Chlorine cycling and fates of 36Cl in terrestrial environments
Year: 2013
Type: report
Abstract: Chlorine-36 (36Cl), 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 36Cl from such repositories to the near surface environment has to be taken into account it is of interest to predict possible fates of 36Cl under various conditions as a part of the safety assessments of repositories for radioactive waste. This report aims to summarize 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 (Clorg) frequently exceeds the abundance of Cl–, particularly in soils. Thereby Clorg 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 (Clorg and biomass) that prolong residence times in the system. Cl– dominates import and export from terrestrial ecosystems while Clorg and biomass Cl can dominate the standing stock Cl within terrestrial ecosystems. Both Cl and Clorg 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 Clorg may be very persistent and resist degradation better than average organic matter. This may lead to selective preservation of some Clorg (with associated low bioavailability). There is a production of Clorg in tissues of e.g. plants and animals and Cl can accumulate as chlorinated fatty acids in organisms. Most other nevertheless important aspects are largely unknown due to lack of data. Key unknowns include: The development over time of major Cl pools and fluxes. As long as such data is lacking we cannot assess net changes over time. How the precesses behind chlorination, dechlorination and transport patterns in terrestrial systems are regulated and affected by environmental factors. The ecological roles of the chlorine cycling in general. The ecological role of the microbial chlorination in particular. The chlorine cycling in aquatic environments – including Cl– and Clorg pools in sediment and water, are largely missing. Given the limited present information available, and particularly the lack of data with a temporal dimension and the lack of process understanding, predictive models are challenging. We also summarize currently available methods to study Cl in the environment.
Language: English
Subfield: Pollution
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W338288308