ISOTOPIC EVIDENCE FOR PALAEOWATERS IN THE BRITISH-ISLES

Before the 20th century, groundwater circulation in the aquifers of th e British Isles had largely adjusted to the temperate maritime climate and sea levels established over the past 10 ka since the end of the P leistocene. However, in the last 100 a this natural regime has been di sturbed by abstraction of water for public supply and industrial use, and palaeowaters from earlier recharge episodes are now becoming a fac tor to be considered in water balance estimates. This paper presents a synthesis of the existing palaeowater distribution in the British Isl es, based on isotopic evidence (delta(18)O, delta(2)H and C-14). As su ch, it has relevance to palaeoclimatic studies in addition to the wate r resource implications. The Triassic basins of England and Northern I reland contain saline waters beyond the range of C-14 dating (> 40 ka) . Stable isotopic ratios show enrichment in some basins and depletion in others, without an overall pattern that would explain all the obser ved compositions. The results for the Wessex basin suggest recharge in pre-quaternary times, but for the other basins some flushing by Pleis tocene or Holocene meteoric waters is indicated. Isolated occurrences of apparently long-residence waters are found elsewhere throughout the British Isles, for example from Carboniferous and Lower Palaeozoic st rata. In such cases, environmental isotopes are more useful as constra ints on hydrogeological models than as indicators of palaeoconditions. Major water supply aquifers are restricted almost entirely to England . The two sandstone formations (Triassic Sherwood Sandstone and Cretac eous Lower Greensand) have a greater range of stable isotopic values b etween phreatic and confined conditions than the two carbonate formati ons (Jurassic Lincolnshire Limestone and Cretaceous Chalk). This indic ates that the sandstone aquifers are better archives of information on palaeoconditions than the carbonate aquifers. They show that atmosphe ric circulation patterns over Britain have probably remained the same since the late Pleistocene. However, C-14 data from all four of the ma jor aquifers emphasise the hiatus in recharge during periglacial condi tions which occurred between the late Pleistocene and early Holocene. (C) 1997 NERC. Published by Elsevier Science Ltd.