Mapping agricultural fields with GPR and EMI to identify offsite movement of agrochemicals

Offsite movement of waterborne agrochemicals is increasingly targeted as a non-point source of water quality degradation. Our research has indicated t hat subsurface water movement is variable and site-specific, and that a sma ll soil volume frequently conducts a large volume of flow, This concentrate d flow is usually caused by soil morphology, and it often results in water moving rapidly offsite from certain areas of fields; little or no lateral s ubsurface flow may occur in other areas. Identifying these subsurface regio ns is difficult using conventional soil survey and vadose zone sampling tec hniques. In this study, traditional surveying is combined with electromagne tic induction (EMI) and ground-penetrating radar (GPR) mapping to identify areas with high potential for subsurface offsite movement of agrochemicals, optimizing these identification techniques, and expanding the mapping proc edures to make them useful at the field-scale for agricultural production p ractices. Conclusions from this research are: (1) EMI mapping provides rapi d identification of areas of soil with a high electrical conductivity and p resumably high potential for offsite movement of subsurface water, (2) GPR mapping of areas identified by EMI mapping provides a means to identify fea tures that are known to conduct concentrated lateral flow of water, and (3) combining the capabilities of EMI and GPR instrumentation makes possible t he surveys of large areas that would otherwise be impossible or unfeasible to characterize. (C) 2001 Elsevier Science B.V. All rights reserved.