Pesticide residues in surface waters of Pagsanjan-Lumban catchment of Laguna de Bay, Philippines

The quality of the water of Laguna de Bay is been threatened by industrial and domestic activities in its western bay and agricultural activities in its eastern bay contribute to nutrient and pesticide discharges affecting the lake. A pesticide monitoring study of surface water of the Lucban River and Salasad Creek in the Pagsanjan-Lumban watershed of Laguna de Bay showed that some of the pesticides used by farmers to protect their crops from pests are being transported in drainage water. In the Lucban River, malathion was detected throughout the monitoring period from 2007 to 2009 at concentrations ranging from 0.005 μg/L to 3.3 μg/L. The detections of pesticides corresponded with times of pesticide application in the field. Profenofos was detected at concentrations of 0.5–15.3 μg/L during 2008–2009 in water of Salasad Creek. The concentrations exceeded the (WHO, 2006) recommended level of 0.1 μg/L for single pesticide in drinking water. There was no direct relationship between total suspended sediments and concentration of pesticides present in the surface water of Lucban River and Salasad Creek. Malathion and profenofos were transported from site of application to water bodies predominantly (>60%) in the soluble (<1.2 μm) phase. Although the concentration of malathion increased in January with increasing rainfall the trend was not consistent throughout the study period. Measures to mitigate the off-site movement of malathion and profenofos to surface water need to be implemented to protect the quality of the eastern bay of the lake. This study shows that pesticide applications to crops very close to the water bodies, a common practice in the Philippines, is responsible for contamination of the creek and river, and a first step in reducing contamination would be improve practices in the areas closest to the drainage system, perhaps by introducing a narrow (3–5 m) riparian buffer zone in which spraying in not permitted. However, this strategy would have limited effectiveness for compounds that move predominantly in a soluble phase.

► Pesticide application to crops very close to rivers can be a source of contamination of creeks and rivers.
► Time of application and distance of field to bodies of water influence concentration of pesticides in rivers.
► Malathion and profenofos were transported off-site in the soluble phase.
► Mitigation strategies must consider mode of transport of pesticides to bodies of water.