Limnol. Oceanogr.. 39(3), 1994, 670-68 I
0 1994, by the American Society of Limnology and Oceanography, Inc.
Design of a seepage meter for measuring groundwater fluxes in the
nonlittoral zones of lakes- Evaluation in a boreal forest lake
D. R. Boyle
Geological
Survey of Canada, 601 Booth St., Ottawa, Ontario
KlA
OE8
Abstract
A lakebed seepage meter for measuring groundwater seepage rates in the nonlittoral zones of lakes was
designed and field tested. Ease of installation and operation, sampling integrity, and year-round stability
in ice-bound lake environments were the main factors governing design.
The design of the meter incorporates a lake-bottom seepage meter connected by a flexible conduit hose
to a sampling station -2 m below lake surface. The operation and design of the meter includes methods
for minimizing and monitoring meter settlement, protecting meter components from trawling fishermen
and nibbling fish (seepage bags), visually monitoring flux levels in seepage bags to determine optimum
time for sampling, and easy detection by a small boat-mounted sonar unit.
Results from a seepage meter survey in a small lake situated in complex glacial stratigraphy show that
the system is effective in measuring very low flux rates and mapping complex inflow and outflow groundwater regimes in lake environments.
Detailed lake studies in glaciated terrains
allow classification of lakes, with regard to
characteristics and relative contributions
of
groundwater and surface water inputs, into the
six main types shown in Fig. 1. Although subtypes can be created within this classification,
Fig. 1 shows the complexity of relationships
that can exist between lakes and surrounding
groundwater regimes. In some cases, lakes cannot be classed as one type year-round; for example, Jaquet’s (1976) studies of the groundwater regime around Snake Lake (Wisconsin)
show that this lake has a hydrology much like
type d (Fig. 1) in late summer, fall, and winter,
but in spring the flow characteristics are represented better by type b. The lake I use to
evaluate a new seepage meter design (Alexander Lake) was, from initial analyses, thought
to be type d or type b. Later detailed seepage
meter analysis showed it to be type b.
Figure 2 schematically depicts two extremes
of limnological regimes and groundwater processes on a continuum of lakes in glaciated
environments.
Although hydrologists generally accept that most groundwater that enters
a lake system does so through the littoral zone,
Acknowledgments
I thank J. Holmes and S. Livingstone for help in carrying
out the field components of this research. R. Forconi contributed to the technical development of the system. L. C.
Chamberlin allowed me to use Alexander Lake as a field
test site. Reviews by D. Lee and two anonymous reviewers
were useful in improving the technical descriptions.
Fig. 2 shows how intermediate and regional
(deep seated) groundwaters may enter the nonlittoral portions of a lake and contribute significantly to inflow.
More detailed studies of both littoral and
nonlittoral groundwater influx into lakes, especially in complex glaciated terrains, are
needed for three reasons. First, in many lake
environments characterized by shallow overburden, intermediate, regional, and structurally controlled groundwater flow may dominate over local flow regimes; second, although
the nonlittoral groundwater seepage flux into
lakes may be less than littoral influx, the nonlittoral groundwaters may have a much different chemical character and, because of their
greater residence time in overburden-bedrock
environments, may contribute a greater chemical loading than their littoral counterparts;
third, both the groundwater flow and chemical
flux patterns in lakes can be quite heterogeneous, and detailed mapping of seepage fluxes
is often required to more fully assess groundwater influence on lakes.
Seepage meter analysis of groundwater inflow into lakes is the most accurate means of
measuring both the magnitude and distribution of groundwater seepage into lake environments. Several seepage meter designs have
been proposed (e.g. Bouwer and Rice 1968;
Zuber 1970; Lee 1977; Carr and Winter 1980;
Cherkauer and McBride 1988), but the most
reliable and cost effective have proven to be
those which use an inverted container with a
670
Groundwater
a
b
Stream
discharge
L
d
671
seepage meter
Stream
discharge
Y
e
4
Surficial
recharge
4
Surficial
Fig. 1. Classification of lake environments into six main classes based on types and relative influences of surface
and groundwater movement. Recharge and discharge are used here to denote surface and groundwater movement into
and out of the lake body.
seepage bag mounted on its top to measure
displacements into the container over a specified area of the lake bottom (Lee 1977; Lee
and Cherry 1978). Most of these designs, however, can be used effectively only in shallowwater environments, such as the littoral zones
of lakes and irrigation channels.
