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Because we get so much of our drinking water from wells that tap groundwater, and because the flow of groundwater has the potential to spread spills of harmful pollutants, it is important to be able to predict and model groundwater movement. Groundwater flow through porous media such as sandstones can be modeled using Darcy’s Law because water interacts with the small particles of rock during slow intergranular fluid flow. Groundwater flows through limestones much more quickly by a process called open channel fluid flow, however, because the groundwater moves through larger cavities formed by dissolution of the limestone. Understanding the geologic controls on where these conduits form is thus critically important to discovering drinking water supplies and predicting flow of pollutants through limestones. In addition to groundwater modeling applications, the ability to predict where dissolution caverns form is also key to mitigating sinkhole hazards and exploring for mineral deposits in limestones.
Several geologic factors have the potential to localize the formation of dissolution features in limestone. These include: 1) limestone strata that are more calcitic and therefore more soluble, 2) limestone strata with a higher initial intergranular porosity (i.e., number of holes between grains), a more favorable grain size, or a more conducive layering characteristic, and 3) fractures related to faulting or folding that focus groundwater flow and dissolution.
In cooperation
with Berks
Products, we are using a multidisciplinary approach to studying the
controls on
cave and conduit formation and groundwater flow near a limestone
aggregate
quarry near Ontelaunee,
Provides one student with paid internship – a continuation of internship sponsored by Berks Products over the past two years
Students manually measure water level in each of 20 monitoring wells on the Ontelaunee Quarry property to detect long-term changes in groundwater flow in the study area
The
shape of the water table is affected by
the permeability of the rocks hosting the groundwater (i.e., the ease
with
which water flows through rock). Over
long periods of time, if there are no changes in the character of the
rock, the
water table establishes a stable, average shape. If,
however,
the permeability of the rocks in
the aquifer changes significantly when new caves open up or when old
caves
collapse, the water table will then shift until a new equilibrium state
is
achieved. Monitoring and making maps of
the water table on a weekly basis highlights sudden shifts in the
overall shape
of the water table and so indicates changes to the system of
dissolution
conduits.
Four undergraduate students involved - Autumn 2003:
Berks Products will pump water from a well drilled in the floor of the quarry at a rate of 10,000 – 15,000 gallons per minute in order to depress the water table in the study area
Automated pressure transducers with data
loggers
(i.e., devices that measure water level and water temperature in wells)
mounted
on ten (10) wells plus a head monitor (i.e., device that measures
pressure)
affixed to an artesian spring that bubbles up onto the floor of the
quarry will
be used to monitor groundwater fluctuations in response to pumping
.
Contour maps of water table will be made for each hour during pumping and for a 24 hour period following pumping
Although the water table establishes a
generally
stable equilibrium configuration over long periods of time, the water
table
experiences short term fluctuations in response to rain storms. Rainwater enters the groundwater system by
infiltration from the surface and by increased infiltration related to
swollen
streams. This effect is amplified in the
case of the Ontelaunee quarry because Berks Products pumps water out of
the
quarry pit, thereby depressing the local water table and creating a
pressure
difference (“head”) that draws groundwater from the streams (Maiden
Creek and
the Schuylkill River) toward the quarry.
The groundwater monitoring well field located between nearby
streams and
the quarry therefore provides an excellent opportunity for studying
groundwater
flow in response to changing head, thereby highlighting principal flow
paths.
Four undergraduate students involved - Autumn 2003:
Automated
pressure
transducers with data
loggers
mounted on ten (10) wells located in the field between the quarry and
the
confluence of two nearby streams (Maiden Creek and the
Water levels and temperature in each well will be monitored every five (5) minutes for a 2-3 week period before, during, and after a rain event
Contour maps and shaded relief maps of the water will be drawn for each 5 minute time frame using ArcMap GIS software
Shaded relief maps for each time frame will be put together into a movie illustrating the response of the water table to changing inflow from the nearby swelling streams
Another way to determine principal
groundwater
flow paths through the aquifer is to measure groundwater levels in the
monitor
wells following a rainstorm. Following a
rainstorm, the water level in nearby streams goes up as the streams
drain the
land of rainwater. Some of the water
from the swollen streams infiltrates into the ground through fractures
and
sinkholes, thereby increasing the pressure (“head”) that drives
groundwater
flow. Rainwater that falls on the
surface also infiltrates down into the ground, further engorging the
aquifer. Water levels in the aquifer
(i.e., the water table) bulge upward in zones of maximum flow. This response to changing pressure is very
rapid in limestone aquifers because the groundwater flows through cave
channels
with relatively little resistance. It is
therefore necessary to measure the water level in each of the 10
monitoring wells
very frequently – on the order of every few minutes – during the rain
event in
order to observe the response of the water table to the storm. Compiling the data into the form of an
animated movie of a shaded relief model of the water table will
compliment the
pump test data and serve as an excellent visual aid for teaching
introductory
geology students in the classroom.
