Heat flow in SW England is well above average for the UK as a whole, but northwards towards Bath and Bristol the values decrease rapidly. However, hot springs occur both in the Bath-Bristol area and in mines in Cornwall. The development of hydrothermal circulation systems is thus not controlled entirely by geothermal gradient: the presence of a suitable fracture permeability is the main requirement. The thermal ‘head’ produced, which nevertheless depends upon the temperature and volume of water in the system, theoretically can exceed local topographic heads even in areas of low geothermal gradient.

Thermal groundwaters usually carry above average quantities of radioelements in solution because of the long residence times involved. 222Rn values are often particularly high. High concentrations of 222Rn in surface waters arise from the discharge of groundwater. The results of a survey of 222Rn in streams in SW England have established areas of high values which are interpreted as rising limbs of convection cells with dimensions in the order of 5–10 km.

In SE Devon γ-ray spectrometry of soils shows two E.-W. belts of high activity. The northern is coincident with the faulted southern margin of the Crediton Trough, while the southern is coincident with the westerly extension of the Abbotsbury fault system. Groundwater movement along deep-seated fracture systems is considered to be the explanation of these features. The horizontal scale of the area involved suggests that a thermal rather than local topographic head is the driving force. Groundwater circulation within fractures, driven by a thermal head, may therefore occur even in areas of low geothermal gradient and should be considered when selecting waste disposal sites.

The concentrations of Br in potable groundwaters in the United Kingdom range from 60 to 340 µg 1-1. The occurrence of Br is described in terms of the Br/Cl weight ratio which enables small changes in bromide concentrations to be assessed in terms of salinity. Median values of Br/Cl in groundwaters range from 2.60 to 5.15 × 10−3 compared with a sea water ratio of 3.47× 10−3. In recent shallow groundwaters the Br/Cl ratio is rather variable in response to a range of natural and anthropogenic inputs (marine and industrial aerosols, industrial and agricultural chemicals including road salt). Some slight enrichment in Br/Cl also occurs naturally during infiltration as a result of biogeochemical processes.

Evolution of Br/Cl along groundwater flow lines reflects the sources of increasing salinity; either the influence of marine sedimentary formations or evaporites. The groundwaters in the Triassic sandstones of the English Midlands show significant Br depletion due to the evaporite source, in contrast to groundwaters in Cumbria. Br/Cl ratios in the Sherwood Sandstone of the East Midlands mainly reflect the natural input sources and can be used to help understand the palaeohydrology.

This paper presents a comprehensive survey of the tritium content of “reference waters”in real working conditions by French laboratories involved in environmental monitoring.Reference waters are virtually tritium-free waters used to determine blanks for theanalytical system and to check for any contamination. It is therefore crucial to be sureof the low tritium content of these waters as this can limit the sensitivity and accuracyof the measurements. Water from the commercial brand “Abatilles” is widely used in Frenchlaboratories and nine samples were analyzed in this study. Three samples from other deepaquifers were also included. The results all range from 0.004 ± 0.004 Bq/kg to 0.175 ± 0.011 Bq/kg, well below the 0.5Bq/kg advised by the French Safety Authority. The spread of the results can mainly beattributed to contamination through plastic during bottle storage. As expected, twosamples from demineralized tap water showed higher tritium activities (0.3–0.35 Bq/kg).Both waters are suitable as reference water for routine monitoring (DL of about 10 Bq/kg)but should be used with caution for activities in the Bq/kg range.

L'établissement du bilan des entrées et sorties de nitrates dans quelques lacs de gravières ainsi que la mesure directe de la dénitrification par émission de N2O (méthode à l'acétylène) ont permis d'évaluer l'activité dépolluante de ces lacs vis-à-vis de la nappe phréatique de la Garonne.L'activité dénitrifiante la plus élevée a été mesurée dans un lac ancien, non colmaté, artificiellement eutrophisé. Par contre, les gravières anciennes colmatées sont sans effet.Des surfaces considérables de plan d'eau sont nécessaires afin d'épurer des débits suffisants en vue de l'alimentation des captages d'eau potable.

La disparition des nitrates d'une nappe superficielle circulant sous un bois d'aulnes et la mesure directe de l'émission de N2O (méthode à l'acétylène) montrent la capacité de ce type de végétation à améliorer la qualité des eaux souterraines. L'activité de ce système semble atteindre une efficacité plus élevée que celle des eaux de surface en raison de la conjonction de facteurs favorables : fourniture de carbone organique dégradable, conditions d'anaérobiose, transport de masse des nitrates.Etant donné l'efficacité du processus, il est suggéré un réaménagement en aulnaie des lacs de gravières situés en amont des captages d'eau potable, afin d'en améliorer la qualité des eaux.