Water Isotope Analysis Research Articles

Composition and flux of groundwater from a California beach aquifer: Implications for nutrient supply to the surf zone

The coastal, unconfined aquifer at Huntington Beach, California contains saline groundwater (33 psu) adjacent to the water line, and a brackish groundwater (BGW) mixing zone (3–9 psu) approximately 50 m from the water line. According to salinity and water isotope analyses, the BGW composition varies spatially in the alongshore direction. Measurements obtained from two BGW wells indicate this water is a mixture of seawater and a freshwater end member (likely infiltrated runoff); results from a third more southerly well suggest the BGW is composed of seawater, freshwater, and water from the confined Talbert Aquifer. Saline groundwater, on the other hand, shares similar salinity and water isotopic composition with seawater. The saline groundwater is enriched with short- and long-lived radium isotopes relative to the surf zone, which in turn is enriched relative to waters further offshore. We derive eddy diffusion coefficients for the nearshore using 223Ra and 224Ra, and in conjunction with 226Ra activities, estimate a submarine groundwater discharge (SGD) rate between 4.2 and 8.9 L min −1m −1 of shoreline, respectively. Using analytical models, we estimate that between 74 and 100% of this discharge represents seawater than has been forced by tides and waves through the beach aquifer. Under one set of model assumptions, results indicate that a portion (up to 56%) of seawater pumped into the beach aquifer by waves and tides is not discharged suggesting salt water intrusion may be occurring. Because saline groundwater is enriched with dissolved inorganic nitrogen and soluble reactive phosphate relative to the coastal ocean, our results suggest that tidally and wave-driven seawater circulated through the beach aquifer represents an important mechanism for nutrient input to the nearshore environment in dry weather. Estimates of nutrient flux to the coastal ocean via SGD along 16–17 km of shoreline are equivalent to nutrient fluxes from two salt water wetlands near the study site during the dry season.

Constraining water sources and hydrologic processes from the isotopic analysis of water and dissolved strontium, Lake Junin, Peru

This paper presents the results of a hydrogeochemical study of the Lake Junin watershed, Peru, to evaluate the controls over chemical composition of water in this remote Andean site. Stream, spring, and lake waters in the Lake Junin basin are all Ca 2+ and HCO 3 − type. Evaporation has enriched the δ 18O of the lake water by about 6‰ compared to that of input water. The 87Sr/ 86Sr ratio of dissolved strontium varies from 0.70777 to 0.72242 in the watershed because of water interactions with limestone and silicate rocks, respectively, in the watershed. The 87Sr/ 86Sr ratios of strontium in water from streams that flow through clastic sedimentary rocks east of the lake is more radiogenic (>0.7100) than elsewhere in the watershed, where the 87Sr/ 86Sr ratios of surface waters range between 0.7076 and 0.7090. These lower ratios are consistent with marine limestone sources found in these watersheds. Synoptic discharge measurements, combined with the strontium isotopic data, show that that three rivers provide over 90% of the surface water to the lake: the Río Chacachimpa, the Río San Juan, and the Río Palcamayo and about 60% of the total water input to the lake, the rest coming from direct precipitation.

Below‐ground competition between trees and grasses may overwhelm the facilitative effects of hydraulic lift

AbstractUnder large East African Acacia trees, which were known to show hydraulic lift, we experimentally tested whether tree roots facilitate grass production or compete with grasses for below‐ground resources. Prevention of tree–grass interactions through root trenching led to increased soil water content indicating that trees took up more water from the topsoil than they exuded via hydraulic lift. Biomass was higher in trenched plots compared to controls probably because of reduced competition for water. Stable isotope analyses of plant and source water showed that grasses which competed with trees used a greater proportion of deep water compared with grasses in trenched plots. Grasses therefore used hydraulically lifted water provided by trees, or took up deep soil water directly by growing deeper roots when competition with trees occurred. We conclude that any facilitative effect of hydraulic lift for neighbouring species may easily be overwhelmed by water competition in (semi‐) arid regions.

