The movement of groundwater and its associated solutes from upland regions was implicated in the degradation of receiving surface water bodies. Current efforts to directly measure this influx of water incorporate manually operated seepage meters which are hindered by severe limitations. A prototype seepage meter was developed by NASA Langley Research Center and Virginia Polytechnic Institute and State University that will allow for the semi-continuous collection and data logging of seepage flux across the sediment water interface. The meter is designed to operate at depths to 40 meters, and alleviate or minimize all disadvantages associated with traditional methods while remaining cost effective. The unit was designed to operate independently for time periods on the order of weeks with adjustable sample sequences depending upon hydrologic conditions. When used in conjunction with commercially available pressure transducers, this seepage meter allows for correlations to be made between groundwater discharge and tidal/sea state conditions in coastal areas. Field data from the Chesapeake Bay and Florida Bay systems are presented

Reay, William G.

Walthall, Harry G.

English

NASA Technical Reports Server

N92-22725
REMOTE SEMI-CONTINUOUS FLOW RATE LOGGING SEEPAGE METER
William G. Reay
Department of Biology
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061
Harry G. Walthall
NASA Langley Research Center
Hampton, Virginia 23665-5225
ABSTRACT
The movement of groundwater and its associated solutes from upland regions has been implicated in
the degradation of receiving surface water bodies. Current efforts to directly measure this influx of water
incorporate manually operated seepage meters which are hindered by severe limitations. A prototype seepage
meter has been developed by NASA Langley Research Center and Virginia Polytechnic Institute and State
University that will allow for the semi-continuous collection and data logging of seepage flux across the sediment-
water interface. The meter is designed to operate at depths to 40 meters, and alleviate or minimize all
disadvantages associated with traditional methods while remaining cost effective. The unit was designed to
operate independently for time periods on the order of weeks with adjustable sample sequences depending upon
hydrologic conditions. When used in conjunction with commercially available pressure transducers, this seepage
meter allows for correlations to be made between groundwater discharge and tidal/sea state conditions in coastal
areas. Field data from the Chesapeake Bay and Florida Bay systems are presented.
INTRODUCTION
Considerable research has been devoted to defining material input into inland and coastal waters.
Advective transport of water and associated solutes across the sediment-water interface has been shown to be
of significant importance in lacustrine, estuarine and marine environments [1,2,3,4,5 i. Mechanisms responsible
for advective transport of solutes within nearshore sediments include: 1) elevated upland hydraulic head (i.e.,
groundwater discharge) [51, 2) convective flows caused by thermal and salinity density differences [6,7,8], 3)
sedimentation [9], 4) spatial variations in sea state (i.e., subtidal pumping) [10], 5) benthic boundary currents [111
and 6) bioadvection. In addition to influencing water quality management efforts, such transport mechanisms
are of biological and geological importance.
Macroscopic seepage rates of water across the sediment-water interface typically range from 0.0 to 5.0
Lm'2hr d. Hydraulic head differences between overlying surface waters and interstitial water is on the order of
millimeters or centimeters for most nearshore environments [12]. Given such low and varied flow rates and
hydraulic head differences, current flow meter technology is inadequate and the measurement of such phenomena
remains a technical problem.
Conventional methods to directly measure water exchange across the sediment-water interface are
limited to manually operated seepage meters [3]. In its simplest form, seepage meters consist of a cylinder
covered by a vented lid that allows a water collection bladder to be attached. In principle, seepage meters are
placed into the sediment and water entering the meter displaces water into the collection bladder. Discharge
is determined by the volume of water displaced per cross sectional area of seepage meter per unit time.
Discharge rates are corrected for anomalous short-term influx of water to the collection bladders [13]. Primary
disadvantages of current meters include: 1) limited time series data, 2) extensive work effort required, 3) limited
to safe diving depths, and 4) anomalous short-term water influx to collection bladders. The remotely operated
seepage meter presented here, Sea Seep I, was designed to alleviate or minimize all disadvantages associated with
traditional methods while remaining cost effective.
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https://ntrs.nasa.gov/search.jsp?R=19920013482 2020-03-17T11:40:45+00:00Z
DESIGN AND CONSTRUCTION
The prototype seepage meter is a remotely operating self contained system consisting of seven major
components, these are: 1) a magnetically controlled proximity switch, 2) motor driven system operating valve,
