Identification and application of new tracers in the discussion of solute transport in the soil on a laboratory scale

Master's thesis in the field of soil science

Abstract:

Today, tracking in water and soil issues has found a much wider application compared to the past. Examining the hydraulic connection and hydrodynamic characteristics of underground water tables, evaluating the origin and spread of pollution are among the most important applications of this method. In our country, along with the development process and with the future growth of the dam building industry and studies of water and soil resources in various basins, the method of tracking in surface and subsurface waters has also increased. Because it is not possible for us to accurately examine and recognize different parts and subsurface layers of the soil. Therefore, the use of early detection methods helps to identify the main path of water flow. The use of new detectors with economic goals and cost-effectiveness on the one hand and being safe for the environment on the other hand have always been the main concern of engineers and researchers. In the introduction of new detectors on a laboratory scale, the ability to detect the detector in low concentrations is an important and fundamental goal, and another is to have the least sensitivity to external parameters such as sunlight, temperature, humidity and other climatic factors. Therefore, the current research seeks to present and introduce new detectors that do not have the disadvantages of old detectors and can be used from a technical, economic and practical point of view. Three types of fine-grained, mixed and coarse-grained soil were used as the porous medium inside the column. Two color detectors Poncio 4-R and Violet Covasol were used as new color detectors in the discussion of solute transfer in this research. The laboratory results obtained from this research, which were in the form of the percentage of mass recovered and the penetration curves, showed that the Ponsio 4-R detector has a better detection capability and had almost no absorption in the three mentioned types of soil, and the Violet Kovasol color detector showed some absorption and was ranked second.

D

Laboratory model and tracking test Seep/W and Ctran/W models were modeled objectively and the modeling results showed that the aforementioned models are able to simulate the flow and solute movement well.  

Key words: transport of solutes, fine-grained soil, tracer, diffusion coefficient, breakthrough curve

Chapter One

Introduction and overview

1-1- Introduction

Today, tracking in water and soil issues has found a much wider application compared to the past. Examining the hydraulic connection and hydrodynamic characteristics of underground water tables, evaluating the origin and spread of pollution are among the most important applications of this method. In our country, along with the development process and with the future growth of the dam building industry and studies of water and soil resources in various basins, the tracking method in surface and subsurface waters has also increased. In the past, the main use of trackers in underground water was to find out things such as the direction, path, speed and time of water passage. Nowadays, due to the increasing trend of pollution of surface and underground water resources, in addition to the above, things like the spread and transfer of pollutants are also considered. In order to know and protect the quality of water resources, it is necessary to have accurate information about the behavior of polluting substances within this system, which can be simulated to a large extent with the help of tracking methods. which is used in the final stages of systematic studies of water and soil resources and geotechnics. In these studies, due to the expansion of the studied area, environmental isotopes and artificial tracers (chemical, color) may be used.It should be noted that if the tracking operation is not carried out according to the instructions and with the necessary precautions, it is not only not useful, but it will also lead to misleading results (Binam, 2018).

Groundwater tracking studies are always done after conducting classic hydrogeology studies and conducting routine geophysical and geotechnical investigations, as well as using tracers in water and soil sources based on their characteristics. The environment and characteristics of the tracker are done. Sometimes the hydrological characteristics of the used place create limitations for the use of some detectors. In addition to the above features, other factors such as physical, chemical and biological effects may also affect the results of the tracking.

In arid and semi-arid areas, water resources with good quality are scarce and these resources are mostly dedicated to urban water supply. For this reason, in these areas, it is possible to use low-quality underground water, drainage effluent and other effluents (Beltran [1], 1999). The use of wastewater in farms may lead to the improvement and sustainability of agricultural production. At the same time, irrigation with wastewater can have risks for production and soil environment. Water shortage in arid and semi-arid areas may increase the risk of soil salinity in these areas. Because there is not enough water to wash the salt. The low quality of water and sewage available in these areas increases the above risk again. Therefore, in order to prevent secondary salinity of the soil due to irrigation with water with high salt concentration, the added salts should be washed out of the upper region of the root, which is active in absorbing water and nutrients. Salts dissolved in deeply infiltrated water can accumulate in the deeper parts of the soil and be discharged through natural drainage or subsurface drainage systems (Beltran, 1999).

Due to the limitation of water resources in arid and semi-arid regions, despite the problem of salinity, it is necessary to use lower quality water sources, but it is necessary to use low quality water sources to avoid environmental pollution and the movement of solutes and chemicals in the soil and the problem of salinity. The soil should be taken into account and the above is not possible with proper irrigation management. Irrigation management can play an important role in the movement of salts and pollutants to the depths of the soil and towards the groundwater, as well as an important role in soil salinity, especially in the area of ??root development of plants, the best irrigation management should consider the amount of salinity in the irrigation water, in the soil and in the root zone of plants and the movement of pollutants to the groundwater (Zhakri[2], 2005). Soil salts are inevitable from an economic, ecological and environmental health point of view. Therefore, the movement of water and solutes in the soil has been of interest since years ago. This issue has attracted the attention of many academic researches in the last few decades, but it has been seriously addressed since the 1950s. In the last two decades, the research in this regard has been raised as one of the significant topics, so that it has recently received a large amount of studies in the field of soil science.

In most of the studies conducted on the movement of solutes and its simulation using models, heavy elements such as cadmium and lead and the movement of pesticides and herbicides in the soil have been discussed. These compounds in the soil are exposed to the phenomena of production and destruction, sedimentation and surface absorption of soil colloids. In other studies, the movement of anions in the soil, which are less exposed to the phenomena of production and destruction in the soil and move faster with water in the soil, especially chlorine and bromine, has been investigated.  Bromide is used more because of its low background value in the soil of arid and semi-arid regions compared to chlorine, its inactivity in the soil and the simplicity of determining its concentration in the soil.

Tracers are types of matter or energy that are used to determine the temporal and spatial distribution of water and its pollutants in water and soil sources.