Selenium speciation studies from Soan-Sakesar valley, Salt Range, Pakistan

A regional study of Selenium (Se) speciation in surface waters, groundwaters, and saturation extracts from Soan-Sakesar valley Salt-Range, Pakistan was started in March 1994 and completed in October 1996. Groundwaters, streams, and springs had median concentrations of total Se 48, 404.5, and 82 mug/l; Se+VI 40, 350 and 51 mug/l, Se+IV 8, 48 and 29 mug/l; Se-II 0, 6.5 and 2.0 mug/l, respectively. The concentration of Se exceeded the recommended water quality guidelines for drinking and irrigation water of 10 and 20 mug/l, respectively. In saturation extracts, median total Se, Se+VI, Se+IV and Se-II were 190, 146 37 and 7.0 mug/l, respectively. Uchhali, Khabbaki and Jahlar lake water samples had the mean concentration of total Se 2103, 670 and 297 mug/l; Se+VI 1777, 470, and 233 mug/l, Se+IV 291, 166 and 39 mug/L; Se-II 35, 34 and 25 mug/l, respectively. The Sakesar limestone Formation of Eocene age rich in shales and fossils (median Se concentration 7.2 mug/kg) are considered to be the source of selenium that have enriched soils of Soan-Sakesar valley. Selenim speciation results show the abundance of selenate in all the environmental samples. A linear relationship between Se+VI and pH (R-2 = 0.84, 0.58, 0.88, 0.82, and 0.91, significant at the 0.05 level) for groundwater, saturation extract, stream, spring, and lake waters were noted, suggesting that the Se+VI in the samples is highly associated with pH. Linear relationships were established between Se+VI and Mg, NO3, and SO4 for groundwater (R-2 = 0.24, 0.16 and 0.64; significant at the 0.01 level) and surface water (R-2, 0.96 0.14, and 0.91, significant at the 0.001 level). This suggests the high concentrations of Mg, NO3 and SO4, which strongly adsorb and compete with selenium for surface site, increases the Se+VI fraction. The results show that the higher percentage of selenate on other species in the analyzed samples is favored by higher pH and oxidizing environments of the valley.