|
Back
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
|
