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The Caspian Sea is a brackish water system and unique in terms of water salinity compositions, for this reason, it is necessary to evaluate the pollution criteria based on the water quality in the past and present. The aim of this study is to determine the concentration of metal elements and petroleum hydrocarbon (PAHs) at water and sediment and organochlorine pesticide and detergent (LAS) at water in the southern Caspian Sea and the causes of their spatial- temporal changes and also comparison with the standard thresholds. Samples were collected from depths of 10 and 30 meter in the vicinity of fish breeding cages in the coasts of Gilan, Mazandaran and Golestan provinces at eight transects in autumn and winter 2018, spring and summer 2019. Water sampling was performed using Neskin sampler at different layers and surface sediments were collected using Van Veen grab. Preparation of heavy metals samples applied acid digestion, organochlorine pesticides, petroleum compounds and detergents (extraction with organic compounds) was performed by standard methods and was measured using atomic absorption, high performance liquid chromatography and spectrophotometer. The results at depths less than 10 m showed that the mean of iron metal samples was 0.20 mg/l in the Caspian Sea. The lowest mean concentration of heavy metals belonged to the mercury element with 2.01 µg/l concentration. The mean annual heavy metals of chromium, nickel, zinc, iron, copper, mercury, cadmium, lead and arsenic in water were observed 0.003, 0.002, 0.037, 0.190, 0.051, 0.02, 0.047, 0.007 and 0.023 mg/l, respectively. The mean values of the examined heavy metals in surface sediment samples were obtained in this order Cr>Ni>Pb>Cd>Hg. Also, the results of measuring heavy metals in sediments showed that the sources of heavy metals in this research are more natural and geochemical than man-made sources. In this study, the amount of some sediments is higher than the global standards. The annual mean of petroleum hydrocarbons in the four seasons of autumn, winter, spring and summer at water were registered 1.278, 0.255, 2.423 and 1.553 µg/l and at sediment were 9.570, 0.658, 1177 and 0.186 µg/kg.dw, respectively. The maximum concentration of water petroleum hydrocarbons was recorded at Astara transect and at surface sediments were at Nowshahr transect. In general, the annual mean of PAHs at water was 1.366 µg/l and among 16 compounds of PAH, phenanthrene isomer had a maximum annual mean concentration. The mean annual PAHs in sediments was 296.9 µg/kg.dw and the highest concentrations were recorded in winter 2018. The annual mean of organochlorine pesticides was 0.462 μg/l in water and among 16 organochlorine compounds, indosulfane compound had the maximum annual mean concentration. The maximum mean annual concentration of organochlorine pesticides (OCPs) was 12.77 μg/l in water, the highest amount was recorded in summer 2019. According to international standards, the organochlorine pesticides concentration were recorded below the standard threshold at most transects. The mean annual detergent (LAS) in water was 0.371 mg/l, which was the maximum value at Sefidrud transect in spring season. The LAS concentration is lower than world standards threshold.
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