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[AHW27-12] Simple method to estimate pollution loads from non-point sources and its applicability
Keywords:landuse, non-point source, pollution loads, groundwater, river water
The study area consisted of 26 watersheds (1,342 km2) covering 72% of Kagawa Prefecture in Japan. We estimated NO3-N concentration factors in groundwater for upland fields, paddy fields, forests and urban landuse types by performance of a multiple regression analysis of the watershed-mean groundwater NO3-N concentrations and the landuse ratios in each of the 26 watersheds. The results showed that the NO3-N concentration factor, which was gained as the partial regression coefficient for the multiple regression analysis, in groundwater was 15.2 mg L-1, 10.3 mg L-1, 2.3 mg L-1, and 2.5 mg L-1 for the upland fields, paddy field, forests, and urban landuse types, respectively. N pollution loads runoff for river water and groundwater were calculated by multiplying total-N concentration factors for river water by river flow rate, and by multiplying NO3-N concentration factors for groundwater by groundwater flow rate. The total N pollution loads runoff were 26.6 kg ha-1 y-1, 12.6 kg ha-1 y-1, 2.8 kg ha-1 y-1, and 8.8 kg ha-1 y-1, for upland fields, paddy fields, forests, and urban areas, respectively.
Applicability was investigated for one river watershed, where spring water quality was measured in the periods 1994-1995 and 2007-2008. NO3-N concentration increased with stream downward in 1994-1995, however, the tendency diminished in 2007-2008. River T-N concentrations were 2.5 mg L-1 and 2.4 mg L-1, and groundwater NO3-N concentrations near the coast were about 8 mg L-1 and 6 mg L-1 for the two periods, respectively. On the other hand, cropland ratios were 23.8% and 21.4%, and surplus-N from croplands (applied-N - absorbed-N by crops) was calculated as 31.1 kg ha-1 and 27.2 kg ha-1 from cropland, and surplus-N from the whole watershed was 43.2 kg ha-1 and 39.7 kg ha-1 for the 2 periods, respectively. The arrival of N loads to the sea by river and groundwater was calculated to be 51% and 48% of the whole surplus-N in the watershed for the 2 periods, respectively. These results are in line with other reports in which pollution load arrival is about half or below half of the pollution load generated.