Environmental Tracers in Subsurface Hydrology

One of the principal uses of environmental tracers is for determining the ages of soil waters and groundwaters. (We may refer to this as ‘hydrochronology’by analogy with the dating of solid materials known as geochronology.) Information on soil water and groundwater age enables timescales for a range of subsurface processes to be determined. For example, ‘groundwater stratigraphy’is used increasingly to decipher past recharge rates and conditions in unconfined aquifers, in much the same way that sedimentary stratigraphy yields information about past depositional environments. The use of environmental tracers to determine water ages allows groundwater recharge rates and flow velocities to be determined independently, and commonly more accurately, than with traditional hydraulic methods where hydraulic properties of aquifers are poorly known or spatially variable. Studies of groundwater residence times in association with groundwater contamination studies can enable historic release rates of contaminants and contaminant transport rates to be determined. Where input rates are known, measurements of groundwater contaminant concentrations, together with groundwater dating, can sometimes be used for estimating chemical reaction rates. The combination of these dating methods with stable isotope measurements has sometimes allowed changes in contaminant sources over time to be determined.

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.

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.

Groundwater is an increasingly important water resource in arid or semi-arid regions, as well as a conjunctive resource in humid environments. Because of the long residence time for groundwater in the hydrologic cycle, the last few decades have seen expanding study of groundwater systems. It is therefore important to continually refine our interpretation of hydrogeologic, geochemical and isotopic data to better understand the spatial and temporal movement of water in the subsurface. With our ever-increasing understanding of the magnitude of climate variations during the last 40 000 years and the impact of our industrialised society on groundwater quality and quantity, hydrogeologists will continue to require more information about the rate of groundwater movement on scales from the subannual to millenium. The 5730 year half-life of ¹⁴C and the ubiquity of carbon (as organic and inorganic forms) in groundwater, makes it a potentially ideal tracer on these timescales.

The occurrence of the heavy radionuclides in the hydrosphere has become increasingly important in the context of today’s emphasis on the measurement of quality and quantity of water resources. The study of the natural aqueous behaviour of uranium, radium, and the shorter-lived daughters serves both as a background for radioactivity pollution studies, and also as a widely applicable method of tracing movements of the groundwater itself.

Uranium-238 (²³⁸U) is a natural radioactive element that is present in all rocks and soils in various concentrations. Decay of ²³⁸U through a series of shorter-lived radionuclides eventually produces radium-226 (²²⁶Ra), which has a half-life of 1620 years. Radium-226 decays by alpha-particle emission directly to radon-222 (²²²Rn), which is short-lived (half-life = 3.82 days). Two other isotopes of radon are formed in natural decay chains, one from thorium-232 and one from uranium-235: radon-220 (²²⁰Rn) with a half-life of 56 seconds and radon-219 (²¹⁹Rn) with a half-life of 4 seconds, respectively. Radon-222 has a long enough half-life to make it useful in geohydrologic studies, whereas the half-lives of ²²⁰Rn and ²¹⁹Rn are too short to make them useful as tracers in environmental investigations.

The origin and the fate of sulphate in groundwater has concerned hydrogeologists for many decades. With increasing industrialisation throughout this century, the differentiation between natural and ‘man-made’sulphate became important. A particular challenge is evaluating potential and existing additions of the latter to groundwater and drinking water supplies. On regional and even global scales, sulphate as a component of ‘acid rain’captured scientific and public awareness in the 1970s and 1980s. Subsequently, intensive research programs on the effects of atmospheric sulphate deposition on terrestrial and aquatic ecosystems were initiated. Today, more than two-thirds of the sulphate in atmospheric deposition in heavily industrialised regions of the northern hemisphere is of anthropogenic origin (Section 7.3.2.4). The rate and the degree to which these increased sulphate inputs have led to acidification of surface water and groundwater has been a matter of considerable scientific debate in the past two decades.

The ⁸⁷Sr/⁸⁶Sr ratio has been extensively used in groundwater studies for several reasons. While classified geochemically as a trace element, Sr is found in easily measurable quantities in a wide variety of rocks. It is soluble in aqueous solution as the +2 ion and is geochemically very similar to Ca, the latter a major alkaline-earth constituent of rocks. The half-life of ⁸⁷Rb, which decays to ⁸⁷Sr, is such that considerable variations exist in the present-day ⁸⁷Sr/⁸⁶Sr ratio in minerals. Finally, Sr isotopes are not measurably fractionated by geological processes, unlike the isotopes of the light elements. All these characteristics potentially make Sr a good tracer of the source rock(s) of the chemical constituents in water. The isotopic value provides an added insight to that achieved by analysing elemental abundances, and this is especially the case when there is precipitation of new mineral phases because this process changes the chemistry of the water, but not the isotopic composition.

