Much of Abu Dhabi's groundwater is too saline to be used for domestic consumption. Fresh groundwater (with salinities up to 1,000 mg/l) makes up only a small proportion of the total groundwater reserve. The two primary sources of fresh groundwater occur in the interior beneath the sand dunes north of the Liwa Crescent and in the shallow aquifers near Al Ain. While the aquifers near Al Ain are periodically recharged by run-off from the Hajar Mountains, those near Liwa are ancient, non-renewable reserves that were formed at a time when the climate of the Emirate was much wetter than it is today.