For the deeper nonlittoral portions of lakes,
where seepage fluxes are generally many orders
of magnitude lower than fluxes in nearshore
zones, the following requirements must be met
for a meter design to be effective: the meter
must be easy to install from a small boat and
must require a minimum of technical expertise
for installation and operation; the meter must
have good operational integrity over a long
period of time (often up to a year or more);
settlement of the meter must be equilibrated,
and a method of measuring any settlement after the meter is activated must be included in
the design; the meter must remain stable in
the lake through periods of ice cover; the sam-
pling station must be easily accessed by a diver
in order to monitor displacement levels in the
seepage bag and eventually determine when
the bags should be sampled; and the meter
installation must be easy to locate in the lake
at time of sampling (preferably by a small boatmounted sonar unit).
I describe instrumentation
and methodology for measuring seepage fluxes into nonlittoral portions of lakes, especially in boreal
regions where lakes are ice-bound from 5 to
12 months of the year.
Seepage meter design
The seepage meter shown in Fig. 3 consists
of an inverted, smooth-surfaced, 77-liter plastic pail with a weighted tubular collar around
its base, a hose fitting for 1O-cm flexible tubing
in the middle of its top, and four gas vents
evenly spaced around the perimeter of its top.
The weighted collar is filled with mediumgrained sand and perforated with small holes
672
Boyle
Areas of thick
overburden
Areas of shallow overburden
Evaporation
Leikare
a$
diffusion
TT
---ifStruciurally
controlled
grouzndwator
+
Limnic
+
zones
’
I
-t
‘\
~-----
Hypolimnion
(periodic
circulation)
--
----
Chemocline
Monomolimnion
(stagnatIon
Fig. 2. Schematic representation
of lake regimes in thick and shallow
various influences of groundwater flow on lake environments.
that allow trapped air to escape before emplacement in the lake bottom; thus the collar
has maximum anchoring weight. If required,
more than one collar can be added to the meter. The top of the meter is slightly concave,
and the hose fitting extends into the interior
of the meter, allowing any gases generated in
the lake sediments to escape through the gas
vents rather than into the sampling bag near
the surface. The gas vents have a 0.9 5 -cm polypropylene ball (less dense than water) housed
within a fitting that has a 0.16-cm vent exit
and a concave profile for seating the ball when
the vent is filled with water. When gas accumulates in the vent, the ball becomes temporarily unseated, allowing the gases to escape.
Flexible, corrugated, high-density polyethylene tubing (1 O-cm sump-pump hose) is used
as a conduit for lakebed seepage flow from the
overburden
terrains
in Canada showing
bottom seepage meter to the near-surface sampling station. Tests showed that this tubing has
a hard smooth surface that resists being hooked
by trawling fisherman. Two complete installations were placed in a popular fishing lake
in northern Ontario in 1990 and have not been
reported as snagged over the last three seasons.
The top of the meter is covered with heavyduty aluminum-foil
to improve its identification by a depth sonar unit during emplacement
and later discovery for sampling.
Sampling station design
Details of the sampling station design are
shown in the inset of Fig. 3. The near-surface
end of the meter hose is equipped with a female
tubing reducer capable of taking a 0.95-cm male
barb fitting on the end of a shutoff valve attached to a flux sampling bag.
Groundwater
673
seepage meter
-2.0 m
0 cm corrugated plastic tubing
Smooth 77 liter plastic pail
J” t ‘i,
Groundwater
Fig. 3.
station.
Lake seepage meter design for nonlittoral
flow
zone applicatons.
The seepage flux bag consists of a 5-liter,
5-mil clear plastic bag, its open end sealed with
a hinge clamp. A plastic tube insert capable of
taking the barbed fitting of a two-way shutoff
valve is also sealed into the open end of the
bag with waterproof sealing compound (RTV).
One side of the seepage bag is painted with
white epoxy paint as a background for measuring water levels in the bag.
