The use of tracers as a hydrogeological tool was reviewed in four papers at this meeting and demonstrated the diversity of their use to the hydrogeologist and groundwater engineer.

The use of radionuclides as groundwater tracers. S. Nixon, (Water Research Centre, Stevenage). The choice of radionuclides is subject largely to a wide range of half-lives, varying from 15 hours (24Na) to 12.26 years (tritium), and to obtaining the necessary authorization to introduce these into an aquifer. The type of equipment used was shown in conjunction with a summary of several case histories including:

(i) injecting radionuclides into an aquifer at controlled depths within an injection well to assess the flow of sea water leaking into a dry dock;

(ii) to prove the connection of leachate in a quarry contaminating a nearby stream;

(iii) to assess groundwater flow beneath a landfill in a limestone quarry;

(iv) to assess the quantity of recharge entering a landslip in comparison to the total quantity of groundwater leaving it.

The use of dissolved gases and natural radioactive isotopes in hydrogeological studies. J. N. Andrews (Bath University). Gases dissolved in the groundwater during recharge provide a means to investigate the climatic conditions existing during recharge and thus enabling limits to be placed on the groundwater age. The gases used in this determination include 4He, 3He/4He, Ne, Ar, Kr, Xe, N2/Ar, 40Ar/36Ar and 14C. Radio-isotopes reviewed by the author were 238U, 234U/238U, 222Rn and 226Rn. The 222Rn and 226Rn were the most significant radioactive isotopes in the

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