Extensive microbial modification of formation water geochemistry: Case study from a Midcontinent sedimentary basin, United States

The Upper Devonian Antrim Shale in the Michigan Basin is an economically significant source of microbially produced methane along the basin margins where meteoric recharge has been focused. Oxygen and hydrogen stable isotope compositions of Antrim formation waters show that fresh waters, recharged from Pleistocene glaciation and modern precipitation, suppressed basinal brine salinity to great depths and enhanced methanogenesis. This paper presents results of integrated elemental and isotope analyses of Antrim Shale formation waters from the margins and center of the Michigan Basin, focusing on solute sources and geochemical modifications associated with regionally extensive microbial methanogenesis. Cl-Br-Na systematics reveal that salinity is controlled not only by mixing between variable amounts of basinal brine and meteoric water, but also by halite dissolution where fluids recharged through underlying Devonian carbonate aquifers with localized evaporite deposits. Divalent cations, carbonate system parameters, and carbon isotope compositions of dissolved inorganic carbon have been systematically and profoundly altered by microbial methanogenesis. Large decreases in formation water Ca/Mg and Ca/Sr ratios accompany increasing carbonate alkalinity values in areas with high rates of microbial gas production. Thermodynamic and reaction-path modeling show that these changes are consistent with calcite precipitation during progressive microbial methanogenesis. Similar variations in fluid chemistry are evident in databases from other sedimentary basins containing black shales and coal beds associated with microbial gas. Microbial methanogenesis may play an important role in the geochemical evolution of divalent cation relationships in crustal fluids and should be considered in models of formation water origin and evolution.

Impact of precipitation seasonality changes on isotopic signals in polar ice cores: a multi-model analysis

For Central Greenland, water isotope analysis indicates a temperature difference of about 10°C since the Last Glacial Maximum (LGM). However, borehole thermometry and gas diffusion thermometry indicate that LGM surface temperatures were about 20°C colder than today. Two general circulation model studies have shown that changes in the seasonal precipitation timing in Central Greenland might have caused a warm bias in the LGM water isotope proxy temperatures, and that this bias could explain the difference in the estimated paleotemperatures. Here we present an analysis of a number of atmospheric general circulation model simulations mostly done within the framework of the Paleoclimate Modeling Intercomparison Project. The models suggest that the seasonal cycle of precipitation and surface mass balance over Central Greenland at the LGM might have been very different from today. This supports the idea that the accuracy of the water isotope thermometry at the LGM in Greenland might be compromised as a result of a modified surface mass balance seasonality. However, the models disagree on the amplitude and sign of the bias. For Central East Antarctica, a strong seasonality effect on the LGM isotopic signal is not simulated by any of the analyzed models. For the mid-Holocene (6 kyr BP) the models suggest relatively weak isotope paleothermometry biases linked to changes in the surface mass balance seasonality over both ice sheets.

IDENTIFYING DISCHARGE ZONES OF ARSENIC IN THE GOOSE RIVER BASIN, MAINE1

ABSTRACT: Water balance modeling and the analysis of stable water isotopes in ground water were conducted to aid the location of ground water discharge areas within the Goose River basin, in mid‐coastal Maine. Previous investigations of drinking water from wells in the fractured crystalline bedrock encountered persistent elevated total arsenic. Such contamination may be related to discharging ground water from fractured zones in the basin. Modeled discharge rates greater than +10 cm/yr and 18O values lighter than‐9.5 per mil VSMOW may indicate recent recharge is mixing with deeper ground water and is focused along some fractured zones in arsenic bearing crystalline rocks.

Isotope analysis of water by means of near infrared dual-wavelength diode laser spectroscopy

A novel diode laser spectrometer was developed using dualwavelength multiplexing, ensuring ideal conditions for high-precision and simultaneous measurements of the 2H/1H, 17O/16O, and 18O/16O isotope ratios in water. A 1.4-m diode laser probed a H16OH / HO2H line pair near 7198 cm-1, while a similar laser observed H16OH, H17OH, and H18OH rovibrational lines around 7183 cm-1, or a H16OH / H18OH line pair near 7200 cm-1. The 1-sigma standard deviation is 0.2 per thousand for 18O/16O, and 0.5 per thousand for the 2H/1H and 17O/16O isotope ratios. Preliminary experiments with repeated injections of a natural abundance sample point to an accuracy of about 1 per thousand for all three isotope ratios in natural samples.

