Chlorine cycling and fates of 36Cl in terrestrial environments

This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the authors. SKB may draw modified conclusions, based on additional literature sources and/or expert opinions. Abstract Chlorine-36 (36 Cl), a radioisotope of chlorine (Cl) with a half-life of 301,000 years, is present in some types of nuclear waste and is disposed in repositories for radioactive waste. As the release of 36 Cl from such repositories to the near surface environment has to be taken into account it is of interest to predict possible fates of 36 Cl under various conditions as a part of the safety assessments of repositories for radioactive waste. This report aims to summarise the state of the art knowledge on Cl cycling in terrestrial environments. The view on Cl cycling in terrestrial environments is changing due to recent research and it is clear that the chloride ion (Cl –) is more reactive than previously believed. We group the major findings in three categories below according to the amount of data in support of the findings. From the result presented in this report it is evident that: • There is an ubiquitous and extensive natural chlorination of organic matter in terrestrial ecosystems. • The abundance of naturally formed chlorinated organic compounds (Cl org) frequently exceeds the abundance of Cl – , particularly in soils. Thereby Cl org in many cases dominates the total Cl pool. • This has important implications for Cl transport. When reaching surface soils Cl – will not be a suitable tracer of water and will instead enter other Cl pools (Cl org and biomass) that prolong residence times in the system. • Cl – dominates import and export from terrestrial ecosystems while Cl org and biomass Cl can dominate the standing stock Cl within terrestrial ecosystems. • Both Cl and Cl org pools have to be considered separately in future monitoring programs addressing Cl cycling. Further, there are also indications (in need of confirmation by additional studies) that: • There is a rapid and large uptake of Cl – by organisms and an accumulation in green plant parts. A surprisingly large proportion of total catchment Cl (up to 60%) can be found in the terrestrial biomass. • Emissions of total volatile organohalogens could be a significant export pathway of Cl from the systems. • Some of the Cl org may be very …