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Baseline groundwater chemistry : the Palaeogene of the Thames Basin

By J.M. Bearcock and P.L. Smedley

Abstract

This report describes the regional geochemistry of groundwater from the Palaeogene aquifer\ud of the Thames Basin. The aim of the study is to assess the likely natural baseline chemistry of\ud the groundwater by identifying the dominant controlling processes. Data have been collected\ud from strategic sampling of 19 new groundwater samples, along with collation, from various\ud sources, of existing groundwater, rainfall, mineralogical and geochemical data.\ud The Palaeogene aquifer results from the cyclical deposition of shallow marine and coastal\ud sediments and comprises sand and clay layers which display large lateral and vertical\ud variation. This variation gives rise to localised regions of productive aquifer, and localised\ud geochemical variations. For these reasons the Palaeogene strata only provide minor aquifers\ud of regional significance. That said, these can be important when managing the water supply in\ud London and the surrounding urbanised areas.\ud The geological differences between formations within the Palaeogene strata mean that the\ud water type is generally a continuum between Ca-HCO3 type to Ca-SO4 type. Other differences\ud in groundwater quality caused by geological variation are evident in the minor and trace\ud element concentrations.\ud The main water quality problems are associated with oxidation of pyrite, which occurs\ud sporadically throughout the Palaeogene beds, and has the ability to produce acidic, hard,\ud metal-rich waters. In beds underlying the confining London Clay, this has occurred as a result\ud of anthropogenic groundwater extraction from the Chalk below. In beds above the London\ud Clay this process can occur entirely naturally, owing to a ready supply of oxygen and water.\ud In addition human impacts are mainly visible in the presence of indicator contaminants, such\ud as nitrate. The widespread presence and increasing concentrations of nitrate indicate the\ud extent to which the unconfined aquifer is influenced by modern farming practices or urban\ud pollution. Trace metals associated with the oxidation of pyrite within the aquifer (Fe, Ni, Co)\ud also affect the water quality. Whilst these are released as a result of natural geochemical\ud reactions, historic pumping regimes have also exerted an influence. Lead is found in elevated\ud concentrations (up to 13.3 μg L-1) in samples taken in close proximity to major roads, and it is\ud likely that these derive from the past usage of leaded petrol in motor vehicles.\ud While some of the groundwaters investigated in this study are of good inorganic quality, and\ud of a soft nature, there are many notable exceptions which contain elevated concentrations of\ud Fe, SO4, Ni and Co. The distribution of these is generally so sporadic that water quality would\ud be difficult to predict prior to drilling a borehole. It is recommended to drill into the\ud underlying Chalk for a groundwater source of more reliable quality and quantity

Publisher: British Geological Survey
Year: 2010
OAI identifier: oai:nora.nerc.ac.uk:12600

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