GROUNDWATER CONTAMINATION BY RICEFIELD PESTICIDES AND SOME INFLUENCING FACTORS

An analysis of 270 groundwater samples collected over 1989-1991 was ma de for determining ricefield pesticide residue concentrations. The sam ples were collected from 53 tubewells located within ricefield boundar ies in irrigated and rainfed areas and used for domestic water supply. Mean concentrations of six commonly used pesticides ranged from 0.002 ppb for chlorpyrifos to 0.209 ppb for monocrotophos. The maximum conc entration Found in a single sample was 3.19 ppb for monocrotophos. End osulfan was present in 79% and monocrotophos in 54% of all samples. Ir rigated environment, lighter soil texture, and wet season rice culture condition contributed to higher concentrations of pesticides in the g roundwater. Increasing tubewell depths generally indicated a decreasin g concentration trend. The study indicates that pesticides in groundwa ter samples had their residues in excess of the limit of 0.1 ppb for s ingle pesticide and 0.5 ppb for multiple pesticides set by internation al agencies. Using these standards, groundwater of the study site migh t be considered to be threatened by excessive use of ricefield pestici des. However, toxicological standards established by FAO/WHO indicate that the water is safe for human consumption. Development of managemen t models capable of predicting residue behaviour within the soil profi le is necessary.