Influence of habitat on the quantity and composition of leachable carbon in the O2 horizon: Potential implications for potable water treatment

Organic material leached from the organic (O) horizon of soils is a major source of natural organic matter (NOM) in surface waters. Dissolved organic carbon (DOC) is a known precursor for the formation of disinfection by-products (DBPs), including trihalomethanes (THMs), formed during chlorination. In this study the concentration and composition of leachable O2 horizon DOC from 5 habitats within a United Kingdom upland reservoir catchment (beech, spruce, larch, and pine forests and blanket peat) were compared with an emphasis on potential treatment implications using XAD fractionation and THM formation potential (THMFP) tests. Statistically significant differences were found between habitats, with pine and larch leachates yielding particularly high DOC concentrations (mean 19.3 and 13.4 mg/L, respectively) and THMFP7d values (mean 1306 and 1527 mu g/L, respectively). The interspecies variation observed suggests that the typical distinction made between deciduous and coniferous species in previous studies is overly simplistic. Interestingly, peat leachate exhibited a surprisingly low DOC concentration (mean 9.0 mg/L), suggesting that the high DOC flux associated with these habitats may be the result of other factors such as depth of organic matter and mineral content. Averaged across all habitats, mean standardized THMFP (STHMFP) was highest in the hydrophobic acid (HPOA) fraction, although substantial differences in the relative reactivities of fractions were found between habitats. Synergistic effects are also likely to complicate the relationship between fractional character and STHMFP.