Heavy Metals in Agricultural Watersheds: Nonpoint Source Contamination

Abstract

Introduction ......................................................................................................... 183 NRCS Technique ................................................................................................. 184Estimation of Runoff Water .......................................................................... 184 Determining Dissolved Elements in Soils and Water .......................... 185 Estimating Elements Loss by Runoff ...................................................... 186 GIS Digital Mapping ................................................................................. 186Application of NRCS Technique ....................................................................... 187 Wagon Train Watershed ................................................................................ 187 Soil and Water Sampling ............................................................................... 188 Soil and Water Analysis ................................................................................. 190 Observed Inˆow for WT Reservoir ............................................................. 190Case Study ............................................................................................................ 192 Runoff and Reservoir Inˆow ........................................................................ 193 Elements in Soil and Water Phase ................................................................ 194 Elements in Stream Water ............................................................................. 195 Elements Loss by Runoff from Soils ............................................................ 201 Elements in Runoff and Loading ................................................................. 212References ............................................................................................................. 215The NRCS (Natural Resources Conservation Service) developed a technique (Elrashidi et al., 2003, 2004, 2005a,b, 2007a,b, 2008, 2009) to estimate element loss by runoff for agricultural watersheds. The NRCS technique applies the USDA runoff curve number model (USDA/SCS, 1991) to estimate loss of runoff water from soils by rainfall. The technique assumes that dissolved inorganic chemicals are lost from a speci"c depth of surface soil that interacts with runoff and leaching water. These chemicals may include any essential plant nutrients (i.e., N, P, Cu, Zn, etc.) and environmentally toxic elements such as Pb, Cd, Ni, and As. Geographical Information Systems (GIS; ESRI, 2006) are used to present data spatially in watershed maps.