214010-020-mtn-home-2019-10-31-wfps-report

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Mountain Home Water Facilities Planning Study Update October 2019
groundwater supply indicate that the surface water may become a more likely option at
some point in the future.
While surface water is unlikely to be part of the City’s 20-year water supply portfolio, given
the declines in the aquifer, the City should consider surface water supply as part of the City’s
long-term supply portfolio. In the future, should the Mountain Home Airforce Base (MHAFB)
elect to proceed with a surface water supply, Keller Associates recommends the City
consider negotiating with the MHAFB to pay for the incremental additional cost to upsize
the transmission pipeline in exchange for a right to convey the incremental additional water
capacity in the pipeline.
Additional Capacity from Existing Wells
The City has previously explored increasing the capacity of existing wells. In 2006, Well 6
was rehabilitated and a new pump was installed capable of delivering approximately 500
gpm more than the previous pump. The Well 14 hole was reamed out in 2005 so that the
capacity of this well could be increased to 1100 gpm. Within the last decade Well 12 was
rehabilitated to increase its well yield and Well 9 was restored to operation.
There is a potential that additional capacity could be obtained from Wells 11 and 13.
Historical records show that one of these wells was pump tested at 3500 gpm with little
additional drawdown. The challenges with increasing the capacity of one of these wells are
threefold. First, the pressures in this region of the system are already much higher when
both wells are operational. Increasing the supply in this region will require substantial
additional off-site transmission improvements. Second, increasing only one well to 3500
gpm would not increase the firm capacity of the system. According to DEQ regulations, the
largest well – now 3500 gpm, instead of 2100 gpm -- would have to be assumed to be offline
in evaluating the system’s firm capacity. Third, the City has a number of 500 hp wells
which allows the City to keep spare motors and other parts for quicker repairs. A 3500 gpm
well would require a substantially larger motor and generator.
Given the nature of groundwater within Mountain Home, it is likely that the only wells that
could have increased capacity without significant drawdown or reduction to the main aquifer
would be Wells 11 and 13, both of which are already pumping at their anticipated capacity.
Because of this, it is not recommended to pursue pulling additional capacity from existing
wells.
New Wells
The City may consider either constructing new wells or acquiring existing private wells. One
advantage to acquiring an existing well is that it removes some of the guesswork with
respect to capacity and quality that are often part of constructing a new well. Keller
Associates recommends that any new well construction be equipped with the capability to
add disinfection in the future even if it is not currently used.
As part of an effort to assess suitable locations for a new well, Brockway Engineering was
retained to perform a hydrogeologic assessment of potential well sites for the City, see
Appendix I for the completed report. The study examined the hydrogeologic conditions at
several sites of interest for a new City well, and summarized the likely maximum yields,
drawdowns, and water levels at each of the sites. Based on this analysis and City input, the
location chosen at the existing City Public Works Shop is recommended as the site for the
new well. This decision was based on the hydrogeologic conditions, the accessibility and
existing security of the site, and the fact that the City already owns the land. Because of
the uncertainty of water quantity, test holes and test pumping are recommended for future
well facilities.
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