## 3 Results
### Pollution Level in Groundwater Due to Localization of Piezometers
#### 3.2.1 Biochemical Oxygen Demand (BOD) in Groundwater  
As presented by [PERSON] and [PERSON] [38], typical BOD values are: 150-300 mg O\({}_{2}\)/L for raw sewage, 200-600 mg O\({}_{2}\)/L for storm water runoff from residential areas, and 20-30 mg O\({}_{2}\)/L for treated sewage. Unpolluted natural waters are characterized by BOD values below 5 mg O\({}_{2}\)/L. It can also be stated that a BOD level between 1 and 2 mg O\({}_{2}\)/L indicates very clean water, 3.0 to 5.0 mg O\({}_{2}\)/L indicates moderately clean water and BOD > 5 mg O\({}_{2}\)/L indicates a nearby pollution source. At BOD levels of 100 mg O\({}_{2}\)/L or higher, the water supply is considered as very polluted with organic waste.  
Monitoring data of BOD in groundwater show that two years before the closure of the vertical barrier, the BOD values exceeded 500 mg O\({}_{2}\)/L in some parts of the monitored area (Figure 3). Such high concentrations referred to the western and northern part of the landfill site, strictly surrounding piezometers P-9, P-7 and P-11A. In the eastern part of the study area, the BOD concentrations were in the range of 100-260 mg O\({}_{2}\)/L. As a result, it can definitely be stated that a large part of the study area was at that time contaminated by organic compounds.  
Figure 3: Distribution of the mean values of the biochemical oxygen demand (BOD) in groundwater in 1998, 2004 and 2016.  
North emphasizing is the fact that, as compared to the survey outcomes from 1998, BOD concentrations were even seven times lower four years after the closure of the vertical barrier (2004). The highest concentrations of BOD recorded at that time in the monitored area did not exceed 70 mg O\({}_{2}\)/L, but still referred to the presence of organic contaminants in groundwater (Figure 3).  
For actual monitoring data (2016), the results show a low BOD level (1.0-7.0 mg O\({}_{2}\)/L), which means that groundwater has not been contaminated with fresh leachate. Increased values of BOD (6-7 mg O\({}_{2}\)/L) concentrations were measured only in the area of the composting plant and the MBT installation.  
Similar results reporting low BOD concentrations in piezometers located at the landfill site were reported, for instance, by [PERSON] and [PERSON] (1958), [PERSON] and [PERSON] (1998), and [PERSON] and [PERSON] (1998). As opposed to these examples, incomparably higher values of BOD were recorded at a municipal solid waste landfill site in Sri Lanka (BOD = 3590 mg O\({}_{2}\)/L) ([PERSON] et al., 2016).  
Additionally, valuable is observation of temporal changes of the BOD level from 1998 to 2016 (Figures 4-6). Apart from the changes before the closure of the vertical barrier (November 2000), the monitoring data show stabilization with some exceptions in case of piezometers located in the area of adjacent facilities. However, in piezometers P-10, P-17 and P-14, data show very low values so even some changes are acceptable and should not be disquieting. It is also worth emphasizing that changes of pH can affect the microorganisms that consume the organic matter and then affect the level of pollution caused by organic compounds. The microorganisms (their amount and activity) have the significant impact on BOD values. In the case, when the amount of microorganisms is small, the processes of biochemical breakdown do not occur or intensity of the biochemical breakdown is insignificant. In natural conditions, such effect is often caused by the presence of toxic compounds (e.g., heavy metals) that adversely affect the enzymatic activity of the microorganisms. In that case, it must be considered that the changed values of BOD do not reflect the actual level of water pollution.  
Figure 4: Temporal changes of the BOD level in piezometers located in the landfill area.