Abstract
Nitrate pollution is an environmental problem in the North China Plain. This paper investigates the variation of groundwater levels and nitrate concentrations in an alluvial fan of the Luanhe river, northeast of the North China Plain. Three transects perpendicular to the riverbank were selected to investigate the exchange between river water and groundwater, and nitrate concentration with its isotopic composition (δ15N-NO3 and δ18O-NO3). The results showed that the groundwater level decreased slightly during the dry season, and increased regularly during the period of river stage rise. The groundwater is recharged by the river over 10 months each year. The nitrate concentration in the groundwater and river water varied with seasons. The nitrate concentration of groundwater in wells near the river is affected by the river water, which varied in basically the same way as the river. The nitrate concentrations in the zone of groundwater level depression cone were lower than those in the wells near the river, due to the long-term pumping of groundwater. However, the nitrate concentrations of river water have little influence on those of groundwater in wells far from the river. The values of δ15N-NO3 and the relationship between the two isotopes (δ15N-NO3 and δ18O-NO3) suggested that NO3-N was mainly attributable to sewage, livestock manure and natural soil organic matter. Due to the existence of a groundwater depression cone near the river, nitrate contamination can be transported into the aquifer with the flow. The average time lag of nitrate migration from the river to the zone of groundwater level depression cone is different in different sections, which shows an increasing trend from the upstream to downstream along the river, with an average of two to six months. It is mainly related to the stratigraphic structure, the migration distance, the hydraulic conductivities of the aquifer and the riverbed sediment. Compared with the case of considering the silt layer, the time lag of nitrate migration is greater than that of the case of ignoring the silt layer. The results will provide useful information for detecting nitrate concentrations in the alluvial fan area of the Luanhe river, northeast of the NCP (North China Plain).
Highlights
Riparian zones are defined as a transition area between the terrestrial and aquatic ecosystems [1,2]
During the dry season from November 2010 to April 2011, the observed groundwater level decreased slightly and the groundwater discharged to the river
The groundwater level decreases from the section A-A’ to section B-B’
Summary
Riparian zones are defined as a transition area between the terrestrial and aquatic ecosystems [1,2]. They play a crucial role in ecological systems and biodiversity because of the interaction of groundwater from inland and the river water from stream. It is essential to understand the dynamics of exchange between surface water and groundwater. Aquifer-stream interaction occurs continuously and can transit between gaining and losing depending on many natural factors, including stream water level, stream bank slope, hydraulic properties of stream bed, groundwater table depth, and aquifer’s hydraulic properties [3,4,5,6]. Understanding the hydraulic connectivity between streams and alluvial aquifers is a requirement for sustainable utilization of water resources, both for groundwater and surface water management
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