Environmental Assessment of Groundwater Chemistry related to Ophiolite complex, Nourabad Lorestan

Some researchers defined environment pollution as any change in properties of the environment members in a way that makes not only natural performance and biological balance disordered but also jeopardizes the benefits and the life of living creatures directly and indirectly. For centuries, different heavy metals, whether anthropogenic or earth-made, have been releasing into water and soil resources through different natural processes and industrial activities. In fact, studying water pollution to heavy metals can provide a good way to improve the quality of water, decrease disease, and increase safety and health. One of the natural water polluting activities is the weathering and oxidation processes on ophiolite rocks of special regions. Controlling chemistry processes and water composition, like weathering and dissolution, ion exchange, and absorption-desorption processes, can have high importance.

After field study and making related maps, the samples from the region’s wells and springs were gathered through a field operation. Since the concentration-aqueous concentration of the elements is affected more by evaporation during the dry season, the samples were gathered at the end of September, the time before precipitation when dry summer is finished. Plastic containers with half-liter capacity were used for the samples. Before gathering, the containers were filled and emptied 3 times. After that, 10cc nitric acid was added to the samples for each liter; and then they were sent to Zar Azma Mineral Company for analysis of their heavy elements. Concentration total of 28 light and heavy elements were measured in the water samples by ICP-MS analysis.

Dendrogram diagram and Pearson correlation were applied to classify the samples based on intergroup similarities, to find the probable source of the elements, and to study the correlation among the elements. It can be seen that Cr has a rather high positive correlation (0.54) with Si indicating the fact that they both have the same source, from Si minerals. This table reveals that there is a high correlation between Ca, S, and Sr on one hand, and a high correlation between Cr and Si and somewhat with Mg, on the other hand. Moreover, there is a rather high positive correlation between Ni and P and somewhat between Ni and Mg. These correlations can prove the fact that the elements have the same source. According to ArcGIS maps, the most concentration of calcium, Sr, and S is recorded for zone 1 including Hezarkhani and Yadabad. Moreover, the lowest concentration of these three elements is recorded for Aliabad and Ch-pavleh. These elements are expected to be originated mostly from conglomerate and limestone deposits. It turns out that the second controlling factor of groundwater chemistry includes the weathering and dissolution of source rock of Cr, Mg, and Si. The same-value maps of sampling points have been classified based on the concentration of Cr, Si, and Mg, revealing that the points of highest and lowest concentration of these elements are geographically the same. This fact proves a high correlation and a similar source of these elements. Therefore it can be seen that the highest concentration of these elements is recorded for Ch-pavleh, while the lowest concentration for Aliabad and Gashour. Since chromium has a high positive correlation with Si, it can be concluded that chromium has a silicate source. Based on the geological map of the region, the important silicate source is peridotite. In fact, peridotites, as a representative of the ophiolite collection of the region, can be considered as the source of Cr entrance to the regional waters. Moreover, as these peridotites include ferromagnesian minerals, they are also regarded as the source of Mg entrance to regional waters. Therefore, peridotites are considered as the common source of Si, Cr, and somewhat Mg entrance to regional waters. The existence of Cr- spinel in the geochemistry of peridotite rocks in Ch-pavleh can also prove silicate and peridotite sources of Cr in the region water.

Pearson correlation among elements was calculated to recognize the elements with the most correlation. According to that, the elements with higher correlation are suggested to have the same source. Factor analysis revealed that controlling factors of water chemistry have the highest correlation with the elements which have the highest correlation. Dissolution and weathering of these elements’ source rock are considered the important factors. Besides, focusing on water-rock interaction, the most essential minerals participating in the actions with water were recognized. Same-value maps were applied to determine the source of the elements. Moreover, according to these maps, it is revealed that the elements with the highest correlation come from the same geographical setting.   Based on these stages, the important controlling factors of water chemistry were recognized as the following: weathering and dissolution of conglomerate deposits to the dissolution of gypsum as the common source of calcium, strontium, and sulfide, weathering and dissolution of peridotites, especially ferromagnesian ( as the common source of magnesium and nickel) and chrome-bearing spinel (a common source of chromium, cilice, and magnesium), dissolution of chalcedony, barite, and calcite as three minerals with the highest saturation index.