Transport of suspended participate matter is widely recognized to occur in subsurface environments. Field data indicate that viruses, bacteria, and clay minerals can migrate considerable distances and that small particles and macromolecules are implicated in the transport of organic contaminants and radio-nuclides. Furthermore, media permeability can be significantly altered by changes in aqueous chemistry through particle release and capture. Quantitative models for predicting particle transport are available within the water filtration literature that account for the mechanisms of particle-media collisions and the conditions for attachment. Predictions from the filtration models are used to analyze particle migration through porous media at typical groundwater flow velocities. As particles accumulate within media pores, available models become less predictive because of the coupling between particle retention and permeability reduction. An examination of filtration data reveals that retention of a relatively small solid volume within media pores can reduce media permeability by orders of magnitude. The fact that contaminants adsorbed to particles are mobile has important implications in understanding and predicting contaminant transport. The design of laboratory experiments and the collection of field samples often neglect contaminants transported by suspended colloids and particles.

Particle transport through porous media

Government agencies at both the state and federal levels now face increasing pressures to assess the likelihood of pesticide occurrence in well‒water supplies. Screening methodologies are required in order to determine which pesticides now in use should receive the greatest attention with respect to groundwater, and in order to determine whether elaborate and expensive groundwater testing should be required in order to register a new pesticide. Several screening techniques have been proposed recently, some based on threshold values for critical physical properties of the pesticide, and others based on mathematical models of the leaching process. A different approach is taken in this paper, whereby an index is derived based entirely on the physical properties of those pesticides that have been found either leachable or essentially immobile. The index is based on graphical examination of a plot formed by two widely available pesticide properties: half‒life in soil (tsoil1/2) and partition coefficient between soil organic carbon and water (Koc). Other physical properties, such as water solubility, octanol/water partition coefficient, and volatility from soil, have often been invoked as indicators of leachability, but they are found in this paper to have no useful power in discriminating between “leachers” and “nonleachers.” Scores assigned with the new screening index agree with the results of several recent well‒water monitoring programs, even though point‒source events are thought to be responsible for some of the observed contamination. A nomogram is given that reduces the task of calculating the index to simply placing a straight edge on a diagram.

Groundwater ubiquity score: a simple method for assessing pesticide leachability

Publisher Summary The chapter provides a state-of-the-art review of advances in fertigation. Topics that are covered include an overview of past trends in fertigation, principles of fertigation, managing crop fertigation, modeling fertigation, monitoring and safety, and future trends and areas needing more research. Monitoring should be advanced on two fronts: (i) development of rapid and reliable methods to determine crop dry-matter weight and nutrient contents, for comparison with the corresponding objective curves; and (ii) improving the methodology of determining nutrient concentrations in the soil solution, by allowing farmers to do it alone in the field, so that correction measures based on soil tests will be timely and effective. A further research objective is to transform reliable soil-crop-atmosphere models into real-time fertigation management models, by introducing input-optimization algorithms based on economic and environmental considerations. Achievement of this objective will depend on prior significant developments in fruit-quality simulation in crop models, without which, crop yield value cannot be evaluated.

Advances in Fertigation

Macroscopic- to more recent molecular scale investigations have enhanced our knowledge of soil phosphorus (P) chemistry, including the retention/release mechanisms in soils and soil components. Phosphate uptake on metal (oxy)hydroxide and phyllosilicate mineral surfaces and in soils generally increases with decreasing pH. Rapid adsorption kinetics is generally observed on many soil adsorbents at acidic pH. P fixation mechanisms such as inner-sphere complexation and intra- and interparticle diffusion often result in slow P release (i.e., hysteresis and irreversible reactions), creating challenges in remediating agricultural soils with high accumulations of P. This chapter covers some of the historical soil P chemical research findings via macroscopic approaches but focuses on more recent molecular scale approaches for elucidating P retention/release mechanisms.

Phosphate Reaction Dynamics in Soils and Soil Components: A Multiscale Approach

The Pesticide Root Zone Model (PRZM): A procedure for evaluating pesticide leaching threats to groundwater

Three simplistic models are developed for evaluating the transport of organic pollutants through soil to ground water. The models consider mobility and first-order degradation. The first calculates linear sorption/desorption of the pollutant and first-order degradation without considering dispersion. The second is similar to the first but also considers dispersion. The third considers nonlinear sorption following a Freundlich equation and first-order degradation but does not consider dispersion. The models are compared to field data for the pesticides aldicarb and DDT. The models projected a lower mobility for DDT than was observed in the field.

Approximating Pollutant Transport to Ground Watera

Phosphorous in concentrations exceeding those generally observed through agricultural activities has been encountered at land application wastewater treatment sites. To predict the environmental impact of these systems, a model was developed to describe the reaction of orthophosphate with mineral soils. The model is based on instantaneous reversible absorption theory plus time-dependent solubility theory. It was assumed, in the model, that first-order kinetics would adequately describe the formation or dissolution of phosphate minerals. Statistical regression analyses were performed on batch sorption data for 14 mineral soils where two equilibrium compounds were considered. The two compounds considered were hydroxyapatite (Ca₁₀HPO₄·2H₂O) and dicalcium phosphate dihydrate (CaHPO₄·2H₂O). Comparisons of correlation coefficients between this model and previous models using a common data set indicate the current model with a mean correlation coefficient of 0.93 is more accurate in describing phosphate reactions in calcareous soils than earlier models with mean correlation coefficients ranging from 0.75 to 0.91.

Kinetic Model for Phosphate Transport and Transformation in Calcareous Soils: I. Kinetics of Transformation

Kinetic Model for Phosphate Transport and Transformation in Calcareous Soils: II. Laboratory and Field Transport1

Computer models for two dimensional subterranean flows and pollutant transport

Both models adequately projected chemical transport velocity and concentration through the vertical soil profile at the relatively homogeneous field site evaluated The two-dimensional solution also projects the transport through the ground water and was able to describe three successive years of application

To Phan

Papers

Approximating Pollutant Transport to Ground Watera

Kinetic Model for Phosphate Transport and Transformation in Calcareous Soils: I. Kinetics of Transformation

Kinetic Model for Phosphate Transport and Transformation in Calcareous Soils: II. Laboratory and Field Transport1

Computer models for two dimensional subterranean flows and pollutant transport

Comparison of a One-Dimensional, Steady-State Hydraulic Model with a Two-Dimensional, Transient Hydraulic Model for Aldicarb Transport Through Soil