Using major ions and δ15N-NO3− to identify nitrate sources and fate in an alluvial aquifer of the Baiyangdian lake watershed, North China Plain

Abstract

In semi-arid regions, most human activities occur in alluvial fan areas; however, NO₃⁻ pollution has greatly threatened the shallow groundwater quality. In this paper, δ¹⁵N-NO₃⁻ and multi-tracers were used to identify the origin and fate of NO₃⁻ in groundwater of the Baiyangdian lake watershed, North China Plain. The investigation was conducted in two typical regions: one is the agricultural area located in the upstream of the watershed and another is the region influenced by urban wastewater in the downstream of the watershed. Results indicate that the high NO₃⁻ concentrations of the upstream shallow groundwater were sourced from fertilizer and manure or sewage leakage, whilst the mixture and denitrification caused the decrease in the NO₃⁻ concentration along the flow path of the groundwater. In the downstream, industrial and domestic effluent has a great impact on groundwater quality. The contaminated rivers contributed from 45% to 76% of the total recharge to the groundwater within a distance of 40 m from the river. The mixture fraction of the wastewater declined with the increasing distance away from the river. However, groundwater with NO₃⁻ concentrations larger than 20 mg l⁻¹ was only distributed in areas near to the polluted river or the sewage irrigation area. It is revealed that the frontier and depression regions of an alluvial fan in a lake watershed with abundant organics, silt and clay sediments have suitable conditions for denitrification in the downstream.