalyptus largiflorens) for soils with different hydraulic properties. The changes in canopy leaf mass and plant available soil water were simulated for the period 1970–1994 using historical daily climate and river level records. The river level records were used to reconstruct the flooding and watertable history of sites where tree water use studies were conducted to calibrate the model. Then the watertable depth and/or flooding frequency was modified and the changes in the canopy leaf mass and soil water availability relative to the historical simulation were evaluated. The simulations suggest that, with the present watertable and flooding regime, very large floods (e.g. 205 days as in 1974–1976) are needed to sustain tree cover on the higher parts of the floodplain where die-back is most severe. They also indicate that soil hydraulic properties have a large influence on the magnitude and time scale of the growth response of salt stressed vegetation to floods and salt accumulation. Infrequently flooded vegetation exhibiting die-back was predicted to increase its canopy leaf area for up to 12 years following the large floods of 1974 and 1976, at sites where the soil was relatively permeable and groundwater highly saline (EC = 55 dS m −1). The changes in canopy leaf area in response to the floods was predicted to be relatively small on sites with heavier clay soils. The growth response of the vegetation to a long term lowering of watertable depth by 1 m was greater than that induced by the small potential increase in flooding frequency which is feasible given the current water storage limitations. The simulations predict that changes in the average annual soil water availability which arise from flood events and soil salinisation, drive a long term cycle in the annual average transpiration rate per unit leaf area suggesting the soil-plant-climate system is adjusting towards a hydrological equilibrium but is not in equilibrium. The proposed management options may control die-back in parts of the floodplain with more slowly salinising heavy clay soils and lower salinity groundwater but are unlikely to prevent die-back on relatively permeable soils with high salinity groundwater. However, they may assist vegetation survival between the long duration flood events which appear to be essential to sustain tree cover on the higher floodplain. The management options need to be evaluated further at the floodplain scale using the understanding from site specific conditions to test simple approaches which can be linked to a geographic information system (GIS) of the floodplain.

Salinization processes in a coastal leaky Aquifer system (Alt Empordà, NE Spain)

Water supply in the Alt Empordà area was obtained from ground water until 1987 when its high salinity contents forced managers to use surface water instead to satisfy the demand. In dry years, however, surface water may not suffice. In this paper, we study the hydrological behavior of this leaky aquifer system to finally estimate the feasibility bility of supplying low salinity ground water to complete local needs. We start on the hypothesis that squitard salt leaching is the main factor of salinity. Hydrogeological parameters of this leaky aquifer system are estimated using standard pumping tests, and fitting simulated drawdown using monthly withdrawal rates with head levels recorded from 1984 to 1987 in selected piezomaters. Based on a mass-balance approach, we conclude that the aquitard did not behave as the only cause of salinity. Seaintruded in the upper unconfined source bed leakedthe athrough the aquitard, and it represented the min origin of chloride.Moreover, sea water intrusion in the main aquifer was likely another source of salinity, as shown by simulated drawdowns.

Geochemistry of submarine warm springs in the limestone cavern of Grotta Azzurra, Capo Palinuro, Italy: evidence for mixing-zone dolomitisation

Subtidal springs in and around the submarine limestone cavern of Grotta Azzurra, at Capo Palinuro, Italy, discharge fluids which are warm (< 25°C), mildly acidic, depleted in Cl −, Na + and Mg 2+, and enriched in Si, alkalinity, Ca 2+, Sr 2+, Mn, NH 4 +, PO 4 3− and H 2S, relative to surrounding seawater. The compositions of the warm fluid samples collected in and around the cave define mixing lines which suggest dilution of a single thermal fluid ( T ≥ 23°C) by cool overlying seawater ( T= 17–17.6°C). The chemical data suggest that the proposed thermal fluid contains two components, one derived from seawater (< 90%) and the other from low-salinity groundwater (> 10%). Excess Si, alkalinity, Ca 2+, Sr 2+ and Mn relative to seawater are likely derived from the groundwater component or dissolution/hydrothermal alteration of the host rocks. Magnesium has been removed from the seawater component in exchange for Ca 2+, due to dolomitisation of the limestone and/or hydrothermal alteration reactions. Saturation-state calculations suggest that the vented fluids are near saturation with respect to calcite and supersaturated with respect to dolomite. This and the presence of dolomite in the host rocks and cave-floor sediments suggest that “mixing-zone” dolomitisation of the limestones is occurring, perhaps kinetically assisted by elevated temperature and/or bacterial mediation in the reducing subseafloor zone. One possible “end-member” condition is considered for the thermal fluid — zero-Mg — which suggests an end-member temperature of 50.5°C and a fluid composition derived from ∼ 38% seawater and ∼ 62% groundwater. The heat source for the circulating fluids is uncertain, but may involve warm underlying igneous rocks or heating via the geothermal gradient. A continuous in-situ record of vent-fluid temperature, salinity, pH and O 2 concentration collected within the cavern is consistent with our interpretation of the fluid origin, and suggests that tidal forcing affects circulation and venting of the warm fluids.

