Sediment accumulation of organic halogens in pristine forest lakes

Abstract

Sediment accumulation of organic halogen was studied in two forest lakes, one pristine and one which received 30 m³ of biologically purified bleaching wastewater from a kraft pulp mill in 1979 equivalent to ca. 2 kg of adsorbable organic halogen (AOX). Lake sediments were dated with²¹⁰Pb,¹³⁴Cs and¹³⁷Cs and the annual deposition rates of organic halogens and organic matter were calculated. Organic bound halogen contents of the sediment aged 150 years was 180 μg Cl g⁻¹ d.w. in both lakes. The concentration of organic bound halogen at the topmost 6 cm of the sediments (less than 20-years-old) ranged from 45 to 80 μg Cl g⁻¹ d.w. This suggests that solvent extractable halogen had enriched in the older sediment layers. The deposition of extractable organic halogen (EOX) in the lakes in 1950’s was 4 to 5 mg Cl m⁻² a⁻¹. Since then, the depositon of EOX doubled in both lakes. The deposition of organic matter increased concomitantly from 50 g m⁻² a⁻¹ to 110 g m⁻² a⁻¹ in Lake Mustalampi and from 35 g m⁻² a⁻¹ to 62 g m⁻² a⁻¹ in Lake Pyylampi suggesting that the increase in the deposition of organic halogen followed the increase in the deposition of organic matter. Of the 2 kg of organic halogen discharged into the lake, 5% or less was detected in the sediment in tetrahydrofuran extractable form 15 years later.