TRACE METALS: ENRICHMENT FACTOR AS A TOOL FOR SEDIMENT QUALITY ASSESSMENT

ABSTRACT

Trace metals in sediments (soils) are of great environmental concern, in order to determine its content in the sediment contamination levels must be evaluated. Trace metals include Cu, Zn, Pb, Fe, Ni, Hg, Cr, As, etc. they can be evaluated using enrichment factor. Assessment of trace metal pollution in sediments requires knowledge of preanthropogenic metal concentration to act as a reference against which measured values can be compared.

CHAPTER ONE

1.0   INTRODUCTION

         Globally, trace metals accumulation in the aquatic environment is of great concern because of their toxicity to the ecosystem and man. Metals such as Cu and Zn are generally regarded as essential trace metals because of their valuable role in metabolic activities in organisms. However, metals like Cd, Pb, Hg and Ni exhibit extreme toxicity even at trace levels, (Merian, 1991).

         Sediments represent the most important sink of trace metals in aquatic environments because more than 90% of trace metals load in marine sediments is bound to suspended particulate matter or sediment (Daskalakes and O’Connor, 1995; Calmano et al, 1993).

Trace metals accumulate in the sediments through complex physical and chemical adsorption mechanisms depending on the nature of sediment matrix and the properties of the adsorbed compounds (Ankley et al, 1992; Leivouri, 1998). These metals could enter the aquatic environment through natural source which involves weathering of minerals and soils or from anthropogenic source (Calmano et al, 1993).

  1. SEDIMENT/SEDIMENTATION  

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice, and by the force of gravity acting on the particles. For example, silt falls out of suspension via sedimentation and forms soil (Nichols, 1999).

Sediment are most often transported by water (fluvial process), wind (Aeolian process) and glacers.  Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow–moving or standing water in lakes and oceans. Desert sand and loess are examples of Aeolian transport and deposition. Glacial moraine deposits and till are ice-transported sediments.

Many of the sediments in our rivers, lakes and oceans have been contaminated by pollutants. Many of the contaminants were released years ago while other of the contaminants enter our water everyday. Some contaminants flow directly from industrial and municipal waste dischargers, while others come from polluted run of in Urban and agricultural areas. Still other contaminants are carried through the air, landing in lakes and streams far from the factories and other facilities that produced them.

Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are suspended or entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: These forces can be due to gravity, centrifugal acceleration or electromagnetism. (Selvara et al, 2004). In geology, sedimentation is often used as the polar opposite erosion, i.e, the terminal end of sediment transport. (Siever, 1988).

Sedimentation is typically used in geology to describe the deposition of sediment which results in the formation of sedimentary rock and in various chemical and environmental fields to describe the motions of often smaller particles and molecules. The process is also used in biotech industry to separate out cells from the culture media.