Chemistry of shallow submarine warm springs in an arc-volcanic setting: Vulcano Island, Aeolian Archipelago, Italy

Abstract Results are presented from a geochemical survey of active submarine warm springs off the island of Vulcano, an active island-arc volcanic centre in the Aeolian Archipelago, Italy. Water samples were collected from submarine ‘fumaroles’ which discharge hot (~ 50–100 °C), acidic, gas-rich, low-chlorinity fluids into the shallow embayment of Porto di Levante, adding dissolved Si, K, Li, Rb, Fe, Mn, NH3 and H2S into surrounding seawater. These fluids are interpreted as a mixture of seawater and low-salinity groundwater which has undergone high-temperature (> 100 °C) hydrothermal alteration, followed by mixing with cool seawater in the sub-seafloor and during venting. The fluid compositions also suggest the chemical ‘overprint’ of reactions resulting from the input of significant amounts of the volcanic gases CO2, SO2 and H2S at this site, specifically, the attack of igneous silicate phases by dissolved CO2, and the hydrolysis and oxidation of SO2 and H2S to SO42−, H+ and elemental sulphur. These overprinting reactions have been proposed for other gas-rich submarine hydrothermal fluids collected from shallow island-arc and hotspot volcanoes, and may be typical of such settings. Several water samples were also collected from a site where warm (

18O studies on recharge of phreatic aquifers and groundwater flow-paths of mixing in the Delhi area

Widespread decline in the groundwater level and growth of salinization are the critical factors controlling future groundwater quality and availability in the Delhi area. 5180 and salinity data have been integrated for studying recharge conditions of phreatic aquifers and tracing the flowpaths of groundwater mixing. Wide range (−2.8%o to −8.6‰) and spatial inhomogeneity of δ 18O in groundwater indicate that the aquifer is characterized by a slow process of natural mixing of groundwater in its lateral extent. There is infiltration of river water to the groundwater, in addition to the recharge from rainfall. Groundwater recharge in the area also occurs through stagnant water pools in low elevation areas where water surface run-off collects. Chloride concentration in groundwater has a wide range from 15 ppm. to 5070 ppm with a log-normal frequency distribution indicating that the salt deposits in the area are not homogeneously distributed. The δ 18OCl relationship clearly suggests mixing of multiple sources of high salinity groundwater with low salinity groundwater or river water along specific pathways. The data also indicates leaching of salts present in the soil during vertical recharge from rainfall and irrigation. Large lateral variation in chloride concentration indicates changes in evapotranspiration rate during recharge. Detailed investigations on identifying the potential recharging zones, flow-paths of mixing and recharge conditions will be useful to protect the groundwater resources from depletion and salinization.

Isotopic study of alluvial groundwaters, south-west lockyer valley, queensland, australia

The isotopic chemistry of alluvial groundwaters from two adjacent valleys are described and hydrological processes within related aquifers are identified as evidenced by oxygen-18, deuterium, tritium and chloride data. A plot of δ18O against δD values reveals isotopic enrichment of the groundwater by the recycling of spray irrigation water. A plot of tritium versus chloride concentrations displays separate linear correlations for alluvial groundwaters within the two valleys. The salinity has a common source, therefore the separate correlations are interpreted as the past transfer of low salinity groundwater from the alluvial aquifers in one valley to the underlying sandstone aquifers.

