Dissolved load of aromatic and halogenated non-methane VOCs in urban sewage during wet and dry seasons

Concentration of dissolved aromatic and halogenated non-methane volatile organic compounds (NMVOCs) was estimated in sewage flowing through the open drainage canal network of Kolkata megacity in India in dry (summer) and wet (post-monsoon) seasons at five locations. Seventeen aromatic and halogenated NMVOC species were studied by headspace solid-phase micro-extraction (HS-SPME) technique followed by gas chromatography–mass spectrometric (GC–MS) analysis. Distinct seasonal variations in the concentration of individual NMVOC species were observed, but spatial variation was negligible. Total dissolved NMVOC (TNMVOC) concentration was higher (16.64 µg l ⁻ ¹) in summer over post-monsoon (12.70 µg l ⁻ ¹). Chloroform and toluene were the most abundant species in both seasons. Principal component analysis indicated contribution from industrial sources (38.8% and 35.5%), solvent usage (35.9% and 35.5%), in situ formation through microbial pathways (22.2% and 11.5%) in dry and wet seasons, respectively. Contribution by gasoline (12.3%) was found in post-monsoon only, possibly due to higher mixing of city's stormwater carrying gasoline residues from roads, garages, and commercial areas. The dynamic load of all quantified NMVOCs combined in the entire canal network was estimated to be 182.2 and 162.0 kg in summer and post-monsoon, respectively. The likely distribution of a few prominent NMVOC species in different environmental compartments, simulated by multimedia mass balance model TaPL3 (3.0), showed that almost the entire dissolved chloroform would be emitted to atmosphere (98%), followed by benzene (71%), in contrast to xylene that would primarily get partitioned into canal sediment (53%). Toluene showed the highest likely atmospheric emission from canal water in summer (63.55 kg), whereas in post-monsoon, chloroform had the highest possible release (48.12 kg) into the atmosphere.