All types of aquifers have been used for ASR, but in general ASR is easier to manage in consolidated aquifers where the formation provides a competent well without the requirement for screen and gravel pack. Carbonate aquifers are often preferred due to the offsetting effects of carbonate dissolution on well clogging. Fractured rock aquifers, even low yielding ones, have been used successfully for ASR with injection rates in some wells exceeding airlift yields. Coarse grained sand and gravel are also very suitable for ASR storage targets, but care needs to be taken with well construction and completion, and to reduce as much as possible the concentrations of organic and colloidal material introduced into the well. Storage in fine-grained unconsolidated media is more problematic, and requires water with very low nutrient and colloidal concentrations in order to avoid chronic and irrecoverable depletion of specific capacity of the ASR well. Generally confined aquifers are preferred to unconfined aquifers because water quality is better protected from overlying land uses and to avoid the potential complications of an undesirable rise in water table elevation.