The natural chlorine cycle – Formation of the carcinogenic and greenhouse gas compound chloroform in drinking water reservoirs

• CHCl3 is formed naturally in the chlorine cycle, beginning and ending with chloride.
• Soil organic carbon is chlorinated to large adsorbable organohalogen molecules.
• CHCl3 is emitted during biotransformation of adsorbable organohalogens.
• The process is influenced by physicochemical factors, mainly by pH & redox potential.
• Ratios of non-chlorinated and chlorinated substrates to CHCl3 influence its formation.

Chlorine cycle in natural ecosystems involves formation of low and high molecular weight organic compounds of living organisms, soil organic matter and atmospherically deposited chloride. Chloroform (CHCl3) and adsorbable organohalogens (AOX) are part of the chlorine cycle. We attempted to characterize the dynamical changes in the levels of total organic carbon (TOC), AOX, chlorine and CHCl3 in a drinking water reservoir and in its tributaries, mainly at its spring, and attempt to relate the presence of AOX and CHCl3 with meteorological, chemical or biological factors.Water temperature and pH influence the formation and accumulation of CHCl3 and affect the conditions for biological processes, which are demonstrated by the correlation between CHCl3 and ΣAOX/Cl− ratio, and also by CHCl3/ΣAOX, CHCl3/AOXLMW, CHCl3/ΣTOC, CHCl3/TOCLMW and CHCl3/Cl− ratios in different microecosystems (e.g. old spruce forest, stagnant acidic water, humid and warm conditions with high biological activity). These processes start with the biotransformation of AOX from TOC, continue via degradation of AOX to smaller molecules and further chlorination, and finish with the formation of small chlorinated molecules, and their subsequent volatilization and mineralization. The determined concentrations of chloroform result from a dynamic equilibrium between its formation and degradation in the water; in the Hamry water reservoir, this results in a total amount of 0.1–0.7 kg chloroform and 5.2–15.4 t chloride. The formation of chloroform is affected by Cl− concentration, by concentrations and ratios of biogenic substrates (TOC and AOX), and by the ratios of the substrates and the product (feedback control by chloroform itself).

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