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Ecological Impacts of Irrigating Native Vegetation with Treated Sewage Wastewater

Background
In 1995, the INL began disposing of treated sewage wastewater at the Central Facilities Area (CFA) by applying it to the surface of soils and native vegetation using a center pivot irrigation system. Research conducted on this disposal method at the INL
 provides an opportunity to determine the benefits and/or hazards of disposal of wastewater on native vegetation in arid and semi-arid regions. Results will be applicable to a wide range of municipal, industrial, and agricultural wastewater disposal needs. Because permits to dispose of agricultural and industrial wastewater may have restriction on application to prevent deep percolation, this research may refine some of the models used to predict the maximum rate of wastewater application possible without percolation below the rooting zone.

The wastewater land application facility at CFA covers approximately 29.5 ha (73 acres). The permit for operating this system limits the application rate to 63.5 cm (25 in.) of water per year, which must be applied such that no more than 7.6 cm (3 in.) of water leaches through the root zone toward groundwater. The 63.5 cm (25 in.) maximum application rate is more than two and one-half times the average annual precipitation and depending on the timing of application, plants may not be able to deplete this in one growing season to prevent leaching. Most of the precipitation in this cool desert biome occurs in the winter and spring, and soil moisture recharge occurs in the spring with snowmelt and rainfall. Therefore, wastewater application must be timed to avoid spring recharge to minimize deep percolation of wastewater. The wastewater also contains organic carbon, nitrogen, other nutrients, and trace metals that may have impacts on the proper functioning of native soil-plant systems.

Different plant species respond differently to addition of water and nutrient elements, especially if those additions come at times of the year that are normally dry. These differences in response can result in some species being favored and others discouraged. Changes in plant community structure can be expected. For example, in arid and semi-arid regions grasses are known to dominate where precipitation occurs mostly in the summer and shrubs tend to dominate in areas where moisture occurs as snow. Summer irrigation may lead to decreases in shrub dominance and increases in grasses.

Changes in plant community structure also mean changing habitats for other organisms such as small mammals, birds, insects, and big game animals. Because the area is relatively small, it is unlikely that decreased habitat quality would have significant impacts on wildlife populations on the INL.  Increases in habitat quality, however, could have substantial impacts on wildlife use pattern in and near this small area.
 

Objectives
The primary objective of the research study was to determine the ecological benefits or hazards of applying wastewater on native vegetation in semiarid regions. Specific objectives were to determine the potential for impacts on rangeland quality, resident wildlife populations, and soil water balance.

Accomplishments through 2004

Plant cover surveys were completed in 39 study plots within the three distinct plant community types (sagebrush steppe, crested wheatgrass, and a transition type) on the application area and in control areas adjacent to the wastewater application area. Soil moisture data was collected once every two weeks at 19 sites in the wastewater application area and 20 control sites throughout the growing season (beginning mid-March and ending mid-October), and a breeding bird survey was conducted according to United States Geological Survey (USGS) guidelines on and around the study site to determine any differences between irrigated and non-irrigated areas in bird usage. Additionally a complete ecological impacts report detailing results from the 2002 growing season was completed in early 2004.

Results

Spring wetting fronts in 2004 ranged from 0.4 m to 1.0 m and did not differ substantially between irrigated and control plots. Subsequent to infiltration, soil moisture decreased steadily throughout the wetted profile through the summer as a result of evapotranspiration. Soils began to approach the lower limit of extraction by early July in 2004. The soil moisture profiles do not indicate an increase in soil moisture at 20 cm or deeper due to wastewater application. If irrigation were to affect soil moisture, it would be expected to see either small wetting fronts in the profile throughout the summer (in the case of pulses in application), or it would be expected that soil moisture in at least some portion of the top of the soil profile to remain elevated (in the case of relatively steady application of water). Neither of these patterns is apparent in the irrigated soil profiles. In fact, those profiles dried down throughout the summer in a manner very similar to that of the control soil profiles. Thus, most of the additional water received by a soil profile through wastewater application was evaporated or transpired before it percolated to a depth of 20 cm within the soil profile. The soil moisture dynamics described here were similar across all plant communities on the application area. Therefore, the probability of water percolating through the rooting zone and continuing to move downward was essentially the same for the wastewater application area and control locations during the 2004 growing season.
 

Investigators

  • Roger D. Blew, Ecologist, Environmental Surveillance, Education and Research Program, S.M. Stoller Corporation, Idaho Falls, ID
  • Amy D. Forman, Plant Ecologist, Environmental Surveillance, Education and Research Program, S.M. Stoller Corporation, Idaho Falls, ID
  • Sue J. Vilord, Wildlife Biologist, Environmental Surveillance, Education and Research Program, S.M. Stoller Corporation, Idaho Falls, ID
  • Jackie R. Hafla, Natural Resource Scientist, Environmental Surveillance, Education and Research Program, S.M. Stoller Corporation, Idaho Falls, ID

Funding Source
U.S. Department of Energy Idaho Operations Office

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