Numerical Simulation Of Groundwater And Salt Transport In Pearl River Delta

The potential groundwater resources and associated environmental issues in Pearl River Delta attract increasingly attention. However, the knowledge of regional groundwater flow and evolution patterns of groundwater chemistry are still weak. The studied area is located nearby South China Sea. The Quaternary sediments have experienced several times between transgression and regression with the changes of sea level. As a result, the Pearl River Delta deposits form an aquifer system with a high permeable confined aquifer composed of gravel sand at the bottom and a low permeable confining unit composed of silty clay, silty sand and sand interbeds. In addition, salinity of groundwater is generally high. On the basis of this background, the patterns of regional groundwater flow and salt transport driven by freshwater infiltration in Pearl River Delta are studied.According to the regional geological structure and Quaternary sedimentary characteristics as well as more than 300 boreholes data, a spatial structure model of the aquifer system is built up. The strata can be divided into 6 layers in considering of sedimentary facies and chronological age. However, the sediments can be classified into 4 layers for numerical modeling in considering of their hydrogeological properties. In these layers, only the gravel sand layer at the bottom has significant for water supply. Distribution of sands in its clayey confining unit is controlled by paleo-rivers. The hydraulic conductivity of clay is obtained through investigation of pumping test data with slug-test analysis method. It is indicated that the hydraulic conductivity of silt is small, with an magnitude of 10^-410^-5 m/d. Along a main direction of paleo-river in Pearl River Delta, a typical reference profile from the upper-stream to a island in the sea is selected and drawn with aquifer systems. The length of the profile is about 180 km. The steady state groundwater flow and salt transport on the profile is simulated with MODFLOW, it is found that multi-flow systems develop on the profile and the maximum penetrating depth of fresh water is 20 m with respect to 1000-years transport. The multi-layers quasi-three-dimensional modeling of regional groundwater flow shows a pattern of flow filed that dominated by distribution of surface water. Hydraulic gradient in horizontal is small. The fresh water front can moves in the gravel sand aquifer to a maximum distance of 66 km, in ten thousand years, from its source area.At last, the impacts of paleo-river channels, the role of sedimentary history in the modeling process and the treatment of submarine aquifer are discussed.