Effect of water diversion on hydro-chemical characteristics of surface water and groundwater in lowland area of the North China Plain: A case study of Nanpi County, Hebei Province

The great grain yield potential of the lowland area of North China Plain have been compromised by regional contradiction between water resources and agricultural production. The combined use of brackish shallow groundwater and diversion water is an effective way to address the regional water issue, which will certainly change the regional water cycle and environment. This study took different seasonal investigations in Nov. 2014, Mar. and Jun. in 2015 after water diversion in Nanpi County, which is located in the lowland area of North China Plain. The effects of water diversion on hydro-chemical characteristics of surface water and groundwater were determined using hydro-geochemical analysis and stable isotopes. The results showed that evaporation increased electrical conductivity (EC), sodium adsorption ratio (SAR) and enrichment of ²H and ¹⁸O isotopes in surface water. Soil sorption and exchange increased Na⁺, Cl^- and SO₄^-2, but decreased HCO3 in surface water, thereby increasing the water salinity in the region. Water diversion changed the interaction between surface water and groundwater. From November to March of the following year, diversion water recharged shallow groundwater near water division channels through directly percolation or irrigation. This decreased EC and depth of shallow groundwater at certain sampling points distributed along the diversion channel. In March 2015, the shallow groundwater types were Na·Mg·Ca-Cl·SO₄, Na·Mg-Cl·SO₄·HCO₃ and Na·Mg-SO₄·Cl·HCO₃, which was as a result of mixing of diversion water (Na·Mg·Ca-SO₄·HCO₃·Cl) with shallow groundwater (Na·Mg-Cl·SO₄) in November 2014. Shallow groundwater recharged channel water in March to July, which decreased groundwater depth. The shallow groundwater type in March was similar to that in July. Water diversion seasonally improved the quality of channel water and shallow groundwater in the vicinity. However, water diversion had no effect on deep groundwater and pool water quality. Water division improved channel water quality immediately after division. However, there was a time lag between diversion operation and shallow groundwater quality improvement. The quality of shallow groundwater improved in March 2015 due to water division in November 2014. Therefore, the combined use of shallow groundwater, division water and pool water for irrigation was critical for the rational development and utilization of both brackish water and freshwater resources, reduction of groundwater exploitation and recovery of deep groundwater level in the study area.