Quantitative comparison of lake throughflow, residency, and catchment runoff using stable isotopes: modelling and results from a regional survey of Boreal lakes

An isotope-based approach for water balance assessment is presented and applied to estimate throughflow, residence time and catchment runoff to 70 headwater lakes on the Boreal plain and uplands of northern and north-central Alberta, Canada. The survey reveals a complex hydrologic regime with systematic variability in water balance due to local site characteristics. On average, runoff to lakes in wetland-dominated catchments is found to be significantly higher than runoff to upland-dominated lakes, with generally higher contributions from catchments with low bog/fen ratios. The isotope method, which relies primarily on water sampling and isotopic analysis, can be easily integrated in routine water quality surveys and is shown to be a practical alternative to conventional hydrological modelling for comparative analysis of water balance controls on hydrochemistry and aquatic ecology of lakes, particularly in low-relief wetland-rich terrain.

Exotic stable isotope compositions of saline waters and brines from the crystalline basement

The overwhelming majority of groundwaters reported in literature show isotopic compositions that place them on or right of the meteoric water line (MWL) in a δ 2H vs. δ 18O diagram. A closer look to the steadily increasing data from the deeper crystalline basement reveals nevertheless that points left of the MWL are not so unusual as has been suggested during the last decades. Their supposed rareness seems to be due to a bias of sampling: The concerned fluids develop only at greater depths and in low permeability environments, which makes access to them so difficult. Paradoxically, “exotic” stable isotope compositions may be the rule in a restricted depth zone in all crystalline basements with slow groundwater movements but not too high temperatures. The present study takes stable isotope data on saline fluids from the deep French granitic basement as a starting point. In order to situate the French data from the Vienne region in a more general context, a literature review has been undertaken assembling more than 1300 stable isotope analyses of waters and brines in crystalline rocks from stable cratons and mountain belts world-wide. The French fluids show some particular features with respect to the majority of other studies as their major ion composition close to seawater or a significant isotopic shift at relatively low salinities. Detailed knowledge on the hydrological and chemical history of the site allowed to develop a conceptual model of isotopic evolution by interaction of fracture fluids with fracture minerals. This model considers interactions of the last fluids flushing the system (marine water, unaltered, evaporated or diluted) with previously precipitated mineral phases and the neoformation of the last generation of fracture minerals. The model has been tested quantitatively for several combinations of dissolution–precipitation reactions involving formation of carbonates, clay minerals and Fe-hydroxides. Most of the modelled scenarios would leave the residual liquid phase depleted in 18O and either enriched or depleted in 2H and produce waters with isotopic signatures corresponding to the observed δ 2H vs. δ 18O correlation. It is therefore suggested that the general phenomenon of an isotopic shift in deep fissured silicate rocks, which seems to be rather independent of lithology, may result from a combination of different reaction mechanisms implying not solely the rock matrix but also the different generations of fracture minerals.

The origin of reservoir fluids in the geothermal field of Los Azufres, Mexico — isotopical and hydrological indications

The calculation of hydrological balance resulted in a potential, average annual infiltration rate of 446±206 mm/m 2 for the Los Azufres geothermal area, which corresponds to a total of 82×10 6 m 3 per a. Due to the highly fractured and faulted structure of the volcanic formations, a considerable potential for the infiltration of recent meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir. Very negative δ 13C values can be explained by the input of organic C from the surface, but the lack of 14C in the deep fluids reflects a pre-historic age for the infiltration event of fossil meteoric water. The dilution of the meteoric water by 14C-free CO 2 gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during the Late Pleistocene or Early Holocene glacial period. Strong water–rock interaction processes, such as sericitization/chloritization, caused the primary brine composition to be camouflaged. A preliminary hydrological model of the reservoir can be postulated as follows: the fossil hydrodynamic system was characterized by the infiltration of meteoric water and mixing with andesitic and/or magmatic water. Strong water–rock interaction processes in the main part of the production zone prove the existence of former active fluid circulation systems. Due to changes in pressure and temperature, the rising fluids get separated into liquid and vapour phases at a depth of 1500 m. After cooling, the main portions of both phases remain within the convective reservoir cycle. Isotope analyses of hot spring waters indicate the direct communication of the reservoir with the surface at some local outcrops. A recent reactivation of the hydrodynamic system is caused by the geothermal production, as indicated by the detection of lateral communication between some production and reinjection wells.

