## 1 Introduction
Often, soils are contaminated with a mixture of many different classes/groups of potentially hazardous substances, including risk elements (REs), polycyclic aromatic hydrocarbons, persistent organic pollutants (POPs) such as polychlorinated biphenyl, chlorinated pesticides, etc., and their potential bioavailability/bioaccessibility for various organisms has already been widely investigated [27, 28, 29, 30]. The negative health effects on animal and human organisms resulting from exposure to each class of pollutant are generally known as well [31, 32, 33]. However, data concerning the mutual effect on the environment of a cocktail of the above-mentioned compounds are scarce. Among the REs group, arsenic (As) deserves special attention. Arsenic is well-known as a 'traditional' environmental contaminant and pollutant of global public health concern. Studies carried out worldwide have shown that the uptake of As from environmental sources and As-contaminated food by both humans and experimental animals can result in various disease conditions [34; 35; 36; 37]. Moreover, inorganic As is present in biological systems as either arsenite or arsenate and can be metabolized via enzymatic methylation into various organometallic compounds such as methylarsonic (MMA) and dimethylarsinic acid (DMA). In this context, the methylated As compounds were proven as less toxic in biological systems compared to the inorganic ones. Therefore, the potential risk of As and its compounds in environmental pollution cannot be underestimated. Arsenic and CBZ at concentrations detected in the environment are known to exert their toxicities in biological systems by causing the generation of excessive reactive oxygen species (ROS), which can overwhelm the organism's antioxidant defence system, leading to oxidative stress [37; 38]. Small terrestrial mammals can be exposed simultaneously to both 'traditional' pollutants (e.g., REs) and ECs such as pharmaceuticals, where the potential mutual effects due to the interaction between both groups of contaminants on these organisms remain currently unknown. In a rare case, exposure of common beet plants to a mixture of REs and ECs resulted in an antagonistic relationship between the two groups of pollutants [39]. The present study prioritised the combined exposure to CBZ and As due to their frequent occurrences in the environment, with the common vote (_Microtus arvalis_ L.) as the selected mammalian model organism for laboratory experiments. The choice of a small terrestrial mammal such as _M. arvalis_ for this research was based on their proven effectiveness as bioindicators for monitoring the terrestrial environment [40; 41]. In addition, they have been effectively utilised as surrogates in previous studies for the assessment of potential health risks to humans and terrestrial animals that were exposed to inorganic and organic contaminants [42; 43]. Arsenic is among the most hazardous pollutants in the Czech Republic due to elevated As content in soils in various areas [44; 45; 46]. The choice of CBZ for this study was based on our previous experiences with animal exposure to this pollutant [47]. We hypothesized potential worsening of the pollutant effects in the case of combined compared to sole exposure. This experiment can provide novel information and add to the general knowledge concerning the potential impact of multiple sources of environmental pollution on wildlife. Therefore, the present study had two objectives:  
1. Assess the impact of the sole and co-exposure effects of CBZ and As oral uptake by _M. arvalis_ by evaluating their accumulation and biotransformation (methylation) within the investigated animal organism as compared to the control group and in relation to each other. In this context, CBZ was present in the drinking water, whereas As-bearing soil was incorporated into the diet of voles to simulate unintentional consumption of soil under natural conditions. 2.