MODELING NITRATE TRANSPORT WITH DISPERSION IN SOIL

ABSTRCT

This study “Modeling Nitrate Transport with Dispersion in Soil”. presented finite element and empirical models to predict transport of   nitrates in a soil.  Three different soil samples were  selected  which  includes  sand,  clay  and  silty  soil.  Each  sample  was  gradually introduced into a fabricated iron column, having a length of horizontal column 30cm and vertical part 60.96cm. A solution of silver nitrate was allowed to pass through the vertical column down to the horizontal part. Samples of soil were collected at a constant distance of

10cm and time interval of 5mins for 60mins .The concentration of nitrate was taken at a constant  distance  of  10cm  .This  was  done  three  times  for  each  of  the  soil  sample. Thereafter, the dispersion coefficient was calculated, and a regression dispersion model developed as a function of   permeability, average diameter of sieve, velocity of flow and time taken to flow. The model was verified with experimental results and found to have a high correlation of 0.958. The prediction of the derived model were compared with those in literature. Data obtained from the experiments were fitted into the two existing models (Shao, 2017; and De Smeldt and Wierenger 1978)  and poor correlation coefficients of – 0.529 and -0.524 were obtained, respectively. The regression dispersion model developed was incorporated into Notodarmojo model. Both the Notodarmojo and the modified models were solved using finite element methods. Then, the two equations were used to predict the variation concentration of nitrate with distance. The results showed that the concentration of nitrate decreases as the distance increases. In addition the modified transport equation gave a better prediction of the variation of nitrate concentration with distance than the Notodarmojo’s equation. The r values for the two ranged from 0.741 to 0.896 and 0.484 to 0.769 for the modified model and Notodarmojo model, respectively. Using the Student t- test, the predicted values were significant to each other  ranging from 0.0074 to 0.331.The research   has established that the   modified model which accounted for variability in dispersion coefficient offered a better approach than the conventional one.

CHAPTER ONE INTRODUCTION

1.1      BACKGROUND OF THE STUDY

Water is very essential for survival of both plants and animals. Two-thirds of the earth surface is covered with water from streams, lakes, seas and oceans. Yet the problem of water scarcity is endemic in many regions of the world. This is because the sources mentioned above are not readily available to man for consumption, industrial activities and for agricultural purposes. Therefore, man has resorted to the use of groundwater as a means of water supply.

Groundwater resource has its inherent problems of pollution. Poor microbiological quality of groundwater systems has caused many diseases outbreak in some communities. Where the source of drinking water is shallow groundwater, there may be a risk of contamination from septic tank effluent. Nitrate sources in groundwater were attributed to bedrock dissolution in the course of groundwater migration and also anthropogenic activities such as on-site sanitation, waste dumpsites and agricultural chemicals (Dan-Hassan et al.,2012).

A more realistic approach to decreasing the potential for contamination of groundwater and waterborne disease by septic tank effluent would be to regulate septic tank placement to minimize the negative impacts on the quality of nearby drinking water. Most states in the United States have attempted to do this by imposing minimum separation distances of

10.7m to 91.4m between drink-water wells and septic tanks, with an average distance of about 15.2m (Plews, 1977).Contamination of drinking water from groundwater is derived by faecal matter from on-site sanitation systems, landfill and agricultural field’s leachate. Household wastes passes through the septic tank into the drain field, then downward through the soil until it reaches the water table (McDowell et al., 2005). Waste materials

found in municipal landfills can be described as heterogeneous porous media, where flow and transport processes of gases and liquids are combined with local material degradation (Kindlein et al., 2006).

It is also possible for untreated waste from septic tanks (which is common in Nigeria) and toxic chemicals from underground storage tanks to contaminate ground water (Asiwaju- Bello, 2004; Benka Coker and Bafer, 1999). Displacement studies are important tools for understanding transport of solutes through soil. These provide insight about contaminant transport processes such as diffusion, dispersion, sorption, retardation and transformation (Shuckla et al., 2002; Shuckla et al., 2003; Fityus et al., 1999).In the soil, biological processes, filtration and adsorption remove most pathogens and some nutrients. However, conventional septic systems are not adequate for removing nitrate, certain pathogens and other compounds, especially where soils or groundwater conditions are marginally suitable or where septic system densities are too high (EPA, 2002). Anything that is not removed by the soil under the drain field will end up in groundwater. In the path of contamination, wastewater leaving the drain field of a septic system trickles first to unsaturated soil above the water table, and eventually to the water table below. All continuously operated septic systems are expected to discharge to groundwater eventually (Woessner, 2000). Where the depth of the water table is shallow and overlying soils are permeable, as is typical in valleys near streams, rivers, or lakes within the inland northwest, recharge from septic systems to groundwater is relatively rapid.