Nitrate contamination, often associated with agricultural activities, is a major problem in some shallow aquifers and is increasingly becoming a threat to groundwater supplies (Gillham and Cherry, 1978; Ronen et al., 1983; Spalding and Exner, 1991). The intake of high levels of nitrate can cause methemoglobinemia in infants, and there is substantial evidence collected from animal experiments that N-nitroso compounds are carcinogens. Similar conclusive evidence is not yet available for humans but many observations suggest that these compounds can function as initiators of human carcinogenesis. These findings are the basis for the maximum permissible limit of 10 ppm nitrate-N (50 ppm as NO₃) in drinking water set by the World Health Organization and the U.S. Environmental Protection Agency. The impact of high loading of nutrients such as nitrate and phosphorous from agricultural practices via groundwater into surface water is also a major environmental concern, causing eutrophication of streams, rivers and lakes (Hill, 1978; Böhlke and Denver, 1995).

The chloride anion is an extraordinarily stable ionic species. Chloride is the thermodynamically favoured form of the element under virtually all terrestrial aqueous conditions. Furthermore, the negative charge of the chloride anion discourages adsorption onto silicate surfaces, which are also typically negatively charged. Due to this behaviour, chloride, once introduced to natural water, is usually advected at the same rate as the water and is not normally removed from the water by geochemical processes. These properties have led to extensive use of Cl as a hydrological tracer (see Herczeg and Edmunds, Chapter 2). The utility of the Cl-tracer has been greatly extended by the recognition that the element has a long-lived radioactive isotope: ³⁶C1. With a half-life of 301 000 ± 4 000 years (Endt and Van der Leun, 1973; Bentley et al., 1986a), ³⁶C1 can be used to date groundwater with subsurface residence times up to one million years, and also has a myriad of uses for tracing subsurface water at shorter time scales. Most of these applications have been realised within the past 20 years.

A large fraction of gases dissolved in surface and groundwater, mainly N₂, O₂ and the noble gases He, Ne, Ar, Kr and Xe, originate from the atmosphere. Whenever water comes into contact with the atmosphere, or other phases such as natural gas, oil and solid organic matter, gases are exchanged and gas concentrations of the individual phases record some characteristics of these processes. Even in the absence of a separate gas phase, dissolved gas concentrations in a water parcel may change as a result of molecular diffusion, and mixing on a variety of space and time scales. However, in many cases the impacts of most of these processes on the dissolved gas concentrations are small and the dominating processes may be reconstructed from the measured concentrations of the dissolved gases. This chapter deals with atmospheric noble gases dissolved in groundwater, which reliably record information on certain physical processes due to the lack of chemical reactions which affect them. As has been shown in studies performed over the past 40 years, atmospheric noble gases dissolved in groundwater yield valuable information on palaeoclimate, in particular temperature at the time of recharge, dynamics of groundwater flow, and denitrification and oxygen consumption rates.

Noble gases are chemically inert and therefore at least some of the complications that are common in interpreting isotope data are absent when working with such tracers. The dependence of their solubility on temperature makes it possible to determine the recharge temperature of recent and paleo groundwaters (Stute and Schlosser; Chapter 11). In this chapter we focus on specific radioactive isotopes of two noble gases, argon and krypton, which occur in trace quantities. These isotopes provide information on when recharge took place, because of their known source functions and decay rates. Four nuclides exist that have been measured in groundwater: ³⁷Ar (half-life 35 days), ⁸⁵Kr (half-life 10.76 years), ³⁹Ar (half-life 269 years) and ⁸¹Kr (half-life 229 000 years). Unfortunately, the concentrations of all four isotopes are very small in subsurface waters and consequently analytical procedures are rather complicated. Nevertheless, very valuable results have been achieved in a number of groundwater studies. Since the number of dating tools is quite limited in hydrogeology, any additional information is welcome even if the analytical effort is considerably higher than for some other tracers. In particular, noble gas radioisotopes have been used in combination with more conventional groundwater dating methods, to investigate mixing of waters of different ages: ³H and ⁸⁵Kr are applicable for young groundwaters (<40 yrs) (this combination is especially useful since the two input functions are different), ³⁹Ar and ¹⁴C are appropriate for older groundwaters (50–20 000 yrs) and ^8lKr and ³⁶C1 for very old groundwaters (up to 10⁶ yrs). Where such mixing occurs, the use of a single tracer will never be sufficient to characterise the age distribution. In general, as many tracers as possible should be used to cover the time ranges considered to be adequate for a specific hydrogeological situation.

