Mathematical modeling of transport of micropollutants in porous medium

Ing. Maria Mursaikova

Disertační práce

Abstract:

Earth's drinking water resources, predominantly dependent on groundwater, are increasingly vulnerable to contamination from various sources. Recent advancements in analytical techniques have enabled the detection of contaminants at exceedingly low concentrations within complex environmental matrices, highlighting the urgency of addressing these emerging threats caused by micropollutants. Micropollutants, also known as emerging contaminants, include a vast and growing range of anthropogenic substances, such as pharmaceuticals, personal care products, industrial chemicals, pesticides and herbicides, detergents and surfactants, etc. Among these micropollutants, artificial sweeteners have become a significant concern due to their increasing prevalence in aquatic environments worldwide. Artificial sweeteners are mainly used as a sugar substitute and are often promoted as healthier options than sugar. However, only a few studies have been conducted on their metabolism and side effects, and there is still much concern regarding their safety. These compounds are primarily excreted unchanged by humans and subsequently enter water bodies through sewage systems, posing a significant environmental threat. Artificial sweeteners have been extensively studied, yet the observed variations in their transport behaviour within porous media still need to be more consistent and better understood. These uncertainties underscore the need for further investigation, as understanding the environmental impact of artificial sweeteners and their behaviour under natural conditions is crucial for effective water management. Of particular concern is Acesulfame-K, an artificial sweetener recently identified as the most common contaminant in various countries, including Switzerland, Germany, and China, with concentration levels exceeding four μg/L in aquatic environments. Recently, Acesulfame-K has been proposed as a specific tracer of wastewater in porous medium because of its persistence. Due to its chemical structures, Acesulfame-K has high biological stability, high solubility in water, low absorption on soil particles, and high polarity. On the other side, 5 Acesulfame-K has been proven to be degraded under certain natural conditions, raising questions about its persistence in porous medium and, thus, its effectiveness as a tracer. In recent years, during an earlier investigation conducted in the Káraný waterworks on the Jizera River, Acesulfame-K was also detected in concentrations exceeding 500 ng/L. It turns out that groundwater contamination caused by Acesulfame-K has recently become a concern, and its transport patterns under natural conditions, along with its environmental impact, remain unclear. To effectively manage and protect groundwater resources, it is crucial to develop enhanced methods for predicting the transport and behaviour of micropollutants in porous media, both spatially and temporally. This requires a comprehensive approach, including using mathematical models to assess groundwater environments' quantitative and qualitative status. The core of this thesis focuses on understanding and characterizing the transport patterns of micropollutants within porous media through mathematical modeling. To achieve this, the study specifically aims to develop a functional two-dimensional numerical model that simulates behaviour of the artificial sweetener Acesulfame-K within the riverbank infiltration system of the Káraný waterworks area in the Czech Republic. The presented model is divided into the following three consecutive steps: • M1 setup (unsteady-state hydraulic model): This step identifies the dominant convective layer and determines its rate. • M2 setup (steady-state contaminant transport): This step searches for an optimal firstorder decay rate, ensuring that, under average transport conditions, the observed average contamination from the river aligns with the contamination observed in th

Abstract:

e well. • M3 setup (unsteady contaminant transport): This step validates the manually calibrated parameters from the previous steps under unsteady-state conditions. Here, the computed well concentrations are compared with those observed in our monitoring campaign. By integrating detailed model simulations with empirical data, this thesis seeks to provide valuable insights into the transport of micropollutants in porous medium. The two-dimensional large-scale numerical model was built using DRUtES open-source software to solve the coupled nonlinear convection–diffusion reaction equation. This model simulates the monitoring campaign between the years 2017–2019, and the total simulation time was 700 days. The temporal discretization was adaptive, with a minimum time step of 10 sec and a maximum time step of 3600 sec. Discrete systems were solved by the LDU direct 6 solver with minimal degree (steady-state problems) and by the conjugate gradient method for normal equations (unsteady problems). The hydraulic and contaminant transport models were governed by the Richards equation, coupled with the advection-dispersion reaction equation. Additionally, this research also proved the ability of Acesulfame-K to degrade within a smallscale laboratory experiment. To support the understanding of the transport process of Acesulfame-K within the riverbank filtration system, the conditions of this laboratory experiment were mimicked with the investigated aquifer conditions. Degradation of Acesulfame-K was confirmed under elevated temperature (12–15 °C) combined with oxic/suboxic redox conditions with a first order-degradation rate of λ = 0.03698 1/d, and the corresponding half-life was T1/2 –19 d. This finding is in satisfactory agreement with the modeled first-order rate constant of λ ∼ 0.0358 ± 0.0022 1/d and a corresponding half-life time of T1/2 –19 ± 1 d. With the results of this research, it is possible to develop a fundamental understanding of the transport principles of selected micropollutants under natural groundwater conditions. Consequently, this research has the potential to support the assessment of environmental risks associated with micropollutants in wastewater and highlight the natural-based solution potentials of riverbank filtration systems used worldwide for drinking water production.

Keywords

micropollutants, mathematical modeling, artificial sweeteners, Acesulfame-K, groundwater contamination

Jazyk práce: angličtina

Datum vytvoření / odevzdání či podání práce: 25. 11. 2024

Obhajoba závěrečné práce

• Vedoucí: doc. Ing. Michal Kuráž, Ph.D.
• Oponent: Ioannis Markonis, Ph.D., doc. Mgr. Ing., Janja Vrzel, PhDr., externi, Daniel Kahuda, externi

Česká zemědělská univerzita v Praze

Fakulta životního prostředí

Doktorský studijní program / obor:
Krajinné inženýrství / Environmentální modelování