The sources and dispersal of nitrate in multiple waters, constrained by multiple isotopes, in the Wudalianchi region, northeast China

Abstract

The Wudalianchi scenic area in NE China has been named an UNESCO "Global Geopark" and "Biosphere Reserve." During this investigation, the sources of nitrate and the hydrologic system through which it is dispersed were assessed using geochemical data and a multiple isotopic approach. The cold waters from the south and north springs originated from the deep subsurface. Isotopically, these waters exhibited relatively negative δD and δ¹⁸O values and nitrate in the water was substantially depleted ¹⁵N, suggesting that the mineral water was primarily derived from depth. Lakes within the Wudalianchi region were primarily composed of water from these deep mineral springs and precipitation. Chemical fertilizers were the primary source of nitrate to the Wudalianchi lakes. Groundwater was found in shallow mineral springs and wells plotted above the local meteoric water line, implying that shallow groundwater was primarily derived from precipitation. Elevated concentrations of nitrate in shallow mineral springs and well waters during the summer, autumn, and winter suggest that shallow groundwater within the Yaoquan volcanic area was also polluted by nitrate from human activities. Denitrification of shallow groundwater is slow, reducing the potential for "self-remediation". The concentration data are supported by nitrogen (N) isotope data; wells and springs exhibited N isotopic ratios between − 5‰ and + 5‰ (typical of fertilizers and precipitation) and exhibited higher oxygen (O) isotope values than water in the Wudalianchi lakes. These relationships suggest that nitrate in shallow mineral springs, wells, and lakes near the Yaoquan volcano was derived from the mixing of chemical fertilizers with local summer rainfall.