Effect of mining rejects on the nutrient chemistry of Mandovi estuary, Goa

Nutrient chemistry in Mandovi estuary during premonsoon is affected by the discharge of mining rejects. Concentrations of nitrate and phosphate, in ge neral, show low levels in this season (nitrate 0.3-3.3 mu M, phosphate 0.11 -0.56 mu M) while silicate show wide range in concentration (1.25-130 mu M) . Elevated concentrations of nitrate (6.1-9.1 mu M) associated with reduced levels of phosphate (<0.1 mu M) and silicate (50-60 mu M) in the mid-estua ry, near the discharge point of mining rejects, suggest that the suspended sediments derived from the mining rejects adsorb phosphate and silicate, an d remove them from the water column while the mining rejects act as a sourc e of nitrate to the estuary. Large quantities (10 tonnes/month) of ammonium nitrate used in explosives for blasting the hard rock at the mining site s eem to be the source of nitrate in mining rejects. The discharged nitrate i s carried both upstream and downstream of the outfall by the tidal currents . Its distribution in the estuary is studied with respect to mixing process es. A plot of predicted nitrate resembled the observed nitrate distribution over similar to 70 km. However, the observed nitrate levels were always lo wer than the predicted ones suggesting biological uptake of this nutrient.