Imaging of contaminant plumes using ERT in Qinhuai River water and its bed caused by urban effluents at Nanjing, China

Abstract

This paper presents the imaging of contaminant plumes in Qinhuai River and its bed for the remediation strategies of the river. Qinhuai meets with Yangtze River at Nanjing area via inner and outer channels, coursing through urban districts of the city. The discharge of urban effluents has badly affected the water quality of the river. Generally, contamination assessment is carried out through traditional point-based sampling methods. However, these methods are often inadequate to estimate accurately the spatial distribution of contaminants and their extension. Therefore, electrical resistivity tomography (ERT) is applied because of its high quality imaging to investigate contamination and its extension in Qinhuai River and its bed. Eight sites were selected at outer and inner channels of the river. A 2-D inversion of the perceived data was carried out and the obtained resistivity values were converted into salinity after temperature correction. The acquired images of resistivity and salinity show that in low streaming conditions the river has large zones of low resistivity in the water body, which are extending toward underlying bed corresponding to high contamination. Conversely, in high streaming conditions, the low resistivity zone shrinks significantly at almost all the studied sites except Xishan Bridge (site E), which still showed high plume of contamination. This phenomenon reveals that in low streaming conditions the contamination level is high while in high streaming conditions salts are flushed out from the river. This study further demonstrates that ERT method can be a useful technique for the environmental studies of rivers, canals and surface water environments.