Evaluation of a Shallow Leaky Water Table Aquifer in a Subhumid Region of France

Abstract

The Arcachon Bay Plio‐Quaternary aquifer, located in a flat poorly‐drained region which receives one m annual precipitation, has a shallow water table which impedes agricultural development. It is composed of a 50‐100 m thick body of well‐sorted clean sand and gravel which has excellent hydrodynamic properties and much fresh ground water in storage. This affluent aquifer is underlain by, and interconnected with, a relatively poor calcareous aquifer of Miocene age. Leakage to the Miocene aquifer, inferred from geologic and hydrogeologic observations, was calculated by water budgeting and Darcy's law to be on the order of 1.5 m3/s. This rate, about 15 percent of the annual precipitation, is nearly half the recharge rate to the Plio‐Quaternary aquifer. Schoeller's chloride method was used to estimate recharge to the Plio‐Quaternary aquifer, and indicated that this recharge amounts to more than 35 percent of the annual precipitation. The calculation of the water budget supported this finding. The evapotranspiration rate, based on Thornthwaite's method, is more than 55 percent of the annual precipitation. High recharge and evapotranspiration rates, a direct consequence of the relief, permeability of soils, and occurrence of hot summers, are coupled with a low surface runoff. Exploitation of the Plio‐Quaternary aquifer can be planned confidently for irrigation and water‐supply purposes. Drainage improvement, needed for extensive farming of the lands, will also result from such exploitation. These development measures are suggested to be tailored and staged in the framework of the master plan aimed to develop and conserve the Arcachon Bay area and the region it is part of. Descriptors: subhumid environment, shallow water table, multiple‐aquifer systems, leaky aquifers, Darcy's law, aquifer recharge, Schoeller's chloride method, water budget, ground‐water quality, environmental hydrogeology, development and conservation. Identifiers: shallow leaky aquifers in subhumid environment.