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Fouling of groundwater injection/recovery systems can lead to performance deterioration and increased maintenance costs. Fouling can be due to the mixing of incompatible waters within a well and/or changes in the contaminant water due to the well environment (pumping, stagnation). The impact of fouling upon monitoring, recovery and reinjection well performance at three sites were investigated. Detachment of a sulphide dominant biofilm in a monitoring well during sampling could potentially have created a significant sampling error. In recovery wells the interval between well rehabilitation was found to be related to the mixing ratio between an alkaline contaminant plume and cation rich groundwater. Declines in injection well rates of 75-94% over 75 days occurred due to iron sulphide precipitates. Control strategies using well design changes, chemical and mechanical treatment and equipment selection are outlined.