Geochemical evolution and salinization of a coastal aquifer via seepage through peaty lenses

The aim of this study was to determine the role of buried peaty lenses in the salinization process of an unconfined coastal aquifer. To unravel the complex biogeochemical processes that occur between the peat matrix and the flowing through groundwater, two monitoring wells were sampled using high-resolution multilevel straddle packers. Moreover, to quantify the salt release in the aquifer from the peat lenses, a 1-m length column experiment was run for 62 days including a long equilibration step of 37 days, an elution step of 5 days, a stop flow of 15 days and a second elution step of 5 days. The column was monitored using 20 cm spaced ports to allow a precise reconstruction of the biogeochemical processes occurring within the matrix. A numerical model with SEAWAT-4.0 accounting for variable density flow and transport was used to simulate the observed salt concentration eluted from the column. To correctly reproduce the observed concentrations, a dual-domain approach coupled with a mass loading rate from the peat layer had to be accounted for. The results of this study highlight the capability of peat lenses to salinize the aquifer, to release large amount of sulfate and phosphate and to increase the chloride–bromide ratio.