2007 Annual Report 1a.Objectives (from AD-416) To determine the relative contributions of on farm deep percolation and associated use of Colorado River water and natural recharge, to the ground water and salt loading from Grand Valley. Comparison to 20 y unpublished data will also allow evaluation of the impact of management changes and land use practices within that time period. 1b.Approach (from AD-416) Ground water samples and samples from washes and the Colorado River will be collected in Grand Valley at the end of the 2005 irrigation season. These samples will all be analyzed for 18O/O16, D/H as well as tritium and other tracers for age dating waters of less than 50 y. using acceptable protocols. Additional samples from the same locations will be analyzed for chemical composition of major dissolved ions and several trace elements. These analyses will be utilized to evaluate the assumption that the ground water composition is integrating local precipitation and irrigation conditions over a series of years rather than reflecting climation conditions of 2005. Calculations will be made to estimate the relative salt loadings from natural recharge and Colorado River derived water. These data will be compared to data collected in 1984 (Suarez, unpublished) to allow evaluation of the impact of land use changes and irrigation project improvements (canal lining, changes in irrigation practices etc) on salt loading to the Colorado River. Documents Reimbursable with Bureau of Reclamation. Log 28824. Formerly 5310-61000-012-01R (FEB 07) 3.Progress Report This report serves to document research conducted under a Reimbursable Agreement between ARS and the Bureau of Reclamation. Additional details can be found in the report of the parent project 5310-61000-013-00D. This research is directed towards determining the impact of irrigation system improvements in Grand Valley on salt loading to the Colorado River. An initial sampling of groundwaters and Colorado River surface water above and below Grand Valley was completed. We analyzed for major ions and trace metals in solution from field filtered samples. Based on the groundwater data from 20 years ago, we simulated the impact of reducing the groundwater recharge from irrigation drainage on the groundwater composition and return flow salt load for a 40 year Interval after several levels of system improvements. We examined the impact on groundwater systems that were noncalcareous, as well as calcite and calcite +gypsum systems. The simulations indicate that changes in management will not be fully reflected in groundwater composition and salt load for 20 to 40 years, depending in part on the changes in recharge volumes from drainage. This research provides a methodology for analysis of the impact of irrigation management on salt loading, essential for determining the effectiveness of existing and proposed irrigation system improvements. We have used teleconferences and meet with the US BoR representatives at regular meetings of the Colorado River Basin Salinity Control Federal Advisory Council and most recently at the Colorado River Basin Salinity Control Program Science Team Meeting July 17. 2007 in Salt Lake City Utah, where results were presented and the project discussed.