Towards a better practice in water sampling: Case studies on used in practice geothermal waters.

The state-of-the-art of sampling procedures is specified in appropriate guidelines, but there is a permanent need to minimise the risk to the water sample quality, in particular for waters with specific characteristics such as geothermal waters. Improper sampling leads to erroneous results and, consequently, e.g. increased uncertainty in models of geochemical processes (scaling, corrosion) in geothermal installations, aquifer geothermometry, or environmental assessments. The impact of the applied filters (nylon, PVDF, MCE), containers (HDPE, borosilicate glass) and preservation on concentration of 34 major, minor and trace elements was studied for the samples of two geothermal waters used in practice. The results of replicated chemical analyses in duplicated samples of geothermal waters and deionized water blanks were subjected to statistical analyses, including nonparametric tests (Kruskal-Wallis, Dunn's, Kolmogorov-Smirnov, Mann-Whitney U, Wald-Wolfowitz). The PVC housing of the used MCE syringe filters contaminates water samples with Sb. Borosilicate glass increases Al, Cu, Pb and Se concentrations in samples. Acidification protects against changes in concentrations of numerous elements (i.a. Al, B, Ba, Cu, Fe, Ni, Pb) in the solution. Results of analysis performed indicated that using nylon membrane filter and HDPE (properly precleaned to wash out extractables) containers for thermal water sampling is recommended.