Jiri Šimůnek

Estimating Unsaturated Soil Hydraulic Properties From Multiple Tension Disc Infiltrometer Data

In a previous study, we showed that the cumulative infiltration rate measured with a tension disc infiltrometer at one particular tension does not provide enough information to estimate van Genuchtens soil-hydraulic parameters by numerical inversion of the Richards equation. In this paper we analyze the possibility of using cumulative infiltration rates obtained at several consecutive tensions for the purpose of estimating soil hydraulic parameters. We also investigate whether additional, easily obtainable information improves identifiability of the unknown parameters. The study is carried out using numerically generated data. The uniqueness problem was analyzed by studying the behavior of response surfaces in the optimized parameter planes. Our parameter estimation procedure combines the Levenberg-Marquardt nonlinear parameter optimization method with a quasi three-dimensional numerical model, HYDRUS-2D, which solves the variably-saturated flow equation. We found that the combination of multiple tension cumulative infiltration data with measured values of the initial and final water contents yielded unique solutions of the inverse problem for the unknown parameters.

Modeling Microorganism Transport and Survival in the Subsurface

An understanding of microbial transport and survival in the subsurface is needed for public health, environmental applications, and industrial processes. Much research has therefore been directed to quantify mechanisms influencing microbial fate, and the results demonstrate a complex coupling among many physical, chemical, and biological factors. Mathematical models can be used to help understand and predict the complexities of microbial transport and survival in the subsurface under given assumptions and conditions. This review highlights existing model formulations that can be used for this purpose. In particular, we discuss models based on the advection-dispersion equation, with terms for kinetic retention to solid-water and/or air-water interfaces; blocking and ripening; release that is dependent on the resident time, diffusion, and transients in solution chemistry, water velocity, and water saturation; and microbial decay (first-order and Weibull) and growth (logistic and Monod) that is dependent on temperature, nutrient concentration, and/or microbial concentration. We highlight a two-region model to account for microbe migration in the vicinity of a solid phase and use it to simulate the coupled transport and survival of species under a variety of environmentally relevant scenarios. This review identifies challenges and limitations of models to describe and predict microbial transport and survival. In particular, many model parameters have to be optimized to simulate a diversity of observed transport, retention, and survival behavior at the laboratory scale. Improved theory and models are needed to predict the fate of microorganisms in natural subsurface systems that are highly dynamic and heterogeneous.

Modeling the release of E. coli D21g with transients in water content

Transients in water content are well known to mobilize colloids that are retained in the vadose zone. However, there is no consensus on the proper model formulation to simulate colloid release during drainage and imbibition. We present a model that relates colloid release to changes in the air-water interfacial area (Aaw) with transients in water content. Colloid release from the solid-water interface (SWI) is modeled in two steps. First, a fraction of the colloids on the SWI partitions to the mobile aqueous phase and air-water interface (AWI) when the Aaw increases during drainage. Second, colloids that are retained on the AWI or at the air-water-solid triple line are released during imbibition as the AWI is destroyed. The developed model was used to describe the release of Escherichia coli D21g during cycles of drainage and imbibition under various saturation conditions. Simulations provided a reasonable description of experimental D21g release results. Only two model parameters were optimized to the D21g release data: (i) the cell fraction that was released from the SWI (fr) and (ii) the cell fraction that partitioned from the SWI to the AWI (fawi). Numerical simulations indicated that cell release was proportional to fr and the initial amount of retention on the SWI and AWI. Drainage to a lower water content enhanced cell release, especially during subsequent imbibition, because more bacteria on the SWI were partitioned to the AWI and/or aqueous phase. Imbibition to a larger water content produced greater colloid release because of higher flow rates, and more destruction of the AWI (smaller Aaw). Variation in the value of fawi was found to have a pronounced influence on the amount of cell release in both drainage and imbibition due to changes in the partitioning of cells from the SWI to the aqueous phase and the AWI.