Changes in DOC treatability: Indications of compositional changes in DOC trends

This study considers long-term records of the nature of water colour and coincident water quality and quantity in order to test hypotheses about increases in release of dissolved organic carbon from peat soils across the northern hemisphere. The study focuses upon the treatment ratio, i.e. the ratio of the amount of coagulant dose required to the water colour of the incoming water, and compares this ratio to possible explanatory variables: pH, conductivity, water temperature, river flow and alkalinity. The study shows that: (i) The annual average increase in treatment ratio is just less than 6.5% over a six year period. (ii) There is a long-term increase in the treatment ratio that is independent of changes in riverflow, pH, conductivity, water temperature and alkalinity and so a real change in DOC composition is occurring. (iii) There is a seasonal cycle in treatment ratio that is also independent of the available water and climate variables. (iv) The upward trend in treatment ratio is declining with time over the study period. The observed trends in DOC composition are consistent with an explanation of increasing DOC concentration and flux based upon changes in flow and temperature but is not consistent with present explanations based upon changes in atmospheric deposition or upon drought unless the effect of the drought are short-lived (1–2 years).