NITRATE IN GROUNDWATERS OF THE CENTRAL VALLEY, COSTA RICA

摘要

Nitrate contamination of ground water under agricultural areas is a well recognized environmental problem, yet few reports describe the problem in tropical regions. This study evaluated concentrations of NO_3-N in shallow aquifers used to provide drinking water to the inhabitants of the Valle Central in Costa Rica. The watershed that recharges these aquifers is an agricultural and urban catchment that may leach nitrate derived mainly from fertilization (that averages 270 kg/ha per annum as nitrogen applied to coffee) and sewage contamination. Aquifers were sampled on an elevation gradient from 780 to 1440 m, along which land use changes from urban and agricultural uses at low and mid-elevations to unfertilized pastures and forests at the highest elevations. Ground water was sampled once during an extensive survey of 56 wells and springs, and during a monthly collection of 14 wells and springs over a 25-month period. Concentrations of NO_3-N were closely related to elevation and averaged 0.37 μg/mL in aquifers at 1600 to 2300 m, 1.70 μg/mL at 1200 to 1300 m, and 4.4S μg/mL at 900 to 940 m. There were significant variations ia concentrations of NO_3-N at the same elevation. Water quality monitoring should be expanded to determine the distribution of wells and springs with high nitrate concentrations throughout the Central Valley and to identify the potential sources of nitrate contamination. Measures directed to improve the efficiency of nitrogen fertilizer and to modernize sewage management practices are needed to help ensure the water quality of these productive aquifers.