## 4 Results and Discussion
### Laboratory Modelling of Biological Groundwater Treatment  
In the first stage, electron donors were selected to stimulate the microbial community of the formation water in Samples 1 and 2 (Table 4). Ordinary substrates and one complex substrate (whey) [36] were used. It has been established that the optimal electron donor for the denitrifying microbial community is whey, a rich substrate containing a variety of organic and mineral substances, including phosphate [37]. Further, whey was used in the in situ experiments. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Electron Donor** & **Sample 1 \({}^{*}\)** & **Sample 2** & **Sample 3** \\ \hline Sodium acetate & 130 & 98 & 108 \\ Sucrose & 127 & 124 & 107 \\ Glucose & 135 & 101 & 95 \\ Ethanol & 34 & 14 & 12 \\ Na lactate & 84 & 74 & 63 \\ Milk whey & 141 & 155 & 120 \\ \hline \hline \end{tabular} \({}^{*}\) Since no nitrate ions were detected in Well 1, in the laboratory experiment, nitrate at a concentration of 1000 mg/L was added, similar to Sample 2.  
\end{table} 
Table 4: Denitrification rate (mg NO3\({}^{-}\)/L/day) using various organic substrates. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Taxon** & **Sample 1** & **Sample 3** \\ \hline Acidomicrobiales & 0.3 & 4.4 \\ Acinetobacter & 2.3 & 0.1 \\ Actinorycelates & 2.2 & 1.3 \\ Alcaligenaceae & 0.4 & 2.6 \\ Anaerobic & 1.3 & 0.9 \\ Bacteroides & 5.3 & 3.4 \\ Chloroflexi & 1.6 & 0.1 \\ Clostrridiales & 1.2 & 1.2 \\ Commandaceae & 7.2 & 1.5 \\ DesulfoDactaceae & 3.7 & 0 \\ Desulfoporosinus & 1.2 & 0 \\ Desulfoviibrio & 5.2 & 0 \\ Faecalibacteriumium & 1.1 & 1.0 \\ Flavobacteriaceae & 2.4 & 3.7 \\ Gallionella & 6.8 & 5.3 \\ Geobacter & 3.4 & 0 \\ Hydrogeompha & 0.6 & 2.8 \\ Lachnospiraceae & 2.6 & 2.2 \\ Methanobacteriaceae & 1.1 & 0.2 \\ Methylophilaceae & 1.3 & 1.8 \\ Methylotorenera & 0.6 & 0 \\ Microococcaceae & 1.6 & 0.1 \\ Mycoplasma & 1.3 & 0.5 \\ Parcubacteria & 0.4 & 5.0 \\ Other & 30.0 & 21.4 \\ Paenibacillus & 0.1 & 1.4 \\ Parvarchaea & 0 & 0.7 \\ Prevotella & 1.4 & 1.4 \\ Pseudomonas & 1.5 & 3.9 \\ Ralstonia & 0.7 & 0.4 \\ Rhizobiales & 1.3 & 0.9 \\ Rhodobacteracea & 0.6 & 1.6 \\ Rhodocyclaceae & 1.6 & 0.2 \\ Rhodospirillaceae & 0 & 1.1 \\ Ruminococcaceae & 2.2 & 1.9 \\ Sulfurimonas & 1.1 & 20.5 \\ Veillonellaceae & 0.8 & 0.1 \\ Verrucomicrobia & 0.9 & 0.4 \\ Xanthomonadaceae & 1.0 & 5.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Heat map of 16S rRNA gene analysis results for Samples 1 and 3 (part 1). (% of the total number of operational taxonomic units—OTUs). the visible removal of sulphates began after 20 days. In Sample 2, nitrate removal and sulphate removal continued for 20 days and 60 days, respectively. In the sample from Well 3 with maximum pollution, nitrate removal continued for 150 days, while a significant accumulation of nitrite was observed. Removal of sulphates lasted 180 days. Thus, in natural water with the original microbial community, purification is very rapid. It should be taken into account that the initial mineralization and redox potential of the background water (Sample 1) were significantly lower, with only 30 mg/L sulphate. To explain the fundamental differences between the nitrate and sulphate removal rates in the laboratory experiments, 16S rRNA gene analysis of the microbial community was conducted at the end of the whey addition experiment (Table 5). The removal of nitrates and sulphates in Sample 1 thus occurred after the addition of whey due to the activity of a metabolically diverse microbial community represented by denittifying and sulphate-reducing bacteria. For highly contaminated Sample 3, nitrate removal was too slow because of the activation of nonspecific nitrate reducers. Such retardation can be explained by toxic nitrite shock, which lasts for a long time and occurs at the initial stage of the denitrification process.