A lo-liter plastic pail with a 4.5-cm hole in
the bottom and a Styrofoam float collar at-
Insert shows details of near-surface
sampling
tached underneath is situated over the meter
hose, and a retainer ring is attached to the hose
below the reducer fitting to prevent the pail
from floating up over the shutoff valve and
flux bag. This pail, which has an aluminum
pie plate between it and the float collar, acts
both as a protective shroud for the flux bag,
preventing it from being snagged by fishermen,
and as a strong sonar target for locating the
meter at time of sampling. A line with a small
float on one end is attached to the inside bot-
674
Boyle
tom of the pail to verify station depth and
detect possible settlement of the meter into the
lakebed (see below). The station pail has a
hinged lid to prevent minnows from nibbling
on the plastic seepage bag.
Emplacement of seepage meter
A sonar unit is used to measure the water
depth at which the meter is to be placed and
to locate an emplacement site with bottom
composition free of large detritus (rocks, logs
etc.). A length of the meter hose equal to that
depth less 2.5 m is attached with a hose clamp
to the fitting on the top of the seepage meter.
Cutting the meter hose 2.5 m shorter than the
measured depth ensures that the sampling station on the near-surface end of the hose is
below both ice level and the effects of wave
action for most medium-sized lakes. The pail
assembly for the near-surface sampling station, minus the float collar, is then slipped over
the meter hose and the retainer ring is fastened
to the end of the hose.
After the seepage meter components (less
the flux bag) have been assembled, the seepage
meter is lowered into the water and held - l2 m above the lake bottom with the station
depth line. The sonar unit is used to ensure
that the lake bottom below the meter is free
of debris. When a good site for emplacement
has been located, the meter is allowed to freefall to the lake bottom to ensure that it is well
seated into the sediments. For gytja-bottomed
lakes, a fall of 1 m with one weighted collar is
sufficient to seat the meter -25 cm into the
bottom. For sandy-bottomed lakes, a fall of 2
m with two weighted collars is generally required for good seating. The sonar unit can be
used to determine approximate bottom penetrations.
The sampling station is held -2 m below
lake surface, and a skin diver inserts the float
collar, which is notched to its center to fit the
lo-cm meter hose, under the station pail. Installation is done in this manner to ensure good
seating of the meter in the bottom sediments
followed by application of a slight upward force
to negate further settling.
The water depth is recorded by the sonar
unit, and the location of the meter is established on a map of the lake with compass bearings and approximate distances from shore.
Later, the sonar unit is used to locate the meter
in the lake area given by these approximate
bearings and distances.
It is advisable to allow the seepage meters,
once installed, to equilibrate for a few weeks
before the flux bags are attached to the sampling stations. This period allows any air
trapped in the hose-meter system during installation to escape and the meter to adjust to
any rebound forces created during bottom penetration. Because the meter has a net positive
buoyancy after the float collar is attached, there
should be no settlement of the meter after rebound adjustments.
When the meters are ready to be fitted with
flux bags, 1 liter of water, dyed with red food
color, is placed in each bag using a small funnel
attached to the other barbed end of the shutoff
valve. As much air as possible is expelled from
the bag, and the shutoff valve is closed. The
bag is then held with the valve pointing down,
and a waterproof felt marker is used to mark
the dyed water level on the unpainted side of
the bag. A skin diver arms the sampling station
with a seepage bag by diving down to the sampling station of each meter, opening the pail
lid, pushing the station pail down over the
meter hose, fitting the barb end of the shutoff
valve of the seepage bag into the tubing reducer
of the meter hose, turning the shutoff valve on,
and shutting the pail lid.
The small amount of air remaining in the
flux bag after most of the air has been expelled
allows the bag to float upright. The diver can
then descend at prescribed time intervals after
installation to check whether the initial water
levels in the bags have increased or decreased
enough in most of bags to obtain meaningful
flux measurements. The groundwater flux into
lake bottoms below the littoral zone can be
very small (1O-5-1O-4 cm3 m-2 s-l), and a
long integration period is often needed to get
good results (see below).
