The variation rule of groundwater salinity in aquitard under different penetration process

Penetration process of groundwater in multilayer aquifer system is an important research content in groundwater resources evolution. In the past, most of researches on the penetration process focus on groundwater flow in aquifer, lacking of water quality research in aquitard, which caused a certain deviation in water quality evaluation.

This study selects a test approach using NaCl solution with different waterhead to penetrate three-layer aquifer system, real-time monitoring the penetration process by using high-accuracy sensors, and got available data. Based on correlation analysis of these test data, including waterhead, salinity, moisture and porosity, it is found that the change of conductivity in soil water penetration is not linear to the distance of penetration path, but is mainly affected by waterhead. The NaCl solution concentration has the second important influence on penetration process. The penetration time of low NaCl solution concentration (1g/l) is 1.2 times of high concentration(10g/l). The change of groundwater conductivity caused by the penetration of NaCl solution with different concentration in aquitard has similar trend, and the influence of concentration gradient is greater than that of hydraulic gradient. The peak value of soil water conductivity in aquitard varies from depth to depth, with cumulative effect. Aquitard has a relative little influence on the composition of deuterium oxygen isotope in soil water, but there is an obvious tendency of dilution. In the process of water penetration, the Cation Exchange Capacity (CEC) value of the soil water in aquifer (silty sand) changes within 8%, while that of aquitard can fluctuate up to 20%.

In general, in the multi-layer aquifer system, aquitard has a strong cation exchange capacity, which partially intercepts the salt in penetration groundwater and reduces the water salinity in the aquifer. These characteristics of aquitard can play an important role in the study of water quality evolution in complex aquifer system.