Effects of Riparian Denitrification on Stream Nitrate-Evidence from Isotope Analysis and Extreme Nitrate Leaching during Rainfall-

The effects of riparian denitrification on stream nitrate were investigated by detailed soil water observations and isotope analysis at a small headwater catchment in an urban area near Tokyo, central Japan. In the base flow period, stream nitrate concentration (<100 µM) was comparable with that of riparian ground water which had less nitrate than unsaturated soil water. Nitrogen isotope analysis showed that the consumption of nitrate by denitrification took place in riparian ground water, suggesting that denitrification is an important process to control nitrate leaching to streams. During rainfall, the concentration of stream nitrate increased up to 400 µM, which was comparable with that of pre-event soil water. The fact that soil water nitrate directly leached to streams indicated that the riparian denitrification process did not work during rainfall because of the rapid discharge of water. A decrease of denitrification effects is a possible reason for high stream nitrate concentration during rainfall.

Fog in the California redwood forest: ecosystem inputs and use by plants.

Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influence the magnitude of fog water input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most frequent, ∼19% of the water within S.sempervirens, and ∼66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S.sempervirens, this fog water input comprised 13-45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, especially in summer when rainfall was absent. Dependence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Interestingly, during the mild El Niño year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Niño year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems and the plants which inhabit them.

An automated system for hydrogen isotope analysis of water

An automated inlet system has been developed for deuterium/hydrogen isotope analysis of water using on-line reduction to hydrogen. A modified HPLC autosampler introduces precise aliquots of water (0.5 to 5 μl) into a heated expansion chamber which is connected via a capillary to hot uranium where the water is reduced to uranium oxide, and hydrogen for analysis. The 600°C uranium reduction furnace is kept at analyzer pressure in the mass spectrometer, which maximizes the lifetime of the reductant to handle more than 4000 injections per 0.5 g of depleted uranium. This is a flow-through system, and unlike zinc or catalyst methods, some `memory' effects are encountered, requiring two injections to obtain a δ D precision of less than ±1‰, and four injections for a precision of less than ±0.5‰. Sample preparation is limited to pipetting water into an autosampler vial. The computer-controlled system provides extended unattended operation allowing, for instance, large runs of samples that can occupy the instrument over entire weekends.

Comparison of two stable hydrogen isotope-ratio measurement techniques on Antarctic surface-water and ice samples

A comparison of the new hydrogen isotope-ratio technique of Vaughn et al. ([Vaughn, B.H., White, J.W.C., Delmotte, M., Trolier, M., Cattani, O., Stievenard, M., 1998. An automated system for hydrogen isotope analysis of water. Chem. Geol. (Isot. Geosci. Sect.), 152, 309–319]; the article immediately preceding this article) for the analysis of water samples utilizing automated on-line reduction by elemental uranium showed that 94% of 165 samples of Antarctic snow, ice, and stream water agreed with the δ 2 H values determined by H 2–H 2O platinum equilibration, exhibiting a bias of +0.5‰ and a 2− σ variation of 1.9‰. The isotopic results of 10 reduction technique samples, however, gave δ 2 H values that differed by 3.5‰ or more, and were too negative by as much as 5.4‰ and too positive by as much as 4.9‰ with respect to those determined using the platinum equilibration technique.

The Vergenoeg magnetite-fluorite deposit, South Africa; support for a hydrothermal model for massive iron oxide deposits

