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Agricultural Chemicals: Sources, Transport, and Fate (ACT)

ACT is trying to answer the following question:
How do environmental processes and agricultural practices interact to affect the transport and fate of agricultural chemicals in the hydrologic system of nationally important agricultural settings, and what are the effects on water quality and implications for management of water resources?

How will ACT answer this question:
The key aspect of the study approach is to investigate the sources, transport, and fate of selected agricultural chemicals in a variety of agricultural settings across the Nation. Each study unit of the ACT study will select an indicator watershed that represents a nationally important agricultural setting. The indicator stream watershed will define the primary area of the ground-water system for these studies. A smaller subbasin will be paired with the indicator stream for the studies. The multiple scales are necessary to understand how the natural hydrologic setting and the superimposed agricultural system interact to affect the fate of agricultural chemicals in the watersheds.

Cycle I data will be used to develop a conceptual and semi-quantitative mass-balance model to describe the sources, transport and fate of selected, important agricultural chemicals used in the watersheds, usually nitrogen and an important pesticide. This model will describe the hydrologic compartments through which transport occurs, important transformations that occur within each compartment, factors that control transformation rates, estimated residence times within each compartment, and fluxes between compartments. Data will be collected from the nested watersheds. Based on these Cycle II data, the conceptual model will be tested, adjusted, and refined by using mass-balance and numerical modeling (process and statistical).

By contrasting and comparing results from a variety of scales and a variety of settings, the study will (1) assess the influence of differing agricultural practices on water quality and the ecosystem, and (2) assess the degree to which methods and results from this work can be successfully transferred to larger scales or to different settings.

Specific objectives of ACT for each agricultural setting:
  1. Develop an annual, mass budget for water and selected agricultural chemicals
  2. Determine the residence times and rates of water and agricultural chemical transport
  3. Identify the important chemical transformation and transport processes for selected agricultural chemicals
  4. Use quantitative methods for the interpretation, extrapolation, and prediction of the fate and transport of water and selected agricultural chemicals
  5. Interpret study results as to the implications for managing the water and water-quality impacts of agricultural systems
Study components for the SANJ ACT
The area of focus for the SANJ ACT is the Merced River watershed. The SANJ ACT is located within the regional ground-water flow model of the SANJ TANC and will benefit from the characterization of the regional ground-water flow system. More detailed surface-water modeling, using TOPMODEL and SWAT, will be carried out on the Mustang Creek subbasin. The study components for the SANJ ACT are broken out in three broad hydrologic compartments: atmosphere, surface water, and ground water.

  1. Atmosphere - The SANJ ACT will collect rain event samples throughout the year at three rainfall sites located within the Mustang Creek subbasin. Samples will be analyzed for targeted pesticides and their transformation products. These data, in combination with spatial rainfall and meteorological data, will be used to calculate the areal distribution of the loads of chemicals to the watershed via rainfall.

  2. Surface water - The SANJ ACT will sample several sites in the small agricultural subbasin of Mustang Creek as well as the downstream Merced River indicator site. The Mustang Creek sites include a culvert receiving overland flow from one almond orchard and three sites along the mainstem of Mustang Creek from the top of the basin to the bottom. The primary surface-water goal will be to identify temporal patterns in the concentrations of selected pesticides and nutrients and to relate these patterns to surface-water transport processes.

  3. Ground water - The SANJ will install and sample lysimeters in the unsaturated zone, along Mustang Creek and near the Merced River. Ground-water wells scattered within the Merced River watershed and located along a flow path to the Merced River will be sampled. The SW-GW interaction site, which will include pairs of piezometers, at different depths, is located at the Merced River, at the terminus of the flow path. Samples from the unsaturated and saturated zones, and the SW-GW interaction site will be analyzed for field parameters, pesticides, nutrients, major ions, and dissolved organic carbon. Selected samples of the SW-GW interaction site will also be analyzed for stable isotopes and parameters that will provide information on the age of the water. The goal of this work is to estimate the flux of water and agricultural chemicals through the unsaturated zone, into the saturated zone, and between surface water and ground water, and to assess the chemical transformations that occur as the water moves through these different zones.

Map showing sampling sites for the ACT study

Study area for the topical study of agricultural chemicals:
sources, transport, and fate, Merced River Basin, California

Sampling activities for SANJ ACT
ACT project sampling activities are shown on the following video clips. Click on photo to view video (Not enabled yet).
Photo of meterological station
Atmosphere, surface water, and vadose zone:
instrumentation and sample collection
Photo of augering into the unsaturated zone
Ground water: drilling and core sample collection
Sampling in Merced River
Ground water and surface water interactions:
instrumentation and sample collection

Results of the SANJ ACT Study

The results from all the ACT studies involving the San Joaquin-Tulare Basins NAWQA are summarized in a special submission to the Journal of Environmental Quality, May 2008 issue. The papers can be accessed at the following URL:


In addition, these papers and more information can be obtained on the national ACT study through the NAWQA ACT website at the following URL:


The specific papers related to work on the San Joaquin study area are listed in the publications.

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