Seasonal and event-based hydrologic variation such as the annual monsoon in the desert southwest are integral to the coupled transport of water, dissolved organic carbon (DOC) and trace metals in the high elevation catchments of the Santa Catalina Mountains Critical Zone Observatory (CZO). Observed increases in the concentration of hydrolyzing cations (Fe, Al, REY) and DOC in stream water during storm events, when discharge is also significantly increased, are consistent with an increased contribution of organic-rich, near-surface soils during monsoonal rains. These same events are characterized by a decrease in non-hydrolyzing cations (Ca, Mg, Na, K), which is consistent with storm-flow dilution of baseflow, the deep groundwater discharge subjected to prolonged parent material contact. The correlation of complexing metals with DOC during storm-flow discharge raises questions as to the nature of these organo-metal associations as well as the dominance of specific size fractions over the course of storm events. We conducted high frequency sampling during a series of storms to provide greater resolution for discharge-dependent hydrochemical signatures in stream water. We hypothesized that increased concentrations of hydrolyzing metals at high flow is due to the entrainment of filterable, nanoparticulate organo-metal complexes, and that theses colloids represent, therefore, a potential signal of ‘bioweathering.’ Sample processing utilized a cascade filtration set-up that separated size fractions at 8 µm, 1.2 µm, 0.4 µm and 0.025 µm. In addition, we used size exclusion chromatography (SEC) coupled in-line with inductively coupled plasma mass spectrometry (ICP-MS) to quantify the distribution of lithogenic solutes among truly dissolved and colloidal forms. In-line UV-vis and refractive index detectors provided a qualitative assessment of organic matter for direct correlation with ICP-MS measurements. Analysis of cascade filtrates by SEC ICP-MS provides greater clarity in separating truly dissolved versus colloidal material contributions to surface water chemical effluxes. Such speciation distinctions are essential for the development of comprehensive process models of chemical weathering in the CZ.
Pohlmann M. A., Perdrial J. N., Prescott-Smith J., Amistadi M., Troch P. A., Chorover J. (2012): Resolving dissolved versus colloidal and particulate weathering product forms across the storm hydrograph. Abstract EP43A-0867 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec (Poster).