Study of Groundwater Occurrence and Mixing in Pushkar (Ajmer) Valley, Rajasthan with d18O and Hydrochemical Data

Abstract Stable isotopic composition of groundwaters and their hydrochemistry have been used for investigating possible existence of both palaeo-water and replenishable groundwater in Pushkar (Ajmer) Valley, Rajasthan. Causes of salinity increase in some of the groundwaters and intermixing of saline and fresh groundwaters have also been studied. The d18O values of water samples show a wide range varying from -6.3‰ to +4.0‰, indicating different sources of origin of these waters. There is a depletion of d18O with increasing depth of water table which may be due to stratification in groundwater with shallower zones being recharged from relatively modern rainfall. The chemical composition of groundwatlbrs shows a wide variation with electrical conductivity ranging from 150μmhos/cm to 8400μmhos/cm. Linear relationship between d18O and Cl- is a clear indication that the groundwaters are freshwater intermixed with different amounts of a saline endmember. Highly depleted, low salinity groundwaters seem to be of palaeo-origin. The most probable genesis of the 18O enriched, high salinity groundwater is from an originally evaporitic environment.

Metal-organic complexes in geochemical processes: Calculation of standard partial molal thermodynamic properties of aqueous acetate complexes at high pressures and temperatures

Estimates of standard partial molal properties at high temperatures and pressures for aqueous acetate complexes of major and trace elements in geologic fluids were made with the aid of experimental data from the literature and correlation algorithms. A system of correlation expressions allows the incorporation of all available experimental data, but also allows estimates in the absence of any measurements. Likely uncertainties for each type of estimate were assessed. Thermodynamic data and equation of state parameters permit calculation of standard partial molal properties including dissociation constants for 114 acetate complexes. Incorporation of many of these dissociation constants with those for hydroxide, chloride, and sulfate complexes demonstrate that acetate complexes are ineffectual in transporting major rock forming elements or trace metals in sedimentary basin brines. In contrast, these complexes could be considerably more efficient in metal transport in low salinity groundwater solutions with elevated concentrations of organic solutes. Calculations indicate that up to 40% of the acetate may be present as sodium and calcium complexes in basin brines with total salinities around 1.0 molal.

The Ag-Mn-Pb-Zn vein, replacement, and skarn deposits of Uchucchacua, Peru; studies of structure, mineralogy, metal zoning, Sr isotopes, and fluid inclusions

