Identifying ‘secure’ drinking water sources — recognizing

WATER SUPPLY WELLS

Because of the low detection limit relative to other environmental tracers, such as tritium, CFCs can play a useful role in lowering the detection of young fractions in well discharge and assessing the susceptibility of public water supplies to contamination from shallow sources. As an example, consider the possibility of a modern, shallow source in equilibrium with North American air recharged at 10°C that mixes with old, pre-CFC water in well discharge. The modern fraction might contain approximately 754, 354 and 97 pg/kg of CFC-11, CFC-12 and CFC-113, respectively. If the minimum detection limit for CFC-11 and CFC-12 is in the range of 0.3–3 pg/kg, it is possible to lower detection of the modern water fraction to the ranges of 0.04–0.4% and 0.08–0.8% in well discharge, based on CFC-11 and CFC-12, respectively. If the shallow, modern

-400 -300 -200 -100 0

sr et e m , ht p e D

3

H = 0.02 - 0.08 TU CFCs = 1965-1967

Land Surface

Water Table Advective Transport of CFCs through UZ

Well

Hypothetical tritium profile

Quaternary Basalts

FIG. 7.4. A schematic illustration is given of an open borehole in Quaternary basalt and sedimentary inter-bed sequences in the Eastern Snake River Plain of south-east Idaho.

Depth to water is more than 300 m. Water from the upper 20 m of the aquifer has a tritium content of 0.02–0.08 TU, with CFC-11 and CFC-12 concentrations consistent with apparent recharge dates from the mid-1960s. The presence of CFCs in the aquifer suggests that infiltration of pre-bomb water contacts old air in the unsaturated zone. In this example, CFCs provided evidence that recharge to the Eastern Snake River Plain aquifer is occurring through deep unsaturated zones in this semi-arid environment.

source is contaminated with CFCs, as often is the case of shallow water in urban environments, the detection of shallow water in well discharge is potentially many orders of magnitude lower than that determined from low level tritium determinations.

The detection limits of CFCs and other volatile halocarbons using purge and trap gas chromatography with an ECD can be several orders of magnitude below most analytical procedures used for agricultural and industrial contami-nants. In many cases, it can be shown that concentrations of halogenated VOCs that are reported as ‘background’, using conventional GC-MS procedures, are in fact not zero when analysed using purge and trap GC-ECD procedures.

Although the concentrations of low level detections of CFCs and other halogenated VOCs in water samples can be orders of magnitude below drinking water standards, the low level detections of CFCs permit a shift in strategy for water supply managers from recognizing water that contains one or more contaminants to screening water supplies that are potentially susceptible to contamination and need to be monitored at higher frequency than those supplies that have lower susceptibility to contamination. By lowering the detection of fractions of shallow water in discharge from public water supplies, long term monitoring has the potential of providing early warning of future contamination problems at the well scale.

There are many ways in which shallow groundwater sources can reach open intervals of public supply wells, including well construction problems, such as leaky casings or leaky seals around the casing. In unconfined, water table aquifers, wells that are not cased deeply enough below the water table are at increased risk of pumping fractions of shallow water, particularly as water levels decline in urban or industrialized areas in which there have been relatively high withdrawals, or as the water table is lowered in the immediate vicinity of the well during use. Hydrogeology also plays a significant role in affecting the extent of shallow water contamination of public supply well discharge. Wells completed in karst or fractured rock can be particularly susceptible to contamination from shallow sources, even at depths of hundreds of metres below the water table. In contrast, public supply wells in sands and gravels that are completed more than 100 m below the water table tend to have low susceptibility to contamination from shallow sources.

There are a growing number of programmes where CFCs have been formally included in source water assessment procedures. In New Zealand, one of the criteria used in drinking water standards for public supply includes residence time estimates based on CFC and/or tritium concentrations in groundwater pumped from wells (Ministry of Health, New Zealand, 2000).

Among a number of criteria used to demonstrate the security of groundwater supplies, the New Zealand Drinking Water Standards permit fitting a (lumped

parameter) dispersion model to two or more CFC or tritium measurements (Zuber, 1986). Groundwater that is not directly affected by surface or climatic influences was defined to contain less than 0.005% of water estimated to be less than one year old (Ministry of Health, New Zealand, 2000).

In the USA, the Department of Health, Source Water Assessment Program of Virginia (Commonwealth of Virginia, 1999) included detection of CFCs in water from public supply wells as a means of determining the intrinsic natural susceptibility of regional aquifers that serve as public water supplies (see Chapter 9, Section 9.9 for further details).

