Salinity-induced increase of the hydraulic conductivity in the hyporheic zone of coastal wetlands

Abstract

In coastal zones globally, salinization of surface water and groundwater is rapidly taking place due to the combined effects of sea level rise, land use change, land subsidence, altered hydrology and climate change. Although increased salinity levels are known to have a great impact on both biogeochemical and hydrological processes in aquatic sediments, only few studies have included both types of processes and their potential interactions. In the present paper, we used a controlled three years experimental mesocosm approach in the surface water of a Dutch coastal wetland to test these interactions as a result of salinization, and to discuss mechanisms explaining the observed hydrological changes. In enclosures (1000 l), surface water salinity was experimentally increased from 14 mmol to 140 mmol Cl l-1 (0.9 and 9 PSU) by adding sea salt. This not only strongly increased pore water salinity, but also increased sulphate reduction rates, leading to higher sulphide and lower methane concentrations. By analysing slug test data with three different slug test analysis methods, we were able to show that hydraulic conductivity of the hyporheic zone increased 2.8 times by salinization. This shows that increased salinity can strongly change the hydrological characteristics of the hyporheic zone in coastal wetlands. Based on our hydrological and biogeochemical measurements, we conclude that the combination of pore dilation and decreased methane production rates were major controls on the observed increase in hydraulic conductivity. The slug test analysis method comparison allowed to conclude that the adjusted Bouwer & Rice method results in the most reliable estimate of the hydraulic conductivity for hyporheic zones. Our work shows that both physical and biogeochemical processes are vital to explain and predict hydrological changes related to the salinization of hyporheic zones in coastal wetlands and provides a robust methodological approach for doing so.