Nitrate pollution of surface water induced by agricultural non-point pollution in the Pocochay watershed, Chile

Diffuse nitrogen pollution caused by irrigated agriculture is quantified for the Pocochay stream located in the Aconcagua watershed in the Valparaíso region, Chile. The study area has an extent of 93 km2 with 36 km2 under irrigation. It is representative for the central part of Chile where intensive irrigation is prevailing and predominantly taking place in the alluvial plains of river systems with water sources from the Andes Mountains. Water quality and discharge were monitored biweekly at several points of the Pocochay main stream from December 2003 to June 2006. The study was supported by an agricultural survey and remote imagery interpretation in order to retrieve data on cropping pattern and actual fertilizer uses in the sub-watersheds. Nitrate – N concentration in the Pocochay stream ranged from 1.3 to 10.1 mg L−1 with a mean of 4.8 mg L−1. Other nitrogen species showed very low concentrations. At average, around 52 kg N ha−1 a−1 are drained from the study area as compared to 167 kg N ha−1 a−1 applied as fertilizer to the fields. Even though nitrate concentrations in the Pocochay do not reach alarming levels yet, the impact of irrigated agriculture on surface water quality is obvious and makes a case for introducing best management practices to avoid aggravation of contamination.

The study reveals a significant impact of irrigated agriculture on N-content in the natural drainage of the study area, the Pocochay stream. Nitrate concentrations were in the range of 2–10 mg L−1 showing, in general, the highest values during the rainy season. N-loadings in the stream were related to the agricultural area and between 36.8 and 67.7 kg ha−1 a−1 are found in surface waters after subtracting other than agricultural sources of nitrogen. If irrigation and fertilizer application would be more intensified in the future a substantial increase in nitrate concentration and load could be expected, as those sub-watersheds with high N-fertilizer input already show elevated concentrations in nitrate concentration and load. The relatively high nitrate loading ratios could be explained through (a) the application of predominantly high soluble N-fertilizers and (b) through high amounts of irrigation water applications. Proper agricultural management practices need to be introduced avoiding over-irrigation by farmers. Here, environmental benefits and economic benefits for farmers due to more efficient fertilizer use could go hand in hand. In continuation, the study of agricultural impacts in the Pocochay watershed will quantify other elements of the hydrological and nitrogen cycle in order to result in more reliable estimates of the sources of N-contamination in the surface water and to include the groundwater system in the analysis of spatio-temporal variability of water and nutrient behaviour.