Numerical simulation of groundwater flow in the Lost Creek Designated Groundwater Basin, Weld and Adams Counties, Colorado

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The Lost Creek Designated Ground Water Basin (LCDGWB) is an important alluvial aquifer for water supply in northeastern Colorado. Increased demand for withdrawals from the LCDGWB has led to a concern about the long-term sustainability of the basin’s ground-water resources.

A numerical ground-water flow model developed using an early version of MODFLOW exists for the LCDGWB. A review and update of the existing model using new data and simulation capabilities of MODFLOW-2000 was needed to advance understanding of ground-water flow in the basin and provide information needed for planning and managing the basin’s ground-water resources.

The Lost Creek Designated Ground Water Basin consists of an alluvial aquifer that is an important source of water for irrigation, public supply, and domestic use in northeastern Colorado. Perennial streams do not exist within the basin, and groundwater in the basin historically has been used primarily for irrigation of agricultural land within the basin’s boundaries.  However, urban growth in the adjacent Front Range urban corridor has increased demand for groundwater in the basin, and potential exportation of groundwater from the basin has raised concerns about the long-term sustainability and management of the basin’s groundwater resources.

This study describes the hydrogeology and a steady-state numerical groundwater flow model of the Lost Creek Designated Ground Water Basin in Weld, Adams, and Arapahoe Counties in Colorado.

  • The description of hydrogeology provides new information, with particular emphasis on aquifer geometry and recharge, that builds upon the work of previous investigators.
  • The steady-state model also builds upon the work of previous investigators to provide an updated tool for simulating the potential effects of various hydrologic stresses on groundwater flow and evaluating possible aquifermanagement strategies.
  • The extent and thickness of regolith (unconsolidated sediments) in the basin were mapped to better characterize the subsurface geometry of the basin.
  • Water-level data were compiled for the period 1990–2001 for use in calibrating the steady-state groundwater flow model.
  • Data concerning infiltration of precipitation, infiltration from stream channels, deep percolation of water applied to irrigated agricultural fields, ditch and reservoir seepage, and subsurface inflow were collected to improve estimates of recharge to the basin.
  • Data concerning well withdrawals, evapotranspiration, and subsurface outflow were used to estimate discharge from the basin.
  • The USGS modular groundwater modeling program MODFLOW–2000 (Harbaugh and others, 2000; Hill and others, 2000) was used to construct and calibrate the updated steady-state groundwater flow model using inversemodeling methods.