Al-Kahlaa River is one of main tributaries of the Tigris River in Missan city and rises from northwest side of Amara city and continues to flow in the direction to the east of city center. Two commercial fish species (Liza abu andCarassius auratus) were collected seasonally (autumn, winter, spring and summer)

The seasonal variations of n-alkanes compounds in molluscs of Shatt Al-Arab river have been studied. The species of molluscs were the snails, Lymnaea auricularia, Theodoxus jordani, Physa acuta, Melanopsis nodosa, Melanoides tuberculata while the bivalves were Corbicula fluminea and Corbicula fluminalis. The molluscs were collected from different locations of Shatt Al-Arab river (along the region extended from Abu-Al-Khasib to Garmat-Ali) during 2004 and 2005. Each species consisted of at least 350 adults of uniform size of individuals. The n-alkanes compounds from species of molluscs were extracted and analyzed by high resolution capillary gas chromatography. The total concentrations of n–alkanes in the molluscs varied from 1.50 μg/g dry weight in the T. jordani to 8.78 μg/g dry weight in the C. fluminea during summer and from 2.26 μg/g dry weight to 12.37 μg/g dry weight during autumn. While ranged from 3.15 μg/g dry weight to 12.44 μg/g dry weight and from 1.78 μg/g dry weight to 5.31 μg/g dry weight during winter and spring. The study confirmed lower concentrations of n-alkanes compounds in the molluscs of Shatt Al-Arab river during summer and spring while higher concentrations were recorded during winter and autumn. This is due to several factors which could act to produce such seasonal variations.

This study includes laboratory uptake and depuration experiments. The bivalve C. fluminea is exposed to 5ml of WSF of Nahran–Umar crude oil for each liter of river water for 48 hours in the static toxicity test system. The exposure water and animals are periodically taken for analysis of total hydrocarbons concentrations by spectroflurometer. After the exposure period is over, the bivalve transferred to clean river water for depuration period of 48 hours. The experiments show that there are no mortalities due to WSF of Nahran–Umar crude oil. The bivalve is usually active during the 12–16 hours of the experiments. Hydrocarbons concentrations in the exposure water decreased are slowly in a linear fashion so that no measurable concentrations remained after 32 hours. Hydrocarbons accumulated by the bivalve are slowly during the first few hours of exposure. After 2 hours of exposure, the concentration of hydrocarbons is approximately 0.38 ppm in water, while in tissues it is 4.8 ppm. After 20 hours, hydrocarbons are present in water at a concentration of 0.09ppm, while the bivalve tissues contained 17.7 ppm hydrocarbons. After 25 hours of exposure, the hydrocarbons concentration decreased in the bivalve tissues. At the end of the exposure period, the bivalve contains approximately 7.4 ppm. After 48 hours in clean river water, tissues burden decreased to 0.22 ppm of hydrocarbons. The bivalve remained contaminated with small concentrations of hydrocarbons for 20 days. The complete depuration occurred at 26 days.