Uchucchacua is an Ag-Mn-Pb-Zn vein, replacement, and skarn mineral district in the central Andes of Peru. Host rocks are massive Jumasha Formation shelf limestones of Turonian age that have been folded into an asymmetric northeast-verging anticline of Andean trend. Strata near the fold crest are cut by minor dacitic intrusions and have been displaced by a conjugate set of steep wrench faults that strike northwest-southeast and northeast-southwest. Most ore occurrences are restricted to host rocks that lie below marly limestone at the top of the middle Jumasha Formation. Vein ores located along the fracture system have formed by fissure infill and by replacement of limestone wall rocks. Larger sheetlike replacement orebodies are parallel and adjacent to a large fault. Replacement is in zones of brecciation adjacent to fault bends which were dilatent during sinistral slip of the master fracture. Such fracture belts may have been subject to paleokarst solution processes before mineralization.Four paragenetic stages have been identified. Fe, Mn, and Si were introduced at the exoskarn stage (I) as the anhydrous silicates ferroan tephroite, johannsenite, rhodonite, and bustamite. During the early main stage (II) ferroan tephroite was replaced by friedelite and magnetite under oxidizing conditions at a relatively low pH. Pb, Zn, Fe, Cu, and B were introduced; principal sulfides are pyrrhotite, Fe-rich sphalerite, Mn-rich wurtzite, alabandite, galena, chalcopyrite, and tetrahedrite. Pyrrhotite was replaced by other sulfides during later stage II. Main gangue minerals were calcite, kutnohorite, rhodochrosite, and quartz. Ag, As, and Sb were introduced during the late stage (III) in the form of sulfosalts, principally pyrargyrite. Redistribution of metals introduced at stage II resulted in the growth of Fe-poor sphalerite and alabandite accompanied by calcite gangue. Decreasing Fe contents of alabandite and sphalerite during late stage II and stage III, together with the appearance of pyrite, indicate an increase in sulfur fugacity and/or decrease in temperature over this period. The supergene stage (IV) affects the upper 30 to 150 m of most veins and involves the growth of Mn hydroxides, goethite, orpiment, marcasite, cerussite, and siderite.Distribution patterns of metal ratios and high metal values define ore bands, with a succession of antiforms and synforms. Ore-band locations are determined by vein width, itself a function of wall-rock reactivity, and fracture permeability. Metal ratio distributions in the Luz vein show symmetric dispersion of metals at right angles to the ore-band axis. Ag shows the least dispersion with progressively greater dispersion of Pb and Zn.Geologic considerations indicate mineralization at a minimum depth of 1,600 m. Most primary fluid inclusions in calcite of probable stage II origin were trapped over a temperature range of about 200 degrees to 337 degrees C (hydrostatic pressure) or about 225 degrees to 362 degrees C (lithostatic). Fluid inclusions from late stage II quartz were trapped at about 165 degrees to 205 degrees C (hydrostatic) or 190 degrees to 230 degrees C (lithostatic). Salinity ranged from 0.5 to 29.7 equiv wt percent NaCl-CaCl 2 with little relation to temperature. Calcite 87 Sr/ 86 Sr values of 0.70743 to 0.71122 extend well outside the range of values both determined for magmatic rocks of the region and estimated for the host limestones but are probably compatible with older sedimentary rocks of the mine district. Salinity levels and NaCl/CaCl 2 ratios are similar to basinal brines associated with Mississippi Valley-type deposits. It is concluded that basinal brine, expelled from the deforming Mesozoic sequence, was a significant component of the ore fluid. This fluid was probably heated by dacitic magmas or cooling dacitic intrusions; a polygenetic high-salinity magmatic-basinal fluid then rose along existing strike-slip fractures toward the mineralization site where it mixed with heated low-salinity ground water.

A reinterpretation of the delta D H 2 O of inclusion fluids in contemporaneous quartz and sphalerite, Creede mining district, Colorado; a generic problem for shallow orebodies?

Water extracted from fluid inclusions in quartz from shallow epithermal ore deposits often has a hydrogen isotope composition (delta D) different from that of water extracted from inclusions in associated minerals. This difference is usually attributed to the involvement of primary fluids from multiple sources. Isotopic and homogenization and freezing temperature determinations on fluid inclusions from contemporaneous quartz and sphalerite from the epithermal, silver and base metal orebodies of the OH vein, Creede district, Colorado, suggest an alternative explanation. In near-surface deposits, differences between delta D (sub H 2 O) of inclusion fluids in ore minerals and quartz may result, instead, from contamination during extraction of the fluids contained in primary inclusions by shallow ground water trapped in pseudosecondary inclusions in quartz.Quartz from the OH vein contains two principal petrographically distinct populations of fluid inclusions: primary and pseudosecondary. The primary inclusions have salinities ranging from 5 to 10 equiv wt percent NaCl, and the salinities of pseudosecondary inclusions cluster between 0 and 1 percent. Primary inclusions in quartz from one locality have a measured delta D (sub H 2 O) value of -69 per mil, while pseudosecondary inclusions at the same locality have a delta D (sub H 2 O) value of -102 per mil. Both salinity and isotopic values for primary inclusions in quartz are similar to those for primary inclusions in contemporaneous sphalerite. Homogenization temperatures for primary and pseudosecondary inclusions in quartz range from 191 degrees to 280 degrees C and from 199 degrees to 278 degrees C, respectively. The delta D (sub H 2 O) value measured on fluid inclusions from bulk crystals ranges between -97 and -85 per mil and represents a mixture of fluids from both primary and pseudosecondary inclusions.We interpret the data to indicate that one or more episodes of abrupt incursion of cooler, overlying ground water into the ore zone caused thermal cracking of the quartz crystals during the time interval of mineralization. Subsequent healing of the fractures trapped heated, low-salinity ground water in pseudosecondary inclusions. The abrupt incursions of overlying ground water are speculated to have resulted from either collapse of a transient vapor-dominated region of the ore zone, or catastrophic venting of the system through hydrothermal eruption(s).The unusually high contrast between the salinities of the ore-depositing fluids and the ground water overlying the ore zone allowed recognition of this phenomenon at Creede. It is likely, however, that Creede is not unique. Similar phenomena may be common in shallow ore zones where rapid fluctuation of an interface between a deep, high-temperature thermal plume and an overlying, cooler ground water may be expected to occur. Careful study of the origins of fluid inclusions, particularly in quartz, is essential to characterize the primary ore fluids and to assess the role of ground water in the hydrology of shallow ore deposits.

