The distribution of hydrochemical types in the Sandringham Sands of west Norfolk is used to test a conceptual model of groundwater recharge and flow. Measurements of dissolved noble gases and strontium (Sr) isotope ratios were integrated into the study as specific tracers of recharge and water:rock interaction processes respectively. Recharge to the unconfined Sandringham Sands aquifer is affected by the patchy distribution of Quaternary sediments, especially glacial till. Groundwater recharged through the till can be distinguished from that recharged to outcrop areas by having higher Ca2+and HCO, concentrations (greater than 80 and 200 mg L \ respectively) and lower Mg2+and K+ concentrations (less than 5 mg L I for both species). Pyrite oxidation during recharge in both settings creates acidity that is consumed by calcite dissolution where the till is present but by glauconite weathering at outcrop, giving rise to the hydrochemical differences observed. Weathering of glauconite releases radiogenic Sr into the outcrop recharged water resulting in H7Sr/H6Sr ratios greater than 0.7087. Groundwater in the unconfined aquifer is reducing and commonly sulphidic as SO/ is reduced by oxidation of sedimentary organic matter disseminated throughout the aquifer matrix. The aquifer is confined to the east of its outcrop by the Gault and Snettisham Clays, separating the Sandringham Sands from the overlying Chalk. The oxygen isotope ratios (8IHO) of water from both the Chalk and the confined Sandringham Sands are depleted relative to those from the unconfined aquifer (::;-7.5%0 and ~-7.3%0 VSMOW respectively) indicating that the confined aquifer is mainly recharged from the Chalk. Two groundwater types are evident in the confined aquifer. The more northerly type is distinguished from the southerly type by having SO/ concentrations less than 20 mg LI and Si02 and Mg2t concentrations greater than 8 and 3 mg LI respectively. The northerly type is significantly supersaturated with respect to atmospheric noble gases, indicating the presence of a large concentration of "excess air". Excess air entrainment occurs during recharge and is proportional to the magnitude of annual water table fluctuation in the recharge area. The high concentrations measured require annual water table fluctuations of between 5 and 8 m, as observed over the drift-free Chalk outcrop in the north of the region. The Gault Clay is absent in this area allowing water recharged to the Chalk to flow into the Sands beneath. Lower concentrations of excess air are found in the southern groundwater type indicating a different recharge mechanism. This groundwater is recharged through the base of an incised channel infilled with Quaternary deposits, which provides a permeable conduit from the Chalk into the Sands, bypassing the confining clays. Therefore, the hydrochemical differences between the types result from different recharge mechanisms. This study is able to directly link observed water table fluctuations with the measured degree of neon supersaturation (~Ne) in groundwater for the first time. The results show excellent agreement to the theoretical relationship.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:435116 |
| Date | January 2005 |
| Creators | Ingram, Richard George Stanley |
| Publisher | University of East Anglia |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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