Nitrate contamination of groundwater: impact from intensive rice-based cropping systems across Ilocos Norte [Philippines]
2001
Castaneda, A.R. | Bouman, B.A.M. | Tuong, T.P. (International Rice Research Inst., Los Banos, Laguna (Philippines). Crop, Soil and Water Science Div.) Bucao, D.S.
The assumption is that nitrate contamination of groundwater, degrades the quality of drinking water in rural areas where farmers have for decades practiced intensified rice-based cropping systems with high rates of fertilizer use. From 1997 to 1999, a study was conducted across Ilocos Norte to evaluate the nitrate-nitrogen (NO3-N) concentration in shallow groundwater. Water samples were collected from field tube-wells used for irrigation and household tube-wells used for drinking. Water samples equipped with porous cups were installed in the field at different depths to trace any nitrate residue movement within the soil profile. Fertilizer use was documented by interviewing randomly selected farmers. Across Ilocos Norte, the contamination differed depending on the intensity of cropping systems. High fertilizer N applications of more than 500 kg N/ha in rice-sweet pepper resulted in higher groundwater contamination, exceeding the allowable maximum contamination level (MCL) of 10 mg/L set by the World Health Organization. Lower concentrations were found with rice-rice and rice-fallow systems, plus or minus 1 mg/L. The concentration was slightly higher during the dry season (DS) than during the wet season (WS) because of the dilution effect of rainfall. Most data across Ilocos Norte ranged from 1.08 mg/L in irrigation water for a WS rice-rice system, to 9.01 mg/L in drinking water in DS. Drinking water contained slightly higher NO3-N concentration than irrigation water probably due to aquifer stability underneath residential areas, resulting in less dilution. Concentration was slightly lower during WS than DS because of rainfall dilution effects. The buildup of groundwater contamination by NO3-N is the result of high fertilizer inputs in intensive and sustained cropping systems since the 1970's. The situation is not too alarming but is enough to cause concern to call for reduced rates of sustained fertilizer N used-based cropping systems
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