Spatial patterns of heavy metals in the deposited sediments at the Cha-Am municipality wastewater treatment ponds system

Vicko Andreas, Mahidol University, Thailand, K.N. Irvine, National Institute of Education and Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore and Ranjna Jindal and Romanee Thongdara, Mahidol University, Thailand


Cha-Am, a popular beach destination approximately 175 km southwest of Bangkok, Thailand, uses an aerated lagoon system with four ponds in series to treat its municipal wastewater. This study investigated the spatial pattern of heavy metals concentrations in the deposited bottom sediment of the four ponds. Based on a randomized grid sampling scheme, 11-14 surface grab samples were collected from each of the four ponds on two different dates in September and October 2016, for a total of 94 samples. A Bruker S1 Titan 600 X-ray Fluorescence (XRF) analyzer was used to determine metals concentrations in the air-dried sediment samples.  Ordinary Kriging in ArcGIS10.1 indicated concentrations of metals were greater in the middle area of each pond but from pond to pond, the metals had different trends in mean concentration. Qualitatively, the mean concentrations decreased from the first to the third pond and increased in the fourth pond. However, t-tests showed that while mean concentrations of As, Cl, and Zn decreased significantly (α=0.05) from the first pond to the third pond and increased significantly in the fourth pond, Cr did not change significantly (α=0.05) between ponds. Mn decreased insignificantly from the first to the second pond, increased significantly from the second pond to the third pond and then decreased significantly to the fourth pond. From the first to second pond, Pb decreased significantly, but there were no significant changes in mean concentration thereafter. Metals concentrations and spatial patterns of the concentrations were similar between the two sampling dates. Concentrations of Cd, Co, Hg and Se were below the XRF Limit of Detection, however, the mean levels of As, Cr, Cu, Pb, and Mn in each of the four ponds frequently exceeded the Ontario Ministry of Environment Lowest Effect Level guidelines for sediment. The system provides a treatment benefit, but any sediment dredged from the ponds should be disposed of with care.

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