Excess air

Numerous observations of dissolved gas concentrations in groundwater have shown that groundwater frequently contains more gas than can be explained by equilibrium solubility with the atmosphere (e.g. Herzberg and Mazor, 1979; Heaton and Vogel, 1981). The excess concentrations of various gases are often in accordance with air composition and this effect is, therefore,

0591 0691 0791 0891 0991 0002

0 10 20 30 40 50

0591 0691 0791 0891 0991 0002

0 10 20 30 40 50

)sraey( ega ledom-CFC)sraey( ega ledom-CFC

Recharge year Recharge year

(a)

(c)

(b)

(d)

FIG. 4.6. The effects of ±5^oC error in estimation of recharge temperature on the CFC model ages derived from CFC concentrations and concentration ratios (upper and lower end of vertical bars) are shown: (a) CFC-11; (b) CFC-12; (c) CFC-113; (d) CFC-11/CFC-12 (before 1980) and CFC-113/CFC-CFC-11/CFC-12 (after 1980). The calculations are based on waters recharged in different years at 9^oC. The CFC model ages calculated from the recharge temperature of 9^oC are represented with horizontal bars.

called excess air (Heaton and Vogel, 1981). This is probably a result of a transient rise in the water table that does not fully displace the gas phase, or can be artificially introduced during well construction, development or purging.

The resulting trapped gas then partitions into groundwater at a pressure that is greater than the local atmospheric pressure. Typically, the amount of excess air is determined by analysis of the concentrations of different noble gases or from the nitrogen/argon ratio in the sample. When such measurements are available, it is possible to correct CFC ages for the presence of excess air.

Several conceptual models exist for accounting for excess air. In the simplest model, the excess air is assumed to be unfractionated, having the same composition as the atmosphere. Stute (1995) observed that in some cases the composition of excess air appears to be fractionated, and to contain a dispro-portionate amount of the heavy noble gases. He proposed a two-step process in which trapped air bubbles completely dissolve in groundwater (to produce unfractionated excess air) but then the excess air starts to diffusionally re-equilibrate with the soil atmosphere. Because the lighter gases have higher diffusion coefficients, the composition of excess air becomes fractionated. The concept of complete dissolution of trapped air is difficult for some systems, as large hydrostatic pressures are required to dissolve the amount of air needed that can then be fractionated to produce observed concentrations in ground-water. Aeschbach-Hertig et al. (2000) introduced a model in which trapped air bubbles partially dissolve into groundwater under closed system conditions.

Because the heavier gases are more soluble than light gases, the water becomes enriched in heavy gases.

Regardless of the formation mechanism, trapped air will contain CFCs and the resulting groundwater will be enriched in CFCs above the level predicted using Henry’s Law. Thus, if any CFC model ages are calculated from measured concentrations and from the recharge temperature, ignoring the presence of excess air, the calculated CFC ages will generally be too low (due to the higher concentration). In general, the effects of excess air on calculated CFC ages are larger:

(1) At higher recharge temperatures;

(2) For CFC-12 rather than for CFC-11 and CFC-113;

(3) For younger waters rather than for older waters (Busenberg and Plummer, 1992).

The sensitivity of CFC ages to excess air is less than 1 year per cm³/kg of excess air for CFC-12 and CFC-113 and is less than 0.05 year per cm³/kg for CFC-11 for modern water at 30°C. When excess air results during well construction or purging, the effect on apparent ages depends on the

groundwater age. Injecting modern air into old groundwater can result in apparent CFC ages that are more than 100% in error.

Wilson and McNeill (1997) reported quantities of excess air in groundwater ranging from 0 to about 50 cm³/kg; however, the majority of values were less than 10 cm³/kg. Furthermore, the maximum amount of excess air that can result from a rising water table is about 1 cm³/kg per m rise in the water table. Since transient water table changes that are greater than 10 m are not common, ‘natural’ excess air values greater than 10 cm³/kg are not likely to be common.

The effects of 30 cm³/kg of excess air on apparent CFC ages is illustrated in Table 4.4. These values were computed assuming complete dissolution (unfractionated) of trapped air. A value of 30 cm³/kg for excess air is not typical of most groundwaters, but could occur under extreme conditions. Hydrogeo-logical environments in which large amounts of excess air might occur in groundwater include fractured rock systems in mountainous terrains, ephemeral streams in arid regions and infiltration basins.