Vergenoeg is a fluorite-bearing massive iron oxide deposit that is genetically related to granites of the Bushveld Complex. It consists of a pipe-shaped body containing primary fayalite, fluorite, apatite, ilmenite, and magnetite that cuts Rooiberg rhyolites and is surrounded by stratiform bodies of felsite and fragmental hematite-fluorite debris. With the exception of its abundant fluorite, Vergenoeg is similar to massive iron oxide deposits such as Kiruna, Pea Ridge, and Cerro Mercado, all of which are related to felsic magmatism. Models suggested for the origin of these deposits can be divided into those invoking separation of an immiscible iron oxide magma from the parent felsic magma and those invoking a hydrothermal solution. Vergenoeg is an excellent place to try to resolve this controversy because the primary fluorite permits study of inclusions of the fluid or magma that formed the ore.Fluorite at Vergenoeg lacks inclusions that might represent an immiscible iron-rich melt but contains abundant aqueous inclusions. Inclusion petrography, heating-freezing measurements, and gas analyses indicate that the primary mineral assemblage at Vergenoeg formed from a high-temperature (>500 degrees C), high-salinity (>67 wt % NaCl equiv) fluid that coexisted with a vapor phase rich in CO 2 . Stable isotope analyses of primary fayalite and titanian magnetite from deep in the Vergenoeg pipe yield calculated water compositions for these temperatures that are typical of magmatic water (delta 18 O (sub H 2 O) = 7-8ppm at 500 degrees C). Alteration of the primary fayalite-fluorite-ilmenite assemblage is widespread in the Vergenoeg pipe. It consists of an early assemblage containing ferroactinolite, grunerite, and titanian magnetite, and later assemblages containing various combinations of siderite, hematite, magnetite, ferropyrosmalite, stilpnomelane, biotite, sphene, quartz, and apatite, which are most common in the upper part of the pipe. Fluid inclusions related to these alteration assemblages homogenize at 150 degrees to 500 degrees C and have salinities of 1 to 35 wt percent NaCl equiv. Stable isotope analyses of hematite and inclusion waters in fluorite suggest that these fluids consist of a mixture of magmatic and meteoric water.These observations suggest that Vergenoeg mineralization formed from hydrothermal fluids of magmatic origin. Comparison of the fluid inclusion record at Vergenoeg to the more fragmentary record at other massive iron oxide deposits supports a magmatic hydrothermal model for this class of deposit.

Quartz-alunite alteration in the western part of the Virginia Range, Washoe and Storey counties, Nevada

Large areas of Miocene volcanic rocks in the western part of the Virginia Range, Washoe and Storey Counties, Nevada, were altered to quartz, alunite, and clay-dominated mineral assemblages, and all significant precious metal and mercury deposits throughout the range, including the Comstock Lode, Ramsey, and Talapoosa districts, occur in or adjacent to extensively altered andesites and dacites of the Miocene Alta and Kate Peak Formations. Radioisotopic analyses of alunite and precious metal vein minerals bracket the ages of quartz-alunite alteration at approximately 16 to 9 Ma, and vein mineralization at approximately 14 to 10 Ma, closely corresponding to Kate Peak Formation eruptive rock ages that range from approximately 15 to 10 Ma. Oxygen, deuterium, and sulfur isotope analyses of alunite, quartz, and water extracted from fluid inclusions in quartz-alunite altered rocks in the western part of the Virginia Range, and fluid inclusion microthermometry, show that the altering fluids were composed of magmatic water and mixtures of meteoric and magmatic water, and that the alteration took place in the temperature range approximately 205 degrees to 265 degrees C. Fluid inclusion water isotopic analyses indicate that Comstock Lode district veins were also deposited by a mixed magmatic-meteoric water fluid, and that subsurface altered intrusions, such as the molybdenite-bearing diorite in the Washington Hill alteration zone, can be identified by isotopic analyses of water in surface samples. Sources and thermal characteristics of fluids involved in quartz-alunite alteration and gold mineralization in the Ramsey district, similar to that at Goldfield, Nevada, and for other deposits in the Virginia Range, have not been determined, but mixingof magmatic and meteoric water at moderate hydrothermal temperatures may have been important in localizing ore.

Genetic and temporal relations between formation waters and biogenic methane: Upper Devonian Antrim Shale, Michigan Basin, USA

Controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane. Natural gas in most conventional deposits has migrated from a source rock to a reservoir, rarely remaining associated with the original or cogenetic formation waters. We investigated an unusual gas reservoir, the Late Devonian Antrim Shale, in which large volumes of variably saline water are coproduced with gas. The Antrim Shale is organic-rich, of relatively low thermal maturity, extensively fractured, and is both source and reservoir for methane that is generated dominantly by microbial activity. This hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas, providing a unique opportunity to characterize methane generating mechanisms. The well-developed fracture network provides a conduit for gas and water mass transport within the Antrim Shale and allows invasion of meteoric water from overlying aquifers in the glacial drift. Steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases; dilute waters grade into dense brines (300,000 ppm) over lateral distances of less than 30 km. Radiogenic ( 14C and 3H) and stable isotope ( 18O and D) analyses of shallow Antrim Shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20,000 yr bp. Carbon isotope compositions of methane from Antrim Shale wells are typical of the established range for thermogenic or mixed gas (δ 13C = −47 to −56‰). However, the unusually high δ 13C values of CO 2 coproduced with methane (∼+22‰) and dissolved inorganic carbon (DIC) in formation waters (∼+28‰) require bacterial mediation. The δD values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis. Methane/[ethane + propane] ratios and δ 13C values for ethane indicate: (1) the presence of a thermogenic gas component that increases basinward and (2) progressive bacterial oxidation of ethane as the Antrim Shale subcrop is approached. Multiple episodes of Pleistocene glaciation over northern Michigan appear critical to the development of these gas deposits. Loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water. The physical erosion cycle of repeated glacial advances and retreats exhumed the Antrim Shale around the northern margin of the Michigan Basin, subjecting it to near-surface physiochemical and biochemical processes. The chemical and hydrologic relations demonstrated in the Antrim Shale reservoir suggest a dynamic connection between Pleistocene glacial history of the midcontinent region and development of recoverable, microbially generated natural gas reserves.

