Tracing infiltration and recharge of thick silt by using D and ¹⁸O isotopes of soil moisture in Xiaogan, Hubei and its ecological efffects

As well known, the permeability coefficient of cohesive soil is extremely low, and the transport rate of water and solute in the cohesive soil is slow and time-consuming. Stable isotopes δD and δ¹⁸O data of soil moisture and rainfall of two boreholes (ZK1, ZK2) in Dabie Mountain Area-Jianghan Plain rainfall-soil moisture-groundwater transformation scientific field test site were studied to trace the characteristics of historical infiltration and recharge of cohesive soil. The results show that there's no obvious layer boundary but just a little difference among the 15 m deep cohesive soil, and soil water moves in the way of "piston flow" vertically. The closer relation of rainfalls in summer and autumn to soil moisture in δD and δ¹⁸O value indicates that the main recharge of soil moisture comes from rainfall infiltration in summer and autumn. The δD and δ¹⁸O values of soil water in ZK1 (sampling interval 0.5-2.7 m, depth 15 m) fluctuate periodically with the increase of buried depth, while the δD and δ¹⁸O values of soil water in ZK2 (sampling interval 0.1m, depth 6.2 m) fluctuate stratified with the increase of buried depth. On the basis of determining the rainfall infiltration recharge years corresponding to the 0-6.2 m depth in the cohesive soil layer, based on ¹⁸O isotopes peak displacement method, it is calculated that the vertical migration velocity of rainfall infiltration recharge in the cohesive soil layer is 10.8-15.0 cm/a, and the annual infiltration recharge is 43.1-58.1 mm, accounting for 4.01% of the annual average rainfall. It takes about 130 years for rainfall infiltration to penetrate the thick cohesive soil for groundwater recharge, which means the thick cohesive soil has good antifouling properties. The historical evolution characteristics of soil water infiltration and migration at the air-soil interface and the inter-annual correspondence of soil water recharge are of great significance for groundwater environmental protection, ecological environment improvement, drought and flood disaster prevention and control in Jianghan Plain.