Busenberg, L.N. Plummer

Reston, Virginia, United States of America

10.1. SULPHUR HEXAFLUORIDE (SF₆)

Sulphur hexafluoride is a colourless, odourless, flammable, non-toxic, stable gas with excellent electrical insulating and arc-quenching properties, and is mainly used as an electrical insulator in high voltage switches and transformers. Industrial production of SF₆ began in 1953 with the intro-duction of gas-filled electrical switches and prointro-duction has increased from nearly zero in 1953 to 85 700 t of SF₆ in 1995 (Maiss and Brenninkmeijer, 1998).

SF₆ has received significant recent interest because of its high greenhouse warming potential which is estimated to be 23 900 times that of CO₂ (Climate Change 1995) and is the second highest value measured. SF₆ was one of the six gases covered by the Kyoto Global Warming Protocol Agreement of 1997.

SF₆ is primarily of anthropogenic origin but also occurs naturally (Harnisch and Eisenhauer, 1998; Busenberg and Plummer, 2000). 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 (Busenberg and Plummer, 1997; 2000). The atmospheric history of SF₆ is now well established (Maiss and Levin, 1994; Maiss and Brenninkmeijer, 1998) and the mixing fraction of SF₆ is currently increasing at a rate of about 6% per year (Fig. 10.1(a)) while those of the CFCs are nearly constant or decreasing (Geller et al., 1997).

The natural background concentration constitutes about 1.0% of the 2001 total atmospheric partial pressure. Small but significant concentrations of SF₆ were measured in 16 minerals and rocks of igneous metamorphic, hydro-thermal and sedimentary origin (Busenberg and Plummer, 2000). Concentra-tions of SF₆ were generally highest in silicic igneous rocks and lowest in mafic rocks. Significant concentrations of SF₆ may be present in some diagenetic fluids. Concentrations of SF₆ significantly higher than equilibrium with modern air–water were measured in groundwater from fractured silicic igneous rocks, from some hot springs, and in some groundwater from volcanic areas

(Busenberg and Plummer, 2000). Concentrations of SF₆ may be a useful natural tracer of igneous and volcanic fluids. Where the terrestrial flux of SF₆ from igneous rocks and mineral grains is high, groundwater cannot be dated by the SF₆ method.

SF₆ has been extensively used in many studies as a natural atmospheric tracer (Lovelock and Ferber, 1982; Levin and Hesshaimer, 1996; Patra et al., 1997; Geller et al., 1997; Hall and Waugh, 1998; Zahn et al., 1999; Reddmann et al., 2001). The gas has been injected into the oceans to determine longitudinal dispersion, diapycnal and isopycnal diffusion, and mixing (Ledwell et al., 1986;

Watson et al., 1987; Ledwell and Watson, 1988; Ledwell and Watson, 1991;

Watson et al., 1991; King and Saltzman, 1995; Law et al., 1998; Ledwell et al., 1998; Vollmer and Weiss, 2002), and air–sea gas exchange and dispersion (Watson et al., 1991; Wanninkhof, 1992; Wanninkhof et al., 1993, 1997; Asher and Wanninkhof, 1998). SF₆ has been used to study longitudinal dispersion, gas exchange and mixing in lakes, rivers and estuaries (Wanninkhof et al., 1985,

0

1940 1950 1960 1970 1980 1990 2000 2010

Year

FS6elom CFC/ria ni soitar

(a)

FIG. 10.1. Shown are: (a) historical concentrations of CFCs and SF₆ in the North American atmosphere; and (b) historical ratios of SF₆ to CFCs in the North American atmosphere.

1987; Clark et al., 1994; Maiss et al., 1994a, 1994b; Clark et al., 1996; Cole and Caraco, 1998; Hibbs et al., 1999; van Bodegom et al., 2001; von Rohden and Ilmberger, 2001; Salhani and Stengel, 2001; Frost and Upstill-Goddard, 2002;

Ho et al., 2002; Vollmer et al., 2002); in the study of soil venting (Olschewski et al., 1995); has been used to label drilling air in fractured rock studies (Davidson, 2002), and as a hydrological tracer (Wilson and Mackay, 1993;

Upstill-Goddard and Wilkins, 1995; Wilson and Mackay, 1995, 1996; Gamlin et al., 2001; Vulava et al., 2002). As with CFCs, SF₆ is measured by gas chromato-graphy with an ECD (Busenberg and Plummer, 2000).

