NEW DELHI, NOV 23: Delhi groundwater is contaminated with harmful fluorides and nitrates, far beyond maximum permissible limits in some pockets, warn scientists.

The heavy contamination is due to leaching of these chemicals along with irrigation water and surface run-off from surrounding areas, says the study by scientists at the Nuclear Research Laboratory (NRL) at the Indian Agricultural Research Institute (IARI) here.

The contaminants, concentrated mostly in West Delhi blocks, are moving towards central urban area along specific flow pathways due to hydraulic gradient, and are polluting freshwater sources too.Almost half of Delhi's groundwater is laced with fluorides and one-third with nitrates, an IARI team headed by Dr P S Datta found. .The high fluorides are mainly due to brick kilns which use fluoride salts for brick manufacture and agro-chemicals that contain fluoride salts.

High nitrate levels have been attributed to fertilisers, domestic sewage, industrial waste, livestock feeding operations and septic tanks. The high amounts could have adverse effects on crop growth and human health.

Groundwater, which lies below the geological strata containing clay, soil and gravel, is being used extensively in Delhi for both irrigation and raw water requirement.

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