The Impact Of Tropical Deforestation On River Chemical Pollution

The objective of this study was to provide a quantitative description of the impact of
converting tropical forests into pastures for cattle grazing in terms of the sediment,
nitrogen and phosphorus concentrations at the outlet of a river basin located in North
West Costa Rica. The Soil and Water Assessment Tool (SWAT) was utilized to model
the watershed. The graphs for pollutant concentration vs. percent area of the watershed
under pasture showed a monotonic increase in concentrations as deforestation
increased. Contaminant levels for the entire watershed as grassland were between 3
and 8 times higher than for the total area as forest, which could put at risk the drinking
water supply and the tourism-based economy of the region. Keeping the current
percentage of area under grassland constant, but restricting pastures to the less fragile
and more fertile lands could decrease the sediment, nitrogen and phosphorus
concentrations at the main watershed outlet by more than 35%, 12% and 21%,
respectively, as compared to the predicted pollutant concentrations for the current land
cover distribution. The effect of varying cattle stocking rates resulted in lower sediment
and nitrogen run-off for areas with higher animal loads in which confined operations are
used part of the year.