The effect of agriculture on water environment in a subtropical island

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The purpose of this study was to analyze the effect of agriculture on water environment in a water-starved subtropical island. Most irrigation ponds were categorized as in eutrophic states and water blooms were observed. It is suggested that runoff from livestock is the main nutrient source. Rainfall runoff from farmland had few eutrophying effects on inflowing loads, because the runoff mainly consisted of roadway discharge and so relevant concentrations were low. Nitrate nitrogen concentrations of groundwater were high, assuming that the nitrate contamination originated from fertilizer and manure. The values and trends of nitrate concentrations at monitoring wells varied greatly. These results imply horizontal groundwater flow is not uniformly distributed and the each well gathers water from vicinal area. It was estimated that around 22% nitrogen output from human activity leached to the groundwater.