Simulated effects of existing and proposed surface-water impoundments and gas-well pads on streamflow and suspended sediment in the Cypress Creek watershed, Arkansas

Cypress Creek is located in central Arkansas and is the main tributary to Brewer Lake, which serves as the primary water supply for Conway, Arkansas, and the surrounding areas. A model of the Cypress Creek watershed was developed and calibrated in cooperation with Southwestern Energy Company using detailed precipitation, streamflow, and discrete suspended-sediment data collected from 2009 through 2012. These data were used with a Hydrologic Simulation Program—FORTRAN model to address different potential gas-extraction activities within the watershed.

The calibrated Hydrologic Simulation Program—FORTRAN model was used to simulate four land-use scenarios and examine the potential effects of these land-use changes on the streamflow and water quality within the Cypress Creek watershed. These simulated scenarios included (1) the conversion of all nonforested land to forest, representing a time period before extensive grazing activities and no gas-extraction activities; (2) a land-use change to that of 1949, representing a time period with some grazing activities and no gas-extraction activities; (3) a time period with current land-use conditions, but without any gas-extraction activities, that is, the exclusion of gas-well pads/pipelines, associated gravel roads, and surface-water impoundments; and (4) a time period with current land-use conditions, but with increased gas-extraction activities (for example, increased gas-well pad and surface-water impoundment activities) to represent a possible future natural gas full-development condition for the area.

A current-conditions simulation also was built and calibrated and represents the current conditions (2013) within the watershed. This simulation was used as the comparison basis for the four land-use scenarios described above. The current-conditions simulation used the 2006 land-use conditions, which consisted primarily of forest and pasture, as well as the current (2013) 35 gas-well pads and pipelines and 6 surface-water impoundments, which account for approximately 1.6 percent of the land use. Simulating a time period before extensive-grazing activities and no gas-extraction activities for scenario 1 resulted in a decrease in suspended-sediment loads and volume of streamflow within the Cypress Creek watershed compared to the current-conditions simulation. Simulating a time period before any gas-extraction activities but with some grazing activities for scenario 2 also resulted in a decrease in suspended-sediment loads and volume of streamflow within the Cypress Creek watershed. Simulating current conditions, but without any natural gas-pad land use or related activities (including pipelines and associated gravel roads), for scenario 3 resulted in mostly unchanged suspended-sediment loads and volume of streamflow within the Cypress Creek watershed, as compared to the current-conditions simulation. Finally, simulating potential future conditions of increased gas-well pad and surface-water impoundment activities for scenario 4 resulted in a decrease (compared to the current-conditions simulation) in suspended-sediment loads and a slight increase of volume of streamflow within the Cypress Creek watershed.

The Arkansas Natural Resources Commission and the Arkansas Department of Environmental Quality list suspended sediment from “poor pastures” as a primary source of nonpoint-source pollution in north-central Arkansas, but unpaved (gravel) roads are another important source of suspended sediment. Because of the high sediment-loading rates associated with gravel roads and the large amount of pasture within the watershed, the factors most responsible for suspended sediment within the Cypress Creek watershed are likely associated more with the pastureland and gravel roads, than factors associated with gas-well pads/pipelines.