Agricultural chemical transport to surface water and the linkage to other hydrological compartments, principally ground water, was investigated at five watersheds in semiarid to humid climatic settings. Chemical transport was affected by storm water runoff, soil drainage, irrigation, and how streams were linked to shallow ground water systems. Irrigation practices and timing of chemical use greatly affected nutrient and pesticide transport in the semiarid basins. Irrigation with imported water tended to increase ground water and chemical transport, whereas the use of locally pumped irrigation water may eliminate connections between streams and ground water, resulting in lower annual loads. Drainage pathways in humid environments are important because the loads may be transported in tile drains, or through varying combinations of ground water discharge, and overland flow. In most cases, overland flow contributed the greatest loads, but a significant portion of the annual load of nitrate and some pesticide degradates can be transported under base‐flow conditions. The highest basin yields for nitrate were measured in a semiarid irrigated system that used imported water and in a stream dominated by tile drainage in a humid environment. Pesticide loads, as a percent of actual use (LAPU), showed the effects of climate and geohydrologic conditions. The LAPU values in the semiarid study basin in Washington were generally low because most of the load was transported in ground water discharge to the stream. When herbicides are applied during the rainy season in a semiarid setting, such as simazine in the California basin, LAPU values are similar to those in the Midwest basins.

An unsaturated-zone transport model was used to examine the transport and fate of metolachlor applied to an agricultural site in Maryland, USA. The study site was instrumented to collect data on soil-water content, soil-water potential, ground water levels, major ions, pesticides, and nutrients from the unsaturated zone during 2002-2004. The data set was enhanced with site-specific information describing weather, soils, and agricultural practices. The Root Zone Water Quality Model was used to simulate physical, chemical, and biological processes occurring in the unsaturated zone. Model calibration to bromide tracer concentrations indicated flow occurred through the soil matrix. Simulated recharge rates were within the measured range of values. The pesticide transport model was calibrated to the intensive data collection period (2002-2004), and the calibrated model was then used to simulate the period 1984 through 2004 to examine the impact of sustained agricultural management practices on the concentrations of metolachlor and its degradates at the study site. Simulation results indicated that metolachlor degrades rapidly in the root zone but that the degradates are transported to depth in measurable quantities. Simulations indicated that degradate transport is strongly related to the duration of sustained use of metolachlor and the extent of biodegradation.

…………………………………………………………………………………… 1 Introduction ………………………………………………………………………………… 1 Purpose and Scope ……………………………………………………………………… 2 Approach …………………………………………………………………………………… 4 Results …………………………………………………………………………………… 6 Merced, CA ……………………………………………………………………………… 6 White River Basin, IN …………………………………………………………………… 7 Model Summaries …………………………………………………………………………… 9 Complex models ………………………………………………………………………… 9 LEACHP …………………………………………………………………………… 9 HYDRUS2D ………………………………………………………………………… 9 RZWQM …………………………………………………………………………… 10 VS2DT ……………………………………………………………………………… 11 Simple models …………………………………………………………………………… 11 CALF ……………………………………………………………………………… 11 GLEAMS …………………………………………………………………………… 12 PRZM …………………………………………………………………………… 12 Summary …………………………………………………………………………………… 12 References …………………………………………………………………………………… 13 Appendix-Published Model Comparison Studies …………………………………………… 15 Tables 1. Model summary ……………………………………………………………………… 3 2. Simulation layers and sediment properties ………………………………………… 4 3. Model parameters associated with chemical transport of bromide, atrazine, and atrazine degradates …………………………………………………………………… 5 AbstractSeven unsaturated-zone solute-transport models were tested with two data sets to select models for use by the Agricultural Chemical Team of the U.S. Geological Survey's National Water-Quality Assessment Program. The data sets were from a bromide tracer test near Merced, California, and an atrazine study in the White River Basin, Indiana. In this study the models are designated either as complex or simple based on the water flux algorithm. The complex models, HYDRUS2D, LEACHP, RZWQM, and VS2DT, use Richards' equation to simulate water flux and are well suited to process understanding. The simple models, CALF, GLEAMS, and PRZM, use a tipping-bucket algorithm and are more amenable to extrapolation because they require fewer input parameters. The purpose of this report is not to endorse a particular model, but to describe useful features, potential capabilities, and possible limitations that emerged from working with the model input data sets. More rigorous assessment of model applicability involves proper calibration, which was beyond the scope of this study.Uncalibrated ("cold") simulations were run using all seven models to predict the transport of bromide (Merced) and the transport and fate of atrazine and three of its transformation products (White River Basin). Among the complex models, HYDRUS2D successfully predicted both the surface retention and accumulation of bromide at depth at the Merced site, whereas RZWQM and VS2DT predicted only the latter. RZWQM predictions of atrazine were closest to observed values at the White River Basin site, where preferential flow has been observed. LEACHP predicted smaller solute concentrations than observed at both the Merced and White River Basin sites. Among the simple models, CALF predicted the highest values of atrazine and deethylatrazine at the measurement depth of 1.5 meters. CALF includes the Addiscott flow option for preferential flow, and also acc...

The study area for the Lake Michigan Basin groundwater-flow model encompasses the entire Michigan Structural Basin centered in the Lower Peninsula of Michigan and extending into parts of Illinois,

shared data and technical observations about the hydrology of the watershed near Ogden Dunes. David Bucaro and Joseph Schulenberg of the U.S. Army Corps of Engineers provided useful resources and feedback for further development of interim model products. Erica Barrow assisted with data collection. The following current and former USGS personnel also helped with this project: Lee Watson (retired; well installation and data collection), David A. Cohen (retired; well and surface-water site installation, data collection, initial model construction and technical advice), Leslie D. Arihood (retired; initial model construction and technical advice), Cheryl Silcox (data collection), and Joseph Sullivan (USGS student employee). The authors would like to thank the following USGS personnel for their review of this report: Jack Eggleston (technical review; USGS Virginia Water Science Center), Martha Nielsen (technical review; USGS Maine Water Science Center), and Mike Eberle (editorial review; USGS Ohio Water Science Center).