This dissertation investigates the sources, sinks, and hydrologic transport of molybdenum (Mo) in experimental watersheds in tropical Panama. The role of preferential flow paths during storm events were first investigated using germanium (Ge) and silicon (Si) as flow path tracers to examine the partitioning of precipitation event waters in catchments with different land covers. The riverine concentrations, weathering yields, and fluxes of Mo, vanadium (V), and uranium (U) were investigated in small mountainous rivers and streams draining high standing ocean islands using archived water samples.
In Chapter 2 of this dissertation, three end-member water types were identified in three experimental catchments — stream baseflow (high [Si], low Ge/Si ratio), dilute event water (low [Si], moderate Ge/Si ratio), and soil pore water (low [Si], high Ge/Si ratio). A three component mixing model was employed as a hydrograph separation tool. During small rain events, storm flow is dominated by baseflow and dilute event water components. During larger events, the third shallow soil water component with high [Ge] and low [Si] is activated, reaching a maximum during the receding limb of the hydrograph. The magnitude of the increase in [Ge] in storm flow is proportional to the size of precipitation event. This component is interpreted to represent the activation of a continuum of long, preferential flow paths in the shallow soil though which event waters acquire an elevated Ge signal.
In Chapter 3, the mixing model from Chapter 2 was applied to the hydrologic cycle of Mo during storm events. Though Mo is considered a rock-derived micronutrient, concentrations were higher in precipitation, canopy throughfall, and shallow soil water than in the groundwater and stream waters in contact with underlying bedrock. A simple mass balance suggests that Mo is being retained within the catchment during storm events. Three and two-component hydrograph separation models consisting of precipitation, soil pore water, and baseflow were applied to predict Mo concentrations in event flow waters resulting from the mixing of these end-members. The modeled fractions vastly over estimated observed Mo concentrations in stormflow, which were modeled better as simple dilution of stream baseflow. Selective chemical soil extractions suggest precipitation-derived Mo is associated with organic material in the soil.
Chapter 4 investigates the riverine concentrations, ocean flux, and weathering yields of Mo, V, and U in a large number small mountainous rivers and streams draining high standing ocean islands. Unlike in large river systems, in which Mo is derived predominately from pyrite dissolution, dissolved Mo concentrations in these rivers do not correlate with sulfate. V correlates strongly with Si in terrains dominated by silicate rocks, but not in sedimentary regions. Fluxes of U and Mo into the ocean from igneous terrains are lower than the global average, while fluxes of V from these regions are higher, and up to two orders of magnitude higher in rivers draining young volcanics. Weathering yields of Mo and V in most regions are above the global mean.