## 1 Introduction  
Nowadays, around 780 million people suffer from water scarcity and the numbers are expected to increase due to population growth, economic development and climate change [1; 2; 3]. By 2025, two billion people will be living in countries or regions with water scarcity and two-thirds of the world's population will be living under water-stressed conditions [4; 5]. The managed recharge of aquifers (MAR) has been successfully applied in different countries for mitigating the effects of water scarcity [6]. The method implies the use of excess surface water to recharge an aquifer under controlled conditions for later use or environmental benefits [7]. MAR application enhances the capacity of existing water supply systems, contributes to the prevention of saltwater intrusion into coastal aquifers and reduces evaporation by storing water in the subsurface layer [3; 7]. Further advantages include the use of soil and aquifer treatment capacity (SAT) for the removal of organic matter, viruses and pathogens from the infiltration water. The technical performance of SAT systems is characterized by two key factors: the maximum amount of water that can be infiltrated and the degree of water purification during water percolation through the soil. These factors depend mainly on the quality of the infiltration water, the operational conditions and the specific properties of the shallowest soil layer in the infiltration basins. In some cases, facility operators embed sand with very low organic matter in this layer. The sand acts as medium for the filtration of suspended particles and as substrate for bacterial development. Otherwise, infiltration takes place through naturally existing sandy soils with similar characteristics. For the efficient degradation of organic matter, the shallowest soil layer must provide sufficient water, nutrients and oxygen to native bacterial communities. The supply capacity is influenced by the properties of the soil matrix such as pore size distribution (PSD) [8; 9; 10]. PSD is defined as the relative abundance of different pore sizes in a representative volume of soil. Pore sizes in loose rock sediments are characterized according to their drainage or water holding capacity and are divided into fine pores (equivalent diameter \(<\) 0.2 \(\upmu\)m), medium-sized pores (from 0.2 \(\upmu\)m to 10 \(\upmu\)m), fine coarse pores (from 10 \(\upmu\)m to 50 \(\upmu\)m) and wide coarse pores (\(>\)50 \(\upmu\)m) [11]. Each pore size depicts a different ability to transport and store water, nutrients and oxygen, which is crucial for microbial activity and further for bioavailability of nutrients [11]. The transport of water containing nutrients and oxygen is poorer in less-drainable, fine pores which form low-porosity media than in well-drainable coarse pores. By contrast, water holding capacity (and implicitly the availability of nutrients) is higher in poorly drainable, fine pores than in coarse pores [9; 12]. The performance of soil microorganisms is largely dependent on water, nutrients, and soil aeration [13; 14; 15; 16]. If the soil moisture content is too low, the water diffusion into the cell (driven by the concentration gradient of the solute) is reversed, which leads to microbial cellular water loss (desiccation stress), and further to diminished microbial activity [17; 18]. Additionally, low soil moisture can also be the reason for insufficient supply of microbes with essential nutrients. In the short term, this may induce states of relative inactivity while in the long term, the starvation of the microbes in unsaturated sites may severely limit the rates of biodegradation [19; 20; 21; 22]. By contrast, a high soil moisture content is correlated with oxygen deficiency in the system and also leads to the reduction of the biodegradation rates [19; 23; 24; 25]. The minimum volume of pore space filled with air should be higher than 10% for the occurrence of aerobic biodegradation [26]. At lower values, the biodegradation process will be anaerobic because of the lack of oxygen.