Determining solute inputs to soil and stream waters in a seasonally snow-covered mountain catchment in northern New Mexico using Ge/Si, 87Sr/86Sr and ion chemistry

The process of mineral weathering releases lithogenic elements to soil and stream waters, which influences the health of catchment ecosystems. Variations in hydrologic conditions between years and climatic seasons may change subsurface flowpaths, modifying the influence of weathering on stream water composition. This two-year study aims to determine the changes in solute sources to stream and soil waters in a seasonally snow-covered headwater catchment in the Jemez Mountains in northern New Mexico by using a multi-tracer approach including major cations, strontium (Sr) isotopes, geranium (Ge)/silica (Si) ratios and trace metals. Climatic forcing was different for the two years studied; 2010 had ample snow accumulation resulting in an increase in stream flow during melt, whereas little snow fall in 2011 caused minimal response in the hydrograph. Stream water base cations display relatively constant concentrations with variations in the hydrograph, suggesting that the rate of solute transport out of the system is directly related to fluxes in the water supply. Strontium isotope ratios of stream waters during 2010 and 2011 snowmelt periods (0.70753) were very similar to the spring Sr-isotope ratio (0.70751) indicating deep soilwater as a single dominant source of major cations to stream waters. However, Ge/Si ratios, Fe, Al, and dissolved organic carbon (DOC) concentrations increase during the 2010 snowmelt. Iron, Al and DOC form complexes with organic material and are therefore enriched in the shallow soils. Ge is enriched in secondary, relative to primary, minerals during incongruent weathering, so it is also enriched in soils relative to parent rock. The Ge/Si ratio peak of 3.11 μmol/mol, in combination with an increase in Al, Fe, and DOC concentrations represents a flushing of shallow soils during the 2010 snow melt. Preceding the snow melt Ge/Si ratios decrease to a minimum ratio of 0.72 μmol/mol in 2010 and continue to decrease into 2011 to a minimum of 0.07 μmol/mol. Aluminum, Fe, and DOC concentrations also decrease after the peak in snow melt and remain stable and low for the remainder of the study period. The lack of an increase in Ge/Si ratios and Al, Fe, and DOC concentrations during the summer rains shows that deeper soil water dominates stream water composition from June-March, following the snowmelt period.