Andrew L. Herczeg

Environmental Tracers in Subsurface Hydrology

List of Contributors. Preface. Acknowledgements. 1. Determining Timescales for Groundwater Flow and Solute Transport P.G. Cook, J.-K. Bohlke. 2. Inorganic Ions as Tracers A.L. Herczeg, W.M. Edmunds. 3. Isotope Engineering - Using stable isotopes of the water to solve practical problems T.B. Coplen, et al. 4. Radiocarbon Dating of Groundwater Systems R.M. Kalin. 5. Uranium-Series Nuclides as Tracers in Groundwater Hydrology J.K. Osmond, J.B. Cowart. 6. Radon-222 L. DeWayne Cecil, J.R. Green. 7. Sulphur and Oxygen Isotopes in Sulphate R. Krouse, B. Mayer. 8. Strontium Isotopes R.H. McNutt. 9. Nitrate Isotopes in Groundwater Systems C. Kendall, R. Aravena. 10. Chlorine-36 F.M. Phillips. 11. Atmospheric Noble Gases M. Stute, P. Schlosser. 12. Noble Gas Radioisotopes: 37Ar, 85Kr, 39Ar, 81Kr H.H. Loosli, et al. 13. 3H and 3He D.K. Solomon, P.G. Cook. 14. 4He in Groundwater D.K. Solomon. 15. Chlorofluorocarbons L.N. Plummer, E. Busenberg. 16. delta11B, Rare Earth Elements, delta37Cl, 32Si, 35S, 129I A. Vengosh, et al. Appendices. Index.

Water balance of a tropical woodland ecosystem, Northern Australia: A combination of micro-meteorological, soil physical and groundwater chemical approaches

A combination of micro-meteorological, soil physical and groundwater chemical methods enabled the water balance of a tropical eucalypt savanna ecosystem in Northern Australia to be estimated. Heat pulse and eddy correlation were used to determine overstory and total evapotranspiration, respectively. Measurements of soil water content, matric suction and water table variations were used to determine changes in soil moisture storage throughout the year. Groundwater dating with chlorofluorocarbons was used to estimate net groundwater recharge rates, and stream gauging was used to determine surface runoff. The wet season rainfall of 1585 mm is distributed as: evapotranspiration 810 mm, surface runoff (and shallow subsurface flow) into the river 410 mm, groundwater recharge 200 mm and increase in soil store 165 mm. Of the groundwater recharge, 160 mm enters the stream as baseflow in the wet season, 20 mm enters as baseflow in the dry season, and the balance (20 mm) is distributed to and used by minor vegetation types within the catchment or discharges to the sea. In the dry season, an evapotranspiration of 300 mm comprises 135 mm rainfall and 165 mm from the soil store. Because of the inherent errors of the different techniques, the water balance surplus (estimated at 20 mm) cannot be clearly distinguished from zero. It may also be as much as 140 mm. To our knowledge, this is the first time that such diverse methods have been combined to estimate all components of a catchments water balance.

Inorganic Ions as Tracers

As water moves into the ground it begins to record information on the history of its recharge source and properties, mainly from rainfall solutes as well as isotopic ratios of the water molecule. The subsurface accepts water at variable rates of movement through the soil, via the unsaturated zone, to the water table. At this stage the groundwater composition undergoes significant modification due to two major processes: an increase in the concentration of atmospheric solutes due to removal of water via plant uptake and evaporation; and reactions between water and rock, leading to the build-up of dissolved substances with different relative ion concentrations to the atmospheric input. The principal and distinctive characteristics of groundwater are mainly established in the unsaturated zone. In the saturated zone the geochemical evolution, though less intense than in the soil and unsaturated zones, follows progressive changes in water quality towards areas of discharge. These processes are time-dependent and the chemical changes as well as isotopic variations may be used to identify this evolution and provide information on water flow paths.