The dating range of water with SF₆ is from 1970 to modern, and the SF₆ method is particularly useful in dating very young (post-1993) groundwater (Busenberg and Plummer, 2000; Śliwka and Lasa, 2000; Bauer et al., 2001;

Zoellmann et al., 2001). It has been used to study the deep water renewal times in Lake Issyk-Kul, Kyrgystan, and the SF₆ model ages agreed closely with the

3H/³He ages (Hofer et al., 2002). The ratios of SF₆ to CFC partial pressures are useful in dating groundwaters that recharged after the 1990s (Fig. 10.1(b)).

The procedure that is used to calculate the Henry’s Law constant for SF₆ is the same as that for CFCs (see Chapter 3). The K_H for SF₆ solubility in pure water and sea water have been determined at 1013.25 hPa total pressure between 273–313 K and for salinities of 0–40‰ (Bullister et al., 2002) (Fig. 10.2). Table 10.1 gives the least squares fitting parameters to the temper-ature, and salinity dependence of K_H for SF₆ concentration units of mol·kg^–1

·(1013.25 hPa)^–1 and mol·L^–1·(1013.25 hPa)^–1:

(10.1.)

where T is the temperature in degrees kelvin and S is the salinity in parts per thousand (‰). Corrections for excess air are very important for SF₆, due to its low solubility in water (Busenberg and Plummer, 2000).

The following example demonstrates the use of SF₆ in dating groundwater in a sand aquifer of the Atlantic Coastal Plain, USA. The Locust Grove watershed, located on the Delmarva Peninsula, USA, is an unconfined, surficial aquifer consisting of sands and gravels, and ranges in thickness from about 25 m at the southern part of the watershed to about 5 m in the northern part. The surficial aquifer is underlain by a confining layer consisting of mainly silt and clay. The geographical location, the location of wells, and the flow paths can be found on maps in Dunkle et al. (1993), Reilly et al. (1994) and Böhlke and Denver (1995). The area is intensely farmed and the groundwater

lnK a a ln

chemistry has been profoundly altered by the agricultural practices (Böhlke and Denver, 1995). The concentrations of agricultural chemicals in the groundwater can be used to estimate the relative ages of the water at this site (Böhlke and Denver, 1995). The SF₆ derived age and nitrate concentrations in the aquifer are shown in cross-section in Fig. 9.4 in Chapter 9. SF₆ ages are in

TABLE 10.1. CONSTANTS FOR CALCULATION OF K_H FOR SF₆ (BULLISTER et al., 2002), Eq. (10.1)

KH a1 a2 a3 b1 b2 b3

Mol·kg^–1

·(1013.25 hPa)^–1

–98.7264 142.803 38.8746 0.0268696 –0.0334407 0.0070843

Mol·L^–1

·(1013.25 hPa)^–1

–96.5975 139.883 37.8193 0.0310693 –0.0356385 0.00743254

1940 1950 1960 1970 1980 1990 2000

Year

0 50 100 150 200 250 300 350 400 450 500 550

FS6tnecnoc rni noitafgk/g

30^oC 25^oC 20^oC 15^oC 10^oC 5^oC 0^oC

FIG. 10.2. Concentration of SF6 (femtograms per kilogram (fg/kg)), in water as a function of temperature in equilibrium with North American air at 1013.25 hPa total pressure.

good agreement with those obtained with other dating methods at the site (see Fig. 9.3 in Chapter 9). Figure 10.3 clearly shows the effect of increased fertilizer use in the past 30 years at this site. The nitrate concentration in most of the upper 10–15 m of the aquifer exceeds the U.S. Environmental Protection Agency (USEPA) maximum contaminated level (MCL) of 10 mg/L as nitrogen.

SF₆ has been successfully used to date groundwaters in a number of multi-tracer studies (Bauer et al., 2001; Zoellman et al., 2001; Katz et al., 2001;

Plummer et al., 2001; Hofer et al., 2002; Vulava et al., 2002). The SF₆ dating method yields excellent results in clastic sediments even in urban areas that are often contaminated with CFCs and other VOCs. However, results have been less reliable for groundwaters obtained from glacial tills, fractured igneous rocks (Busenberg et al., 1998, 2001) and from carbonate aquifers (Katz et al., 2001; Busenberg and Plummer, 2000) where the natural background SF₆ was often significant, and in some cases, exceeded the anthropogenic fraction. SF₆ can be elevated in urban air (Santella et al., 2003).

0 10 20 30 40 50

Model SF₆ Age in Years 0

10 20 30 40 50 60 70 80 90

ON3-tnecnoC rgm ni noita/L

KeBe 53

FIG. 10.3. Shown are concentrations of nitrate in groundwater, as mg NO3 per litre, from the Locust Grove, Maryland, watershed as a function of the SF6 age. The results indicate an increase in fertilizer application in the past 35 years. The nitrate in KeBe 53 was reduced to N2.