Tritium (³H) is the only radioactive isotope of hydrogen, and has a half-life of 12.43 years (Unterweger et al., 1980). Large quantities of tritium were introduced into the hydrological cycle by atmospheric thermonuclear testing in the 1950s and 1960s, providing a useful environmental tracer for water originating from this period. Tritium decays by beta-emission to ³He, the rare, stable isotope of helium. Under favourable conditions, measurements of both ³H and ³He in groundwater allow the reconstruction of tritium concentrations in precipitation and the determination of water flow paths. Ratios of ³H to ³He can be applied to quantify the extent of radioactive decay, and hence determine subsurface water residence times up to 40 years.

Helium-4 is produced within the Earth by the decay of ²³⁸U, ²³⁵U and ²³²Th. Shortly after the discovery of the radioactivity of U and Th, the idea of using the accumulation of He in minerals as a dating tool was proposed by Ernest Rutherford (Hurley, 1954). The U-He dating method for rocks is based on the assumption that U- and Th- bearing minerals quantitatively retain the He produced within them. However, comparison of U-He dates with other methods (e.g., K-Ar) has shown that U-He ages frequently underestimate the true age of the sample as a result of incomplete He retention.

Chlorofluorocarbons (CFCs) are stable, synthetic, halogenated alkanes, developed in the early 1930s as safe alternatives to ammonia and sulphur dioxide in refrigeration. Production of CFC-12 (dichlorodifluoromethane, CF₂C1₂) began in 1931 followed by CFC-11 (trichlorofluoromethane, CFC1₃) in 1936. Many other CFC compounds have since been produced, most notably CFC-113 (trichlorotrifluoroethane, C₂F₃C1₃). CFCs are nonflammable, noncorrosive, nonexplosive, very low in toxicity, and have physical properties conducive to a wide range of industrial and refrigerant applications. Primary uses of CFC-11 and CFC-12 include coolants in airconditioning and refrigeration, blowing agents in foams, insulation, and packing materials, propellants in aerosol cans, and as solvents. CFC-113 has been used primarily by the electronics industry in manufacture of semiconductor chips, in vapour degreasing and cold immersion cleaning of microelectronic components, and as a solvent in surface cleaning procedures (Jackson et al., 1992). Release of CFCs to the atmosphere and subsequent incorporation into the Earth’s hydrologie cycle has closely followed production. For example, it has been estimated that CFC-11 and CFC-12 produced for aerosol propellants were released, on average, within 6 months of sale (Gamlen et al., 1986), and emissions of CFCs used as blowing agents in open-cell foams and extruded foams took place within less than 1 year (Midgley and Fisher, 1993). CFCs used in refrigeration and airconditioning have somewhat greater storage times, being released on average within 1 to 10 years, and CFCs used as blowing agents in closed-cell thermoset foams are released after more than 10 years (Midgley and Fisher, 1993). Current estimates of the atmospheric lifetimes of CFC-11, CFC-12, and CFC-113 are 45 ± 7, 87 ± 17, and 100 ± 32 years (Volk et al., 1997).

Boron isotope analyses have been increasingly used in hydrogeological studies during the past decade, due to recognition of large variations in the natural isotopic composition of boron (at least 90%_o), and the development and refinement of mass spectrometric techniques (e.g., Spivack and Edmond, 1986; Vengosh et al., 1989; Eisenhut et al., 1996). The wide range in isotopic composition of the boron sources in water resources, both natural (e.g., sea water, fossil brines, hydrothermal fluids) and anthropogenic (sewage effluents, boron fertilisers, fly ash leachate), as well as the reactivity of boron with the aquifer matrix, make boron a useful natural isotopic tracer in groundwaters.