Insights into Colloids-mediated Transport in Catchments: a Metadata Analysis

Mobile soil colloids play an important role in carrying earth elements in freshwater systems, including carbon, nutrients, and pollutants. The associations of these elements with colloids change their transport behavior and reactivity and hence affects water quality and their functions in the downstream environments. We performed a metadata analysis on measurements from two small catchments, Upper Hafren and Lower Hafren at Plynlimon, Wales, to identify the temporal and spatial variability in the partition of elements in colloidal and dissolved phases from upper to lower streams. Based on Pearsons correlation distance measurement, we identified the temporal variation of 47 elements within a two-year period and clustered them into different groups. At upper and lower streams, we found that the elements clustered into four and three major groups, respectively, that differentiate their transport behaviors. We also found higher similarity in the transport behavior of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), Fe, Al, V, La, Ce, Pr at the upper stream and DOC, Fe, V, La, Ce, Pr, Cu at the lower stream with Pearsons correlation distance < 0.3. The similarity of transport behavior between DOC, Fe, and Al suggests the potential roles of colloidal Fe and Al oxyhydroxides in carbon retention. Additionally, the flow regime, indicated by water flux, was found important in regulating the movement of those elements with Pearsons correlation distance < 0.5. Along with the positive rolling correlation found between DOC, Fe concentration under higher water flux conditions, we propose that high flow rates during storm events can promote better mixing and hence enhance Fe association with DOC due to the formation of colloidal Fe oxyhydroxides and organo-mineral complexation.