Surface sediment zonation of the Zayandehroud Dam reservoir based on geochemical indices

Abstract

AbstractDam reservoirs serve as suitable places to settle sedimentary materials carried by rivers. Pollutants absorbed into these sediments may lead to water contamination, increased aquatic animal mortality, and loss of biodiversity. Potentially toxic elements (PTEs) are classified as highly hazardous pollutants due to their long persistence in the environment. Due to their destructive effects on humans and the environment, it is important to examine sensitive and special areas for contamination with them. Therefore, spatial zoning of these pollutants in surface sediments of aquatic ecosystems is very effective in identifying the entry routes and determining their origin, as well as in identifying sedimentation conditions and determining sensitive zones for pollution reduction management. The sampling of surface sediments with the aim of zoning the sediments of the Zayandehroud Dam reservoir in terms of the level of contamination with PTEs based on geochemical indicators was carried out from 20 stations in this lake in December 2022. Then, the physical and chemical variables of the sediments were measured, including pH, EC, Eh, percentage of moisture, bulk density, dry density, grain size, percentage of organic matter, percentage of calcium carbonate, and concentrations of 26 elements. The results showed overall increasing changes in pH, bulk density, dry density, percentage of organic matter, percentage of calcium carbonate, and sand-sized particles from the dam crest to the lake entrance, while the percentage of moisture and Eh showed an overall decreasing trend. Only some elements, including silver, arsenic, chromium, copper, nickel, and lead, exceeded the US sediment quality guidelines (ERL and ERM) at some stations, among the 26 elements measured in the 20 surface sediment samples. Which was found towards the dam crest for arsenic and lead, chromium except for a few points, nickel and copper were found throughout the study area, and silver was found towards the lake entrance. Also, the Enrichment Factor (EF) of lead and copper was lowest at the lake entrance (EF 40) at one station at the lake entrance. The Geoaccumulation Index also showed non-polluted (Igeo1) only near the dam crest.Keywords: Zonation, Surface sediments, Elements, Geochemical indices, Zayandehroud Dam Introduction Potentially toxic elements (PTEs) are classified as highly hazardous pollutants due to their long persistence in the environment. Due to their destructive effects on humans and the environment, it is important to examine sensitive and special areas for contamination with them. Therefore, spatial zoning of these pollutants in surface sediments of reservoir dames is very effective in identifying the entry routes and determining their origin, as well as in identifying sedimentation conditions and determining sensitive zones for pollution reduction management (Kabata et al. 2007). Due to the increasing population growth and corresponding increasing drinking water need, along with the development of agricultural and industrial sectors in the Gavkhooni Basin in arid to semi-arid region of the central plateau of Iran, the optimal utilization of the Zayandehroud Dam is crucial. For this purpose, determining the amount and spatial distribution pattern of PTEs in the surface sediments of the Zayandehroud Dam reservoir using geochemical indices was determined as the main goal of this study. Material & MethodsThe sampling of surface sediments of the Zayandehroud Dam reservoir was carried out using Ekman grab sampler from 20 stations in December 2022. Sampling of surface sediments was carried out along the dam lake and in nine rows on the left and right sides of the old course of the Zayandehroud River. Sediment samples were placed in one-kilogram plastic containers with lids, coded, and transported to the laboratory. Then, the physical and chemical variables of the sediments were measured, including pH, EC, Eh, percentage of moisture, bulk density, dry density, grain size, percentage of organic matter, percentage of calcium carbonate, and concentrations of 26 elements.In order to determine the contamination levels of various elements, a comparison was made with the US sediment quality guidelines (ERL and ERM; Long et al. 1995). Moreover, in order to determine the level of sediment pollution, the EF was used as a suitable indicator to indicate the degree of pollution (Li et al. 2021), and the geoaccumulation index (Igeo) was used to measure the intensity of pollution (Muller 1969), and the pollution load index (PLI) was used (Weissmannová & Pavlovský 2017).The normality of the data distribution and the presentation of a summary of the statistical information of the physical and chemical variables of the sediments and the calculated indices were performed in SPSS software, version 18. Due to the non-normality of most of the variables, the Spearman correlation test was used to determine the relationship between the different variables. Correlation coefficients of 0.7 to 0.85 and greater than 0.85 were considered strong and very strong correlation coefficients, respectively (Dawson et al. 2007). Also, in order to prepare a zoning map of the surface sediments of the Zayandehroud Dam reservoir in terms of the Pollution Load Index (PLI), ArcMap software and the interpolation command using the inverse distance weighting (IDW) method were used. Discussion of Results & Conclusions The results showed overall increasing changes in pH, bulk density, dry density, percentage of organic matter, percentage of calcium carbonate, and sand-sized particles from the dam crest to the lake entrance, while the percentage of moisture and Eh showed an overall decreasing trend. The results of the sediment size gradation generally showed similarities with the spatial pattern reported in previous studies conducted on surface sediments of the Zayandehroud Dam reservoir (DaryaTarsim Consulting Engineers 2011).In general, changes in element concentrations in surface sediments can be divided into two groups. Calcium and thorium elements are in the first group and other elements are in the second group. Only some elements, including silver, arsenic, chromium, copper, nickel, and lead, exceeded the US sediment quality guidelines (ERL and ERM) at some stations, among the 26 elements measured in the 20 surface sediment samples. Which was found towards the dam crest for arsenic and lead, chromium except for a few points, nickel and copper were found throughout the study area, and silver was found towards the lake entrance. Also, the EF of lead and copper was lowest at the lake entrance (EF 40) at one station at the lake entrance. The Geoaccumulation Index also showed non-polluted (Igeo1) only near the dam crest.Based on the significant positive correlation coefficients between most elements of the surface sediments of the Zayandehroud Dam reservoir, it can be stated that the majority of the studied elements are of natural origin. The enrichment of elements such as chromium, nickel, and copper observed at most stations is probably of natural origin and is consistent with the upstream geology of the Zayandehroud Dam Basin. The increased concentration and enrichment of elements such as silver and lead observed in some stations show the effect of human activities.

Keywords

• zonation
• surface sediments
• elements
• geochemical indices
• zayandehroud dam