Water contaminated with one or more CFCs

Samples 8–16 (Table 8.2) demonstrate different ways in which groundwater has been contaminated with one or more CFCs, i.e. CFCs have been added in addition to that determined by air–water equilibrium at recharge. Sample 8 is from a domestic well completed in a poorly confined part of the Floridan aquifer near Albany, Georgia, USA. The water is recharged by aerial infiltration into a karstic limestone aquifer, where land use is mostly rural

and agricultural. The apparent ages based on CFC-11, CFC-113 and ³H/³He are similar, but the sample is clearly contaminated with CFC-12. The source of the CFC-12 contamination is unknown, but effluent from domestic septic tanks is a possibility. One of the strengths of CFC dating is that there are three compounds capable of defining apparent age and, depending on local land use practices, contamination with respect to one or two CFCs does not necessarily preclude the possibility of obtaining a reliable age. The agreement in apparent age from CFC-11, CFC-113 and ³H/³He suggests this sample is not significantly mixed with old water.

Sample 9 is from a monitoring well located near a closed and reclaimed landfill in Albuquerque, New Mexico, USA. The water is contaminated with all three CFCs and cannot be dated using CFCs. Although not harmful, the relatively high CFC concentrations indicate that the groundwater near the landfill is probably being affected by anthropogenic sources. Although landfills are anoxic, and microbial degradation of CFCs probably occurs there, landfills are a source of CFC contamination to groundwater. This landfill was constructed within the unsaturated zone, and water has apparently leaked through the landfill to the aerobic aquifer beneath. High concentrations of all three CFCs occur in the aquifer. Because of their low detection limit and generally high concentrations in landfills and other waste sources, a potential application of CFCs is their use as an early warning in assessing susceptibility of wells to contaminant sources.

Samples 10 and 11 are from monitoring wells near a 0.17 km², unlined landfill that was in operation from 1962 to 1994 in New York, USA. The landfill was capped over approximately 75% of its area in 1996 and the remainder was capped in 2000. The landfill was constructed in glacial till, and is in direct contact with the shallow aquifer. Sample 10 was taken from a monitoring well in the shallow aquifer within the contaminate plume approximately 30 m downgradient of the landfill. Sample 11 is from a monitoring well approxi-mately 366 m downgradient from the landfill. Based on monitoring of water chemistry, there was little evidence that water at the furthest point downgra-dient (at sample 11) was being influenced by leachate from the landfill. The N₂–Ar recharge temperature of sample 11 is indicative of natural recharge in the area, and the absence of dissolved methane suggests the sample is not affected by the landfill. However, the water is probably undergoing iron reduction, as indicated by the low dissolved oxygen and elevated dissolved Fe(II).

Sample 11 has CFC-11 and CFC-113 concentrations in the ‘normal’ range, suggesting apparent ages of 22 and 19 years, respectively, and are only slightly younger than that based on dissolved SF₆ (26 years). SF₆ does not degrade in anoxic waters (Busenberg and Plummer, 2000), and the SF₆ apparent age is consistent with the dates of operation of the landfill, while the CFC-11 and

CFC-113 apparent ages may have been affected both by contamination and degradation. Sample 10, located nearest the landfill, has been affected by degassing as indicated by low concentrations of dissolved N₂ and Ar, and an apparent recharge temperature of 38.5^oC, more than 30^oC warmer than the recharge temperature determined for sample 11. This sample has probably been gas stripped by methane gas formation (9.0 mg/L dissolved methane).

Consequently, the CFCs and SF₆ have probably been gas stripped and have concentrations lower than that of samples that have not been degassed. Even with gas stripping, the CFC-12 concentration is approximately 100-fold that of modern air–water equilibrium. Traces of methylbutane and isopropyl alcohol were detected by purge and trap GC-MS procedures in sample 11. More than 26 VOCs were identified in sample 10 using purge and trap GC-MS procedures.

Sample 12 is from a domestic well in piedmont sands and gravels along the eastern margin of the Middle Rio Grande Basin in central New Mexico, in an area where the basin receives inflow of old palaeowater from outside the basin. The ¹⁴C activity of the dissolved inorganic carbon is only 22.0 pMc, indicating a radiocarbon age of more than 10 ka. The sample is aerobic and contains very low concentrations of CFC-11 and CFC-113. In contrast, the sample contains more than 10 000 pg/kg of 12. A likely source of the CFC-12 is effluent from septic tanks in the area. The CFC-11 and CFC-113 concen-trations are too low to be used to date the young fraction in the sample.

Although none of the CFCs can be used to date the sample, the presence of high levels of CFC-12 indicates the water is being influenced by anthropogenic sources and is susceptible to contamination.

