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SLT00110 - EFFECT OF SOIL TEMPERATURE ON SOIL MOISTURE VARIABILITY


ABSRACT

Soils can process and hold considerable amount of water. They can take in water, and will keep doing so until they are full, or until the rate at which they can transmit water into and through the pores is exceeded, the effect of temperature on soil moisture variability for the Month of May, 2018.

CHAPTER ONE

INTRODUCTION

1.1 Background of the Study

Humans are exposed to ionising radiation from many sources, including naturally occurring radionuclides (cosmic and terrestrial, e.g. radon gas), certain occupations (e.g., cardiologists and power plant workers), various diagnostic tests and medical therapies (e.g., x-rays or radiotherapy) as well as artificially produced radionuclides released to the environment following nuclear accidents (Rodgers and Holmes,2008).

The risk management of radiation protection of humans is based on a model that assumes a linear relationship with no threshold between radiation dose and health risk. This linear no threshold (LNT) model is based on data from high dose/high dose rate experiments and epidemiological studies, such as those of atomic bomb survivors1. It is known that acute radiation gives rise to DNA lesions and causes genotoxic effects at high doses. The effect of chronic exposure to low dose rates (LDR) is less clear, despite its relevance for humans. (Suzuki and Yamashita, 2012)

While the LNT model has guided risk assessment for decades, scientists have expressed a number of concerns regarding the applicability of the model in the low dose area (reviewed in ref. 2). For long term exposures below a total dose of 100 mGy, increased cancer risks above background rates are difficult to detect in populations. Hence, there are still large uncertainties about the health risks in the low dose area that need to be addressed. One of the controversial assumptions of the LNT model is that it does not take biological defence mechanisms (e.g. DNA repair) into consideration – mechanisms that could conceivably modify the risk of cancer at low doses. Additionally, very little is known about effects of chronic exposures at low dose rates, despite their human relevance.( Blaisdell et al., 2001)

Background radiation consists of three primary types: Primordial, cosmogenic and anthropogenic. Primordial radionuclides are present in the earth’s crust and found throughout the environment. Cosmogenic radionuclides are produced when cosmic radiation interacts with elements present in the atmosphere and are deposited through both wet and dry deposition. Anthropogenic sources of radiation result from human activities, but are considered background because their presence is ubiquitous. 

 Primordial radionuclides include isotopes of uranium, isotopes of thorium and 40K. The natural concentrations of these radionuclides in soils vary regionally and depend upon the type of rock from which the soils were formed. The extent to which these radionuclides are taken up by plants and incorporated in animal tissues depends on the levels present in the environment, the characteristics of the native soils and the chemical behavior of the elements or compounds involved (Shankland, 1960).