Impacts of the 2013 Extreme Flood in Northeast China on Regional Groundwater Depth and Quality

Flooding's impact on shallow groundwater is not well investigated. In this study, we analyzed changes in the depth and quality of a regional shallow aquifer in the 10.9 x 10(4) km(2) Sanjiang Plain, Northeast China, following a large flood in the summer of 2013. Pre- (2008-2012) and post-flood records on groundwater table depth and groundwater chemistry were gathered from 20 wells across the region. Spatial variability of groundwater recharge after the flood was assessed and the changes in groundwater quality in the post-flood period were determined. The study found a considerable increase in the groundwater table after the 2013 summer flood across the region, with the largest (3.20 m) and fastest (0.80 m center dot s(-1)) rising height occurring in western Sanjiang Plain. The rising height and velocity gradually declined from the west to the east of the plain. For the entire region, we estimated an average recharge height of 1.24 m for the four flood months (June to September) of 2013. Furthermore, we found that the extreme flood reduced nitrate (NO3-) and chloride (Cl-) concentrations and electrical conductivity (EC) in shallow groundwater in the areas that were close to rivers, but increased NO3- and Cl- concentrations and EC in the areas that were under intensive agricultural practices. As the region's groundwater storage and quality have been declining due to the rapidly increasing rice cultivation, this study shows that floods should be managed as water resources to ease the local water shortage as well as shallow groundwater pollution.