U.S. Geological Survey, Reston, Virginia, USA
During the past 40 years, a variety of methods have been developed that can provide information on the age of young groundwater (0-50 year timescale) [1]. Groundwater age refers to the time elapsed since recharge, but is model dependent, being based on an interpretation of measured concentrations of environmental tracers in groundwater samples.
As a reference point, an “apparent age”, which assumes unmixed samples (piston flow) is often reported, although a number of mathematical models have been developed that can be used to interpret mean age (residence time) of water that discharges from a groundwater reservoir. Other applications incorporate environmental tracer data in the calibration of numerical models of groundwater flow.
Environmental tracers that have proven most useful in providing groundwater age information have an atmospheric source and can be grouped according to (1) those based on measurement of the concentrations of both parent and daughter isotopes, such as in applications of 3H/3He in groundwater, (2) those based on the measurement of the activity of a single radionuclide in groundwater, such as in applications of 3H and 85Kr in groundwater dating, and (3) those based on measurement of the concentration of anthropogenic gases in groundwater, such as in applications of chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6). In the first case, the initial concentration of the radionuclide is reconstructed from the measured concentrations of the parent and daughter isotopes and age is then determined from the decay equation. The second case requires a priori definition of the initial concentration of the radionuclide recharged to the aquifer, and then age is estimated from the measured concentration and the decay equation. In the third case, age information is derived from a prior knowledge of the atmospheric input function of an anthropogenic gas, its solubility in water, and the measured concentration in the water sample. Each method has advantages and limitations, and therefore, a multi-tracer approach is recommended.
The 3H/3He age is based on an isotope mass-balance calculation that determines the amount of tritiogenic 3He in the water sample, and has been applied to dating waters from the late 1960s to modern. Several conditions are necessary to calculate and interpret the age, including detectable 3H (greater than approximately 0.5 TU), and relatively low concentrations of terrigenic helium.
The source for atmospheric 85Kr is primarily from reprocessing of fuel rods from nuclear reactors. Because of difficulties in collection and analysis, 85Kr has not yet been widely used in groundwater studies, though it has considerable potential for dating on the 0-30 year timescale. In CFC-contaminated environments, or in anoxic environments, noble-gas dating techniques such as those based on 3H/3He and 85Kr measurements are usually more reliable than those based on CFCs. However, because of their low detection limit, CFCs can be detected in water from the late 1940s.
Sulfur hexafluoride (SF6) is primarily of anthropogenic origin but also occurs naturally in some aquifers that contact crystalline rocks. The troposphere mixing ratio of SF6 has increased from a steady-state value of 0.054±0.009 to about 5 parts per trillion during the
past 40 years. The history of SF6 mixing ratios in the atmosphere is now well established, with values currently increasing at about 6 percent per year; whereas mixing ratios of CFCs are nearly constant or decreasing, resulting in ambiguity in ages based on CFCs in waters recharged since the mid-1990s. CFCs have a useful dating range of about 1950 to the early 1990s. The potential dating range of water with SF6 is from about 1970 to the present, and the SF6 method is particularly useful in dating very young (post-1993) groundwater, if there are no terrigenic sources of SF6.
In bimodal (binary) mixtures of young and old (pre-tracer) water, the first case (such as for
3H/3He dating) yields directly the age of the young fraction in the mixture, but with the second (3H, 85Kr) and third cases (CFCs SF6, etc.), the measured concentrations must be corrected for dilution before the age of the young fraction can be estimated. Ratios of CFCs or SF6/CFC ratios have proved useful in dating young fractions in some groundwater mixtures. Other models with other age distribution functions have been applied to interpret groundwater residence times in aquifers.
Knowledge about the age of groundwater can be used to define recharge rates, refine hydrologic models of groundwater systems, reconstruct historical loadings of contaminants to aquifers, predict contamination potential, and estimate the time needed to flush contaminates from groundwater systems. Detection of environmental tracers in groundwater can be used to trace seepage from rivers into groundwater systems, provide diagnostic tools for detection and early warning of leakage from landfills and septic tanks, and can be used to assess susceptibility of water-supply wells to contamination from near-surface sources.
REFERENCE:
[1] COOK, P. and HERCZEG, A. (Eds.) “Environmental Tracers in Subsurface Hydrology”, Kluwer Academic Publishers, 529p.