One of the concerns about the accuracy and
reproducibility
of seepage rates for this and
other similar meter designs is the need to minimize and monitor settlement of the meter after it has been armed with a seepage flux bag.
Significant settlement of a meter will cause
positive water displacements in the seepage
bag, thus producing erroneously high seepage
rates for areas of groundwater inflow and low
Groundwater
seepage rates for areas of groundwater outflow.
Meter settlement with the present system has
been minimized in three ways: the meter is
allowed to freefall for a short distance above
the lake bottom to ensure penetration and seating into the sediment; a float collar is attached
under the station pail after the freefall to produce a slight net positive buoyancy (upward
force), which is not, however, enough to lift
the meter out of the sediments; and meters are
not armed with seepage bags for l-2 months.
Although I believe that the above measures
will negate any further meter settlement, it is
advisable to carry out a second meter measurement to test reproducibility
of results. As
shown in the following example, this process
of initial measurement and data validation can
take up to 2 yr for some lakes. As with all
seepage meters, the key to eliminating the effects of settlement on seepage measurements
is to negate or minimize settlement by technological innovation and then to validate results by repeated measurement.
To evaluate the meter in the test lake described below, I instituted another method of
assessing whether settlement has occurred.
When the system is initially installed, the depth
line is detached from its clip in the interior of
the pail, and the depth from the bottom of the
pail to the water surface is marked on the depth
line with a permanent marker or tape. At the
same time, a datum for correcting water surface elevations when meter depths are rechecked during arming and collecting of the
seepage flux bags is set up as follows. A steel
stake (1.5 m long x 1.5-cm diam), with a round
metal disk welded 15 cm from one end, is
driven 1.2 m into the lakeshore sediments of
a sheltered portion of the lake in water - 1 m
deep. A 2.5-cm-diameter steel pipe is slipped
over the disk end of the steel stake. The disk,
which acts as a datum platform, is located - 30
cm from the lake bottom, below ice level. When
the seepage meter is first installed, the depth
of the water at the datum stake is measured
and recorded with a calibrated 2.5-cm-diameter PVC pipe which fits over the stake and
rests on the disk surface. Each time the meters
are armed with the seepage bags and each time
the seepage bags are collected, the datum water
depth is measured, and any differences in water
surface elevations are applied to the meter
seepage meter
675
depth line measurement taken at the seepage
meter before calculating settlement depth. The
establishment of a lake level datum, initial
marking of the seepage station depths on the
station depth line, and final measurement of
settling below lake level datum at time of sampling of the seepage bags should be done on a
calm day to ensure accuracy. The method is
considered accurate to within + 2 cm for smallmedium-sized lakes (determined by repeated
measurements over a 2-d period by four different workers). This method is preferred to
the use of commercial lake level recorders because level recorders have to be removed before winter and reestablished after spring
breakup, thus causing considerable error in
measuring true datum levels.
Eficiency factors
Lee (1977), Erickson (198 I), and Cherkauer
and McBride (1988) have all noted a drop in
the ability of their meters to measure true seepage fluxes when tested in laboratory tank studies. This drop in efficiency (which can be between 15 and 40%) is largely a factor of design
features of the meter, which cause resistance
to fluid flow, and the lack of proper equilibration time of the meter in the lake sediments
before arming with a seepage bag. Lee (1977)
has also noted that silt sedimentation,
disturbed during installation
of the meter, may
also cause reductions in efficiency. For the littoral meter installations used here, an average
correction of 1.50 taken from Erickson’s tank
tests has been applied to seepage measurements, since the meter design closely approximates that used by Lee.
The very low flux rates the nonlittoral seepage meter has been designed to measure make
proper tank experiments to determine an efficiency factor virtually impossible. However,
the long equilibration time before arming with
a seepage bag (-2 months) and the use of a
premeasured amount of water in the bag, shown
by Erickson (198 1) to increase efficiency, suggest an efficiency on the order of 85-95%. In
the absence of a measured efficiency factor for
the nonlittoral meter system, the seepage fluxes obtained should be considered minimum
values. When littoral and nonlittoral
seepage
rates are compared, the lack of application of
an efficiency correction factor to the latter will,
676
Boyle
\i,2
ONTARIO
i
0- LlbmFlerS
-22w
160
-
32
N5
6
2
100
-
TOTAL
-----
SNOW
-
IE
b
kl
Fig. 4.
however,
tation.
