AbstractThe decreasing of groundwater quality has been the major issue in Tangerang area. One of the key process is the interaction between groundwater and Cisadane river water, which flows over volcanic deposits of Bojongmanik Fm, Genteng Fm, Tuf Banten, and Alluvial Fan. The objective of this study is to unravel such interactions based on the potentiometric mapping in the riverbank. We had 60 stop sites along the riverbank for groundwater and river water level observations, and chemical measurements (TDS, EC, temp, and pH). Three river water gauge were also analyzed to see the fluctuations. We identified three types of hydrodynamic relationships with fairly low flow gradients: effluent flow at Segmen I (Kranggan - Batuceper) with 0.2-0.25 gradient, perched flow at Segmen II (Batuceper-Kalibaru) with gradient 0.2-0.25, and influent flow at Segmen III (Kalibaru-Tanjungburung) with gradient 0.15-0.20. Such low flow gradient is controlled by the moderate to low morphological slope in the area. The gaining and losing stream model were also supported by the river water fluctuation data. TDS and EC readings increased more than 40% from upstream to downstream. At some points the both measurements were two times higher than the permissible limits, along with the drops of pH values at those areas. This study shows the very close interaction between Cisadane river water and groundwater in the riverbank. Therefore the authorities need to be managed the areas with a very strict regulations related to the small and large scale industries located near by the river.