Linking Soils and Streams: Hydrological Controls on Organic and Inorganic Solute Transport in two Mediterranean Catchments

The dissolved chemical load in a particular stream is the result of a complex interplay between upland soil biogeochemistry, hydrology and downstream biogeochemical cycling. Because few studies have coupled biogeochemical and hydrological studies at the sub-catchment scale, we still do not fully understand the processes controlling transport and retention of nutrients as they interact with different hydrologic pathways. To better understand the processes behind observed seasonal trends in both organic (dissolved organic matter (DOM)) and inorganic (NO3, HCO3, Si, Al, Fe, Ca, Mg, ...) solute concentrations entering the stream network, we monitored changes in solution chemistry as rainwater moved through the soils and out into the streams of two small (<2 ha) coastal California catchments of differing rainfalls. In the steeper, more humid and deeply weathered site where vertically infiltrating throughflow dominates everywhere except in the immediate vicinity of the channel head, concentrations of DOM drop rapidly with depth primarily due to adsorption with the fine textured soil resulting in very low concentrations in stream water. At the gentler sloping site, saturated subsurface and saturated overland flow occur during most large storms leading to a rapid transfer of DOM and other biologically active solutes from the surface soils directly to the stream bypassing the deeper fine textured zone where adsorption and other removal mechanisms could occur. Due to this short-circuiting of typical elemental removal mechanisms, we observe a significant jump in both organic and inorganic solute concentrations from base flow levels during these large storm events as the spatial extent of saturated flow greatly expands upslope from the channel head. In the two ecosystems in this study, the hydrologic routing of water from soil to stream plays a critical role in determining the stream water chemistry.