DOI: 10.1594/pangaea.921881
title: Grain size, organic characteristics and heavy metals in surface sediment from Densu Estuary, Ghana

description: Urbanization and agriculture along the coastal regions of Ghana contribute to increasing environmental pollution. Ecological assessment of heavy metals in surface sediment of Densu Delta was used to determine the state of ecological environmental quality for its sustainable management. Also, organic pollution is a major environmental problem. The data provides spatial distribution on grain size, organic properties and metals of surface sediment in a tropical estuary, West Africa, Ghana.
description: The sediment particle sizes was determined via hydrometer method (Gee & Bauder 1986). The sizes classes (% of sand, silt and clay) were determined using Shepard's classification scheme (Shepard 1957, Poppe et al. 2003). Sediment organic properties such as organic carbon, total nitrogen, total phosphate, carbon-to-nitrogen ratio were determined by standards methods (Jackson 1958, Sommers & Nelson 1972, Tiessen & Moir 1993). The samples were pretreated with 10% HCl to remove the inorganic carbonates. Sediment samples were then oven-dried at 60°C for 24 hours to get rid of moisture (Bianchi et al. 2008).The sediment fraction, 5 to 2 mm was used for organic compounds determination. Organic carbon was determined using the rapid-dichromate oxidation technique (Walkley Black method) with a correction factor of 1.3 (Walkely 1934, Nelson & Sommers 1996). Organic carbon and total nitrogen were measured using an automatic CHN elemental analyzer (Vario El II) and Kjeldahl total digestion technique (Smart et al. 1981) respectively. The results were expressed in percentage of organic carbon and total nitrogen in dry sediment. Total phosphorus was determined by the ascorbic persulfate method and absorbances read in a UV/VIS spectrophotometer at 880 nm (Langner & Hendrix 1982) (Johnes & Heathwait 1992, Ma et al. 2017).The quantity of sediment organic matter (SOM) was estimated by determining sediment organic carbon and using a factor, which assumes that 58% of the SOM was formed by carbon (Jackson 1964, Bianchi et al. 2008). The percentage of organic carbon multiplied by 1.724 gives the % of sediment organic matter determined by the Walkley-Black titration method (Jackson 1964). Organic matter was determined also by dry combustion technique (Ball 1964, Ben-Dor & Banin 1989, Bianchi et al. 2008). The loss-on-ignition assumes mass loss is due to loss of organic matter (Miyazawa et al. 2000).Heavy metalsThe wet sediment samples were dried at 60°C to a constant weight (Ismail 1993). The dried sediments were grounded into powder using mortar and pestle. The homogenized sediment was sieved through 200 µm and 63 µm mesh-size sieve to obtain the fine-grained size for analysis of metal availability using Kjeldahl total digestion (Hseu 2004). About 0.5 g of fine fraction (63 µm fraction) was weighed and placed in a 100 ml polytetrafluoroethylene beaker. 5 ml of 65 % HNO3 was added and then the mixture was boiled gently for 30 to 45 minutes in a fume chamber to oxidize oxidizable matter (Edgell 1989, Watson 1994, Sastre et al. 2002, Tuzen et al. 2011). After cooling, 2.5 ml of 70 % HClO4 was also added and the mixture was gently boiled until dense white fumes appeared (Hseu 2004). After cooling, 20 ml of distilled water was added and the mixture boiled to release any fumes and filtered through 0.45-μm Whatman membrane filter (Hseu 2004). Acid dissolved solutions were analyzed for the concentration of metals, Zinc (Zn), Lead (Pb), Copper (Cu), Mercury (Hg), Manganese (Mn), Iron (Fe), Cadmium (Cd) and Arsenic (As). Reference and standard materials were employed to measure the metals via Perkin Elmer Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) Analysis at the Ecological Laboratory, Institute of Environment and Sanitation Studies, University of Ghana, Legon-Accra, Ghana.This study was part of the biological monitoring of coastal waters in Ghana funded by Volkswagen Foundation (Grant no. 89371).