Borehole Tracer Study: Ogallala Formation - northern High Plains
Focused flow and chemical migration near irrigation wells
Objective:
Test a conceptual model hypothesizing fast-path water and chemical flux in response to
seasonal ponding adjacent to irrigation wells, and to extrapolate results of site-specific
simulations to quantify High Plains aquifer susceptibility to focused flow phenomena
surrounding irrigation wells.
Approach:
The USGS code VS2DT was used to simulate 2-D radial flow and transport in simplified
scenarios near irrigation wells. Three separate simulation phases were run.
Phase I simulation tested the potential for chemical transport via the gravel pack
annulus of the irrigation well during season ponding adjacent to the well pad.
Phase II simulation tested the sensitivity and importance of a variety of factors on
enhanced solute transport beneath seasonal ponding adjacent to irrigation wells.
Phase III simulations tested various permutations of controlling factors from Phase
II to evaluate conditions that are conducive to conservative chemical migration to
ground water in < 50 years.
Soil and vadose zone cores were collected for soil moisture and water-extractable
concentrations of Cl- and NO3- to provide a ground-truth of assumption used during the
VS2DT simulations.
Phase III results will provide the foundation for development of an artificial neural
network model for prediction of enhanced chemical transport near the 128,700 irrigation
wells across the High Plains.
Timing:
Field coring, laboratory analyses and numerical simulations were completed, 2005.
Analysis and writing will be completed, 2006.
Targeted Results and Benefits
Identify the potential and assess validity of hypothesis that poorly sealed annuli of
irrigation wells offer focused and rapid pathways for water and chemical to reach the water
table, which have important implications for ground-water quality and agricultural
management practices.
Identify the potential and assess validity of hypothesis that poorly sealed
annuli of irrigation wells offer focused and rapid pathways for water and
chemical to reach the water table.
Complement the unsaturated zone studies already completed in the southern, central,
and northern High Plains.
Extend our understanding of water and chemical fluxes from land-surface to the underlying
aquifer.
