Disentangling water transport and tracer mixing mechanism in mountainous environments influenced by volcanic features

Abstract

Landscape features of volcanic origin influence the transport of water and solutes across high-elevation environments. Nevertheless, knowledge regarding how these features affect subsurface hydrological behavior at different spatial scales (from plot to catchment) is scarce. The effect of the soils originated from volcanic ash, such as Andosols (or Andisols), and the influence of highly fractured geology of volcanic origin on subsurface hydrological behavior and water flow path delineation are poorly understood. To fill this knowledge gap, I took as the main objective in the doctoral project the analysis of how Andosols and fractured volcanic geology influence flow transport and tracer mixing mechanisms. Laboratory, experimental, and field measurements of the water retention curve (WRC) of Andosols in combination with data extracted from the published literature shows that standard laboratory methods resemble well a small portion of the wet range of the WRC, specifically, from saturation to the matric potentials 3 to 5 kPa (pF 1.5-1.7). For higher matric potentials, standard laboratory methods substantially overestimate the water content of the soils in comparison to experimental and field measurements. Further, a unique set of hydrometric, stable isotope, and soil hydraulic properties data were evaluated to investigate how Andosols influence water transport and tracer mixing mechanisms at a steep tropical hillslope. The results from this analysis point to the dominance of vertical flow paths within the soil matrix, despite the formation of a perched water layer below the root zone, which mimics the hydraulic behavior of a wet, layered sloping sponge. Last, I used a tracer-aided hydrological model (TraSPAN) calibrated for the stable isotopes of water and electrical conductivity (or specific conductance) during a rainstorm event for the analysis of the role of the fractured volcanic geology on flow transport and tracer mixing at the catchment scale. The model structure that best simulated the streamflow hydrograph and the tracers concentrations during a rainfall event consisted of two water reservoirs representing the soils with high infiltration capacity and the groundwater system formed in the fractured bedrock. During the monitored event, only 13% of total precipitation was converted into runoff, with a major proportion (75-81%) corresponding to pre-event water stored in the catchment prior to the event. These findings indicate a large water storage capacity of the system in the fractured volcanic geology.