Although, it is possible for wastewater to be absorbed by plant roots, in reality this should not happen because properly-designed drain fields are installed below the root zone of grasses and outside the rooting areas of trees. Therefore, most septic effluent reaches the water table. This water carries with it some of the soluble contaminants of effluent that are not absorbed by soil, including nitrogen, various bacteria and viruses which pollute the

groundwater. Microbiological contaminants found in groundwater occur naturally in the environment from soils, plants and in the intestines of humans and even warm-blooded animals. Coliform bacteria are used as indicator organisms for the presence of pathogenic bacteria, viruses and parasites from domestic sewage, animal waste or plant and soil materials. Bacteria, viruses and parasites in groundwater can cause diseases such as polio, cholera,  typhoid  fever,  methemoglobinemia  or  blue-baby  syndrome  and  infectious hepatitis. Hence, a study relating to groundwater contamination by nitrogen on the soil was felt highly desirable and important. The study would also aid in determining the safe location of sub-surface wells from septic system.

1.2 STATEMENT OF THE PROBLEM

Nitrate is a problem as a contaminant in drinking water (primarily from groundwater and wells) due to its harmful biological effects. High concentrations can cause methemoglobinemia  and  have  been  cited  as  a  risk  factor  in  developing  gastric  and intestinal cancer. Due to these health risks, a great deal of emphasis has been placed on finding effective treatment processes to reduce nitrate concentrations to safe levels. Assessing  the  likelihood  and  magnitude  of  this  risk  is  a  formidable  and  complex challenge. Septic systems tend to contaminate groundwater when pollutants from septic system effluent enter receiving waters. Therefore, acute water shortage from lack of urban water supply has forced individuals to use untreated water obtained from surface and underground sources within the environment, on the common assumption that it is “always clean” thereby exposing them to hazardous chemicals and infectious organisms from such water sources. For instance, excessive amounts of nitrate in water, causes ill health effects in infants less than six months old and susceptible adults.  It also causes “blue baby syndrome” or methemoglobinemia in infants which can lead to brain damage and sometimes death.

1.3 OBJECTIVES OF STUDY

The aim of the study is modeling nitrate transport with dispersion in soil

The specific objectives are to:

i.      Collect samples from the borehole source for a period of one year and analyze it in the laboratory to determine the concentration of nitrates.

ii.      Characterize the soil in terms of size particle, bulk density and moisture content will be determined.

iii.      Collect soil samples from septic tank next to borehole source so as  to quantify the extent of   pollutant along the transport system.

iv.       To develop  a  regression dispersion model

v.        Modify and solve one-dimensional solute transport equation, thereafter calibrate it using measured data.

vi.      Verify the model and compare it with the existing model.

1.4 SCOPE OF STUDY

The modeling of groundwater with emphasis on nitrate is an important topic based on its harmful effects on water. Nitrate is a well-known contaminant of both ground and surface water. It is an important environmental and human health analyte, and thus, its detection and quantification are considered to be essential. Mathematical models for groundwater flow and contaminant transport should be established to predict future conditions of the groundwater levels and contamination values.

The results of the model are more meaningful and reliable only if all parameters for flow and transport processes are known. Permeability and average diameter size particles are the most important parameters for transport modeling. In this study, permeability and average diameter size particles of soils were determined, for different sizes of soil by means of an experimental system. The focus in this research is to model movement of

nitrate in the soil through laboratory analysis after which a model will be developed based on the results from the laboratory analysis and investigation.

1.5 JUSTIFICATION OF THE STUDY

The significance of this research is to study the movement of nitrate from septic system to groundwater  and  to  develop  a  transport  model  to  understand  better,  the  source  and transport of nitrates in the aquifer system. Improper maintenance of septic system causes groundwater pollution if too many septic systems are located in an area or if a system is overloaded or not working properly, it can contaminate groundwater with bacteria, viruses and hazardous cleaning materials or household chemicals. Even properly working well- maintained septic systems can contribute nitrates to groundwater. These can show up in well water around the septic system which  could give  rise to many problems which include drastic reduction of aquatic life, public health hazards etc. Therefore, the study would help in determining the safe location of sub-surface wells from a septic system in order to ensure the protection of public and environmental health.

1.6 LIMITATIONS OF THE STUDY

Several studies conducted on the dispersion of nitrate in soil have shown that dispersion of nitrate  in  soil  is  very  difficult  to  determine.  Because  it  was  difficult  to  determine dispersion of nitrate in soil directly, concentration of nitrate was determined with respect to time at a constant distance, thereafter dispersion was calculated using Tinker’s equation. Also, the research work was limited to 3-Dimensional equation.

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