Weiser, a community of
approximately 5000 residents lies near the confluence of the Weiser
and
Snake
Rivers
in
Washington County
,
Idaho
. Unacceptably high levels of nitrate influence the shallow ground
water system that lies below the valleys of these two rivers near the
community. The Idaho Water Resources Research Institute, Technical
Assistance to Rural Ground Water Development In Idaho project team
undertook this study in order to define the hydrogeological parameters
of the ground water system, and to provide these data to the Weiser
focus group, a consortium of organizations that works to mitigate the
nitrate problem.
The community of Weiser lies
at the northwestern end of the Western Snake River Plain, a regional
fault-bound structural depression formed during Neogene time.
Clay-rich sediments deposited from a prehistoric lake fill the
depression and form the floor of the shallow ground water system in
the study area. The aquifer consists of sands and gravels that reside
above the clays on the floor of the Snake and
Weiser
River
Valleys
.
Buried paleo-river channels
meander through the aquifer-forming strata and describe a complicated
array of high-conductivity zones. The axial, coarse-grained parts of
this fluvial system could divert ground water flow along the old
channels in preference to a more straightforward flow path. Surface
contaminants entering this aquifer will reflect this convoluted flow
path if it exists.
Most monitoring wells show
elevated levels of nitrate, indicating that research should focus on
locating the plume boundary. Delineating the plume footprint will help
identify sources. Possible sources include agricultural fields,
confined animal feeding operations, septic systems, and onion dumps.
Nitrate concentrated in canal
water contributes to the total as well, as leakage recharges much of
the shallow ground water system. Canal water may accumulate nitrate in
either of two ways. Sheet wash during flash flood events may add a
substantial quantity of nitrate-laden particulate in the areas of
confined animal feeding operations. Also the constant recycling of
water from canals, across fields, and back to canals probably adds
nitrate to the water. The latter process may help explain nitrate
concentrations in ground water on Weiser flat.
We recommend collecting
geochemical samples from an array of wells distributed more broadly
over the shallow ground water system in order to determine where and
how nitrate is added. Samples of water from canals following intense
storm events will provide an understanding of the influence from
surface sheet wash events.
