Chemical Transport From Soil Into Surface Runoff And Mixing Zone Depth

Mixing zone depth (MZD) has been an interdisciplinary field focused in hydrology, soil water dynamic, soil erosion, soil science, soil & water conservation, and related environmental science, including the mechanisms of chemical transport and transfer from soil to surface runoff, soil properties, hydraulic gradient and soil erosion. The depth of mixing zone has also been used in the chemical transport model and non-point source pollution models widely, e.g. SWAT, AnnAGNPs, and so on. The mechanisms of chemical transport from soil to surface runoff are great important for improving soil and groundwater qualities in watershed, and increasing agricultural productivity both in theory and practice.Selecting bromide as chemical tracer, the paper has discussed the mechanisms of chemical transport processes by simulated surface runoff and rainfall. It was been discovered that Bernoulli Effect should be neglected in chemical transport from soil to surface runoff. We developed a laboratory flow cell and experimental procedures to quantify chemical transport from soil to runoff by each of individual processes:i.e.,1) erosion; 2) convection under a vertical hydraulic gradient; 3) convection from surface flow or the Bernoulli Effect; and diffusion. According our data, the mixing zone depth is gotten by theories under different conditions. The results will be used to improve the water quality model.The main conclusions are as follows:1. The results of simulated runoff experiments in laboratory showed that with the increasing of velocity of surface runoff, the soil nutrient flux from soil into surface runoff is enhanced by Bernoulli Effect and erosion under different soil hydrologic condition. This means soil nutrient loss in the soil will rise. With the augmentation of absolute Value of static head, soil nutrient losses, soil erosion and sediment loss add under artesian seepage condition, on the contrary, they decrease under free drainage condition. These results show that artesian seepage condition could make greater contributions to water quality problems, soil nutrients losses problems and soil erosion problem than saturation and free drainage conditions. In early period of surface runoff, under saturation condition, the losses amount of soil nutrient is less than that under free drainage condition (FDC). But if the duration of surface runoff is rather long, it is just on the contrary. At the same time, under artesian seepage condition, the losses amount of soil nutrient is more several times or dozens of times than that under free drainage condition. Under free drainage and saturation condition (SC), soil erosion, which causes bromide mass flux into surface runoff, makes a significant contribution to chemical loss in the soil. Under artesian seepage condition (ASC), convection possesses considerable percentage of bromide mass flux into surface runoff. So soil erosion intensity under artesian seepage condition is more powerful than those under saturation and free drainage condition. And soil erosion and the associated Bromide flux occurred in the first instance of flow introduction. Erosion increased as the hydraulic gradient shifted from free drainage to artesian seepage and as the flow rate was increased.2. The results of simulated rainfall-runoff experiments in laboratory indicated that different chemical loss flux could be caused by different hydraulic gradient under the same rainfall intensity by the following sequence as ASC> SC> FDC. Under the same hydraulic gradient, different chemical loss flux was gotten by different rainfall intensity, it was exponential relationship between chemical loss flux and rainfall intensity. Under FDC, raindrop erosion zone concept is a gross simplification and can not be used adequately to scribe chemical loading processes from soil to runoff water. It was only a model concept; it couldn't be represented by a not only compact surface soil which can prevent surface water into soil layer but also accelerated diffusion coefficient. Under SC, convection had a great contribution to chemical loss from soil to runoff. Therefore, more chemical loss mass from soil to runoff was led to for enlarging diffusion coefficient by rainfall intensity. Also, chemical loss by rainfall under SC was more obvious than under FDC. In addition, rainfall not only caused accelerated diffusion coefficient but sever soil loss by under ASC. In rainfall-runoff simulated experiment, accelerated diffusion coefficient was equal to 1.11 to 6.98 times the molecular diffusion coefficient, which enlarged with increasing of rainfall intension and hydraulic gradient. Although, soil erosion was weak for a little the runoff flow rate, rainfall aggravated soil loss when rainfall intensity gets to 90 mm/h. Consequently, under ASC chemical loss from soil to runoff was more serious than under SC and FDC. Our data proved that there is a linear relation between rainfall intensity and raindrop energy under FDC, i.e., it is a constant that the same bromide loss mass can be caused by per milliliter rainfall. And we found a simple two-term diffusive and convective model for chemical loading.3. Our data indicated that chemical transport process from soil to surface runoff water was not a simple process. Mixing zone was the soil depth that chemicals entry into surface water via interaction with diffusion, convection, erosion and Bernoulli Effect. It has close relationship with soil texture, static head, and flow rate and runoff time. Results of our experiment showed the variability of mixing zone depth under different hydraulic head and runoff flow rate. It is definitely not a constant value of 10 mm as assumed in most of the models. With soil water increasing from FDC to SC to ASC. static head rises gradually from blew to above soil surface. Results of our experiment showed there was exponentially relationship between mixing zone depth and runoff flow rate, there exists increasing relationship linearly between MZD and hydraulic gradient. At the same runoff time, more bromide flux means deeper MZD. At the same flow rate, under FDC, diffusive flux controlled chemical transport. Different MZD displayed different diffusion under different static head. The maximum different of MDZ is less than 0.8 mm with different static head. Under ASC, convective flux controlled chemical transport. Different MZD displayed different convection under different static head. It has a great contribution to chemical loss from soil into surface runoff. From SC to ASC, at the same of flow rate, MZD enhances 7.7 mm with static head increases per 10 mm. In runoff and rainfall-runoff simulated experiment, MZD is less than 2.5 mm under FDC, is less than 6 mm under SC. But the whole soil layer is mixing zone under ASC.