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Environmental isotope hydrogeology 135

Fig. 4.9 Variations in mean monthly
tritium concentrations in precipitation
since 1953 at four IAEA stations
(IAEA/WMO 1998): Ottawa, Canada
(northern hemisphere, continental);
Valencia, Ireland (northern hemisphere,
marine); Harare, Zimbabwe (southern
hemisphere, continental); Kaitoke, New
Zealand (southern hemisphere, marine).

Although the bomb peak has decayed away, slow downward migration of water through the low-
making tritium less useful as a dating tool, earlier permeability, saturated Chalk matrix. The tritium
studies in the 1970s and 1980s were able to relate appears to migrate with a ‘piston-like’ displacement
large quantities of tritium in a sample to groundwater with only limited dispersion of the peak concentra-
recharge in the 1960s. For example, in Fig. 4.8b, mod- tions. Detailed analysis of the profiles in terms of their
ern water with a tritium concentration up to 57 TU mass balance and peak movement revealed the possi-
(Table 4.1) was sampled in the recharge area of the bility of rapid water movement, or ‘by-pass’ flow. It is
Chalk outcrop on the northern rim of the London estimated that up to 15% of the total water move-
Basin. Away from the Chalk outcrop, tritium values ment occurs through the numerous fissures that
decline rapidly below the confining Eocene London comprise the secondary porosity of the Chalk once
Clay. Another area of interest is the River Lea valley the fluid pressure in the unsaturated zone (Section
where a tongue of water containing measurable 5.4.1) increases to the range −5 to 0 kPa (Gardner
tritium (53 TU maximum) extends south from the et al. 1991). At other locations, a ‘forward tailing’
Chalk outcrop. This area coincides with the region and broadening of the tritium peaks are observed
of the North London Artificial Recharge Scheme in sequential tritium profiles providing evidence of
(see Box 8.2) and it appears likely that the tritium greater dispersion (Parker et al. 1991).
anomaly is due to artificial recharge of treated mains Laboratory measurements have shown that
−
water containing a component of modern water. tritiated water and NO have similar diffusion coef-
3
Another application for tritium is providing a ficients and so similar concentration profiles might
− −
tracer for modern pollutant inputs to aquifers such as be expected for NO (Fig. 4.10b). Dispersion of NO
3 3
agricultural nitrate and landfill leachate. For example, is suggested where there has been less downward
sequential profiling of tritium in porewater contained movement than would be expected if piston flow
in the unsaturated zone of the English Chalk has were occurring. An apparent flattening of the original
−
helped in the understanding of the transport pro- NO peaks leads to higher concentrations than ex-
3
perties of this dual-porosity aquifer. As shown in pected in the deeper part of the profiles. The rate of
Fig. 4.10a for a site in west Norfolk, sequential re- solute movement in the Chalk implied by the tritium
− −1
drilling and profiling of tritium concentrations in the and NO profiles is between 0.4 and 1.1 m a . The
3
Chalk matrix revealed the preservation of the tritium actual rate of movement at a specific site will depend
bomb peak. The results are interpreted as indicating on many factors, including the physical properties