Dissolved organic matter and the dissipation of chlorine in estuarine water and seawater

Dissolved organic matter in estuarine water and seawater collected in the summers of 1980 and 1981 in the James River, Virginia and the mouth of Chesapeake Bay were separated into fractions according to their nominal molecular weights (NMW) by ultrafiltration. Estuarine waters contained higher concentrations of dissolved organic carbon (DOC). Among the fractions, between 66–89% of the DOC was found in the fraction with NMW below 10,000. Estuarine waters also had higher chlorine demands. At a dose of 5 mg l⁻¹, in 23 h, about 90% of the added chlorine disappeared in estuarine waters, whereas, in seawater, only 60–75% of the chlorine had dissipated. At least two-thirds of the chlorine demand occurred in the first 5 h. About 10–30% of the chlorine demand may be attributed to the fraction with NMW above 10,000. The remaining chlorine demand was distributed almost equally between the fractions with ranges of NMW of 1000–10,000 and below 1000. If reactivity is measured in terms of organic chlorine demand (ΔClo) per unit weight of DOC, the fractions with lower NMW (< 1000 and 1000–10,000) always had a higher reactivity towards chlorine. Between these two fractions, the one with NMW between 1000 and 10,000 exhibited higher reactivity more frequently. The highest reactivity found was 1.4 mg ΔClo mg⁻¹ DOC.