Stable isotope evidence for the hydrogeological characteristics of clay-rich till in northern East Anglia

The intrinsic vulnerability of the Chalk aquifer in East Anglia in eastern England to surface-derived contamination is dependent on the nature and juxtaposition of overlying Quaternary deposits. The study presented here is a direct investigation of the physical characteristics and pore water isotope composition of the glacial deposits found overlying the Chalk at five drilled locations in north Norfolk. The pore water isotopic composition demonstrated a range of d18O values from -8.13 to -6.47‰, similar to the range for Chalk groundwater in the area of -8.18 to -7.01‰. A lithology-dependent model is developed that recognizes the presence of isotopically depleted palaeowater of late Pleistocene origin contained within layers of clay-rich till, adjacent to pore water containing modern meteoric water within layers of sand-rich till. The presence of (weathered) vertical fractures and intercalated lenses of sand-and gravel-rich layers is inferred to explain the occurrence of isotopically enriched water within the general mass of clay-rich till. It is apparent that groundwater movement in the heterogeneous lodgement till is controlled by advection in the more permeable sand-rich layers, whereas in the mass of clay-rich till, diffusion is more dominant. At one confined site, the observed pore water isotopic profile is modelled as having developed by downward diffusion of modern meteoric water during the past 5-10 ka.