Effect of natural flood spreading of March 2019 on the quantitative and qualitative characteristics of groundwater in floodplain at the end of Gorganroud basin

In order to investigate the effect of flood in March 2019 on quantitative and qualitative changes of groundwater, the floodplain at the end of Gorganroud river basin, which was naturally exposed to floods, was selected. To investigate changes in groundwater quantity and quality, 17 and 11 observation wells with suitable distribution in the study area were selected, respectively. Then, quantitative (groundwater level) and qualitative data (11 physicochemical parameters) of wells were collected in two periods before and after the floods. First, groundwater level changes in two periods were evaluated by paired T-test and average groundwater level of the plain. Then, physicochemical characteristics changes in two periods were evaluated using paired T-test. Finally, hydrogeochemical changes were assessed using Gibbs, Durov and Piper diagrams, and the water quality for agricultural and drinking purposes were investigated using Wilcox and Schoeller diagrams. Paired T-test results showed that the groundwater level in 88.2% of the observation wells after the flood had a significant increase. This has increased the groundwater level as well as the unit hydrograph of the plain (about 2 meters) after the flood. The results of changes in groundwater quality parameters showed that the amount of EC, anions and cations (except NO3) were decreased in most of the observed wells after the flood. This has reduced the hardness of groundwater but has not had a significant effect on the type and hydrogeochemical facies of floodplain groundwater. The reason can be the hydrochemical similarity of infiltrated water and groundwater of the plain aquifer.