S. Puri writes: In his studies Irving (1982) has used chemical analyses obtained from pumped wells over a period 1883 to mid 1970's to suggest that the chloride ion concentration is an aid to estimating recharge derived from rainfall or via leakage through rivers. He has applied this method to the aquifers of the whole of Lower Greensand formation in the Anglian region: Woburn Sands is a local name for the ‘silver sands’, found in only a limited area in the Anglian Region, and is a member within the Lower Greensand formation.

A more detailed study of the hydrochemistry of this acquifer (Binnie & Partners 1982) reveals a rather complicated system which casts doubt on this technique. On the author's own admission there are many unexplained observations of anomalous chloride concentrations, e.g. Clophill and Histon. (Southill, according to the grid reference given, falls well off the unconfined outcrop.) Fig. 1 shows the Piper plot for the chemistry of Bir-chmoor pumping station, which is situated on the unconfined aquifer. There are eight abstraction holes to differing depths within an area of less than 1 km2. The Cl concentration range is between 20% and 60% of the total cations. We consider these chemical variations to reflect the layered hydrochemical nature of the water in the acquifer. Thus, if a pumped well were abstracting from a greater saturated thickness, its chloride ion content would be different from that abstracting a smaller thickness. Application of the author's method would give

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