[Hydrochemical Characteristics and Drivers of Inner Lakes Fed by Ecological Water Replenishment and Irrigation Return Flows in Qingtongxia Irrigation District]

Abstract

Inner lakes in irrigation districts serve as critical hydrological nodes for water-salt dynamics in arid and semi-arid regions, with their hydrochemical signatures providing sensitive indicators of regional water quality. This study comprehensively investigated the hydrochemical characteristics and controlling mechanisms of nine representative inner lakes in Ningxia's Qingtongxia Irrigation District under three recharge regimes (ecological water replenishment, irrigation return flows, and mixed sources). Combining hydrochemical analysis (Piper and Gibbs diagrams, ionic ratios) with principal component analysis (PCA), key findings revealed that: ① Inner lake waters were weakly alkaline (mean pH of 8.9 ±0.3) and brackish [mean total dissolved solids (TDS) concentration of (1.3±0.7) g·L^-1], dominated by Na⁺ (51%) and SO₄^2- (46%). Pronounced seasonal variations were observed, with summer irrigation period (July) showing 20% and 50% higher ionic mass concentrations than spring (May) and post-autumn (October) periods, respectively. ② Recharge regimes significantly influenced hydrochemical characteristics of inner lakes. Comparative analysis revealed that lakes fed by irrigation return flows and mixed sources showed significantly higher TDS (P<0.05) relative to ecological-water replenished lakes. The total ionic mass concentration in irrigation-fed lakes was approximately double that observed in ecological-water replenished lakes. Within the irrigation district, SO₄·Cl-Na water types predominated in the inner lakes recharged by irrigation return flows and mixed sources, while ecological-water replenished lakes displayed mixed Cl-Ca·Mg and SO₄·Cl-Na water types. ③ Both natural processes (evaporation-crystallization and rock weathering) and anthropogenic activities drove the hydrochemical evolution of the lake water. The anthropogenic activities were quantitatively assessed through principal component analysis, revealing that human activities accounted for 77.0% of the observed hydrochemical variations. The major anthropogenic sources were identified as: pesticide application/livestock and poultry farming/industrial wastewater (33.1%), nitrogen fertilizer/pesticide application (20.4%), potassium fertilizer application (12.3%), and ecological water replenishment (11.2%). These findings demonstrate that human activities predominantly control lacustrine hydrochemical variability within the irrigation district, providing a scientific basis for precision water-salt management and ecosystem rehabilitation in irrigated areas.

Keywords: Qingtongxia Irrigation District; controlling factors; ecological water replenishment; hydrochemical characteristics; inner lakes; irrigation return flows.