The water in sample 13 is from a shallow monitoring well in a sand aquifer in southern New Jersey, USA, sampled in April 1992, in an area where atmospheric mixing ratio is influenced by air from two large metropolitan areas in the USA: Philadelphia, Pennsylvania and Wilmington, Delaware. It has been suggested that the water shown in sample 13 represents equilibration with urban air, that is, air somewhat enriched in CFCs having mixing ratios greater than that observed at Niwot Ridge, Colorado, USA (Szabo et al., 1996). The land use is rural and agricultural, yet the CFC-11 and CFC-12 concentrations in the water are higher than possible for equilibrium with Niwot Ridge air at that time. At the assumed recharge temperature of 13^oC, the measured CFC-11 and CFC-12 concentrations are 110 and 104 pg/kg higher than possible for equilibrium with 1992 Niwot Ridge air. And using the lowest possible recharge temperature of 11^oC, the CFC-11 and CFC-12 concentrations still exceed 1992 air–water equilibrium values by 44 and 77 pg/kg. At this site, the CFC-113 was apparently not contaminated, indicating an apparent age of 2 years (13^oC recharge temperature), which compares to 2.5 years based on ³H/³He dating. It was suggested by Szabo et al. (1996) that the contamination of CFC-11 and

CFC-12 in shallow groundwater in the area was due to contamination from urban air, as has been observed in parts of north-eastern USA (Ho et al., 1998).

Samples 14 and 15 are examples of waters that were probably contami-nated during drilling and/or well completion. Sample 14 is from a deep monitoring well that was drilled along the eastern mountain front in piedmont sands and gravels near Albuquerque, New Mexico. The well was completed in July 1997 and sampled one year later. The rotary drilling used drilling fluid that circulated through open tanks at the land surface. In completing the well, high pressure air was injected at shallow depths, which lowered the head over the water column and caused the well to be flushed from depth. The sample contains high ⁴He and SF₆ of terrigenic origin, indicating old water. The ¹⁴C activity of the dissolved inorganic carbon is 35.4 pMc, indicating an unadjusted radiocarbon age of more than 8 ka. CFCs were low or not detectible in most other deep monitoring wells sampled in the vicinity of Albuquerque, but water from the deep completion at the Matheson piezometer nest (sample 14, Table 8.2) contained more than 240 pg/kg of CFC-11, 50 pg/kg of CFC-12 and 14 pg/

kg of CFC-113. The concentrations suggest that this sample was contaminated with some drilling fluid that was nearly in equilibrium with 1998 atmosphere.

The ratios of the various CFCs in the sample are significantly different from those of the 1998 atmosphere and, thus, excess air was not the source of CFC contamination in sample 14. Because of the shallow depth for air injection, it is unlikely that the air contamination was due to well development. In this case, the CFC data provided a useful tool to detect monitoring wells that may still contain water in the vicinity of the well screen that contains fractions of drilling fluid.

Sample 15 is from a monitoring well drilled in low permeability mudstones/siltstones in Nebraska, USA. The well was drilled in June 1999, and sampled on two occasions prior to CFC collection in May 2000. It was reported that after removal of the water standing in the well, several days are required for the water level to recover. The ¹⁴C activity of the dissolved inorganic carbon is only 13 pMc and the water contains < 0.8 TU. The well was developed by air injection, apparently within the 3 m screened interval. Air injection would probably not significantly affect the ³H or ¹⁴C activity of the sample but would likely introduce significant amounts of CFCs in the vicinity of the well screen, which would subsequently dissolve under hydrostatic pressure. Further evidence of air contamination is the relatively high dissolved gas content and high, calculated, excess air (Table 8.2). If 11.2 cm³ of 1999 air dissolved per kg of groundwater (Table 8.2), then this excess air would have contributed 302, 168 and 75 pg/kg of CFC-12, CFC-11 and CFC-113, respectively. The concen-trations observed, with the exception of the CFC-113, are similar to the ones calculated from the excess air, which suggests that all CFCs were introduced

during the development of the well. In this case, both the CFCs and dissolved gases were useful in recognizing air contaminated groundwater.

Sample 16 is included in Table 8.1 as an example of a water sample that was probably contaminated by the sampling equipment. The sample is from the sand aquifer on the Delmarva Peninsula of Maryland, USA, sampled in November 1991 (Dunkle et al., 1993). The very low tritium content (-0.2 ± 0.3 TU) is consistent with the age of nearly 45 years determined from CFC-12.

However, the CFC-11 concentration indicates a recharge date of 1964, and apparent age of 27 years. Water recharged in 1964 on the Delmarva Peninsula would have contained more than 100 TU of tritium in 1991 (Dunkle et al., 1993). The sample was collected using a pump that had a moving internal rubber rotor. Dunkle et al. (1993) noted a young bias of CFC-11 apparent ages relative to CFC-12 in many of the older water samples from Delmarva. The addition of small amounts of CFC-11 from sampling equipment would have less effect on younger water with higher CFC-11 concentrations. Subsequent tests comparing water pumped from deep wells with the pump used by Dunkle et al. (1993) and another pump made of all metal parts showed that the sample pump used earlier introduced CFC-11. Other materials sometimes used in sampling that have been found to contaminate water samples with CFCs are some silicone rubber and plastic tubing used in peristaltic pumps, and plastic impellers and plastic discharge lines used in some pumps.