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The metal contamination has become a global problem by affecting the natural functions of water bodies. Sediment, as the largest storage and resources of heavy metal, plays a rather important role in metal transformations. Under the influence of human activities, large amounts of uncontrolled heavy metal inputs from urban and industrial sources entered the mainstream and tributaries and accumulated in the sediments. They can be suspended and returned to the overlying water under many circumstances creating secondary metal ion pollution in bottom water.
In Padaviya Reservoir the significant difference in Fe, Mn, Zn, Cd and As concentration, the surface and bottom waters observed. The average Fe concentration in surface and bottom was 539 84 µg/L, 3,226 503 µg/L respectively. The average Mn concentration in surface and bottom was 295 56 µg/L, 1,650 306 µg/L respectively. The average Zn concentration in surface and bottom was 166 39 µg/L, 884 85 µg/L respectively. The average Cd concentration in surface and bottom was 1.
66 0.19 µg/L, 2.33 0.30 µg/L respectively. The average As concentration in surface and bottom was 1.08 0.05 µg/L, 2.11 0.23 µg/L respectively. It suggests high bottom loading from the sediment it is acting as secondary pollution in the system. So these values are compared with physicochemical parameters as pH, DO, Orp, phosphate, BOD, and COD in each layer for identification of factors affecting metal ion release. We can clearly see bottom water pollution and loading, which will be a significant cause of environmental degradation and water resource contamination.
Water quality degradation in aquatic ecosystems has become a severe environmental issue which is caused by a variety of processes, including agricultural runoff, domestic sewage discharge, and industrial runoff (Meffe and de Bustamante 2014 Ginger et al.
2017 Hudspith et al. 2017 Huang et al. 2018). Among the many types of pollutants that can be found in water bodies, toxic metals pose a particularly severe threat to aquatic ecosystems (Liu et al. 2003 Maanan 2008 Mekonnen and Hoekstra 2015 Firmansyah et al. 2017 Ding et al. 2018). Under the influence of human activities, large amounts of uncontrolled heavy metal inputs from urban and industrial sources entered the main and tributary streams and accumulated in the sediments (Yang et al. 2009). A good example is intense agricultural practices which can lead to spatiotemporal changes to the significant ions trends and hydrochemical properties of water (Li et al. 2009 Ako et al. 2012 Cuculic´ et al. 2018).
Toxic metals include individual metal ions and their compounds that negatively affect human health. These metal ions can have irregular and significant accumulation patterns controlled by the physical, chemical, and biological processes found in the water body (Chae et al. 2014 Devarajan et al. 2015 Shinohara et al. 2016 Ginger et al. 2017 Zhang et al. 2017). Sediments in aquatic ecosystems play an important role as a sink and source for metal ions. An only a small fraction of metals found is dissolved and transported. Of these, most are deposited in bottom sediments (Audry et al. 2004 Bai et al. 2011 Ding et al. 2015 Shinohara et al. 2016 Ginger et al. 2017). Sediments can store the metals in many forms, and also they can be suspended and returned to the overlying water under certain circumstances. This storage and resuspension can consequence in secondary metal ion pollution with further ecological risk to water body (Wang et al. 2004 Devarajan et al. 2015 Ginger et al. 2017 Huang et al. 2018 Rajeshkumar et al. 2018) and affect the environment and ecosystem health due to its characteristics of persistence and toxicity (Payán et al. 2012). Furthermore, metal equilibrium and their transportation at the confluence between the lake water and sediment are greatly influenced by the multiple physicochemical properties, hydrological variations as such as pH, dissolved oxygen (DO), suspended solids, sediment particle size, etc. (Butler 2009, Atkinson et al. 2007).
The toxicity of specific metal ion-dependent on several factors, Besides the metal content, the chemical species of the metals often change simultaneously and exhibit different physical and chemical behaviors in terms of chemical interaction, mobility, bioavailability, and potential toxicity (Wang et al. 2014, Ibragimow et al. 2013,Karlsson et al. 2010 Mekonnen and Hoekstra 2015 Ding et al. 2015 Hudspith et al. 2017). To this end, literature has indicated that dissolved metals found in overlying waters can be significantly more toxic than metal ions absorbed or deposited in sediment (Zabetoglou et al. 2002 Pejman et al. 2015 Rajeshkumar et al. 2018).
So analyses of these conditions have been critical for the assessment of the potential ecological and public health impacts of this particular type of pollution (Arain et al. 2008 Tao et al. 2012 Devarajan et al. 2015 Ma et al. 2016 Rajeshkumar et al. 2018). This research investigated the influence of the overlying water parameters pH, DO ,Orp, phosphate, BOD and COD on heavy metal release from a typical river sediments identification of any anthropogenic and/or natural sources of these metals pollution and Gaining a better understanding of the key factors affecting the release of metals from sediments can provide better predictions regarding changes to metal ion bioavailability as well as a better understanding of the shifting metal ion cycling on this ecosystem. It would be useful to predict the changes in metal availability and the fate of the metals.
Padaviya Reservoir located in the North-Central Province of Sri Lanka (8°49’30.6′ N 80°46’2.05’E) is a shallow man-made irrigation reservoir. It is generally assumed that the reservoir was constructed during King Mahasena’s reign from 274 to 301 A.D. by impounding Mora Oya and Mukunu Oya seasonal streams by an earthen dam. It was completely restored in 1954 to maximum water storage of 0.1 km3 and a maximum depth of 8 m.
The water sampling stations were randomly selected based on the shape of the reservoir and the locations of the two streams that fed water. The sampling sites are numbered from S1 to S7. Water samples were collected in 10 sampling campaigns 5 campaigns in 2017 (February, May, July, October and December) and five campains in 2018 (January, March, May, July and September).
Figure 1: Water sampling locations at the Padaviya Reservoir numbered from S1 to S7.
The water quality parameters pH, oxydation reduction potential (ORP), and dissolved oxygen (DO) were analyzed onsite at 0.25 m depth intervals from top to bottom of the water column using a YSI EXO2 SONDE (Yellow Springs Instruments, Yellow Springs, OH, USA) at each location. SONDE was calibrated in the laboratory before field deployments. Approximately one liter (1 L) samples were collected at 0.25 m depth intervals using a van Dorn sampler at each location. A portion of the unfiltered sample was used for COD and BOD determination using the Cr2O72- reflux method and 5-day incubation method respectively as described in APHA 1998. The other portion of the sample was filtered immediately through 0.45 nylon filter papers and acidified to pH > 2 using high concentrated purity HNO3 acid for the determination of Fe, Mn, Zn, Cd, As and phosphates using a Thermo Scientific 3000 series atomic absorption spectrometer in both flame (Fe and Mn) and graphite furnace (Zn, Cd and As) modes. Before the analysis, the acidified samples were dried at 70°C (Smith et al, 1996) and then digested with 5 mL concentrated HN03. Total phosphate concentrations were measured using the digested sample and molybdate method. All the filtered and unfiltered samples were kept in the cool storage at 4°C duing transport to the lab.
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