On the N side of the London basin the principal representative of the Lower Greensand is the Woburn Sands, which outcrops in Bedfordshire and Cambridgeshire.
Where the outcrop is covered with till, recharging waters contain higher concentrations of chloride than those recharged in drift-free areas. This is considered to be due to higher evapotranspiration rates where the soil is underlain by boulder clay. Indirect recharge from influent streams and mixing with old saline waters also cause localized increases in chloride concentrations.
Average, annual, natural direct recharge for till-free areas, calculated from average annual rainfall and average chloride concentrations in the recharge and rain waters, agrees closely with published figures calculated using Penman and Grind-ley's equations. Approximate values for recharge through boulder clay are calculated in this way and the merits and limitations of this method for estimating recharge are discussed. The amount of indirect recharge from influent streams is also estimated.