Cambridge Journals Online

GROUNDWATER DYNAMICS AND QUALITY UNDER INTENSIVE CROPPING SYSTEMS

A study was conducted from October 1994 to March 1996 to assess groundwater dynamics and quality in relation to landuse and farm input of nitrogen fertilizer in a highly diversified and intensive agricultural area at Magnuang, Batac, Ilocos Norte. Monthly groundwater depths, nitrate-nitrogen (NO₃-N), chloride (Cl^{[minus sign]}), bicarbonate (HCO₃^{[minus sign]}), electrical conductivity (EC) and pH were determined in 19 agricultural and domestic wells. In the dry season, sweet pepper (Capsicum annuum) had a higher irrigation requirement and caused more groundwater level decline than other crops. EC (700–3000 [mu]mho cm^{[minus sign]1}) and HCO₃^{[minus sign]} (90–500 ppm) in all wells exceeded the FAO threshold quality for irrigation but were not related to farm management practices. Eight wells showed near or above the World Health Organization NO₃-N limit (10 ppm) for drinking water. High nitrogen fertilizer input increased the mean NO₃-N (r² = 0.45, p < 0.002). The percentage of the wells' service area under rice cultivation in the wet season accounted for 84% (p < 0.001) of the variation in NO₃-N among the wells. The mean NO₃-N declined as the percentage of service area under rice increased. This was related to the denitrification process in the flooded fields and the lower levels of nitrogen fertilizer for rice compared with other crops.