Changes in domestic water supply appearance and quality linked with increasing manganese amount and iron at a lesser degree, having been found by the inhabitants of Beaucaire (Rhone Valley), a study was made by chemists, hydrogeologists and biologists to detect the origin. The problem is not specific to this town and can frequently be found in the Sa#xF4;ne and Rhone valleys. The results show that we have to deal with an aquifer with semi-captive groundwater whose supply is double: the Rhone and calcareous mountain masses. The low-oxygen waters are enriched, by dissolution from gravel, with manganese and iron which are preferentially fixed on porous and cracked surface elements. The conditions for manganese liberation are connected with the presence of organic matters, bacteria (capable to reduce the manganic compounds) and anaerobiosis. The supply does not seem to be the only cause in the low-oxygen contents; the action of heterotrophic microorganism using organic matter and consuming oxygen, at the groundwater periphery, must be taken into account. Thus the central part of the groundwater system is a reducing environment which allows the modification of the bacterial metabolism towards a ferric and manganic compounds reduction process. The result is the transformation of insoluble manganic and ferric compounds into manganous and ferrous compounds which are liberated into the groundwater.
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