IAEA-CN-104/154 STABLE HYDROGEN- AND OXYGEN-ISOTOPE BEHAVIOUR OF SOIL WATER IN SAND DUNES, AND ITS RELATIONSHIP TO SHALLOW GROUNDWATER, SOUTHERN GREAT LAKES REGION, CANADA
F.J. LONGSTAFFE, K.L. GAGE
Department of Earth Sciences, The University of Western Ontario, London, Ontario, Canada E.A. WEBB
School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada
The stable hydrogen- and oxygen-isotope compositions of soil water from seven profiles, up to 200 cm in depth, were measured ten times over fourteen months for a sand dune complex located on the southeastern shore of Lake Huron, southwestern Ontario, Canada. The profiles represent settings ranging from stabilized dunes covered by oak savannah to unstabilized dunes (both barren and grass-covered, with separate sites for C3 versus C4 vegetation), and topography varying from dune crest to slope to slack (the low area between dunes).
The stable hydrogen- and oxygen-isotope compositions of the soil water commonly show measureable differences from incoming precipitation each month. The soil water is generally, but not always, enriched in 2H and 18O relative to precipitation. The soil-water compositions fluctuate widely with depth and season (δ2H = -132 to –13‰; δ 18O = -18.0 to +0.6‰). By comparison, monthly composite precipitation samples range from delta2H = -97 to –26‰, and δ 18O = -14.1 to –4.8‰, and describe a local meteoric water line of δ2H = 7.8(δ
18O) + 9.1. During the year, the stable hydrogen and oxygen isotopic compositions of Lake Huron (δ 2H = -57±3‰; δ18O = -7.3±0.3‰) and shallow groundwater (δ2H = -77±4‰; δ 18O
= -11.1‰) remained relatively constant.
The results illustrate the complexity in the seasonal distribution of soil-water stable isotope compositions in a temperate, humid, continental setting, and the difficulty in estimating precipitation compositions from such data. Understanding seasonal variability is particularly important when assessing the stable isotopic compositions of proxies for climate that utilize soil water in their formation. For example, average annual stable isotopic compositions for soil water may be more relevant to interpretation of results for pedogenic carbonates and clay minerals than they are to silica phytoliths, which form only during certain parts of the growing season. The depth and timing of pedogenic mineral formation is also important, given that soil water can show significant depth-dependent variations in stable hydrogen and oxygen isotopic compositions. Likewise, rooting depth can be important to understanding the stable hydrogen and oxygen isotopic compositions of phytoliths and other phases formed in plant tissues.
The stable hydrogen and oxygen isotopic compositions of soil water in the study area reflect mixing between precipitation recharge and antecedent soil water, plus the processes of evaporation and (to a lesser extent) transpiration, particularly during summer months. Mixing causes the wide seasonal variations in the isotopic composition of precipitation to be attenuated with depth in the soil profile, but the downward movement of significant precipitation events can still be tracked from month to month. For most months, enrichment in 2H and 18O from evaporation is evident only in the top 10 cm of each profile. However, during the periods of highest average temperature (~25°C) and average minimum daily
relative humidity (~50%), some effects of evaporation can be discerned to depths of 60 cm, depending on the amount of monthly rainfall and the timing of sampling relative to the last major rainfall event. Uptake of soil water by plants does not result directly in a change in the isotopic composition of the soil water, but it has a modest indirect effect by reducing the volume of soil water, rendering it more susceptible to evaporative enrichment in 2H and 18O.
The presence of vegetation tends to decrease the amount of direct evaporation from the soil surface, and through its subsurface root and rhizome network, promote mixing within the soil- water system. Systematic variations in soil-water content are most closely associated with rainfall amount, slope, vegetation extent and tree canopy cover. Some variations are also associated with seasonal differences in productivity of C3 versus C4 grasses. However, variations in the stable isotopic composition of soil water between heavily vegetated and lightly vegetated sites are not as large or as systematic as the differences in water content.
Instead, the stable hydrogen- and oxygen-isotope compositions of soil water from all profiles describe similar patterns for a given month. For each season, there is a distinctive shape to the depth-dependent variation in soil-water stable isotopic values. This reflects characteristic seasonal differences in the relative influence of spring snowmelt, precipitation, antecedent soil water, evaporation and transpiration on soil-water composition.
IAEA-CN-104-155 STABLE ISOTOPES ON SEASONAL TO MILLENNIAL TIME SCALES AS RECORDED IN LOW-LATITUDE, HIGH-ALTITUDE ICE CORES
L. G. THOMPSON, M.E. DAVIS, E. MOSLEY-THOMPSON, P-N. LIN, T.A.
MASHIOTTA, K. HENDERSON
Department of Geological Sciences, Byrd Polar Research Centre, The Ohio State University, Columbus, OH 43210, USA
This paper examines the stable isotopic ratios, 18O/16O (δ18Oice) and 2H/1H (δDice), preserved in mid to low-latitude glaciers as tools for paleoclimate reconstruction. Ice cores are particularly valuable as they contain additional data (such as dust concentrations, aerosol chemistry, and accumulation rates) that can be combined with the isotopic information to assist with inferences about the regional climate conditions prevailing at the time of deposition. We use a collection of multi-proxy ice core histories to explore the stable isotopic climate relationship on seasonal, decadal, centennial, and millennial timescales. The paper looks closely at the relationship between temperature-precipitation and stable isotopes over these diverse time perspectives. Stable isotopic variations in ice cores from the tropics are highly correlated with sea surface temperatures (SSTs) across the equatorial Pacific Ocean, which are closely linked to ENSO variability. Therefore, a network of ice cores from selected locations offers the prospect of reconstructing low-latitude circulation.
