Titanium in UK rural, agricultural and urban/industrial rivers: Geogenic and anthropogenic colloidal/sub-colloidal sources and the significance of within-river retention

Operationally defined dissolved Titanium [Ti] (the b0.45 μm filtered fraction) in rivers draining rural, agricultural, urban and industrial land-use types in the UK averaged 2.1 μg/l with a range in average of 0.55 to 6.48 μg/l. The lowest averages occurred for the upland areas of mid-Wales the highest just downstream of major sewage treatment works (STWs). [Ti] in rainfall and cloud water in mid-Wales averaged 0.2 and 0.7 μg/l, respectively. Average, baseflow and stormflow [Ti] were compared with two markers of sewage effluent and thus human population: soluble reactive phosphorus (SRP) and boron (B). While B reflects chemically conservative mixing, SRP declined downstreamof STWinputs due to in-streamphysico-chemical and biological uptake. The results are related to colloidal and sub-colloidal Ti inputs from urban/industrial conurbations coupled with diffuse background (geological) sources and within-river removal/retention under low flows as a result of processes of aggregation and sedimentation. The urban/industrial inputs increased background [Ti] by up to eleven fold, but the total anthropogenic Ti input might well have been underestimated owing to withinriver retention. A baseline survey using cross-flow ultrafiltration revealed that up to 79% of the [Ti]was colloidal/nanoparticulate (N1 kDa i.e. Nc. 1–2 nm) for the rural areas, but as lowas 28% for the urban/industrial rivers. This raises fundamental issues of the pollutant inputs of Ti,with the possibility of significant complexation of Ti in the sewage effluents and subsequent breakdown within the rivers, as well as the physical dispersion of fine colloids down to the macro-molecular scale. Although not directly measured, the particulate Ti can make an important contribution to the net Ti flux.