The salinity of well water has trebled over the last six years in Chennai city outskirts owing to increased sea water intrusion, CPR Centre Honorary Director Nandhita Krishna said on Saturday.The intrusion is due to over-exploitation of ground water and and inadequate recharge, Nandhita Krishna explained at a seminar on rain water harvesting (RWH) organised by the Indian Officers' Association.This is the finding of a survey of the CPR Centre, on the quality of ground water in 150 bore wells and open wells from Ennore to Injambakkam since 1996, she said.