Location
of Alexander
60
-
Lake.
have little or no effect on interpre-
Evaluation of the seepage meter
Field site-The
site chosen to carry out the
initial evaluations of the system (Alexander
Lake) is a small lake situated on thick glacial
drift in the boreal forest region of northern
Ontario (Fig. 4). Alexander Lake was chosen
for several reasons. There is no visible surface
recharge to the lake or surface discharge from
it (types b or d; Fig. l), and the lake is small
enough (0.25 km2) to allow easy assessment
with a small number of seepage meters (lo15). The lake is located in complex sandy esker, lacustrine clay, and glacial till overburden,
which would possibly give rise to a complexity
of littoral and nonlittoral
groundwater flow
characteristics. The Secchi depth of the lake
(8.80 m) is deep enough to see the near-surface
sampling stations and thus test the effectiveness of various designs for detection by sonar.
The lake bottom is sandy to organic-rich and
largely free of coarse debris. The lake is a fishstocking sanctuary and therefore free from human activity, although, as described above,
one of the objectives of the study was to develop a design that was largely ‘Yishermanproof.” The lake is warm enough in summer
to allow skin diving with wet suits. Good supporting information
such as precipitation records (Fig. 5) and bathymetry (Fig. 6a) are available, and the lake is easily accessible by road.
Physiography and geology-Alexander
Lake
is situated in thick (30-l 00 m) glacial overburden largely of Wisconsin or younger age.
The east side of the lake is immediately ad-
1
1990
1
I
1991
___-
1992
I
MONTH
Fig. 5. Plot of average monthly precipitation
for the
Alexander Lake region from September 1990 to September 1992. Station is 20 km north of the lake. (Data from
climate office of Ontario Department of the Environment.)
jacent to the Watebeag esker system comprised
mainly of outwash fans and horizontally bedded sands and pebble gravels (Richard and
McClenaghan 1984). Surface deposits on the
west side of the lake consist mainly of glaciofluvial
silty-fine to coarse-grained sands.
Although stratigraphic information from drilling is not available for the immediate area of
the lake, drilling just to the north of the area
strongly suggests that the fluvial sediments described above are underlain by laminated to
varved silts and clays of the Barlow-Ojibway
Formation, the most extensive surficial unit in
the Watebeag area (McClenaghan 1992). These
lacustrine sediments may in turn be underlain
by the Matheson Till, the youngest till sheet
in the area (Hughes 1965).
The main axis of the lake parallels the principal structural trend of the area, characterized
by NNW-trending
major fault and shear zones
and numerous mafic intrusive dykes (Pyke
1976). Many of the lakes in this region parallel
or are situated on these structures; it can be
surmised, therefore, that some receive input
from groundwaters moving along these structures up into the overlying overburden and
lake bottoms.
The bathymetry for the lake is shown in Fig.
6a. Two basins are separated by an east-west-
Groundwater
677
seepage meter
b
1,
J
0
-
Intermittent seepage. . . . / - - - ’
Flat slope. . . . . . . . . . . . . . . . . A
<30%slope
..............4
30 - 60 % slope . . . . . . . . . . . . A
Bathymetry contour (m) . . . -4~
Short term seepage (m) . . . . . . . l
Seepage meter site . . . . . . . . . . l
Seepage meter flux
(cm 3 m -2 s-l ) x 104 . . . . . 1+O.W
Seepage flux contour
(cm3 mm2 5-l) x 104. . . .rw.u
A
Stagnation zone . . . . . . . . . ///I//////
9
Fig. 6. Bathymetry (a) and contoured seepage flux data (b) for Alexander Lake. Arrows around panel b refer to
land slopes. Seepage contours are for nonlittoral groundwater flow only (199 l-l 992 measurements). Negative seepage
flux denotes groundwater movement into lakebed. Cross-section for schematic in Fig. 7 shown as line AB in panel b.
trending ridge, the west end of which has a
small island. The north basin is about twice
as deep as the south basin. The shelf areas
surrounding the south basin are much wider
and more gentle in slope than those around
the north basin. From examination of the bathymetry, topography and regional surface flow
characteristics, I suspected that the northern
half of the lake was an area of groundwater
recharge and the southern half an area of dis-
charge. Studies with the meter described here
confirmed this (see below), but the observed
groundwater flow characteristics were more
complex than anticipated.
