THE CHEMISTRY AND ORIGIN OF GROUNDWATER IN TRIASSIC SANDSTONE AND QUATERNARY DEPOSITS, NORTHWEST ENGLAND AND SOME UK COMPARISONS

The Triassic Sherwood Sandstone Group is a major UK aquifer. In this study the groundwater geochemistry of a 60 km(2) area has been investigated to improve the understanding of the hydrogeology of the area and to determine the origin of groundwater. Archived analyses of groundwater samples were interpreted to determine the dominant geochemical processes and subsequently samples were taken from selected sites for stable and radio-isotope measurements. Near areas of direct recharge of the Sherwood Sandstone, groundwaters are unsaturated with respect to carbonate minerals. Sulphate and chloride are contributed to the groundwater in the Sherwood Sandstone from the overlying Quaternary glacial drift deposits and Mercia Mudstone. The groundwater chemistry and isotopes of sulphate indicate that the source of sulphate is gypsum in the Triassic sequence and the Triassic-derived glacial deposits. A significant proportion of the calcium in groundwater is shown to be exchanged for sodium in the aquifer matrix. However, in the more mineralised waters, carbonate minerals are precipitated because of contributions to the groundwater of calcium from dissolution of gypsum. Up piezometric gradient, groundwaters are modern and have relatively high C-14 and tritium contents. The oldest water which was sampled for C-14 is estimated to be about 4000 years older than the modern waters which were sampled from near recharge zones. The groundwater regime in the study area may be compared and contrasted with Triassic sandstone aquifers elsewhere in the UK. Incongruent carbonate dissolution occurs, as described in the Triassic of the East Midlands, the Lower Mersey Basin and southwest England. Exchange reactions are important in determining the groundwater chemistry as found elsewhere in the Triassic of northwest England, and in contrast to the East Midlands and southwest England.