Colloid Mobilization and Seasonal Variability in a Semiarid Headwater Stream

Abstract

Colloids can be important vectors for the transport of contaminants in the environment, but little is known about colloid mobilization at the watershed scale. We present colloid concentration, composition, and flux data over a large range of hydrologic conditions from a small watershed (Gordon Gulch) in the foothills of the Colorado Front Range. Colloids, consisting predominantly of Si, Fe, and Al, were present in most stream samples but were not detected in groundwater samples. Mineralogical and morphological analysis indicated that the colloids were composed of kaolinite and illite clays with lesser amounts of amorphous Fe-hydroxides. Although colloid composition remained relatively constant over the sampled flow conditions, colloid concentrations varied considerably and increased as ionic strength of stream water decreased. The highest concentrations occurred during precipitation events after extended dry periods. These observations are consistent with laboratory studies that have shown colloids can be mobilized by decreases in pore-water ionic strength, which likely occurs during precipitation events. Colloidal particles constituted 30 to 35% of the Si mass flux and 93 to 97% of the Fe and Al mass fluxes in the <0.45-µm fraction in the stream. Colloids are therefore a significant and often overlooked component of mass fluxes whose temporal variations may yield insight into hydrologic flowpaths in this semiarid catchment.

Colloidal transport in the Gordon Gulch catchment of the Boulder Creek CZO and its effect on C‐Q relationships for silicon

“…Stream discharge data was collected from a permanent stream gauge using pressure‐transducers. Streamflow measurements below 3 L s −1 are estimated values due to issues with the pressure‐transducers during low‐flow conditions [ Mills et al ., ]. Some discharge values in the data indicate a value of zero L s −1 .…”

“…A 12 L stream sample was filtered through a high‐capacity 0.45 µm Geotech dispos‐a‐filters TM . The filtrate was centrifuged for 4.5 h at 8000 rpm to remove colloids >0.02 µm [ Mills et al ., ]. After centrifuging the filtrate, the supernatant was decanted and the centrifuged pellets of colloids (>0.02 d eff < 0.45 µm) were combined for analysis.…”

“…After centrifuging the filtrate, the supernatant was decanted and the centrifuged pellets of colloids (>0.02 d eff < 0.45 µm) were combined for analysis. For the soil colloids, a soil sample 10 m from the stream channel and 200 m upstream from the gauge was collected by bucket auger until refusal at 50 cm [ Mills et al ., ]. The soil sample was mixed with 4 L of deionized (DI) water and placed in a shaker table for 1 h. Separation of soil colloids >0.02 µm was accomplished using the same procedure as described above.…”

“…The soil sample was mixed with 4 L of deionized (DI) water and placed in a shaker table for 1 h. Separation of soil colloids >0.02 µm was accomplished using the same procedure as described above. The sampling procedure operationally defines two element fractions; the dissolved plus colloidal (<0.45 µm) and the dissolved fraction (<0.02 µm) [ Mills et al ., ].…”

“…Recent work in the Gordon Gulch catchment at the Boulder Creek CZO has documented “chemostatic” behavior for Si in this small montane catchment. An intriguing observation about silica chemistry from Gordon Gulch is the recognition of significant colloidal mobilization of Si in stream water [ Mills et al ., ]. In the following study, a geochemical perspective is used to understand the sources of silicon to the stream and how that changes in response to variations in discharge…”