Two small intermittent
seepage channels at
the northeast side of the lake (Fig. 6) are evident from topography. During the two summers of study, both of these channels were dry,
indicating that they are probably meltways for
spring runoff.
678
Boyle
Climate-Alexander
Lake is in a temperate
climatic zone; precipitation
falls for -6-7
months as rain and 5-6 months as snow. The
1990-l 992 precipitation
record for the Timmins area (- 80 km to the northeast of the lake)
is shown in Fig. 5. Precipitation levels are usually greatest in spring (April-June)
and fall
(September-October)
months. Both the levels
and the patterns of precipitation can, however,
vary from year to year as can be seen by a
comparison of 1990 and 199 1 data in Fig. 5.
During the period when the seepage meters in
the nonlittoral
portions of the lake were installed (August 1990-August 1991) the total
precipitation for the area (as equivalent water)
was 102 cm.
Lakes in this region generally freeze over in
late October-early November, and breakup is
normally in late April-early
May. Ice thicknesses are generally on the order of 50 cm.
Seepage flux-The
calculated groundwater
seepage flux or macroscopic seepage velocity
is a function of seepage meter design and the
integrated time over which the meter is armed
with a seepage bag. It can be represented by
the equation:
Seepage flux (cm3 m-2 s-l)
of water displaced=I[Vol.to/from
bag (cm’)
L
Area of meter (m2)
I/
time (s).
(1)
A negative seepage flux would indicate
groundwater discharge through the lakebed.
Equation 1 can also be expressed in units of
velocity (cm3 m-2 s-l = pm s-l = 8.64 cm
d- l), generally termed macroscopic seepage
velocity (MSV) (Lee 1977). MSV is equal to
the average sediment linear interstitial velocity
times the sediment porosity expressed as a
fraction.
In the nonlittoral portion of lakes, seepage
rates are generally co.00 1 cm3 m-2 s-l. Here,
I use the cm3 m-2 s-l notation to quantify
groundwater seepage through lakebeds; to make
an easier comparison between rates in both the
littoral and nonlittoral zones, I express all rates
to the power of 10e4.
is generally accepted that
Littoral zone--It
the majority of groundwater enters most lakes
through the littoral zone and lesser amounts
enter through the deeper nonlittoral zones. Exceptions, however, may occur where there are
strong point sources (e.g. springs) of groundwater influx in the nonlittoral portions of lakes
or where overburden stratigraphy (e.g. clays
over sands) controls groundwater
flow. As
mentioned earlier, the chemical load to a lake
through groundwater inflow may be greater in
magnitude for deeper nonlittoral groundwater
input than for nearshore influxes.
Groundwater seepage fluxes into the littoral
zone of Alexander Lake were compared with
fluxes for the rest of the nonlittoral portion of
the lake by installing six seepage meters similar
in design to the meter described by Lee ( 1977)
in the northern and southern littoral zones of
the lake (Fig. 6). The meters were installed for
1 week each in June, July, and August 1990;
results are shown in Table 1. Seepage measurements in the littoral zone confirmed that
the northern littoral zones of the lake are areas
of groundwater influx and the southwestern
littoral zones are regions of groundwater discharge. The line of meters (A, B, C) installed
in the northern littoral zone from 8 to 50 m
from shore shows an almost threefold drop in
seepage flux between meters A and B and a
twofold drop between B and C. This pattern
has also been observed at other lakes (McBride
and Pfannkuch 1975; Lee et al. 1980; Erickson
198 1; Cherkauer and Zager 1989). Although
this pattern of seepage flux in the littoral zones
is probably typical of most lakes, reversals (i.e.
increased flux with distance from shore) have
been noted (Woessner and Sullivan 1984).
