Hydrogeochemical characteristics and processes of groundwater in an over 2260 year irrigation district: A comparison between irrigated and nonirrigated areas

Abstract

A comprehensive understanding of groundwater chemistry and its evolution in irrigation districts is essential for irrigation management. In this study, with emphasis on a comparison between irrigated and nonirrigated areas, the groundwater chemistry and hydrogeochemical processes of a typical irrigation district with an irrigation history of over 2260 years were studied to clarify the effects of long-term irrigation on groundwater quality. Based on 107 water samples collected from across the study area, a comprehensive analysis was conducted using multivariate statistics, stable isotope analysis, and hydrogeochemical modeling. The results showed that the variation range and average concentrations of almost all the ions in the irrigation district are much greater than those in the nonirrigated areas. The groundwater in the nonirrigated areas could be characterized by low TDS and HCO3 or HCO3·SO4 types, whereas the groundwater in the irrigation district could be characterized by complex water types and high TDS. Stable isotopes of hydrogen and oxygen indicated that the groundwater in the irrigation district experienced strong evaporation. The calculated groundwater residence time showed that it takes approximately 2180 years for the groundwater to flow through the irrigation district. Some old irrigation water was present, and it influenced the current groundwater chemistry in the study area. The processes forming the current groundwater chemistry in the irrigation district can be formulated as: mixing → evaporation → water–rock interaction. For mixing, the proportions of rainfall, irrigation water from the Jing River, and lateral recharge from the nonirrigated areas were found to be approximately 30%, 49%, and 21%, respectively. For evaporation, the ratio based on the TDS was 3.83 when comparing the groundwater in the irrigation district with the mixed water after evaporation. Hydrogeochemical modeling showed that the irrigation area is a potential carbon source. The dissolution of halite and gypsum, the precipitation of calcite and dolomite, CO2 degassing and Na-Ca exchange are the main chemical reactions in the geochemical evolution of groundwater. The lesson learned from this irrigation district is that long-term irrigation will lead to salinization and complexity of the groundwater. Hence, in the water resource management of irrigation districts, attention should not only be paid to the balance of water quantity, but also to the balance of salts and the hydrochemical evolution of groundwater.