Chlorine transport in a small catchment in southeast Sweden during two years

Previous studies have revealed that chlorine participates in a complex biogeochemical cycle in soil, which suggests that the transport of chloride through catchments may also be influenced. The present study is based on field observations of organic carbon, chloride (Cl_in), and chlorinated organic carbon (Cl_org) in precipitation, soil, and runoff over a 2-year period from a small, forested catchment in southeast Sweden. The study reveals that (1) the soil pool is dominated by Cl_org, (2) the input via wet deposition and output of Cl_in via runoff is 30 times smaller than the total storage of chlorine (Cl_in + Cl_org) in soil, and (3) the transport is dominated by Cl_in. The organic matter that entered the outlet of the catchment was more chlorinated in the autumn than during the rest of the year, and rain events taking place in low-flow periods had a greater influence on TOC, Cl_org, and Cl_in than did rain events taking place in high-flow periods. The seasonal pattern in combination with the low-flow versus high-flow pattern and previous findings of increasing chlorine-to-carbon ratios with soil depth suggests that the chlorine-to-carbon ratio variation in the leached organic matter is due that water preferentially comes from deeper layers in low-flow conditions. This study provides well-founded estimates of Cl_org and Cl_in storage and fluxes for the studied catchment; however, the processes underlying the observed seasonal Cl_org variations and transportation processes need further study.