Catalyzes the regioselective dehydration of galactarate into 2-keto-D-threo-4,5-dihydroxyadipate ((2S,3R)-dihydroxy-5-oxohexanedioate). Is not active on other acid sugars. galactarate = (2S,3R)-dihydroxy-5-oxohexanedioate + H2O Binds 2 magnesium ions per subunit. kcat is 6.8 sec(-1). The enzyme product is the enantiomer of the product obtained in the galactarate dehydratase reaction catalyzed by STM3697; the enzymes thus differ in their regiochemistry of dehydration. Belongs to the mandelate racemase/muconate lactonizing enzyme family.