Sandy soils drain rapidly and thus are susceptible to leaching problems. Clayey soil is less porous, so water will pool at the soil surface and run off instead of infiltrating. For instance it can be difficult to distinguish transport processes from transformation processes in the field. Usually these are detections of the older, banned organochlorines, but there are also frequent detections of insecticides used for pest control in residential settings. Ag-products are infrequently detected.
Most models neglect preferential flow.
PRZM is capable of simulating transport and transformation of the parent compound and as many as two daughter species. The user may make use of constitutive relationships between pressure, water content, and hydraulic conductivity to solve the flow equations. Tritium has been used, but also with little success. Compounds started in use too recently or areas of high susceptibility have high recharge rates. Too many parameters influence.