The near-constant Si concentration under varying discharge observed in Gordon Gulch of the Boulder Critical Zone Observatory (CZO) indicates that the silica fluxes are strongly controlled by discharge. To identify the mechanisms supplying increased Si at high discharge (Q), we examine Si-Al-Fe-Ge in soils, streams, and groundwaters. We identify bedrock weathering in groundwater and colloidal transport as the two end-members that supply Si to the system. During base flow, streamflow is derived from groundwater, and weathering of feldspar is the main source of Si with a Ge/Si ratio 0.2–0.5 µmol/mol and low dissolved Al and Fe. The groundwater data are consistent with incongruent weathering of feldspar as the main source of dissolved Si. As discharge increases, Si-Al-Fe-bearing colloids are mobilized and the Ge/Si ratio of the stream rises to 3.0 µmol/mol. Mineralogical analysis using XRD identified Al-Si phases including kaolinite, illite, and quartz in the colloidal size fraction (<0.45 µm). The Ge/Si ratio of stream and soil colloids is ≈3.8 µmol/mol. The mechanism of colloidal transport with increasing discharge can account for the concentration-discharge (C-Q) patterns of Si, Al, and Fe with near-zero or positive power law slopes observed in the Gordon Gulch catchment. Anomalously high Ge/Si ratios also identified a third end-member resulting from coal fly ash deposition during the winter and spring. Wind trajectories during 2012, correlation between Ge/Si and math formula, and comparison to atmospheric deposition data imply contamination from nearby coal fired power plant operations.
Aguirre A.A., Derry L.A., Mills J.T., Anderson S.P. (2017): Colloidal transport in the Gordon Gulch catchment of the Boulder Creek CZO and its effect on C‐Q relationships for silicon. Water Resources Research 53, 2368–2383, . DOI: 10.1002/2016WR019730
This Paper/Book acknowledges NSF CZO grant support.