Research in hydrogeology
in the Department of Geological Sciences broadly includes field-based studies
and mathematical modeling of ground-water flow, mass transport, and reactions
in the subsurface. Past and present research topics include:
Controls on ground-water composition
in regional sedimentary aquifers
High Plains (Ogallala)
Aquifer: The High Plains aquifer is the largest aquifer in the USA
and an important resource for agricultural, industrial, and municipal uses.
Ground-water withdrawals have caused the water table to fall as much as
1 meter per year in some areas since the 1950s. Consequently, numerous
studies of recharge have been conducted, but relatively few studies have
examined controls on water quality in this semi-arid region. Potential
water-quality problems include nitrate contamination and salinization by
brines from underlying rocks. Sunil Mehta (PhD 2000) documented that areas
of salinization northeast of Amarillo, Texas, result mainly from upward
seepage of relatively shallow brines into the High Plains aquifer and not
leakage of deeper brines produced with oil and gas. Detailed mapping of
water quality in pilot borings can guide the placement of wells and thus
limit the impact of brine seepage in municipal wellfields. Elsewhere in
the region, the composition of ground water in the High Plains aquifer
is controlled primarily by processes occurring during recharge, including
depression-focused infiltration, evapotranspiration, carbon cycling in
the vadose zone, and (probably to a lesser extent) silicate weathering
and cation exchange. Denitrification in the soil zone limits nitrate loading
to ground water and thus facilitates the use of waste water for artificial
recharge, which promotes water conservation.
(Left: Abhijit Mukherjee uses a colorimetric technique
to measure the concentration of ferrous iron in the Canning public-supply
well, June 2005.
Sediment and pathogen transport
in Inner Bluegrass karst ground-water basins
Ground-water/surface-water interactions
in the Gulf Coastal Plain of western Kentucky
Ledbetter Creek:
With colleagues at Murray State's Center
for Reservoir Research, Karen Thompson (MS 2002) and Todd Aseltyne
(PhD candidate) have monitored and modeled the influence of reservoir-level
manipulation on ground-water flow within the Ledbetter Creek watershed
in Calloway County, Kentucky. Ledbetter Creek is a tributary to Kentucky
Lake, the terminal reservoir on the Tennessee River. Water-level monitoring
of piezometers and wells and gaging of stream and spring flow indicates
that ground water discharges to Ledbetter Creek and within the creek's
embayment. As at PGDP, discharge rates and water levels in wells tend to
be highest in late spring and lowest in early winter. Within the embayment
itself, the hydraulic gradient is reversed from upward to downward when
the embayment is submerged between April and September.
(Left: Andrea Hougham [undergraduate assistant] and Danita
LaSage collect samples from a sand boil [submerged spring] in Little Bayou
Creek, May 2000.
Natural attenuation of trichloroethene
in ground water and surface water
Bengal Basin: An
estimated 50 million residents of the Bengal basin in eastern India and
Bangladesh face health risks from ingesting arsenic in shallow ground water.
The arsenic appears to be released from sediments transported down the
valleys of the Ganges and other rivers draining the Himalayas; land-use
practices may accelerate the rate of arsenic release. Abhijit Mukherjee
(PhD candidate) is studying the quality and sustainability of deep ground
water in the Indian state of West Bengal. We are using data from the network
of water-supply wells operated by the state Public Health Engineering Directorate
to develop models of ground-water flow and chemical evolution at the regional
scale. Interim results indicate that some deep public wells are contaminated
with arsenic. High iron and low dissolved oxygen concentrations are consistent
with results of studies in Bangladesh; arsenic appears to be liberated
as iron oxides dissolve under anoxic conditions. Inferring ground-water
flow paths and accompanying trends in water quality is complicated because
of the heterogeneous delta-plain sediments and extensive pumpage for irrigation.
Right: Sunil Mehta collects brine from an oil well in
Carson County, Texas, August 1997.)
Ground-water flow along
bedding planes and fractures in limestone results in dissolution of the
rock, evolution of conduits, and development of an integrated surface and
subsurface drainage network. Because of the size of these conduits, particle
transport is much more significant in karst than in other types of aquifers.
The transport of pathogens (such as bacteria and viruses) can be facilitated
by sorption to fine sediment mobilized during storms. With colleagues in
Agronomy, Civil Engineering, and the Kentucky Geological Survey, Todd McFarland
(MS 2003), Tom Reed (MS student), and James Ward (PhD student) have monitored
two springs in Woodford County, Kentucky, for discharge, water chemistry,
pathogen concentrations, and sediment characteristics during both base
flow and storm flow. Blue Hole spring in the city of Versailles drains
a mixed urban and agricultural basin, whereas spring SP-2 at the UK Animal
Research Center drains an agricultural basin. Microbial results are consistent
with differences in land use. Suspended sediment consists mainly of silt-sized
detrital quartz, and Blue Hole spring appears to respond more rapidly to
precipitation than SP-2 does.
(Blue
Hole spring, May 2002.)
Paducah Gaseous Diffusion
Plant (PGDP): Researchers and regulators are emphasizing the need to
view water and chemical fluxes within an integrated (watershed) framework,
rather than decoupling studies of subsurface and surface processes. We
hypothesized that contaminant plumes from the U.S. Department of Energy's
Paducah Gaseous Diffusion Plant (PGDP), a Superfund
site in western Kentucky, could discharge to wetlands and streams in
the Ohio River flood plain. Contaminants of concern include trichloroethene
(TCE) and technetium-99, a radionuclide associated with uranium reprocessing.
Eric Wallin (MS 1998), Abhijit Mukherjee (MS 2003), and Danita LaSage (PhD
2004) quantified spatial and temporal variability in seepage along Bayou
and Little Bayou Creeks, which bracket PGDP. We located contaminated springs
along Little Bayou Creek and documented that discharge to tributaries can
short-circuit plume migration to (and dilution by) rivers. Spring-flow
rates and contaminant concentrations in the stream tend to decrease from
May through January, then rebound. Josh Sexton (MS student) is developing
a GIS-based stratigraphic framework model of the study area.
Right: Danita and Andrea monitor water levels in piezometers
in Ledbetter embayment, May 2000.)
Monitored natural attenuation
can be a practical, cost-effective option for remediating ground-water
contamination, particularly for chlorinated hydrocarbons such as TCE, a
common solvent and suspected carcinogen. Ground-water remediation at PGDP
focuses on zones of the Regional Gravel Aquifer (RGA) with TCE concentrations
> 1 mg/L. Although TCE does not undergo significant natural attenuation
within the RGA, processes in overlying or underlying strata might limit
TCE impacts on surface-water ecosystems or deeper ground-water flow systems.
David Butler (MS 1999) and Nadege Etienne (MS [Plant and Soil Science]
1999) identified methanogenic bacteria, which have been shown to be capable
of degrading TCE at other sites, in soils and sediments like those bounding
the RGA. However, biodegradation was insignificant in batch cultures over
periods as long as 10 months, perhaps because of a lack of nutrients or
a lack of acclimation to TCE. Chris Sweat (MS 2000) found that TCE sorption
to soils and sediments appears to depend upon both the amount and the polarity
of organic matter. Using tracer tests, Abhijit Mukherjee (MS 2003) found
that dilution and volatilization appear to limit TCE concentrations in
Little Bayou Creek.
(Abhijit
Mukherjee collects samples during Little Bayou Creek tracer test, January
2002.)
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