3) seepage collection bladder, 4) discharge adjustable metering pump, 5) data logger, 5) rechargeable battery
powered electrical system, and 7) dual chamber seepage meter housing (Figures 1 and 2). Total weight of the
seepage meter and components is approximately 50 kilograms.
System Operation
The main body of the prototype seepage meter is a cylinder made of 16 gauge stainless steel with dual
compartments having a cross sectional area of 0.25 m 2. Batteries, system operating electrical circuit, data logger,
metering pump, motorized valve and vented bladder isolation chamber are housed in the water proof upper
compartment of the meter base. Compartment reenforcement allows the device to be used at depths to 40
meters. A ceramic magnet attached to the collection bladder decreases in proximity to the reed switch as the
bladder fills with displaced water from the main body. When the reed switch is closed by the magnet, an
electrical circuit initiates to sequentially rotate the 3-way valve from the sampling position to the input of the
metering pump. The pump discharges water from the collection bladder until the proximity of the magnet
increases and opens the reed switch. Subsequently, the pump is deactivated, data is logged, the motor resets the
valve to the sampling position, and the system shuts down to conserve electrical power. The valve operation,
pump out and data logging cycle interrupts sampling for only 15 seconds. Depending upon hydrologic conditions
and data requirements, pump out volumes can be modified over a wide range by varying bladder size, proximity
switch adjustment and pump cycle timer. The seepage meter is initiated and deactivated by an external magnet
and internal magnetic switch.
l!d_
._ry I-
..."_'.-" 'L.
_" "='
Figure 1. Photograph of remote semi-continuous flow rate logging seepage meter.
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Pump
discharge-_
. /,- Bagchamberopento sea water
_-Inlet _-Sampling bag
I. 22.25 -
I
T
11l
Figure 2. Schematic drawing and major components and overall dimensions of remote semi-continuous flow
rate logging seepage meter.
_24V
-_.-
T
m
_o_%_o,o,
Computer
power
RYLSPDT
"DPDTCO
o(
I
$1 - Power switch (Reed)
S2 - Proximity switch (Reed)
* - Cam operated with valve
3-way
valve
motor
e,
RYLDPDT
Computer
cycle
switch
Figure 3. Electrical schematic of major circuits of remote semi-continuous flow rate logging seepage meter.
447
RESULTS
Calibration Tests
Due to the flow resistance caused by the components inherent to seepage meter designs and alteration
of water flow paths induced by meter placement, experiments were conducted to compare actual versus measured
discharge rates. Efficiency experiments were conducted in a constant head flow tank (1.0 m2 cross sectional
area) filled with a well-sorted free sand. The constant head flow tank was allowed to equilibrate to a specific
discharge for I hour and calibration experiments were conducted for a 2 hour period. The seepage meter was
reinstalled for each individual test. Measured discharges (Q') were determined by equation (1):
Where:
Q'= (C,*V)/(A*T)
O' = Measured discharge (ML'2T "1)
Cc = Cumulative counts
V = Pump out sequence volume (L3)
A Area enclosed by seepage meter (L2)
T = Time period (T)
(1)
The ratio of Q':Q, where Q is the actual discharge rate, was compared to determine seepage meter efficiency
and accuracy [14]. Experimental results comparing seepage meter efficiency under varying discharge rates are
given in Table 1. Mean efficiency of standard manual meter design has been shown to be approximately 60
percent [14]. Following initial installation, a specific time period is required to equilibrate hydraulic pressures
between ambient surface water and water enclosed within the meter system, however this is generally on the
order of minutes.
Table 1. Seepage meter efficiency under varying discharge rates.
Actual Discharge
Rate
(Lm-2hr-1)
Measured Discharge
Rate
(Lm-2hr-1)
Percent Mean
Efficiency
6.00 1.26 21
10.32 262.52
Even though the seepage meter displays a slight negative buoyancy, settling of the meter into the
sediment may occur resulting in an increase of pressure inside the meter and displacement of water into the
collection bladder (i.e., 1 mm of settling would result in a 250 ml displacement of water). The effect of settling
was determined by installing the meter into the sediment under no-flow conditions. Results for the well sorted
sand indicated no significant settling effect. However, caution should be exercised in high porosity, low dry bulk
density unconsolidated sediments (i.e, silt-clay mixes).
Field Application
Sea Seep I was field tested at two locations varying in sediment type and seepage discharge. The first
site was located in a tidal creek on Virginia's Eastern Shore and characterized by a tidal range of approximately
1.0 meter. Nearshore surficial (upper 20 cm) quartz sandy sediments were conducive for water transport,
exhibiting a mean porosity and vertical hydraulic conductivity of 0.45 and 10 -2.4 cmsec .1 , respectively. The
second site was located in the nearshore zone of Florida Bay which exhibits a tidal range on the order of 0.10
meters. Vertical hydraulic conductivity and porosity of surficial carbonate sediments were on the order of 10-2.0
cmsec "1 and 0.40 respectively. Field test results for the Virginia Eastern Shore and Florida Bay sites are