Decadally-averaged stable isotopes records from three Andean and three Tibetan ice cores are combined in a composite in order to present a low-latitude stable isotope history for the last two millennium. Comparisons of this composite over the last millennium are made with the Northern Hemisphere proxy record (1000-2000 A.D.) reconstructed by Mann et al. (1999) and measured temperatures (1856-2000 A.D.) reported by Jones et al. (1999). The ice cores evidently have captured a great deal of the decadal-scale variability in the global temperature trends. The ice core record shows a 20th century isotopic enrichment that suggests that a large scale warming is underway at low latitudes. The rate of this isotopically-inferred warming is amplified at higher elevations over the Tibetan Plateau, while amplification in the Andes is latitude-dependent with enrichment (warming) increasing equator-ward. In concert with this apparent warming, in situ observations reveal that tropical glaciers are currently disappearing.
A brief overview of the loss of these tropical data archives over the last 30 years is presented, along with evaluation of recent changes in mean stable isotopic values. The isotopic composition of precipitation should be viewed not only as a powerful proxy indicator of climate change, but also as an additional parameter to aid our understanding of the linkages between changes in the hydrological cycle and global climate.
IAEA-CN-104/156 INTERACTION BETWEEN THE GEOTHERMAL OUTFLOW OF SOUTHERN NEGROS GEOTHERMAL FIELD AND THE SHALLOW GROUNDWATER AQUIFER IN DUMAGUETE CITY, NEGROS ORIENTAL, PHILIPPINES
J.A. CARANTO, P.I. PAMATIAN and M.S. OGENA
PNOC-Energy Development Corporation, Geoscientific Department, Geothermal Division, Fort Bonifacio, Makati City 1201 Philippines
Chemical and isotopic data indicate that significant quantity of mineralized thermal fluids are present downstream of the Palinpinon thermal spring areas that are naturally migrating into the shallow groundwater aquifer. Water district wells 49, 53, 54 and 55 located east from the Palinpinon springs, are tapping fluids that are relatively enriched in Na, K, Cl, SO4, B and Li ions. Fluids appear to be diluted towards the Sibulan area as waters become diluted to Ca+Mg-Cl+SO4 type. Shallow water southeast from the Palinpinon hot springs in the area of well 47 is composed of typical Ca+Mg-HCO3 groundwater. Isotopically, the shallow groundwater in the vicinity of well 54 is relatively enriched in δ18O and δ2H while in the well 47 area the waters are relatively depleted. Isotopic dilution lines reveal that well 55, which is receiving groundwater recharged at relatively lower elevations, mixes with thermal waters from springs Pal3 and 40. The other wells 49, 53 and 54 are being recharged by groundwater at relatively the same elevation, and this groundwater mixes with the other Palinpinon hot springs resulting to the enrichment of heavy isotopes. Slight variations in the stable isotope composition of the shallow groundwater were observed from 1999 to 2002, but the difference is not as distinct as the variations of the heavy isotopes in rainfall. Isotopic altitude gradient for δ18O and δ2H are 0.28 and 2.1°/oo per 100 meters change in elevation, respectively. These correspond to a calculated recharge elevation of at least 1000masl for the shallow groundwater. Relative age dating using chlorofluorocarbon (CFC) reveals relative ages from 10 to older than 60 years old, which partly confirm the previous Tritium age of 50 to 100 years old.
Numerical simulation models confirm the migration of the thermal fluids from Palinpinon hot springs to the groundwater wells in the vicinity of well 54. Seasonal variation in isotope, Cl and SO4 concentration indicate only minor dilution effect from precipitation. Drawdown in the deep geothermal reservoir have induced more than 500 meters of drawdown in the center of the resource but not enough to revert the naturally outflowing fluids from the Palinpinon thermal springs. Hence, there exists continuous natural migration of slightly mineralized geothermal fluids into the shallow groundwater aquifer of Dumaguete City.
IAEA-CN-104/157 AUSTRIAN NETWORK OF ISOTOPES IN PRECIPITATION (ANIP): QUALITY ASSURANCE AND CLIMATOLOGICAL PHENOMENON IN ONE OF THE OLDEST AND DENSEST NETWORKS IN THE WORLD