Spatial distribution of nitrate sources of rivers in the lake Biwa Watershed, Japan: Controlling factors revealed by nitrogen and oxygen isotope values

Abstract

To clarify the pathways and origins of dissolved nitrate (NO-3) in rivers flowing into Lake Biwa, Japan, three types of coordinated surveys of concentrations and isotope compositions of NO-3 were conducted: (1) synoptic river sampling of 32 representative inflow rivers, (2) two rivers in catchments with different land uses, and (3) headwater catchment samplings. The data set used in this paper is the first comprehensive collection of isotopic composition of NO-3 in rivers of a large-scale basin in Asia. While the NO-3 spatial concentration pattern showed significant seasonal variation, the spatial pattern of the δ15N of dissolved NO-3 (δ15NNO3) was more consistent throughout the year. The δ15NNO3 was significantly positively correlated with the population density of each catchment. A mass balance model assuming the δ15NNO3 and the flow rate of sewage effluent was developed. The model simulated the relationship between the population density and the δ15NNO3 reasonably well, suggesting that the dominant source contributing to the increase in δ15NNO3 was the sewage effluent. The spatiotemporal distribution of δ18ONO3 suggested the possibility of the addition of atmospherically derived NO-3 through precipitation and was indicated by the hydrological pathways for NO-3 exports in different land uses. In forest-dominated catchments with natural drainage systems, a slightly elevated x03B4;18ONO3 signal remained in the stream water even during base flow conditions. This study demonstrated that multiscale, multi-isotopic investigation is a promising strategy for describing the spatial distribution of NO-3 sources synoptically and is useful for evaluating the influences of land use change.