Evaluating the contribution of point and non-point sources of nitrogen pollution in stream water in a rural area of Central Hokkaido, Japan

Abstract There is an increasing concern about nitrogen (N) pollution of surface water for which agricultural activities can be identified as non-point sources. Drainage from manure storage sites in large-scale livestock husbandry operations and drainage from landfill sites are considered to be point sources of N pollution that severely affect the quality of stream water. We evaluated the contribution of point as well as non-point sources of N pollution in stream water in the Etanbetsu River drainage basin, which is located in Asahikawa city, Central Hokkaido, Japan. Water sampling was carried out at 17 sites in the river both in the snowmelt and in the post-rice-planting seasons in 1997. Various drainage areas such as tributaries, open-ditches, and tile-drains from agricultural fields, wastewater from a pig farm, and a landfill leachate were also sampled at 47 and 49 sites in total in the snowmelt and post-rice-planting seasons, respectively. The results showed that the total N (T-N) concentration in the upper reaches of the Etanbetsu River was as low as <0,7 mg L−1 in the snowmelt season and <0.3 mg L−1 in the post-rice-planting season. The concentration increased at the outlet of the Etanbetsu River to values as high as 1,6 mg L−1 in the snowmelt season and 2,0 mg L−1 in the post-rice-planting season. This increase in the TN concentration was mainly due to landfill effluent and wastewater from a pig farm. The landfill leachate showed a remarkably higher TN concentration of 96 mg L−1 in the snowmelt season and 162 mg L−1 in the post-rice-planting season, followed by a TN concentration of 33 and 34 mg L−1 in the pig farm wastewater in the respective seasons. The TN load showed a similar pattern to that of the TN concentrations. In the upper reaches of the Etanbetsu River, the N load was as low as <4 g s in the snowmelt season and <0.4 g S−1 in the postrice-planting season. The N loads became as high as 26 and 4.8 g S−1 in the respective seasons at the downstream site due to the landfill effluent and the wastewater from a pig farm. These results indicated that the landfill site and the pig farm acted as major N pollution sources affecting the stream water quality. We estimated the contribution of these two point sources together with that of forested and agricultural lands to the stream water TN load, The results showed that, at the upstream sites, the contribution of agricultural land was 44–69% in the snowmelt season and 56–63% in the post-rice-planting season, indicating that agricultural non-point sources play a major role in the stream TN load. In the entire Etanbetsu River basin, however, the contribution of the point sources of landfill effluent and wastewater from a pig farm was estimated to be extremely high. Estimates of the riverine N load were as high as 90% and 95% in the snowmelt season and in the postrice-planting season, respectively. If these point sources were removed, the stream TN concentration would be as low as 0.19-0.2 mg L−1 in the snowmelt season, which may allow the TN concentration to be maintained within the range of TN concentrations commonly found in forested streams in Hokkaido. <