Observation and modeling of catchment-scale solute transport in the hydrologic response: A tracer study

The coherent description of water flow and solute transport within heterogeneous hydrologic media (e.g., hillslopes or entire catchments) in response to external rainfall forcings represents a challenge in hydrological modeling. In this paper the mechanisms determining the mobilization and transport of solutes in soils through the paths of runoff formation are investigated by means of a tracer experiment conducted within an instrumented hillslope draining into a tributary of the Dese river basin (northeastern Italy). The response of the test catchment during a rainfall event occurred at the beginning of December 2006 has been analyzed by employing two different chemical tracers: nitrates from diffuse agricultural sources (NO 3-) and lithium from a point injection (Li+). Rainfall depths, streamflows, and pressure heads within different soil horizons were also collected. The observed hydro-chemical response of the Piovega Tre Comuni has been reproduced by a Mass Response Function solute transport model. Our results evidence the origins of the chromatographic effects seen in the system's response. In particular, the deep subsurface component of runoff is found to dominate the long-term behavior of the hydrograph and proves responsible for exporting relatively large amounts of solutes from soil. Experimental evidences and modeling results also suggest that the behavior of the breakthrough curve in stream waters may be strongly affected by the persistence of the rainfall forcing and by the relative magnitude of the rainfall-driven hydrologic contribution with respect to the background noise unrelated to soil moisture dynamics.