Hydrodynamic disconnectivity between surface water and groundwater is common in arid environments. It is also prone to affect shallow streams in wetter climate. Sediment layers with low permeability, owing to clogging, for instance, reduce hydrodynamic connectivity. The resistance of this clogging layer, results in unsaturated infiltration, which is characterized by the non-linear Richards equation. Either due to lack of field information, parsimony regarding computational resources or mere misunderstanding of the system, infiltration is often assumed saturated or drastically simplified in hydrogeological models. Here we show the existence of three simple generic asymptotic solutions to the unsaturated problem of vertical steady-state infiltration through a clogged profile, which we associate to three regimes: one dominated by the clogging layer, one by the underlying sediments and one depending on both layers. We also argue that infiltration rate roughly grows linearly with ponding depth. These observations motivate a refined definition of clogging potentially helpful in model selection as well as two novel approximate infiltration formulas. Our refined generic analytic framework is useful to better understand and formalize infiltration.

Streams and rivers are important components of the carbon cycle as they simultaneously transport and transform terrigenous dissolved organic matter (DOM). The time DOM spends in a stream network is an important constraint on the biogeochemical processes that act upon DOM. We used high-resolution Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) to study the spatial distribution of DOM at the molecular level at more than 100 sites within an Alpine stream network during summer and winter baseflow. We developed a model approximating the time DOM spent in the fluvial network. Discharge-weighted travel time (DWTT) explained the compositional changes of DOM, which differed markedly in summer and winter. We attribute the seasonal differences to differences in source material. Hydrological mixing at confluences was an important driver of the spatial dynamics of DOM. From the spatial patterns of individual DOM compounds we inferred the distribution of sources within the catchment, which differed seasonally. Finally, we estimated the apparent mass transfer coefficients of individual DOM compounds at the network level and identified the oxidative state of DOM as an important factor explaining uptake efficiency. This work contributes to our understanding of the spatial processes, temporal constraints and chemical properties of DOM that regulate the transformation and diagenesis of DOM at the fluvial network scale.

Quantification of the rate of gas exchange across the air-water interface is essential in understanding the biogeochemical cycling of carbon in mountain streams. However, estimating the gas transfer velocity (k) is not trivial, due to high turbulence and subsequent bubble-mediated gas transfer. Schmidt scaling is often used to estimate gas transfer velocities of climate relevant gases (e.g. CO2) from tracer gases (e.g. argon (Ar)), but this method has high uncertainty when scaling between gases of different solubilities in streams with bubble-mediated gas transfer. Here we explore a method for the estimation of gas exchange of CO2 from Ar by performing dual tracer gas additions in mountain streams. Ar and CO2 gas were simultaneously and continuously injected into streams and gas exchange rates were estimated using an exponential decline model. The mean ratio of gas exchange of Ar to CO2 (a) was 1.7 (95% credible interval of 1.3 to 2.3), approximately equal to the theoretical value of 1.7 (based both on Schmidt scaling and solubility). This result indicates that Ar can be used to estimate gas transfer of CO2 with scaling but with some uncertainty. Finally, modeled results suggest that the use CO2 as a tracer gas to measure gas exchange in streams with environmental conditions favoring interconversion to bicarbonate (i.e, high pH and alkalinity), can result in an overestimation of the gas transfer velocity k.