River surface size fractioned sediments pollution with heavy metals Case study: Sefidroud river

Introduction
In recent years, restrictions of fresh water resources in the world and protection of the quality of these resources cause attention of many scientists about pollution of aquatic fresh waters like rivers. One of the most river pollutants is heavy metals. Unlike the organic compounds, these elements do not disintegrate by natural processes and have toxic potential risks for living organisms and environment. Metals are the natural components of water ecosystems and most of them are necessary for organisms. Only when the contents exceed from the limitation, they can water pollutants. It is general phenomenon that metal contaminants in aquatics accumulated mostly in fine sediments. Accordingly, few studies have been done about heavy metals pollution in size fractioned sediments. This study aims to examine the changes of heavy metals concentration
in Sefidroud size fractionated sediments as one of the most important and largest river in the country using some common pollution indices. Finally, It also assessed the sources of heavy metals by using multivariate statistical analysis.
Materials and Methods
The study area was Sefidroud river in south west of Caspian Sea ,that have been located within Gilan Province .Its catchment area is about 14041km2 and is located between 50° 36' 00"and 48° 34' 00"E longitude and 38° 27' 00"and 36° 34' 00"N latitude. According to Iran Meteorological Organization reports, Gilan Province is the most rainy province with an average of 580 mm rain per year in Iran.
The Talesh Mountains and the western Alborz belt as gigantic barrier is located between this territory and Iran inland. This is the only natural connection of the Gilan territory with Iran inside plateau, and is through the Sefidrood valley. Gilan Province is composed by two following regions: The lowlands, adjacent to Caspian Sea and the mountainous region.
Moreover, important and interesting particularities of Gilan Rivers include a massive hydrographic network with a large number of rivers as well as high range of water flow in the rivers. The inundating rivers, is created by transporting circular stone pieces and blocks through under-washing the ridges overlooking the farms and orchards, threaten the cultivated areas and gardens.
According to the Iran Ministry of Industries and Mines data 60 active and 19 abounded mines are located in study area that may release heavy metals into the environment. Coal mining activities are of the examples of mine type in the catchment area.
Sampling sites were located on different geological formations in the catchment areas of the Sefidroud river. Five surface sediment samples were taken from this river from upstream to estuaries during June to July 2013.Surface sediment samples were collected by mini Ekman type grab sampler. All samples were transferred to the laboratory in sealed plastic bags under 1 to 4 °C. Grain-size analysis was carried out using wet standard sieving methods for particles larger than 38µm using sieve shaker (Analysette 3 Pro, Fritsch) and laser grain size analyzer for particles less than 38 µm (Analysette 22, Fritsch) at the Research center for applied geology, Geological Survey of Iran, Sediment logy Laboratory.The 5 collected river samples were separated into six particle size ranges; <38µm, 38-63µm, 63-125µm, 125-250µm, 250-500µm and 500µm-1mm by wet sieve shaker for chemical analysis.
Homogenous and powdered samples, bulk and fractional, (0.5 g) were treated with HNO3/HCl/ HF according to ASTM (ASTM-D4698-92-2013). Metal concentrations (Cu, Zn, Cr, Fe, Mn, Pb, Ni and Cd) in solution were determined by an Inductively Coupled Plasma/Optical Emission Spectroscopy (ICP-OES-730,Varian). For assessing heavy metals pollutions in Sefidroud size fractionated sediments, some common indices(enrichment factor, modified degree of contamination, risk index, degree of sediment toxicity)and were used.In the present research, PCA and HCA were run to interpret obtained data , identify the contaminants probable origins to metal pollution by some common and reliable sediment quality indices and interrelationship size fractioned river sediment with metal risk assessment in aquatic fresh water.
- Assessment methods of sediment pollution
-Potential ecological risk(RI)
Potential ecological risk index(RI)developed by Hakanson to assess ecological risk in lake sediments. It has been used for assessing the degree of heavy metal pollution in aquatic sediments by considering the toxicity of heavy metals and the relation between aquatic systems and heavy metals. So, for assessing aquatic systems, the risk index(IR) has been introduced as a useful instrument in some researches.
- Modified degree of contamination (mCd)
Modified degree of contamination was first proposed by Abrahim and Parker (2008) to modify degree of contamination suggested by Hakanson .Using mC, overall contamination of a sediment sample by multi metal can be assessed instead of assessment of contamination caused by one metal in one sample.
- Introducing the index of sediment toxicity degree STd
In the present study a new index of metal pollution in sediment is proposed based on the results of multivariate statistical analysis of the data from the study area. Based on STd equation, increase in contaminant concentrations causes an increase in STd value, which reveals more degree of contamination.
Discussion of Results
- Size fractioned river sediment
Generally, size fractioned river sediments is used for a preliminary physical characterization of sediment samples. This phenomenon is emphasized by more researchers that because of higher specific surface area in fine particles, larger pollutants such as heavy metals can present them. It was found that more than 29.1% of the river sediment particles at all of the sampling sites are between 250-500 µm.
- Size fractioned sediments and metal pollution
Total metal concentrations (Cu, Zn, Cr, Fe, Mn, Pb, Ni and Cd) and statistical parameters of 5 samples across the six particle size ranges, <38µm, 38-63µm, 63-125µm, 125-250µm, 250-500µm and 500-1000 µm have been determined. As well as to identify more precisely the status of stations, the obtained values compared with average amount of sediments and Earth’s crust. In particles less than 38 µm, concentration of all heavy metals are much higher than average amount of sediments and Earth’s crust. In particles 38-63 µm, except Cu and Ni, concentration of all heavy metals are higher than average amount of sediments and Earth’s crust.
- Potential ecological risk(RI)
In all stations, except for particles 63- 125 µm, the ecological risk index was medium to significant. Particle size of 500 µm to 1 mm has been low ecological risk index.The particle size 63-125 µm had high ecological risk index.
- Modified contamination degree(mCd)
It can distinguished that all size fractioned sediments except 63-125µm, have been low and medium contamination. High modified contamination degree has been find in 63-125 µm particles. Also, in these particles, the stations near the estuarine have more pollution index.
- Sediment toxicity degree(STd)
It has acknowledged that by decreasing the sediment size, sediment toxicity degrees become higher.
Conclusion
The aim of this research is recognizing the contamination of Sefidroud river sediments and the effects of particle size.
To reach the purpose of these research, heavy metals concentration in six sediment sizes were measured.
Generally, the results of analysis showed that by reducing the particle size, the heavy metals concentration were more than average amount of sediments and Earth’s crust.
Degree of sediment toxicity index showed that with decreasing of sizes, the indexes increasing. Finally, multivariate analysis showed that concentration of some metals like Cr and Ni were high because of anthropogenic sources. Other metals like Fe and Mn have natural sources.