7.6. RECONSTRUCTING THE HISTORY OF CONTAMINANT LOADING TO AN AQUIFER

In dating groundwater with environmental tracers, it is assumed that the aquifer records a history of the tracer concentration recharged over the timescale investigated. Similarly, other substances recharged may also be archived in the groundwater reservoir. When the mixing effects of dispersion are low and transport of the solute is not retarded, a history of solute loading to the aquifer can be reconstructed by measuring solute concentration in dated samples. Groundwater systems are likely to hold valuable records of past environmental change that occurred in recharge areas of the aquifer over periods that are otherwise unobtainable. For example, numerous studies have retrieved records of recharge temperature and excess air achieved in concen-trations of noble gases (Mazor, 1972; Andrews and Lee, 1979; Andrews, 1991;

Stute and Sonntag, 1992; Stute et al., 1992; 1995; Andrews et al., 1994; Blavoux et al., Clark et al., 1997; Stute and Schlosser, 1999) and N₂-Ar (Heaton, 1981;

Heaton and Vogel, 1981; Heaton et al., 1986) in groundwater on the radiocarbon timescale. As a test of various dating methods, initial tritium concentrations in recharge waters are often reconstructed by correcting the measured tritium for radioactive decay over the age of the sample (or determining initial ³H from the measured ³H and tritiogenic ³He), and comparing the reconstructed initial tritium to local records of tritium in recharge waters (see, for example, Dunkle et al. 1993; Ekwurzel et al., 1994;

Plummer et al., 2000).

Groundwater dating permits the retrieval of environmental records from aquifers only over the timescale valid for the particular tracer. Because of their relatively short (recent) timescale, CFCs and ³H/³He are particularly well suited for retrieving records of human impacts on aquifers. For example, CFC dating of shallow groundwater in agricultural areas of the Atlantic coastal plain of parts of Maryland and New Jersey, USA (Dunkle et al., 1993; Böhlke and

Denver, 1995; Modica et al., 1998; and see Chapter 9, Section 9.1) indicates fairly low rates of nitrate loading to groundwater in the study areas prior to the early 1970s, and rapid loading of nitrate beginning in the early to mid-1970s in Maryland and early 1980s in the New Jersey study. The rapid rise in nitrate concentrations in groundwater apparently reflects changes in the local application rate of fertilizers (Böhlke and Denver, 1995). In many samples from agricultural areas of the Atlantic coastal plain, the maximum nitrate concentration exceeds the USEPA maximum contaminant level (MCL) of 10 mg/L as N. There is considerable interest in retrieving dated records of nitrate loading to groundwater because, unless denitrified in the groundwater system, the groundwater nitrate will eventually discharge to surface waters and estuaries adding nutrients that degrade the aquatic health of the region (Bachman et al., 1998). Using the reconstructed nitrate–age relation for groundwater in Maryland and an exponential model, Böhlke and Denver (1995) found that base flow concentrations of nitrate in a stream receiving discharge from the aquifer could be predicted if the average residence time of water in the aquifer was 20 years. This information can be used to predict the future response of surface waters in the area to changes in land use and to application rates of fertilizers. Spurlock et al. (2000) used CFC dating in conjunction with a one-dimensional transport model to estimate the age and history of herbicides in water from wells in California, USA. Böhlke et al.

(2002) used CFC dating to retrieve a record of nitrate recharge rates over a period of several decades beneath changing agricultural land use in Minnesota, USA. Delin et al. (2001) used CFCs and other methods to document local variations in recharge rates of water and agricultural chemicals caused by topographical effects.

Many of the contaminants of interest added to groundwater may be unstable and can degrade, which can result in minimum estimates of historical loading to the aquifer. However, if the degradation product(s) can be measured, the initial concentrations can be reconstructed. For example, Böhlke and Denver (1995) and Böhlke et al. (2002) used the quantity of excess N₂ from denitrification to reconstruct the initial nitrate concentration in anaerobic, denitrified samples when developing groundwater records of changing agricultural contamination in recharge. Finally, in samples that are mixtures of young and old, it is usually reasonable to assume that the contaminant is associated with the young fraction. Therefore, in reconstructing the contaminant concentration in the young fraction, the measured concentration must be divided by the fraction of young water in the mixture. Approaches for combining groundwater dating with studies of agricultural recharge records and discharge mixtures are summarized in Böhlke (2002, 2003).

Another example of reconstructing contaminant loading to aquifers from river recharge of a contaminated river is presented in Chapter 9, Section 9.7.