Streams draining mountain catchments are an important pathway for carbon and weathering products to leave the critical zone (CZ). During intense events such as North American monsoon related storms, shallow flow paths may dominate and introduce soil-derived particulate organic matter (POM), mineral particles and organo-mineral heteroaggregates, into the streams. However, it is not yet well understood how the composition of colloidal and particulate matter (PM) changes during the storm-fed hydrograph. We hypothesized that during small, low intensity storms (small hydrograph response) both organic and organo-mineral aggregates will dominate the suspended particulate load, and that during larger high intensity storms (distinct rise of stream water levels) there will be a significant increase in organic polysaccharide particulates during the rising limb and peak of the storm, with higher levels of minerals being re-introduced during the falling limb of the hydrograph. A headwater stream draining a small (1.3km2) watershed in the Santa Catalina Mountain Critical Zone Observatory (SCM-CZO) was sampled at high resolution (5 minute) intervals during monsoon storms, and solutions were cascade-filtered through polycarbonate filters of 8, 1.2, 0.4 and 0.025 µm pore size. The PM mass was determined and particles >8 µm were further analyzed using Fourier Transform Infrared (FTIR) microscopy. These qualitative spectral results were supplemented by the ultra-violet/visible and fluorescence spectra of the colloidal and dissolved (<1.2 µm and <0.4 µm) matter in the same sample. Preliminary results for a low intensity storm suggest that three main types of particles dominated: i) POM, ii) silicates and hydroxide minerals and iii) OM-mineral aggregates. POM was most abundant and chemical functional group composition resembled that of the <1.2 µm and <0.4 µm filtrates (e.g. distinct phenolic-OH and aliphatic C-H stretching bands were observed in both sets of spectra). Bands characteristic of silicate minerals were present in the particulate load (>8 µm) but not in the <1.2 µm and <0.4 µm filtrates. However, the composition of colloidal and particulate load did not change significantly over the course of the hydrograph. Additionally, the PM (>8 µm) mass per liter did not show a consistent trend of change over the course of the hydrograph. This suggests that the low intensity storm may not have moved enough particulate material into the stream to detect a shift in flow path using this method. Additional data are being collected to determine the extent to which high intensity storms alter the distribution of colloidal and particulate matter of stream flow.
Prescott-Smith J., Pohlmann M. A., Perdrial J. N., Perdrial N., Troch P. A., Chorover J. (2012): Does the composition of streamwater colloidal and particulate matter change during monsoon storms?. Abstract EP43A-0866 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec (Poster). .