Numerical simulation of the cumulative effects of land-use change and aggregate mines on ground-water flow and wetlands along the South Platte River, Brighton to Fort Lupton, Colorado

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The South Platte River valley between the City of Brighton in Adams County and the town of Fort Lupton in Weld County, Colorado, contains many wetland areas that provide habitat for wildlife in the Front Range Urban Corridor. The extent of aggregate mining along this reach of the South Platte River valley continues to increase as the demand for aggregate grows in response to urban development.

The presence of aggregate pits can affect the direction of ground-water flow and water levels in the alluvial aquifer adjacent to the South Platte River. Because wetlands are sensitive to changes in ground-water levels, the hydrologic effects of aggregate pits also may affect vulnerable wetlands near pits. Changes in land use and land cover can have significant influence on economic and environmental quality at multiple scales, and mapping land-use changes is fundamental to the health and viability of the Nation’s natural and developed environments. 

To improve understanding of land-use change and the potential effects of land-use change and aggregate mining on groundwater flow, the U.S. Geological Survey, in cooperation with the cities of Brighton and Fort Lupton, analyzed socioeconomic and land-use trends and constructed a numerical groundwater flow model of the South Platte alluvial aquifer in the Brighton–Fort Lupton area.

The numerical groundwater flow model was used to simulate:

  1. steady-state hydrologic effects of predicted land-use conditions in 2020 and 2040
  2. transient cumulative hydrologic effects of the potential extent of reclaimed aggregate pits in 2020 and 2040
  3. transient hydrologic effects of actively dewatered aggregate pits
  4. effects of different hypothetical pit spacings and configurations on groundwater levels.

The SLEUTH (Slope, Land cover, Exclusion, Urbanization, Transportation, and Hillshade) urban-growth modeling program was used to predict the extent of urban area in 2020 and 2040. Wetlands in the Brighton–Fort Lupton area were mapped as part of the study, and mapped wetland locations and areas of riparian herbaceous vegetation previously mapped by the Colorado Division of Wildlife were compared to simulation results to indicate areas where wetlands or riparian herbaceous vegetation might be affected by groundwater-level changes resulting from land-use change or aggregate mining.

Analysis of land-use conditions in 1957, 1977, and 2000 indicated that the general distribution of irrigated land and non-irrigated land remained similar from 1957 to 2000, but both land uses decreased as urban area increased. Urban area increased about 165 percent from 1957 to 1977 and about 56 percent from 1977 to 2000 with most urban growth occurring east of Brighton and Fort Lupton and along major transportation corridors. Land-use conditions in 2020 and 2040 predicted by the SLEUTH modeling program indicated urban growth will continue to develop primarily east of Brighton and Fort Lupton and along major transportation routes, but substantial urban growth also is predicted south and west of Brighton.

Steady-state simulations of the hydrologic effects of predicted land-use conditions in 2020 and 2040 indicated groundwater levels declined less than 2 feet relative to simulated groundwater levels in 2000. Groundwater levels declined most where irrigated land was converted to urban area and least where non-irrigated land was converted to urban area. Simulated groundwater-level declines resulting from land-use conditions in 2020 and 2040 are not predicted to substantially affect wetlands or riparian herbaceous vegetation in the study area because the declines are small and wetlands and riparian herbaceous vegetation generally are not located where simulated declines occur.