The monthly magnitudes of both the littoral
recharge and discharge seepage fluxes in 1990
correlate well with the monthly precipitation
loadings (Table 1 and Fig. 5). In the nonlittoral
zone, the seepage rate drops by an average of
20% over the two periods of measurement
(September 1990-September
199 1 and September 199 l-September
1992), which compares very well with an average decrease in
precipitation loading of 25% between these two
periods.
Nonlittoral zone-Fifteen
deep-water seepage meters (Fig. 6; Table 2) were installed in
the nonlittoral portions of Alexander Lake in
June 1990 and armed with seepage bags in
August of that year. At the time of installation
and again at the time of arming with the seepage bags 2 months later, the depths of the sam-
Groundwater
679
seepage meter
Table 1. Seepage fluxes for meters placed in the littoral zone of Alexander Lake during June, July, and August
1990. Each meter was armed five times with a seepage bag for 24 h over a 1-week period in each month.
Secpagc
meter
Depth
(m)
Distance from
shoreline (m)
A
B
C
D
E
F
1.0
2.2
3.5
1.5
1.8
1.8
8
20
50
10
7
8
Seepageflux* k SD
(cm3 m-2 s-l X 10--4)
Jul
Jun
AW
+520+40
+115+15
+12&4
+640*56
-280+24
-35Ok38
+ 1,175+ 180
+460+74
+26+6
+ 1,360+: 115
-575k68
-925k87
$725485
+220+28
+18+-4
+890+-95
-330+-28
-460+-32
* The arca for these meters was 0.256 mz.
pling stations below lake level were recorded
and the data adjusted with the nearshore datum stake method described above. For all
meters except 2, 3, and 11, there was no discernible settlement of the meters between the
time of installation
and the time of arming
with seepage bags. Meters 2, 3, and 11 showed
settlements of between 1.5 and 2.0 cm-at or
just below the magnitude of possible error
(+2.0 cm). When all of the meters were sampled 12 months after arming with seepage bags,
station depth measurements were well within
the estimated datum correction error, indicating that both the physical emplacement methods and the time delay before arming (-2
months) are important steps for ensuring relatively error-free measurements of seepage flux.
Comparison of seepage fluxes into the littoral zones with those observed for the nonlittoral portions of the lake (Tables 1 and 2)
shows the former to be one to two orders of
magnitude greater than the latter, depending
on distance of the littoral meter from shore.
A contour map of seepage fluxes in the nonlittoral zone of the lake is shown in Fig. 6b.
As stated earlier, the northeastern littoral zone
of this lake (represented by data for meters A,
B, and C, Table 1) is a major area of groundwater inflow. Downslope from this zone, however, meters 9 and 10 show no seepage flux.
Along the meter line A-B-C-10, the seepage
flux drops sharply from 500 to 1,200 cm3 rnd2
s-l down to 0. Within the northern basin of
the lake, the seepage flux contours increase in
a similar fashion to the depth contours, and
maximum seepage is in the deepest portion of
the basin (meter 8). The area between the basin
area of influx and the littoral inflow zone is
interpreted in Fig. 6b as a groundwater stagnation zone. Such zones have been noted in
other lake environments (stagnation point of
Winter 1976; transition zone of Karnauskas
and Anderson 1978; hinge line of Cherkauer
and Zager 1989) and may either reflect the
presence of a junction zone between inflowing
and outflowing groundwaters or a significant
change in overburden stratigraphy, especially
with regards to hydraulic conductivities.
Since
both the littoral and nonlittoral zones in the
northern half of this lake are regions of groundwater inflow, the inferred stagnation zone
probably represents a bed of the Barlow-Ojibway Clay Formation under the shoreline glaciofluvial sediments. The deep northern basin
would appear to penetrate lower basal tills unTable 2. Lakebed seepage fluxes for meters installed
in the nonlittoral portion of Alexander Lake. Eight meters
outside stock netting areas in the lake wcrc rearmed with
secpagc bags and resampled in 1992.