High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba, Canada

High concentrations of U and 226Ra, and elevated 234U/ 238U activity ratios have been measured in groundwater samples collected from water supply wells and exploratory boreholes in the area surrounding the Underground Research Laboratory (URL) of Atomic Energy of Canada Limited, in southeastern Manitoba. All groundwaters come from the Lac du Bonnet granite batholith or sediments overlying the batholith. Uranium concentrations attain almost 1 mg/l in some shallow, low-salinity groundwaters, whereas 226Ra tends to be high (up to 38 Bq/l) in deeper, saline waters. The U concentrations are some of the highest observed in global groundwaters, yet no significant ore body or mineralization is known in the area. Analyses of unaltered rock samples of the Lac du Bonnet granite show slight U enrichment over average Canadian Shield granites (6.5 μg/g vs 4 μg/g), and altered wall rock in fracture zones is enriched in U by up to an order of magnitude compared to adjacent bedrock. Low 234U/ 238U activity ratios in this altered rock indicate active and recent leaching of U by groundwater. The key control on U concentration appears to be redox potential. Concentrations of U in rock, residence time and groundwater composition are of lesser importance. Geochemical modelling of the shallower, oxidized waters indicates that U speciation consists mainly of anionic carbonate complexes of the uranyl ion. This is supported by the remarkable efficiency of an anionic filter developed to remove high levels of U from drinking water in the area. In more reducing groundwaters, U concentrations are similar to those determined in recent experimental work on uraninite solubility in the pH range 7–8.5. Colloidal U is <10% of total U and organic complexation is unlikely to be significant because of low dissolved organic concentrations. The results emphasize the significance of redox potential in controlling U mobility in both oxidizing and reducing environments and indicate the usefulness of U concentration in estimating groundwater Eh.

Two new amphipod crustaceans of the genusBahadziafrom ‘blue holes’ in the Bahamas and some remarks on the origin of the insular stygofaunas of the Atlantic

Bahadzia setimana sp. nov. from South Andros, and B. obliqua sp. nov. from Cat Island, both in the Bahamas, are described. The recently described genus Bahadzia now contains five closely related taxa, each apparently confined to a single island or isolated group of islands, in a restricted area of the West Indies, ranging from Haiti in the south to the northern Bahamas. The present discoveries show not only that low-salinity groundwaters of the West Indies have a marked degree of insular endemism, but that the high-salinity anchihaline habitats possess endemics in certain groups as well. Current models which seek to explain the evolution of stygobionts on Atlantic Islands are reviewed critically and the most likely synthesis is provided.

Radiocarbon dating of groundwater in tertiary sediments of the Eastern Murray basin

The Tertiary sediments located in the eastern part of the Murray Basin contain one of the most important low salinity groundwater resources in New South Wales. This has been especially apparent during recent years as increasing demands on the resource are being made by irrigators and municipal water suppliers. It is therefore imperative that the hydrogeoloical environment in which the groundwater occurs be thoroughly understood to allow adequate management of the resource. As part of a systematic exploration of the Tertiary aquifer systems, the Australian Atomic Energy Commission, in association with the Water Resources Commission of New South Wales, carried out a radiocarbon dating project on 37 groundwater samples from bores screened in these unconsolidated sediments. The results indicate water ages in the range 'modern' to 15 800 years. Groundwater recharge areas are indicated and rates of groundwater recharge and movement determined. The latter shows close correlation with velocity values quantitatively determined by Darcy's law.

