Specifications

Section 7 Environmental Effects of the Selected Plan
EAA Storage Reservoirs Revised Draft PIR and EIS February 2006
7-2
sediment and plant material settle post-construction and produce peat/mucky
soils over time. Soils within the buffer area and STA are anticipated to remain
hydric and retain muck properties or revert to muck properties post-
construction.
The soils on the project site are classified as Unique Farmland by the NRCS.
Coordination was completed with USDA/NRCS and a determination was made
that 31,473 acres of Unique Farmland would irretrievably be lost through
conversion of the project site to an open water reservoir. This represents
approximately 8% of farmland in the county of this quality to be converted.
Refer to the Pertinent Correspondence Annex H, correspondence with NRCS for
details.
The project may benefit soils regionally, as better water management practices
may reduce the rate of soil subsidence in areas surrounding the reservoir. This
proposed reservoir, in combination with other CERP water storage elements, is
expected to ultimately improve hydroperiods in the Water Conservation Areas
and reduce subsidence of soils there. Overall, significant impacts to soils are
anticipated to be minor and localized. Some beneficial impacts are associated
with implementation of the proposed project. However, the conversion of prime
farmland is anticipated to be significant and irretrievable.
7.2 HYDROLOGY
Construction and operation of the proposed reservoir, canals and STA would
permanently affect surface hydrology. At present and under the no-action
alternative, water flows are/would be controlled by agricultural water
management practices. Post construction water will be pumped into the surface
reservoir and contained within the reservoir, the STA or the adjacent canals.
Some of the impounded water would seep under cut-off walls surrounding the
impoundment and re-emerge within the seepage buffer or seepage canal
collection system. The exception is within the southwest corner of the project
site. The areas that are anticipated to be affected are the Holey Land WMA and
Rotenberger WMA. Modflow modeling indicates that at extreme conditions
(water impounded at 12 feet within the reservoir), ground water stages could
increase by up to +1.0 feet, with stages lowering into a southward direction,
away from the reservoir. The associated affects on the water stages within these
WMAs is currently under investigation and will be updated in the final PIR.
The groundwater aquifers may be impacted by the proposed action. The
reservoir would act to recharge the surficial aquifer. The reservoir water would
be in direct contact with the surficial aquifer through the surface soils. The
surficial aquifer will be recharged by the reservoir and it is expected to show
higher post-construction levels over the EAA region.