2.5. Hg analysis Water and sediment samples collected from each shrimp pond were used to determine the amount of total mercury (Hgr). All tubes and bottles used for analysis were first cleaned by heating to 65 oC in 4 N HCI for 12 h, cooled and rinsed three times with deionized water, and then dried in a clean oven at 60 oC overnight. Method 1631 2001) and method 1631 (2002) of the U.S. Environmental Protection Agency (USEPA) were used to prepare the sediment and water samples, respectively for detecting Hgr and then analyzed by the Perkin-Elmer Flow Injection Mercury System 400 using NaBH4 as the reducing reagent and 3% (v/v) HCl for the carrier solution.