Evaluation of dissolved and acid-leachable trace element concentrations in relation to practical water quality standards in the Syr Darya, Aral Sea Basin, South Kazakhstan

The Syr Darya is one of the major rivers supplying the Aral Sea with freshwater. Soviet programs aimed at maximizing agricultural productivity in the Syr Darya basin increased diversion of water drastically affecting its water quality with significant consequences to its suitability for irrigation, fisheries and other uses. While water quality standards for trace elements are typically measured in the dissolved phase, there is evidence that adsorbed phases may also be relevant. Here we report potentially available heavy metals and metalloid con-centrations in the Syr Darya water through the treatment of unfiltered waters samples with dilute nitric acid. Significant differences were found for most studied elements (Mann-Whitney U Test, p < 0.05) between their dissolved and acid-leachable concentrations. For Sr and Se in both sampling campaigns, no significant differences were found between their dissolved and acid-leachable concentrations, indicating their low geochemical reac-tivity. Dissolved V concentrations and acid-leachable Ni and Zn were found to exceed Kazakhstan Maximum Permissible Concentrations (MPC) values for the protection of fishery water quality. Our study evaluates the importance of considering regulatory issues of measuring trace metal concentrations to assess the water suit-ability for fisheries and irrigation.

Automated summary

The Syr Darya river, a major water source for the Aral Sea, has been heavily impacted by Soviet agricultural programs that increased water diversion, leading to a decrease in water quality and affecting irrigation, fisheries, and other uses. Our study found significant differences in the concentrations of trace elements when measured in the dissolved phase compared to the acid-leachable phase. Elevated concentrations of dissolved V and acid-leachable Ni and Zn exceeded Kazakhstan's Maximum Permissible Concentration (MPC) values for fishery water quality protection.