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WETLAND HYDROLOGIC ASSESSMENT:
FIELD MAPPING
Kristen Meidt
Timothy Alton
November 3, 2010
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Table of Contents
Introduction 3
Methods 3
ResultsWatertable map
44a
Conclusion 5-6
AppendixWetland Hydrologic Assessment:Field Mapping Lab Sheet
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7-8
References 6
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INTRODUCTION
The hydrologic characteristics of a vernal pond on the Richard Stockton College campus
were assessed. At five localities within the basin, soil logs and measurements of the water
table and hydraulic head were made. The Darcyan velocity and groundwater flow velocity
were calculated and a water table map of the site was also constructed. The soil evidencesuggests the source of water forming the vernal pond is from surface runoff rather than
groundwater emergence.
The vernal pond was located south of the Richard Stockton Police Station and north of
Moss Mill Stream. The topography of the vernal pond was a concave, five foot deep basin. It
lied within Oak-Pine uplands and a ring of Red Maple trees and tall dense shrubs
surrounded the basin. The ground cover was a thick layer of dried sphagnum moss.
METHODS
The hydrology of the vernal pond was assessed on site by four different teams at eachcardinal point and in the basin center. The distances between stations to the center point
were recorded by surveyors. Each survey group used a soil auger to drill to the water table
while analyzing the soil types at the depths of every withdrawal. The soils were
characterized generally by color, texture, thickness and moisture content. Once the water
table was visualized, the depth to the water was measured in feet.
Raw elevations were measured at each assessment point with optical survey equipment.
The center of the vernal pond was selected as the universal datum point with an elevation
of 5.35 feet. The adjusted surface elevations at each position were then calculated by
subtracting the raw elevation values from the universal datum point. The hydraulic heads,
or water table elevations, were also calculated by subtracting the depths to water from theadjusted surface elevations for each point.
A water table map was then constructed with the collective data of water table elevations.
Contour lines were first drawn between the center and adjacent cardinal points. Then,
contour intervals of the next lowest tenth unit were added between existing elevation
points. Finally, construction lines were drawn between corresponding values to
demonstrate groundwater flow.
The Darcyan velocity of the vernal pond was calculated by multiplying the hydraulic
gradient by the hydraulic conductivity constant. The hydraulic gradient was determined by
dividing the difference of highest and lowest water table elevations by the distancebetween them. The hydraulic conductivity constant used was 0.01 cm/second which is a
typical value for the Kirkwood-Cohancey Aquifer. The groundwater flow velocity was then
calculated by dividing the Darcyan velocity by a typical porosity of 30%.
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RESULTS
The soil types of each marginal station were evaluated during the process of drilling to the
water table. The center of the basin had an initial soil layer of approximately 1.5 feet of
gravely, clayey sand, rich in organic matter. The next layer was 0.5 feet of medium brown
sand. The final soil layer was a light gray, gravely, clayey sand.
The soil qualities of the East surveying point are documented in detail. The following chart
demonstrates the soil characteristics by depth starting from the surface layer. The
increments are of each auger withdrawal consisting of approximately five inches each and
do not describe soil horizon layers.
Color Texture Other Soil Properties
Black Sandy loam Rich organic matter
Dark Gray More sandy and clayey
Dark Gray Clayey sand
Dark Gray Clayey sandBrown Sandy clay
Light Brown Clayey Moist
Orange Clayey sand Coarse grained
Orange Sand Gravely, high moisture
content, iron rich
Yellow Sandy mud Gravely, very wet
Data collected from each marginal station is pooled into the table below. It displays the
calculations in feet of the surface and water table elevations as well as the depth to the
water table. The water table elevation values, or hydraulic heads, were used to construct
the water table map presented on following page.
Site Name Surface Elevation Water Table Depth Water Table Elev.
Center 0 2.10 -2.10
North 1.39 3.75 -2.36
South 0.55 2.80 -2.25
West 0.39 2.65 -2.26
East 1.73 3.75 -2.02
The Darcyan velocity was found by calculating the hydraulic gradient and multiplying it by
the hydraulic conductivity. The station with the highest water table elevation was the East site with -2.02 feet. The North site had the lowest, or deepest, water elevation of -2.36 feet.
The difference between the aforementioned water table elevations was 0.34 feet and was
then divided by the length between the two stations which was 41.87 feet. The length was
calculated using the Pythagorean Theorem and the measured distances of the stations from
the center of the basin. The hydraulic gradient therefore equaled 0.008 and was multiplied
by the constant hydraulic conductivity value of 0.01 cm/second. The Darcyan velocity
consequently equaled 0.00008 cm/second. The groundwater flow velocity calculated was
0.00027 cm/second.
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CONCLUSION
The hydraulic assessment of the vernal pond yielded data that was critical in determining
the direction of groundwater flow and the source of the wetland ’s water. Data collected
from the wells drilled at five survey points encompassing the vernal pond included soil
analyses, the water table depth and hydraulic head of the site.
Probably, the source of water that forms the vernal pond is from surface runoff and not
ground water emergence. When the storage and infiltration capacity of the soils in the
surrounding uplands is exceeded, surface runoff occurs. The excess water then flows from
surfaces of highest elevation to the basin which has the lowest elevation. Surface runoff
also tends to erode small particles of sediment, such as clay, that accumulate within the
basin.
Clay layers were documented at each survey point within the vernal pond. Clay tends to
obstruct ground water infiltration and holds the water within the basin for some time.
Although clay has relatively high porosity of 50%, the small grain and pore sizes make it difficult for water to pass through. It is also less likely that water would pass through the
clay layer from the bottom due to gravitational forces. The only expected way for
groundwater emergence to bypass the clay is if the water table was higher than the clay
layer.
The hydraulic heads that were calculated at each point made it possible to investigate the
spatial characteristics of the watertable. The direction of groundwater flow was
determined by analyzing the hydraulic heads in the watertable map. These measurements
were used to find flow direction instead of depth because watertables tend to mimic
topography. The contour lines of the map represent the hydraulic head flowing northwest,
from the highest to lowest elevations. The figures below represent the water tableelevations and the vernal pond in cross section.
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