presented in Figures 4 and 5, respectively.
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260
Cumulative Counts Elevation (ore)
] 140
r%t,, o
/ ,...,,,,,,.o,./ ,/1,oo
% 80
- 60
4O
2O
2OO
180
100
5O
0 0
0 8 10 15 20
Time (hr)
--'- Counts _ Tide
Figure 4. Graphic representation of Sea Seep I field test of tidal creek site in Chesapeake Bay.
Dlsohmrge Lm-2hr-1 Elevation (ore)
10 18
8
6
4
2
14
13
12
11
0 - ' i , i 10
10 12 14 1e 18 20 22 24 2B 28 30 32 34 3B
Time (hr)
--'-- Dleoherge --e- Tide
Figure 5. Graphic representation of Sea Seep ! field test of nearshore site in Florida Bay.
449
Discharge (i.e., counts) rates show an inverse relation to tidal elevation at the Chesapeake Bay site.
During high tides, water levels within the tidal creek caused hydraulic gradients between a point 0.7 meters below
the sediment-water interface and surface waters to approach and equal 0.00, thereby effectively shutting down
submarine groundwater discharge. Hydraulic gradient (dh/dl) is the change in hydraulic head per unit distance
(!), where hydraulic head (h) describes the total energy in a moving mass of water at a particular point.
Conversely, as the tide recedes, vertical hydraulic gradients (up to 0.19 at ebb tide) and discharge rates increase
in concert. The remotely controlled seepage meter allowed for the collection of 246 discharge data points as
compared to the 4-10 time integrated data points that would have normally been collected using manual methods.
As with the Chesapeake Bay site, the Florida Bay study site displayed a strong correlation with tidal stage (Fig.
5). It should be noted that the data set for Florida Bay was summarized and consists of 1166 data points
collected over a 22 hour time period.
SUMMARY AND DISCUSSION
The reported remotely operated semi-continuous flow rate data logging seepage meter was developed
to, 1) operate over extended periods of time without the need for recurrent monitoring, 2) measure relatively
low flow rates over short periods of time, 3) log flow rate data in addition to tidal and sea conditions, and 4) to
remain cost effective. To accomplish this, the design employed a modification of the collapsible bag method used
in manual seepage meters and commercially available components such as rechargeable batteries, electrical valves
and pumping systems, scientific data loggers, and pressure transducers.
Development of this prototype remote seepage meter was driven by an environmental concern for inland
and coastal water bodies. The data presented for the Chesapeake Bay and Florida Bay region exempfifies the
geographic universality and potential importance of the phenomena, submarine groundwater discharge. Time
series data allows for comparison with hydrologic conditions such as tidal influence on discharge rates.
Current uses for this instrument are generally limited to research activities and water quality
management applications. This basic concept in flow meter technology does provide an alternative to
applications that address extremely low flow rates in conjunction with low pressures. When used in conjunction
with proper hardware material, potential industrial uses of this flow meter concept may include flow rate
measurements of caustic and corrosive fluids. A second remote seepage meter has been designed with
modifications that increase meter efficiency and sensitivity, and allow for deeper water deployment.
We are proceeding to protect this technology with a patent.
ACKNOWLEDGEMENTS
Numerous friends and colleagues made significant contributions throughout the development, calibration
and field testing phases of Sea Seep I. This project could not have been possible without the guidance and high
morale of Dr. George Simmons, Jr. of VPI&SU. The authors would like to thank Dr. Daniel Gallagher of
VPI&SU and Gary Clow of the U.S. Geological Survey for providing their expertise with regards to the data
storage and access aspects of this project. Fabrication was performed at NASA Langley Research Center with
the technicial expertise and assistance of Gene Fullcrton, Noland Liddle and Earl Knight of the Metals
Technology Development Section, Mark Simonton and Ellis Hogge of the Advanced Machining Development
Section, Dennis Noe and James Bell of the Electronics Fabrication Development Section. We also thank John
Samos, Joseph Mathis and Dr. Franklin Farmer at NASA Langley's Technology Utilization and Applications
Office for continued support throughout the development phase of this project. We also gratefully acknowledge
Dr. Alan Hulbert and Dr. Steven Miller of NOAA's Undersea Research Center/UNC-Wilmington, for their
support during initial field testing. We are grateful to Jake Waller, Scott Smedley and Eduardo Miles for support
during field testing, and Karen Reay and Mark Kidd of VPI&SU for assistance during calibration experiments.
A special thanks is given to Hartmut Beck of the Fachhochschule Hamburg, who spent numerous hours
investigating various concepts and low flow rate sensor designs while on internship at VPI&SU. We would also
like to acknowledge Jim SherriU, who provided technical support during the early stages of our endeavor.
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Remote semi-continuous flow rate logging seepage meter

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