2.4.3. Water Sampling and Analyses

Periodic water sampling was conducted continuously once a week for up to two months. At each sampling time, the pH, dissolved oxygen (DO), electrical conductivity (EC), oxidation-reduction potential (ORP) were measured one by one initially. DO of the overlying water was directly measured by a DO meter (EXStik DO600, Nashua, NH, USA). pH value was measured by a pH meter (SunTex SR-2300, Hsi-Chih, Taipei, Taiwan). The electrical conductivity (EC) was measured by a conductivity meter (Taina EZDO-6021, Taiwan). The oxidation–reduction potential (ORP) of sediment within the depth of 1–3 cm of caps was measured by an ORP meter (SunTex SR-2300, Hsi-Chih, Taipei, Taiwan). After the basic measurement, 100 mL of water was collected from each column by disposable syringes and filtered with 0.45 μm filter membranes. The supernatant was kept in 20 mL glass bottles in a refrigerator at 4 ◦C before analyses.

The samples prepared for Hg analysis were preserved with 0.5% BrCl. For Ni, Cr, Cu, and Zn analyses, samples were preserved with 0.5% HNO3 and determined by inductively coupled plasma optical emission spectrometry (Agilent ICP-OES 700 series, Santa Clara, CA, USA). For MeHg analysis, samples were put in brown bottles in the shadows to avoid light and analyzed immediately by ethylation, purge and trap, and gas chromatography/CVAFS (Brooks Rand Automated Total Mercury System, Seattle, WA, USA). Total organic carbon (TOC) was analyzed by a TOC analyzer (OI Analytical Aurora 1030W, College Station, TX, USA). QA/QC were regularly checked by analyzing the quality control samples from each batch.