One undergraduate student involved - Autumn 2003:
When groundwater flows through limestone, the water dissolves a tiny amount of rock, slowly enlarging the dissolution conduits in the rock. As a result of rock dissolution, elements characteristic of the dissolved rock become dissolved in the groundwater. Calcitic limestone is rich in calcite (CaCO3), so interaction with groundwater with calcitic limestone would enrich the water with calcium ion. Dolomitic limestone, on the other hand, is rich in the mineral dolomite (CaMg(CO3)2), so interaction with groundwater with dolomitic limestone enriches the water in both calcium and magnesium. The dissolved rock therefore leaves a sort of geochemical fingerprint in the water that passes through it. By comparing chemical analyses of the groundwater, we hope to identify chemical tracers indicating which rock beds have the most dissolution cavities.
Student will collect samples of water using a bailer from each well and analyze the composition of the water using the Graphite Furnace Atomic Absorption Spectrometer to identify chemical markers that record water-rock interactions. The calcium:magnesium ratio will be interpreted in terms of the relative degrees of interaction of the water with calcitic and dolomitic limestones.
Student
will
sample the well field several
times
to identify changes in the chemical trends in the aquifer that might
suggest
changing flow paths in the limestone.
Two undergraduate students involved - Autumn 2003:
Systematic mapping of the orientations and abundance of fractures in different rock formations and the relationship between folding of the rocks and rock fracture
The
intergranular porosity of fine-grained
limestones like those in eastern
Two undergraduates student involved – part of rock fracture project:
Collect systematic suite of rock samples every 5 feet across the quarry
X-ray diffraction (XRD) study of samples to get semi-quantitative estimate of relative proportions of the minerals calcite, dolomite, quartz silt, and clay by comparing characteristic peak heights of each mineral on XRD scan
X-ray fluorescence (XRF) study of samples to get quantitative analyses of Ca, Mg, Si, Al, Fe content of rocks using Evansville Cement facility (their machine is already calibrated for these types of rocks), from which we can calculate theoretical mineral proportions more precisely
Thin sections of samples will be studied to document the porosity, grain size, and fine-scale textures in each rock type
Some limestone beds may be more soluble in rainwater than others and so be more vulnerable to cave formation. Limestone is composed of varying proportions of the minerals calcite (CaCO3), dolomite (CaMg(CO3)2), quartz (SiO2), and clay (Al2Si2O5(OH)4). Calcite is by far the most soluble of the four, followed by dolomite, whereas quartz and clay are essentially insoluble. The size of the mineral grains can also play a big role in determining the solubility of a rock – fine-grained rocks are generally more soluble than coarser-grained ones. Finally, rocks that contain pores through which some water can flow allow more water-rock interaction and thus potentially more dissolution. Rock outcrops exposed in the quarry show what appear to be dissolution channels that follow bedding, occurring only within specific rock layers. A combined approach using XRD to characterize the minerals present, XRF to determine the chemical composition of the rocks, and analysis using petrographic microscopic will help us to evaluate the potential differences in the solubility of different limestone layers.
The students studying mineralogical and geochemical variations in the sedimentary rocks will also be doing the rock fracture study with Dr. Tindall. The students will sample rock as they measure fractures. Integrating the rock fracture and the mineralogical study will allow for us to say something about the interplay between a rock’s composition and propensity to fracture.
This project is an excellent example of how scientific research can bring together undergraduate students, several professors of differing specialty, and local industry in a collaborative effort to solve some fundamental questions.
By selecting a large group of student participants, more students have opportunity to gain experience doing more things, while still maintaining a sense of “ownership” of their own part of the study. Working as a group also requires students to learn to cooperate be responsible for their part of the work.
Because the project involves such a tight
collaboration with local industry, students develop connections in the
field
that may lead to jobs when they graduate.
In addition to Berks Products,
Cooperation with local industries promotes
By involving multiple Kutztown professors, the project encourages faculty interactions and builds group cohesion within the department.
