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

The East Midlands Triassic (Sherwood Sandstone) aquifer which has been the subject of detailed radiometric age studies, is used to investigate both in ert and reactive constituents of groundwater as indicators of residence tim e. Detailed resampling of the aquifer in 1992 has provided a considerable b ody 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, P r, F, I) and their element ratios. These form a group of essentially unreac tive tracers primarily reflecting changing rainfall inputs and palaeoclimat ic 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 indicator s, 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. Howe ver the concentrations of certain trace elements - Li, Rb, Cs, Mn and Mo - which are not limited by solubility constraints show linear trends along th e 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 wit hin the aquifer since high salinity groundwaters are found below the Sherwo od 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 r elationship 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 elemen ts. The age of the fresh groundwater is thus shown to be up to 100 ka BP, i ndicating the likelihood of semi-continuous recharge during the Devensian g lacial period preceding the glacial maximum, when no recharge occurred (10- 20 ka BP). This approach may be of value in extending groundwater dating be yond the radiocarbon timescale as well as interpreting sites where no radio carbon data are available. The scatter from the mean age line indicates tho se groundwaters which are derived from either rapid or slower than average flow zones within the aquifer, as well as age stratification. This stratifi cation is borne out by a depth profile of groundwater from a new borehole n ear to outcrop which is shown, by a combination of chemical and isotopic tr acers, to contain Holocene overlying late Pleistocene water. (C) 2000 NERC. Published by Elsevier Science Ltd. All rights reserved.