The study and comparison of Fried-Combernous and Brigham mathematical models for calculating Nitrate dispersivity in saturated conditions

In recent decades, the study and move pollutants in saturation zone due to the many chemical and physical effects that is subsurface water quality has become a significant issue. The increasing use of nitrogen fertilizers due to high dynamics of nitrate in the soil would be a serious threat to groundwater, and therefore, to human health. Brigham and Fried-Combernous models are important models of persistent contaminant transport in homogeneous saturated porous media under constant flow, which are proposed for calculating the dispersivity in short and long travel distance, respectively. The purpose of this study is evaluation and comparison of Fried-Combernous and Brigham mathematical models for calculating nitrate dispersivity in homogeneous saturated sandy soils under laboratory conditions. For the study of dispersivity with Fried-Combernous and Brigham models, pure potassium nitrate salt solution under the sustainable regime with concentration of 160 (mg⁄lit) was added in the sandy soil columns ( in three sizes coarse, medium and fine - distance transmission at 80, 40 and 20 cm ) as persistent contaminants, then for extracting the needed parameters the concentration of output nitrate in three different porosities volumes were measured and Breakthrough curves were plotted for each column. Laboratory analyze data showed that dispersivity increases as the hydraulic conductivity and soil particle size increases. The results also demonstrated that the obtained dispersivity for fine sandy soil in both Fried-Combernous and Brigham models has significant difference with obtained dispersivity for coarse and medium sandy soil. So that by increasing the average travel distance in medium and coarse sand the nitrate dispersivity increased. But the dispersivity of fine sand decreased as the travel distance increased. Check calculated values of β parameter for fine sand showed the value of this parameter increases dramatically with increasing travel distance. This represents a higher pollutants velocity transition than the water velocity in the pores at the shorter distances in fine sand, and therefore the travel distance is increased with increasing dispersivity. The findings showed that the results of Fried-Combernous and Brigham models in the short travel distance is different, and it also cannot be generalized to long travel distances. Thereforre, the Fried-Combernous model was not efficient enough for short travel distance.