An holistic approach to modelling the removal of particles and chemicals from natural waters.

Abstract

Production of food and fiber for use by people is a vital necessity, yet a highly controversial one. The rise of modern agriculture has greatly increased the yield of crops, but the benefit of increased production has come at the cost of environmental degradation. The intensified use of land has led to both increased soil erosion and increased use of pesticides and fertilizers. If mobilized off the agricultural field, soil and chemicals have the potential to contaminate both surface water and groundwater. There are many methods used to mathematically model the movement and fate of eroded soil and mobilized chemicals. These methods cover a wide range of spatial and temporal scales, ranging from simulating one storm event on one field to simulating a watershed over the course of years. It would, therefore, be helpful to have a common method of measuring soil and chemical removal in various water bodies. For this reason, a new expression is introduced, a half-life of removal. This term represents a standard quantification of the residence time of a chemical or particle in water. The half-life measure is versatile and universal. Not only can it be calculated for any particle or chemical in any aquatic environment for any removal mechanism, but it can also be compared against any other chemical or particle half-life for many different situations. The versatility of the half-life measure is illustrated by using a variety of equations and models to compute the half-life of removal for soil particles and chemicals in three water compartments: surface water, groundwater, and run-off. The particles considered are size 300 #22;m and smaller while the chemicals have Kd values ranging from 10-1 to 107 L/kg. The half-lives obtained for the ranges of particles and chemicals in the three water compartments are compared. The universal nature of the half-life measure is demonstrated by this comparison of particle and chemical half-lives in the three compartments.