A comparison of groundwater dating with 81Kr, 36Cl and 4He in four wells of the Great Artesian Basin, Australia

The isotopic ratios 81 Kr/Kr and 36 Cl/Cl and the 4 He concentrations measured in groundwater from four artesian wells in the western part of the Great Artesian Basin (GAB) in Australia are discussed. Based on radioactive decay along a water flow path the 81 Kr/Kr ratios are directly converted to groundwater residence times. Results are in a range of 225^400 kyr with error bars in the order of 15% primarily due to counting statistics in the cyclotron accelerator mass spectrometer measurement. Additional uncertainties from subsurface production and/or exchange with stagnant porewaters in the confining shales appear to be of the same order of magnitude. These 81 Kr ages are then used to calibrate the 36 Cl and the 4 He dating methods. Based on elemental analyses of rock samples from the sandstone aquifer as well as from the confining Bulldog shale the in situ flux of thermal neutrons and the corresponding 3 He/ 4 He and 36 Cl/Cl ratios are calculated. From a comparison of: (i) the 3 He/ 4 He ratios measured in the groundwater samples with the calculated in situ ratios in rocks and (ii) the measured N 37 Cl ratios with the 4 He concentrations measured in groundwater it is concluded that both helium and chloride are most likely added to the aquifer from sources in the stagnant porewaters of the confining shale by diffusion and/or mixing. Based on this ‘working hypothesis’ the 36 Cl transport equation in groundwater is solved taking into account: (i) radioactive decay, (ii) subsurface production in the sandstone aquifer (with an in situ 36 Cl/Cl ratio of 6U10 315 ) and (iii) addition of chloride from a source in the confining shale (with a 36 Cl/Cl ratio of 13U10 315 ). Lacking better information it is assumed that the chloride concentration increased linearly with time from an (unknown) initial value Ci to its

Controls on δ34S and δ18O of dissolved sulfate in aquifers of the Murray Basin, Australia and their use as indicators of flow processes

The usefulness of stable isotopes of dissolved SO4 (δ34S and δ18O) to study recharge processes and to identify areas of significant inter-aquifer mixing was evaluated in a large, semi-arid groundwater basin in south-eastern Australia (the Murray Basin). The distinct isotopic signatures in the oxidizing unconfined Murray Group Aquifer and the deeper reducing Renmark Group confined aquifer may be more sensitive than conventional chemical tracers in establishing aquifer connections. δ34S values in the unconfined Murray Group Aquifer in the south and central part of the study area decrease along the hydraulic gradient from 20.8 to 0.3‰. The concomitant increasing SO4/Cl ratios, as well as relatively low δ18OSO4 values, suggest that vertical input of biogenically derived SO4 via diffuse recharge is the predominant source of dissolved SO4 to the aquifer. Further along the hydraulic gradient towards the discharge area near the River Murray, δ34S values in the unconfined Murray Group Aquifer increase, and SO4/Cl ratios decrease, due to upward leakage of waters from the confined Renmark Group Aquifer which has a distinctly low SO4/Cl and high δ34S (14.9–56.4‰). Relatively positive δ34S and δ18OSO4 values, and low SO4/Cl in the Renmark Group Aquifer is typical of SO4 removal by bacterial reduction. The S isotope fractionation between SO4 and HS− of ∼24‰ estimated for the confined aquifer is similar to the experimentally determined chemical fractionation factor for the reduction process but much lower than the equilibrium fractionation (∼70‰) even though the confined groundwater residence time is >300 Ka years. Mapping the spatial distribution of δ34S and SO4/Cl of the unconfined Murray Group Aquifer provides an indicative tool for identifying the approximate extent of mixing, however the poorly defined end-member isotopic signatures precludes quantitative estimates of mixing fractions.

Isotope Engineering—Using Stable Isotopes of the Water Molecule to Solve Practical Problems

This chapter discusses the use of the stable isotope ratios of hydrogen and oxygen (2H/1H and 18O/16O) to address problems related to groundwater as a sustainable resource, and in particular to recharge, delineation of flow systems and quantification of mass-balance relationships (relative amounts of water from various sources) in applied hydrologic investigations. We will attempt to cover many examples from throughout the world. This chapter is written for the hydrologist who needs to solve a problem. We present just enough theory to make this chapter a stand-alone document, followed by several real-world examples of the uses of stable H and O isotope ratios for solving practical hydrologic problems—thus, the title, isotope engineering.

Isotope studies in large river basins: A new global research focus

Rivers are an important linkage in the global hydrological cycle, returning about 35%of continental precipitation to the oceans. Rivers are also the most important source of water for human use. Much of the worlds population lives along large rivers, relying on them for trade, transportation, industry, agriculture, and domestic water supplies. The resulting pressure has led to the extreme regulation of some river systems, and often a degradation of water quantity and quality For sustainable management of water supply agriculture, flood-drought cycles, and ecosystem and human health, there is a basic need for improving the scientific understanding of water cycling processes in river basins, and the ability to detect and predict impacts of climate change and water resources development.

