Wetland Hydrologic Assessment

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WETLAND HYDROLOGIC ASSESSMENT:

FIELD MAPPING

Kristen Meidt

Timothy Alton

November 3, 2010



2

Table of Contents

Introduction 3

Methods 3

ResultsWatertable map

44a

Conclusion 5-6

AppendixWetland Hydrologic Assessment:Field Mapping Lab Sheet

6

7-8

References 6



3

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%.



4

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.



5

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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Wetland Hydrologic Assessment