Assessment of the impact of agricultural pesticides on water quality of Laguna de Bay [Philippines]

Laguna de Bay is the second largest freshwater lake in Southeast Asia covering a surface area of 90,000 ha. The lake is composed of three bays, each of which is threatened by varying degrees of pollution. The western bay is threatened by urban point source pollution. The eastern bay is affected by agricultural use in its catchments. It receives drainage from the agricultural land uses in Pagsanjan-Lumban catchments located in the southeastern part of the Laguna de Bay. The Pagsanjan river discharge to the Eastern Bay and accounts from 35% of the lake's total fresh water inflow. Intensive agriculture covers an estimated 50% of the sub catchment area. Risk assessment of pesticides using the Pesticide Impact Rating Index (PIRI) showed that all pesticides used in tomato production were rated by PIRI to have a very low surface water concentration at the range of 3-5% OM and 3-18% slope. In terms of toxicity, the assessment ranged from very high for L-cyholothrin at 3-5% OM and 8-18% slope, high impact for cypermethrin, low impact for malathion and very low impact for the rest of the pesticides used by tomato growers. All the pesticides used by farmers for bittergourd production were rat ed by PIRI to have a very low impact on surface water contamination at 3-5% organic matter and at slope of 3 and 8%. However, L-cyhalothrin had a very high toxicity, cypermethrin with medium toxicity impact and a very low toxicity impact of profenofos. These can be attributed to the dense growth of weeds and other plants at the sides of the channel thereby reducing the movement of pesticides into the canal. Residues of insecticides used by vegetable farmers in Majayjay, Laguna were detected in the surface water and sediment at the three sampling sites along the irrigation channel diverted from the Balitian River. It was difficult to correlate the levels detected with time of spray application as the various crops were present at different growth stages at any given time during the monitoring period. The presence of these pesticides reflects the degree of its water solubility and its hydrophobicity. The frequency of detection of these pesticides as well as its concentration may be due to agricultural activities occurring in the vicinity, water solubility, environmental persistence and analytical detection. The effect of drift during the application of these insecticides cannot be discounted however since PIRI assessment predicted that these chemicals would have low surface water mobility. In-situ bioaasay demonstrated the effect of presence of residues in water as mortality to Tilapia fingerlings were observed at the three sites. The study demonstrated that use of pesticides in agricultural activities can lead to non-point source of water pollution by pesticides. Practices to mitigate movement of pesticides to bodies of water must be in place.