The importance of silicate weathering of a sedimentary aquifer in arid Central Australia indicated by very high 87Sr/86Sr ratios

With the exception of brine systems and crystalline-rock aquifers, groundwater 87Sr/86Sr ratios rarely differ by more than ±0.01 from the Phanerozoic seawater value of 0.70923. Groundwater 87Sr/86Sr ratios from Central Australia (0.72562–0.76248) are the highest ever recorded for a low salinity, sedimentary aquifer. We propose a model for the evolution of these 87Sr/86Sr ratios and demonstrate how the lowest 87Sr/86Sr ratios reflect water–rock interactions with predominantly carbonate minerals, while the highest 87Sr/86Sr ratios are ultimately controlled by weathering of old primary silicate minerals. Detailed analysis of aquifer core material identified albite and microcline as the most prevalent primary silicates and where these minerals were abundant, relatively high Rb concentrations were measured. High Rb combined with long time scales since deposition has produced very high 87Sr/86Sr ratios in the rocks and subsequently the groundwater. Therefore, strontium isotopes can provide further constraints for chemical models of arid zone groundwater systems, particularly when silicate hydrolysis is believed to significantly contribute to dissolved solute compositions.

Past changes to isotopic and solute balances in a continental playa: clues from stable isotopes of lacustrine carbonates

In many salt lakes around the world, the relative abundance of preserved authigenic minerals is different from that predicted from solute mass balance calculations. Conventional mass balance models assume that chloride behaves conservatively over long periods of time and fail to take into account the role of diffusion, deflation and fractional crystallisation/dissolution of salts. An alternative approach is to use oxygen isotopes as these reflect directly water molecule rather than solute concentrations and have the added advantage of providing a palaeohydrological record in lacustrine carbonates. We present a steady-state, stable-isotope model, in conjunction with stable isotopic measurements of sub-surface brines, regional groundwaters and carbonate deposits from the Lake Malata–Lake Greenly playa complex in South Australia, to estimate the apparent leakage and palaeoleakage from these superficially closed playa lakes. The steady-state model calculations, using the present and compositions of the lake brines and inflowing groundwater, suggest that the apparent present-day leakage for the complex is 75 to 90% of inflow (∼35 times that calculated from Cl− and Br−). Under such conditions, only low magnesian calcite precipitates and the lake water experiences reduced effects of evaporation, gas and vapour exchange and, consequently, reduced isotopic and chemical enrichment. Further, our model shows that calcite becomes increasingly Mg-rich until leakage is reduced to ∼55 to 70% of inflow — a condition favourable for dolomitisation. and of the lacustrine carbonates show excursions on the order of 5‰ over the length of a 2.3 m core, indicating that the lake complex has varied from being throughflow dominated (presence of low Mg-calcite relatively depleted in and ) to evaporation dominated (high Mg-calcite/dolomite relatively enriched in and ) throughout late Quaternary. Our estimates of leakage fractions are consistent with the observed mineralogical suite, but there remains a discrepancy between apparent closure indicated by the presence of the highly saline brine reservoir (<1% leakage) and high rates of throughflow inferred from stable isotope data. We propose that the brine was formed by winter time re-solution of a seasonal halite crust which forms during summer dominated evaporative discharge. Recirculation of the secondary brine, and mixing with regional groundwater may decouple the solute cycle from the water cycle in many playa lakes. The end result is a partially mixed brine characterised by nearly conserved solutes but with isotopic signatures indicative of brine–rainfall–groundwater interactions.

Global network is launched to monitor isotopes in rivers

The Global Network of Isotopes in Rivers (GNIR), launched by the International Atomic Energy Agency (IAEA) in 2007, compiles water isotope data on rivers to complement the 45-year-old IAEA/World Meteorological Organization (WMO) global network of isotopes in precipitation (GNIP). River runoff carries an integrated memory of hydrological processes in a basin. Recent studies [e.g., Vorosmarty and Meybeck, 2004] suggest that the impacts of storages, diversions, and redirection of streamflow for water supply, hydropower, and irrigation might surpass the impact of recent and anticipated future climate changes on river runoff. Consequences of these effects include changes in the frequency and extent of flooding, increased sediment load, altered groundwater recharge, and degradation of water quality and riparian ecosystems. These consequences often result in political disputes or upstream-downstream inequities. GNIR aims to provide an improved understanding of stream/aquifer interactions, the impacts of climate changes on river runoff, and especially human impacts on river discharge with the use of isotope data.