Application of hydrogeochemistry to uranium exploration in the pine creek geosyncline, northern territory, Australia

In the Pine Creek Geosyncline, fast moving, annually recharged, low-salinity ground waters dissolve uranium- and magnesium-enriched gangue minerals from mineralized aquifer rocks. The level of dissolved uranium depends on prevailing pH, Eh, salinity and degree of adsorption, which limits its effectiveness as an exploration indicator. Near each known deposit, leaching of magnesium-enriched gangue minerals produces ground waters with very similar major-element concentration plots, the shape of which constitutes a mineralized aquifer “signature”. Gangue minerals also supply high levels of Mg 2+ (expressed as NMg = [Mg 2+]/[Ca 2+ + Mg 2+ + Na + + K +] in milliequivalents per litre) to contained ground waters, NMg > 0.8 being common in ground waters from mineralized aquifers at each Pine Creek Geosyncline deposit. Data from Ranger One No. 3 ore body illustrates how progressive mixing of waters from mineralized and unmineralized aquifers causes graded reductions in NMg, which, when plotted onto a ground plan, delineate a hydrogeochemical aureole. High NMg (> 0.8) coincides with high uranium concentration (> 20 μg/l of U) in ground waters near Nabarlek and Ranger. Because pH-Eh conditions in aquifers at Jabiluka depress uranium solution, < 10 μg/l of U is present, although NMg values are generally > 0.8. To date NMg has always been < 0.8 in nonmineralized aquifer waters, whereas uranium may be > 50 μg/l in ground waters from felsic igneous aquifers, which can be identified as uneconomic by low (< 0.4) NMg, and by a fixed relationship between uranium and co-leached species such as F - and soluble salts. Measurements of pH, Eh, salinity, Fe(II), Ca, Mg, Na, K, Cl, SO 4, total carbonate, phosphate, F -, Cu, Pb, Zn and U in waters from 48 percussion holes in and near the Koongarra ore bodies have been related to mineralogy recorded in drill logs. The composition of waters from 20 holes near and along strike from known mineralization, fitted the mineralized aquifer “signature”, had NMg > 0.8 and uranium up to 4100 μ/l. These data confirm the use in this region of NMg as a hydrogeochemical indicator of uranium mineralization; they also indicate additional zones of possible mineralization.

Cyclic Hydrologic and Diagenetic Events in San Andres Formation: Geologic Implications: ABSTRACT

In the San Andreas Formation in Roosevelt, Curry, Quay, and DeBaca Counties, New Mexico, cyclic and diagenetic events reflect rhythmic fluctuations in sea level, climate, and sedimentation. Depositional cycles in the San Andres record: (1) rapid transgressions which accompanied eustatic rises in sea level; (2) seaward progradations of subtidal, intertidal, and supratidal (sabkha) facies during stillstands of sea; (3) subaerial exposure during lowstands of sea level; and (4) continuous subsidence of the area. Effects of fluctuation in sea level, climate, and sedimentation were transmitted to San Andres carbonates through changes in hydrology which produced distinctive diagenetic overprints. The following cyclic hydrological events are recorded in many San Andres intervals: (1) dolomitizing fluids dolomitized initially unstable carbonates; (2) hypersaline brines emplaced anhydrite as partial to nearly complete replacement of carbonate (anhydritization) and as cement; and (3) low-salinity ground waters dissolved much anhydrite, brought about replacement of anhydrite by gypsum, hemihydrate, calcite, and silica and also caused dedolomitization. Many San Andres carbonate intervals bear evidence of having been subjected to overprints of two cyclic fluid invasion sequences. Nearly all primary and secondary porosity in dolostone has been occluded by dolomite, anhydrite, and calcite cements. In limestones (exclusive of dedolostones) nearly all primary and secondary porosity has been occluded by calcite and anhydrite cements. Many San Andres limestones were formed by paramorphic replacement of dolomite by calcite (dedolomitization). Secondary intercrystalline and moldic porosity in the original dolostone have been preserved after dedolomitization. Diagenesis and porosity relations within the San Andres reflect the relative flux of dolomitizing, anhydritizing, and low-salinity fluids within a given interval. Tertiary porosity is extensively preserved in intervals in which the low-salinity stage of a hydrologic cycle has not had its overprint superseded by dolomitizing or anhydritizing fluids of a subsequent cycle. The most highly porous intervals contain both secondary intercrystalline and anhdrite moldic porosity. End_of_Article - Last_Page 763------------