Major ion concentrations and the inorganic carbon chemistry of the Humber rivers

Measurements of major ion concentrations in the main rivers draining into the Humber estuary show two dominant spatial patterns, related to anthropogenic sources from catchments draining urban/industrial areas and background weathering sources from the rural catchments. Most major ions exhibit dilution effects with flow, with higher concentrations at baseflow compared with stormflow conditions. This suggests a predominance of point (effluent) and/or groundwater (weathering) sources of major ions. An exception is NO3−, which exhibits higher concentrations under stormflow conditions in certain rural catchments, suggesting diffuse (catchment) sources, possibly derived from agricultural runoff. Inter-ion relationships were used for endmember mixing analysis to identify whether or not components were exhibiting conservative behaviour within the river. The dominant relationship between major ions is a straight line between baseflow and stormflow endmembers, indicative of conservative mixing processes. The chemical mixing patterns appear to be controlled by hydrology and the existence of distinct inter-ion ratios between baseflow and stormflow endmembers. In terms of the inorganic carbon system, HCO3− is the major component of dissolved inorganic carbon (DIC), with dissolved CO2 (H2CO30 and CO2(aq)) representing approximately 10% and CO32− less than 1% of DIC. All river waters are oversaturated with respect to CO2, with excess partial pressures of carbon dioxide (EpCO2) typically between 5 and 20 times atmospheric partial pressure. The elevated levels of EpCO2 in the river water are sufficient to decouple the relationship between pH and alkalinity. EpCO2 is related to microbial production of CO2 by breakdown of organic carbon and the availability of nutrients (particularly NO3−) and is generally highest in the rivers draining urban and industrial catchments, together with the Derwent which receives high rates of agricultural runoff. EpCO2 and pH do not exhibit simple mixing relationships and both determinands are inherently non-conservative in behaviour. These results provide important background information on which to base more detailed process studies and modelling work. The results imply that most of the major ions can be modelled using simple mixing relationships. In contrast the non-conservative behaviour of EpCO2 and pH may require more thorough process evaluation for modelling the in-stream carbon system.