Role of riverine colloids in macronutrient and metal partitioning and transport, along an upland–lowland land-use continuum, under low-flow conditions

An assessment is made of the role of riverine colloids in macronutrient (nitrogen, phosphorus and carbon), metal and trace element partitioning and transport, for five rivers in the Ribble and Wyre catchments in north-western England, under baseflow/near-baseflow conditions. Cross-flow ultrafiltration was used to separate colloidal (< 0.45 µm > 1 kDa) and truly dissolved (< 1 kDa) fractions from river water. Clear patterns were observed, along the upland–lowland land use continuum, in the partitioning and transport of macronutrients and metals between the colloidal, truly dissolved and acid-available particulate (> 0.45 μm, suspended) fractions. Of these operationally-defined fractions measured, colloids were generally more important for both macronutrient and metal transport in the upland than in the lowland rivers. The results suggest that organic moieties in truly dissolved form from sewage effluent may have a greater capacity to chelate metals. Organic-rich colloids in the upland moorlands and metal oxide colloidal precipitates in the industrial rivers had a higher capacity for binding metals than the colloidal fractions in the urban and agricultural lowland rivers. Aggregation of these colloids may provide an important mechanism for formation of larger suspended particulates, accounting for a higher degree of metal enrichment in the acid-available particulate fractions of the upland moorland and lowland industrial rivers, than in the lowland agricultural and urban rivers. This mechanism of transfer of contaminants to larger aggregates via colloidal intermediates, known as ‘colloidal pumping’ may also provide a mechanism for particulate P formation and the high proportion of P being transported in the particulate fraction in the uplands. The cross-flow ultrafiltration data also allowed refinement of partition coefficients, by accounting for colloids within the solids phase and replacing the filtered (< 0.45 μm) fraction with the truly dissolved (< 1 kDa) concentrations. These provided a clearer description of the controls on metal and P partitioning along the upland-lowland continuum.

► Using cross-flow ultrafiltration, we assess the role of colloids in macronutrient and metal partitioning and transport.
► Rivers draining a continuum of land use types were sampled, from upland moorland to lowland agricultural and urban.
► For many metals and macronutrients, colloids dominated the filtered (< 0.45 μm) river water fraction in the upland rivers.
► Truly dissolved (< 1 kDa) fractions were proportionally greater in the lowland rivers, linked to sewage effluent inputs.
► Quantifying colloidal/truly dissolved fractions allowed clearer description of controls on metal partitioning.