Nitrate and pesticide contamination of groundwater under rice-based cropping systems:: past and current evidence from the Philippines

The quality of shallow groundwater used for domestic purposes by the rural poor in developing countries is threatened by contamination with nitrate and residues of pesticides used in agriculture. In Asia, many of the rural poor live in rice-producing areas where the continuous flooding of rice (Oryza sativa) fields may lead to high leaching rates of groundwater contaminants. This study presents results of measurements of nitrate and pesticide concentrations in shallow groundwater in 54 domestic wells under rice-based production systems in Luzon, Philippines, from 1989 to 2000. In irrigated double rice cropping areas, seasonal-mean nitrate concentrations were 0-2 mg 1(-1), which is below the WHO limit of 10 mg 1(-1) for drinking water. In wet season (WS) rainfed rice and dry season (DS) irrigated sweet pepper (Capsicum annuum) double cropping areas, monthly mean nitrate concentrations were 5-12 mg 1(-1). Nitrate concentrations were lower under other, less fertilized upland crops, and with a high percentage of the area under rice in the WS. Mean pesticide concentrations were generally one to two orders of magnitude below the WHO single (0.1 mug 1(-1)) and multiple pesticide (0.5 mug 1(-1)) limits, although temporary peak concentrations of 1.14-4.17 mug 1(-1) were measured. For both nitrate and pesticides, the high leaching potential appears to be offset by the particular transformation and loss processes taking place under tropical, anaerobic conditions. The agro-ecological environment and history of the study sites suggest that the results may be characteristic of many parts of tropical Asia where rice-based cropping has intensified since the Green Revolution in the mid-1960s. (C) 2002 Elsevier Science B.V. All rights reserved.