Vadose zone air as a biogenic source of methane in Nerja Cave system (South of Spain)

Subterranean air in karst cavities has often low methane contents in comparison to atmosphere and so karst systems have been considered a sink of atmospheric methane. Methane-oxidizing bacteria have been hypothesized as responsible for CH4 depletion in many caves around the world, although ionization radiation was also proposed as possible mechanism for this process. We measured CO2 and CH4 concentration and their C isotopic composition (d13C-CO2 and d13C-CH4) over 2 years within the Nerja cave system (South Spain) and in 9 boreholes drilled into the vadose zone (Triassic carbonate aquifer) surrounding the cave. According to d13C-CO2 and d13C-CH4 vadose zone of this karst system is a source of biogenic methane, produced both by acetate fermentation and CO2 reduction. Biogenic and atmospheric methane flows (along fractures from the vadose zone, and through ventilation, respectively) into the cave, where it is oxidized by methanotrophic bacteria that we detected on soil samples into the cave.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

A numerical method for efficient 3D inversions using Richards equation

Fluid flow in the vadose zone is governed by Richards equation; it is parameterized by hydraulic conductivity, which is a nonlinear function of pressure head. Investigations in the vadose zone typically require characterizing distributed hydraulic properties. Saturation or pressure head data may include direct measurements made from boreholes. Increasingly, proxy measurements from hydrogeophysics are being used to supply more spatially and temporally dense data sets. Inferring hydraulic parameters from such datasets requires the ability to efficiently solve and deterministically optimize the nonlinear time domain Richards equation. This is particularly important as the number of parameters to be estimated in a vadose zone inversion continues to grow. In this paper, we describe an efficient technique to invert for distributed hydraulic properties in 1D, 2D, and 3D. Our algorithm does not store the Jacobian, but rather computes the product with a vector, which allows the size of the inversion problem to become much larger than methods such as finite difference or automatic differentiation; which are constrained by computation and memory, respectively. We show our algorithm in practice for a 3D inversion of saturated hydraulic conductivity using saturation data through time. The code to run our examples is open source and the algorithm presented allows this inversion process to run on modest computational resources

Estimation of Unsaturated Flow Parameters by Inverse Modeling and GPR Tomography

The main goal of this work was to evaluate the possibility of estimating the flow parameters and geological structure of the unsaturated zone, also called vadose zone, using both geophysical and hydrological data and methods. The vadose zone at Moreppen field site located near Oslo’s Gardermoen airport was used as the case study. Moreppen field site has been the subject of numerous studies related to sedimentological, hydrological, geophysical and geochemical processes in the saturated and vadose zone. However, in the field of hydrology none of the previous studies at Moreppen used spatially continuous geophysical data to estimate the flow parameters at the field site. In this study, cross well GPR travel time tomography for the first time was used at Moreppen to map the spatial and temporal distribution of the electromagnetic (EM) wave velocity at the field site. The EM wave velocities were converted to the soil water content using a petrophysical relationship. Then using an inverse flow modeling conditioned on volumetric soil water content, we estimated hydrological parameters in the field site. Since snowmelt is the main groundwater recharge at Gardermoen, we focused our study to the water flow through the vadose zone during the snowmelt

Illuminating hydrological processes at the soil-vegetation-atmosphere interface with water stable isotopes

Funded by DFG research project “From Catchments as Organised Systems to Models based on Functional Units” (FOR 1Peer reviewedPublisher PDFPublisher PD

Catchment-scale vulnerability assessment of groundwater pollution from diffuse sources using the DRASTIC method : a case study

The catchment-scale groundwater vulnerability assessment that delineates zones representing different levels of groundwater susceptibility to contaminants from diffuse agricultural sources has become an important element in groundwater pollution prevention for the implementation of the EUWater Framework Directive (WFD). This paper evaluates the DRASTIC method using an ArcGIS platform for assessing groundwater vulnerability in the Upper Bann catchment, Northern Ireland. Groundwater vulnerability maps of both general pollutants and pesticides in the study area were generated by using data on the factors depth to water, net recharge, aquifer media, soil media, topography, impact of vadose zone, and hydraulic conductivity, as defined in DRASTIC. The mountain areas in the study area have “high” (in 4.5% of the study area) or “moderate” (in 25.5%) vulnerability for general pollutants due to high rainfall, net recharge and soil permeability. However, by considering the diffuse agricultural sources, the mountain areas are actually at low groundwater pollution risk. The results of overlaying the maps of land use and the groundwater vulnerability are closer to the reality. This study shows that the DRASTIC method is helpful for guiding the prevention practices of groundwater pollution at the catchment scale in the UK

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

Analytical models were developed that simulate stable isotope ratios of volatile organic compounds (VOCs) near a point source contamination in the unsaturated zone. The models describe diffusive transport of VOCs, biodegradation and source ageing. The mass transport is governed by Fick's law for diffusion, and the equation for reactive transport of VOCs in the soil gas phase was solved for different source geometries and for different boundary conditions. Model results were compared to experimental data from a one-dimensional laboratory column and a radial-symmetric field experiment, and the comparison yielded a satisfying agreement. The model results clearly illustrate the significant isotope fractionation by gas-phase diffusion under transient state conditions. This leads to an initial depletion of heavy isotopes with increasing distance from the source. The isotope evolution of the source is governed by the combined effects of isotope fractionation due to vaporization, diffusion and biodegradation. The net effect can lead to an enrichment or depletion of the heavy isotope in the remaining organic phase depending on the compound and element considered. Finally, the isotope evolution of molecules migrating away from the source and undergoing degradation is governed by a combined degradation and diffusion isotope effect. This suggests that in the unsaturated zone, the interpretation of biodegradation based on isotope data must always be based on a model combining gas-phase diffusion and degradation.Comment: 11 pages, 6 figure