1 - paducah environmental information center

Annual Site Environmental Report for 1999
systems at Paducah. The pump-and-treat system
installed northwest of the plant also controls the
highest concentrations of dissolved 99'fc that
would otherwise migrate offsite. Continued
groundwater monitoring serves to identify the
extent of contamination, predict the possible fate
of the contaminants, and determine the
movement of groundwater near the plant. To
date, three groundwater plumes have been
identified emanating from the Paducah Site
(Figure 9.1). Appendix D provides additional
information about these plumes.
Groundwater monitoring at Paducah
complies with one or more federal or state
regulations and permit conditions and includes
perimeter exitpathway monitoring and offsite
water well monitoring. A more detailed
description of groundwater monitoring is found
in this Section under the heading, of
Groundwater Monitoring Program. Figures 9.2
and 9.3 show the locations of all wells sampled
during 1999. Analytical results from the
sampling described in this section are available
upon request from the BJC Public Affairs Group.
Groundwater Hydrology
A portion of rainwater accumulates as
groundwater by soaking into the ground,
infiltrating porous soil and rock; The
accumulation of groundwater in pore spaces of
sediments creates a source of useable water - an
aquifer (Figure 9.4). Water from the surface
moving down through the soil makes its way by
percolating ,downward through the pore spaces
between soil grains (Figure 9.5)~ The, smaller the
pore spaces, the slower'the flow, of water through
the' sediment. The physical .propenythat
describes the ease with which water can mov.e
through the pore spaces and fractures in a
materiall is called' hydraulic conductivity; or
permeability. Permeability is ~etermined not
only by the volume and size of the' pore spaces
but also by how weU the pore Ispaces are
connected. Aquifers are found in permeable
sediments (such as sand and gravel) and rocks
(such as sandstone and 'fractured limestone).
Less permeable sediments (such as clay) and
rocks (such as shale and dense limestone) make
up aquitards that restrict groundwater movement.
The boundary between the unsaturated and the
saturated zones is known as the water table. This
boundary usually, but not always, gently mirrors
the surface topography, rising above natural
exits such as springs, swamps, and beds of
streams and rivers, where groundwater is
discharged to the surface.
Groundwater movement is detennined by
differences in hydraulic head (a function of the
energy associated with the water's elevation
above sea level and the pressures exerted on it by
surrounding water). Water will rise in a well
casing in response to the pressure of the water
surrounding the well's screened zone. The
depth to water in the well is measured and the
elevation calculated to determine the hydraulic
head of the water in the monitored zone (Figure
9.6). The hydraulic gradient measures :the
difference in hydraulic head over a specified
distance. By comparing the water levels' in
adjacent wells screened in the same zone, a
horizontal hydraulic gradient can be detennined
and the lateral direction of groundwater flow can
be predicted. Only wells screened in the same
zones are considered when determining the
horizontal gradient. Wells screened above and
below an . aquitard can also have different
hydraulic heads, thus defining a vertical
gradient. If the water levels in deeper wells are
lower than those in shallower wells, the vertical
component of flow is downward.
Permeability of the su1;>surface strata
containing the aquifer also plays an essential role
inthe direction of groundwater flow through an
aquifer system. Because the earth's sediments
and their permeability vary greatly, groundwater
flowing through sob surface strata does not travel
at a ,constant rate or without impediment. As
i
groundwater moves in the downgradient
direction, it has both a horizontal and a vertical
component, just as a household drain moves tap
water both horizontally and vertically, seeking
the lowest point of exit. Aquitards deflect
groundwater movement as drainpipe walls
control the direction of tap water movement. In
an aquifer constrained by aquitards such as
Groundwater
9-3