Seepage Depth
met&
(m)
1
2a
2b
3
4
5
6
7
8a
8b
9
10a
lob
11
12
13
14
15
8.2
11.0
11.0
19.2
4.8
3.0
7.8
3.8
21.2
21.2
3.5
9.5
9.5
6.0
6.2
3.2
8.4
14.0
Volume (cm’) displacement
to/from seepagebag
90-9 1
-100
-200
-150
+590
+505
0
-420
+160
+1,105
+1,310
0
0
25
75
+200
0
+55
+115
91-92
Seepageflux
(cm’m -* s-l x 10--4)
90-9 1
-135
-120
+465
+425
-375
0
0
+85
-0.19
-0.39
-0.29
+1.07
t-o.97
0.00
-0.8 1
+0.31
+2.13
+2.54
0.00
0.00
0.05
0.15
+0.39
0.00
+0.10
+0.22
91-92
-0.25
-0.23
+0.88
+0.82
-0.73
0.00
0.00
+0.16
* Meters a and b represent duplicate sampling sites with meters placed -3
m apart.
Boyle
A
6
6
‘bob
b
b
Basal tills
Flow from dasement (?)
Basement
Fig. 7. Schematic cross-section of northern basin in Alexander Lake showing possible stratigraphic
flow conditions that may give rise to the lakebed stagnation zone outlined in Fig. 6b.
derlying the clays. This stratigraphic sequence
is common in this region (McClenaghan 1992).
A schematic cross-sectional diagram across
the northern basin is shown in Fig. 7; it depicts
shallow surface groundwaters flowing into the
nearshore zone of the lake, with deeper
groundwater flowing up into the basin controlled by basal tills and overlying clays. The
basin can be thought of as a cone penetrating
sediments that have vertical hydraulic gradients which increase with depth. Artesian flow
conditions in the basal till units of this area
are well known to those carrying out drilling
operations in the region. The available data do
not seem to indicate the presence of a stagnation zone on the western side of the northern
basin.
Both the littoral and nonlittoral meter data
confirm earlier theories that the southern portion of the lake is a region of groundwater outflow through the lakebed. Two of the meters
in this region (5 and 13) show zero flux, and
meter 12 has a small positive flux. It is not
and groundwater
clear whether some of the groundwater influx
in the northern basin moves through the ridge
sediments between the two basins and then
down through the south basin sediments, or
the northern groundwaters are generated entirely in the north basin and move through the
lake water column to exit in the south basin.
Meter 14 shows a small positive flux right up
to the southern edge of the north basin, and it
is most likely that both processes are operative.
The data for the southern half of the lake suggest that a stagnation zone exists between the
two basins (Fig. 6b) similar in origin to the first
explanation of stagnation mentioned above, in
which groundwater inflow potentials cancel
outflow potentials.
Reproducibility-Reproducibility
of results
for the meter design was tested in two ways.
First, duplicate meters were installed -3 m
apart at three sites (2, 8, and 10); second, eight
meters were left in the lake at sites considered
safe from netting operations for fish stocks,
and were rearmed in August 199 1 to be sam-
Groundwater
pled in August 1992. The results of these tests
are presented in Table 2. For the duplicate
sites, the sampling error is < f 15%. All of the
meters resampled a year later showed lower
secpagc rates than those initially measured for
1990-199 1, but the relative differences in
seepage flux remained csscntially the same. The
higher lakebed seepage rates for initial sampling in 1990-l 99 1 probably reflect the effect
of higher groundwater potentials caused by
greater overburden infiltration rates during that
period.
Conclusions
Although methods of quantifying
groundwater inflow into the littoral portions of lakes
are now well established, those for measuring
nonlittoral influxes have, until recently, been
lacking. The nonlittoral meter described here,
which addresses major problems of application (such as settlement, reproducibility,
ease
of use, system stability, and protection from
disturbance), has been shown to be effective
in measuring very low flux rates into lakes and,
when used with littoral meters, in mapping
complex inflow and outflow groundwater regimes within lake environments.
Our knowledge of the influence of groundwaters on water and nutrient budgets of lakes
is still rudimentary.
The nonlittoral
seepage
meter described here should help greatly in
studies of the behavior of groundwaters in lake
environments, especially in boreal and tundra
regions of the northern hemisphere.
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Submitted: 23 December 1992
Accepted: 23 August 1993
Amended: 19 October 1993