TABLE 4.4. DIFFERENCES BETWEEN THE ACTUAL GROUND-WATER AGES AND THE APPARENT CFC MODEL AGES BASED ON CALCULATIONS THAT IGNORE THE PRESENCE OF 30 CM³/KG OF EXCESS AIR IN A WATER SAMPLE FORMED AT 30^oC

Recharge year

Age offsets from recharge year calculated from CFC concentrations and ratios (a)

CFC-11 CFC-12 CFC-113 CFC-11/

CFC-12

CFC-113/

CFC-11

CFC-113/

CFC-12

1950 0 2 N/A^a –1 N/A^a N/A^a

1955 0 3 N/A^a –1 N/A^a N/A^a

1960 0 3 N/A^a –4 N/A^a N/A^a

1965 0 3 N/A^a –5 N/A^a N/A^a

1970 1 4 N/A^a –7 N/A^a N/A^a

1975 1 5 1 N/A^a 0 0

1980 3 8 3 N/A^a 2 0

1985 4 11 3 N/A^a 2 0

1990 C^b C^b C^b N/A^a C^b 0

1995 C^b C^b C^b N/A^a C^b 0

a N/A indicates not applicable (below one year offset).

b C: CFC concentrations or ratios higher than the surface water which is at equilibrium with the modern atmosphere, indicating contamination effects.

The program QCFC allows the effects of excess air (as well as other relevant parameters) on apparent CFC ages to be evaluated (see Appendix III).

4.5. SORPTION

A few studies have suggested that groundwater dating with CFCs may be difficult in aquifers with high organic carbon content because of sorption of CFCs to the aquifer solid phase.

Under the assumption of linear, reversible, instantaneous adsorption, the velocity of a sorbed tracer, V_t, is related to the groundwater velocity, V_w, by:

(4.2)

where R is termed the retardation factor, rs is the density of the solid phase, q is the water content, and K_d is the ratio of the mass of solute on the solid phase per unit mass of solid phase, to the concentration of solute in solution.

The sorption of CFCs on the solid phase of soils has been studied in column experiments by Ciccioli et al. (1980), Brown (1980) and Jackson et al.

(1992). Ciccioli et al. (1980) found negligible retardation for CFC-11, CFC-12 or CFC-113 during passage through a column of ground Ottawa sand. For transport through ground limestone, retardation factors were measured to be 1.0, 1.1 and 1.5 for CFC-12, CFC-11 and CFC-113, respectively. From these data, partition coefficients are estimated to be K_d = 0.00, 0.01 and 0.07, respec-tively. Brown (1980) measured the retardation of CFC-11 to be 1.4 (K_d = 0.05) and 2.2 (K_d = 0.16) during passage through columns of (unground) Ottawa sand and Yolo sandy loam, respectively, and Jackson et al. (1992) measured a retardation factor of 12 (K_d = 1.45) for CFC-113 during passage through a column of Gloucester aquifer sediments (fine sand, f_oc = 0.06%; velocity 0.03 cm/min).

Difference in sorption characteristics for Ottawa sand in the two studies may be due in part to a difference in flow velocity. In the study of Brown (1980), the water velocity was 2.9 cm/min, while in the study of Ciccioli et al.

(1980) it was 21 cm/min, suggesting that in the latter case, equilibrium parti-tioning between the liquid and solid phases may not have occurred.

Field measurements of partition coefficients under natural groundwater flow rates have been obtained by Cook et al. (1995) and Bauer et al. (2001).

Cook et al. (1995) measured CFC-11, CFC-12 and CFC-113 concentrations in a piezometer nest near a groundwater flow divide (vertical flow predominating)

R V

V^w K

t

s d

= = + Ê

-ËÁ ˆ 1 1 ¯˜

r q

q

in a silty sand aquifer at Sturgeon Falls, Ontario, Canada. Thirteen piezometers were sampled between the surface and 20 m depth. Tritium concentrations on the same piezometer nest had been measured previously (Solomon et al., 1993). Simulation of tracer concentrations using a groundwater flow and transport model indicated a partition coefficient of K_d = 0.14 (R = 1.70) for CFC-113 relative to ³H. A partition coefficient of K_d = 0.03 (R = 1.15) for CFC-12 was estimated, although this may not have been significantly different from zero. A partition coefficient could not be determined for CFC-11, because this tracer was removed by microbial degradation. Bauer et al. (2001) estimated a retardation factor of 1.5 for CFC-113 (with respect to SF₆, ³H and

85Kr) in a fractured basalt aquifer, by simulating tracer concentrations using a two-dimensional solute transport model. In contrast, Busenberg and Plummer (1993) found good agreement between CFC-12 and CFC-113 ages in a shallow, coarse sand aquifer on the Delmarva Peninsula (Delaware, Maryland, and Virginia, USA), indicating minimal sorption of CFC-113.