Estimation of natural background level of Cr(VI) in ultramafic rock related alluvial aquifers of central Greece: Comparison of results by statistical and deterministic preselection method approaches

The definition of natural background concentration levels (NBLs) of geogenic trace metals in groundwater is a challenging issue, particularly in areas where anthropogenic activities are also present. The estimation of NBLs, in combination with environmental quality standards, in such areas is particularly important for the establishment of relevant groundwater threshold values. Over 100 groundwater samples were collected and analysed from four Cr(VI) impacted, alluvial groundwater bodies of central Greece during two consecutive hydrologic years. A common feature of the examined aquifers is the presence of weathered ultramafic rock material in the alluvial sediments. Most sampled boreholes (79 %) are used for irrigation, whereas 21 % of them are used for domestic drinking water supply. Hexavalent Cr concentrations in groundwater, ranging from below detection limit to 430 μg/L, have been attributed to both geogenic and anthropogenic factors. The scope of the present study is to estimate the NBL of Cr(VI) by using a classical statistical approach and a deterministic preselection method and test the comparability of results. In the statistical approach the distribution of samples versus Cr(VI) concentrations has been explored by using probability plots. In this way, the concentration variations within the examined groundwater bodies can be studied and the presence of sub-populations becomes evident by breaks in the slope. In the instance of the preselection method, the concentrations of a set of additional analyzed parameters in ground water, including major water ions and nitrate as well as dissolved oxygen, have been taken into account in order to categorize the samples into two groups of low and high anthropogenic influence, respectively. The comparability of the results derived by the two approaches are discussed in the context of EU Water Framework Directive.