GCM analysis of local influences on ice core δ signals

A high resolution GCM is used to examine the effect of changes in local surface climate parameters on the ice sheets that can influence the interpretation of the isotopic signal of the ice from deep cores. The model suggests that the 10°C difference between the LGM surface temperature deduced from borehole thermometry and that deduced from the water isotope analysis to a great extent may be due to a modification of the precipitation seasonality in central Greenland. For central East Antarctica, the model tends to suggest a weak opposite bias.

Determining the recharge mode of Sahelian aquifers using water isotopes

It is proposed that the drainage network plays an important role in the recharge process of the fractured aquifers in the African Precambrian shield and that the fractured aquifer system is likely to be hydraulically continuous; this contrasts with most previous studies, which suggested direct recharge by rainwater percolation. Two areas were selected in Niger for the study of the aquifer recharge process using isotopic analyses of water ( 2H, 18O, 3H). The first area, centred on the village of Kobio, is the 21-km 2 drainage basin of the Lomona intermittent stream, some 60 km southwest of Niamey (the capital city of Niger). The second area, in the vicinity of Niamey, represents a portion of the Niger basin, draining a surface area many orders of magnitude larger than the Lomona basin. The mean 18O composition of water from all wells in the Kobio aquifer provides evidence for recharge by evaporated water. This is confirmed by the concomitant increase of 18O content with rising static water levels as recharge proceeded. Tritium data suggest progressive aging of the Kobio aquifer water in the flow direction inferred from static water levels, with a down-gradient depletion of 18O composition, suggesting that `enriched' recharge water is progressively mixed with `depleted' aquifer water. Recharge by a reach of the Lomona is proposed to explain these results. In the Niamey area wells, the 18O time series clearly define an injection of evaporated water from the surface into the fractured aquifer. This recharging plume of evaporated water most likely originates from the Niger River. Thus, isotope data for two drainage basins of very different sizes indicate that aquifers are recharged by water from the rivers and that the flow regime of surface waters controls the recharge process.

High‐precision continuous‐flow isotope ratio mass spectrometry

Although high-precision isotope determinations are routine in many areas of natural science, the instrument principles for their measurements have remained remarkably unchanged for four decades. The introduction of continuous-flow techniques to isotope ratio mass spectrometry (IRMS) instrumentation has precipitated a rapid expansion in capabilities for high-precision measurement of C, N, O, S, and H isotopes in the 1990s. Elemental analyzers, based on the flash combustion of solid organic samples, are interfaced to IRMS to facilitate routine C and N isotopic analysis of unprocessed samples. Gas/liquid equilibrators have automated O and H isotopic analysis of water in untreated aqueous fluids as complex as urine. Automated cryogenic concentrators permit analysis at part-per-million concentrations in environmental samples. Capillary gas chromatography interfaced to IRMS via on-line microchemistry facilitates compound-specific isotope analysis (CSIA) for purified organic analytes of 1 nmol of C, N, or O. GC-based CSIA for hydrogen and liquid chromatography-based interfaces to IRMS have both been demonstrated, and continuing progress promises to bring these advances to routine use. Automated position-specific isotope analysis (PSIA) using noncatalytic pyrolysis has been shown to produce fragments without appreciable carbon scrambling or major isotopic fractionation, and shows great promise for intramolecular isotope ratio analysis. Finally, IRMS notation and useful elementary isotopic relationships derived from the fundamental mass balance equation are presented. © 1998 John Wiley & Sons, Inc., Mass Spectrom Rev 16, 227–258, 1997