Residence time indicators in groundwater: the East Midlands Triassic sandstone aquifer

摘要

The East Midlands Triassic (Sherwood Sandstone) aquifer which has been the subject of detailed radiometric age studies, is used to investigate both inert and reactive constituents of groundwater as indicators of residence time. Detailed resampling of the aquifer in 1992 has provided a considerable body of new inorganic geochemistry data, though without radiocarbon. Several inert indicators are defined including the isotopic ratios delta(18)O, 62H, delta(2)H, Cl-36 noble gas ratios, as well as the halogen elements (Cl, Pr, F, I) and their element ratios. These form a group of essentially unreactive tracers primarily reflecting changing rainfall inputs and palaeoclimatic conditions, except at outcrop where human impacts are also seen clearly. The concentrations of Cl, mainly from atmospheric sources, remain below 25 mg l(-1) Cl over a distance of some 30 km from outcrop. Reactive indicators, the result of time-dependent water-rock interactions, include delta(13)C, Mg/Ca, Sr/Ca, Na/Cl and show diagnostic trends along the flow lines. However the concentrations of certain trace elements - Li, Rb, Cs, Mn and Mo - which are not limited by solubility constraints show linear trends along the present day flow gradient. This water-rock interaction is taking place in groundwaters with low total mineralisation and it can be demonstrated that reactions involving these elements and isotopes are occurring entirely within the aquifer since high salinity groundwaters are found below the Sherwood Sandstone. There is good correlation between some of the elements and C-14 activities in the 1977 data set and this has been used to derive a concentration-age relationship for the 1992 set of data. A chemical timescale for the aquifer is then established using (a) Li and (b) a combination of five trace elements. The age of the fresh groundwater is thus shown to be up to 100 ka BP, indicating the likelihood of semi-continuous recharge during the Devensian glacial period preceding the glacial maximum, when no recharge occurred (10-20 ka BP). This approach may be of value in extending groundwater dating beyond the radiocarbon timescale as well as interpreting sites where no radiocarbon data are available. The scatter from the mean age line indicates those groundwaters which are derived from either rapid or slower than average flow zones within the aquifer, as well as age stratification. This stratification is borne out by a depth profile of groundwater from a new borehole near to outcrop which is shown, by a combination of chemical and isotopic tracers, to contain Holocene overlying late Pleistocene water. (C) 2000 NERC. Published by Elsevier Science Ltd. All rights reserved. [References: 38]