## 3 Results and Discussion
### Preservation Studies of As(III/V) Stability in Natural Water Samples  
Arsenic occurs in groundwater predominately in its inorganic forms and the Eh and pH are the most important factors predicting inorganic As speciation. Under oxidizing conditions, increased concentrations of As(V) in groundwaters can be derived from the oxidation of sulfide minerals, particularly arsenopyrite. In reducing conditions at near-neutral pH, As(III) is the dominant As species, derived from desorption of As bound to Fe and Mn mineral oxides [34]. The area of Osijek, in eastern Croatia, is located in the southern part of the Pannonian Basin. In the Pannonian Basin (Hungary, Romania, Croatia, and northern Serbia), groundwater resources have naturally high levels of arsenic, which makes it the largest region in Europe that is affected by high concentrations of this element in drinking water [35]. The comprehensive overview of the problems of natural arsenic pollution in eastern Croatia was given by [PERSON] et al. [36]. The authors analysed groundwater samples taken from 56 wells. Arsenic was detected in 46 of 56 samples and \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Sample**} & \multicolumn{6}{c}{**Time after Sampling**} \\ \hline  & **3 h** & **24 h** & **48 h** & **3 days** & **7 days** & **9 days** & **10 days** \\ \hline  & \multicolumn{6}{c}{1: Unpreserved} \\ \hline As(III) & 10.01 & 10.91 & 9.85 & 11.00 & 14.30 & a & 17.01 \\ As(V) & 9.48 & 10.52 & 10.13 & 9.66 & 6.20 & a & 3.00 \\ \hline  & \multicolumn{6}{c}{2: CA, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\)} \\ \hline As(III) & 9.84 & 9.11 & a & 9.14 & 10.14 & 18.10 & 16.50 \\ As(V) & 10.24 & 12.89 & a & 10.86 & 10.00 & 1.90 & 3.50 \\ \hline  & \multicolumn{6}{c}{3: CA, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\) + HAc} \\ \hline As(III) & 8.74 & 9.81 & 11.70 & 9.31 & 9.87 & a & 13.30 \\ As(V) & 11.42 & 10.19 & 8.13 & 10.89 & 9.50 & a & 6.70 \\ \hline  & \multicolumn{6}{c}{4: NaC, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\)} \\ \hline As(III) & 10.09 & 9.46 & 9.83 & 3.4 & 3.79 & a & a \\ As(V) & 8.92 & 10.55 & 9.67 & 20.40 & 23.41 & a & a \\ \hline  & \multicolumn{6}{c}{5: NaC, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\) + HAc} \\ \hline As(III) & 9.75 & 9.46 & 8.40 & 9.71 & 9.57 & 9.90 & 5.50 \\ As(V) & 9.95 & 11.57 & 12.20 & 10.29 & 10.22 & 10.10 & 14.52 \\ \hline  & \multicolumn{6}{c}{6: TAR, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\)} \\ \hline As(III) & 10.48 & 10.56 & a & 14.80 & 13.77 & a & 20.01 \\ As(V) & 7.62 & 8.16 & a & 5.80 & 6.33 & a & 0 \\ \hline  & \multicolumn{6}{c}{7: TAR, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\) + HAc} \\ \hline As(III) & 11.00 & 10.55 & 9.76 & 10.15 & 9.91 & a & 12.90 \\ As(V) & 9.54 & 9.63 & 10.84 & 9.47 & 10.74 & a & 7.10 \\ \hline  & \multicolumn{6}{c}{8: OX, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\)} \\ \hline As(III) & 10.30 & 10.15 & 10.60 & 10.10 & 10.40 & a & 11.08 \\ As(V) & 9.71 & 9.41 & 8.30 & 10.02 & 10.60 & a & 8.92 \\ \hline  & \multicolumn{6}{c}{9: OX, c = 2 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\) + HAc} \\ \hline As(III) & 10.06 & 9.60 & 6.75 & 6.87 & 7.40 & 6.05 & a \\ As(V) & 11.73 & 11.07 & 16.25 & 16.16 & 13.04 & 13.95 & a \\ \hline  & \multicolumn{6}{c}{10: HAc} \\ \hline As(III) & 9.60 & 9.52 & 6.20 & a & 7.60 & a & 14.30 \\ As(V) & 10.55 & 10.48 & 14.75 & a & 12.43 & a & 6.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Influence of preservation methods on stability of arsenic speciation in As(III)/As(V) 10 \(\upmu\) L\({}^{-1}\) model samples. concentrations were between 1.3 and 491 \(\upmu\)g L\({}^{-1}\). In approximately 64% of the sampled arsenic concentrations exceeded 10 \(\upmu\)g L\({}^{-1}\) and in 20 wells the concentrations were above 100 \(\upmu\)g L\({}^{-1}\). Preservation studies in four natural samples collected from Croatia (Zagreb county and Osijek county) were conducted.