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Groundwater attenuation of summer stream temperatures favors deeper intrusion depths into Lake George, NY

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Groundwater inputs to two major streams along the southern end of Lake George attenuate summer temperatures resulting in deeper lake intrusion depths relative to other major streams. Between late April and early October, East and West Brook baseflow water temperatures generally were cooler than other major streams by similar to 4 degrees C in mid-summer. Historical data for West Brook confirmed that the trend occurred as far back as 1970. As a consequence of cooler spring and summer temperatures coupled with higher salinity, deeper lake intrusion from these streams was hypothesized based on density calculations. Warmer streams entered the lake as overflow through late spring while East and West Brook intruded into the lake at depth. Upon stratification, East and West Brook intrude at or below the metalimnion while other monitored streams generally intrude at or above the metalimnion; by mid-August/early September all streams intruded below the metalimnion. Highresolution profiler data identified the presence of underflow during a fall storm event in 2014. Deeper intrusion depths of East and West Brook would supply organics and oxygen to the Caldwell Sub-basin hypolimnion which can potentially have both negative and positive effects on hypolimnetic oxygen depletion.
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Groundwater attenuation of summer stream
temperatures favors deeper intrusion depths into Lake
George, NY
Mark W. Swinton, Lawrence W. Eichler, Sandra A. Nierzwicki-Bauer,
Jeremy L. Farrell, James W. Sutherland, Michael R. Kelly
and Charles W. Boylen
ABSTRACT
Groundwater inputs to two major streams along the southern end of Lake George attenuate summer
temperatures resulting in deeper lake intrusion depths relative to other major streams. Between late
April and early October, East and West Brook baseow water temperatures generally were cooler
than other major streams by 4WC in mid-summer. Historical data for West Brook conrmed that the
trend occurred as far back as 1970. As a consequence of cooler spring and summer temperatures
coupled with higher salinity, deeper lake intrusion from these streams was hypothesized based on
density calculations. Warmer streams entered the lake as overow through late spring while East and
West Brook intruded into the lake at depth. Upon stratication, East and West Brook intrude at or
below the metalimnion while other monitored streams generally intrude at or above the
metalimnion; by mid-August/early September all streams intruded below the metalimnion. High-
resolution proler data identied the presence of underow during a fall storm event in 2014. Deeper
intrusion depths of East and West Brook would supply organics and oxygen to the Caldwell Sub-basin
hypolimnion which can potentially have both negative and positive effects on hypolimnetic oxygen
depletion.
Mark W. Swinton (corresponding author)
Lawrence W. Eichler
Sandra A. Nierzwicki-Bauer
Jeremy L. Farrell
Charles W. Boylen
Darrin Fresh Water Institute,
5060 Lake Shore Drive,
Bolton Landing, NY 12814,
USA
E-mail: swintm@rpi.edu
Sandra A. Nierzwicki-Bauer
Charles W. Boylen
Department of Biological Sciences,
Rensselaer Polytechnic Institute,
110 8th Street,
Troy, NY 12180,
USA
James W. Sutherland (retired)
NYS Department of Environmental Conservation,
625 Broadway,
Albany, NY 12233,
USA
Michael R. Kelly
IBM Research,
1101 Route 134 Kitchawan Road,
Yorktown Heights, NY 10598,
USA
Key words |baseow, groundwater, intrusion depth, Lake George, NY, temperature
INTRODUCTION
Temperature is one of the most important factors to con-
sider when evaluating how streams function because of its
inuence on physical, chemical, and biological processes.
Temperature is negatively related to water density and vis-
cosity, which can impact stream intrusion depth into a
lake (Laborde et al. ;Cortés et al. ), sediment inl-
tration rates (Constantz & Murphy ), and stream
discharge (Constantz et al. ). Warmer water reduces
gas solubility while enhancing biological oxygen demand
(BOD), which can mobilize phosphorus (Liikanen et al.
) and trace metals from sediment in reduced conditions
(Von Gunten et al. ). Since nearly all aquatic organisms
are ectothermic, temperature inuences metabolic rates
similarly throughout trophic levels (Gillooly et al. )
with enzymatic activity approximately doubling with each
10 WC rise in temperature (Black ).
Many streams derive the majority of their discharge
from groundwater and thus the headwater temperatures
are similar to groundwater with water temperature trending
toward air temperature with downstream ow (Sullivan
et al. ). The extent of the temperature change depends
on climate, riparian vegetation, stream morphology, and
groundwater inputs (Sullivan & Adams ). Climate
often is the dominating factor inuencing water temperature
177 © IWA Publishing 2017 Hydrology Research |48.1 |2017
doi: 10.2166/nh.2016.188
with absorption of solar radiation being the primary source
of warming (Morin & Couillard ;Webb & Zhang ;
Evans et al. ;Johnson ). Riparian vegetation can
insulate small streams by shading the water and the adjacent
landscape (Sweeney ;Dong et al. ;Johnson );
however, the insulating effect of shading diminishes as
stream width increases (Poole & Berman ). Stream mor-
phology changes the surface area to volume ratio and can
promote the exchange of stream water with the hyporheic
zone (Brunke & Gonser ;Poole & Berman ;
Webb et al. ). Hyporheic water temperatures generally
are warmer than stream temperatures during the winter
and cooler during the summer because a portion of the
water originates from groundwater, which maintains a con-
sistent year-round temperature that approximates the mean
annual air temperature (Brunke & Gonser ;Hayashi
& Rosenberry ). Streams heavily inuenced by ground-
water inputs exhibit attenuated year-round temperatures
(Holmes ).
Ultimately, it is the combination of these factors that
determine stream temperature, and the temperature differ-
ence between a stream and its receiving waterbody will
dictate the stream intrusion depth (Fischer et al. ;Kill-
worth & Carmack ). When stream inputs are warmer
and less dense than the lake, they intrude as overow.
When stream inputs are cooler and thus denser, they intrude
as underow until a level of neutral buoyancy is reached
(Alavian et al. ). In a well-mixed lake, all inputs will
be mixed regardless of intrusion depth. During periods of
stratication, however, intrusion depth will dictate at what
depth in the water column stream constituents become
available (MacIntyre et al. ;Cortés et al. ).
Intrusion depth in Lake George is of great interest
because it may impact the regular occurrence of hypolim-
netic oxygen depletion in the Caldwell Sub-basin, the
southernmost sub-basin of the lake. The land surrounding
the Caldwell Sub-basin is the most developed area in the
Lake George watershed, based on property tax records
from the eight towns and villages within the Lake George
watershed; with roughly half of the 9,000 total buildings
in the Lake George watershed located in this area. Greater
urbanization coupled with the south to north ow of the
lake has resulted in a consistent phosphorus and chlorophyll
gradient in the lake that decreases as water ows north
(Boylen et al. ). The higher nutrient concentration in
the Caldwell Sub-basin and subsequent increased phyto-
plankton biomass has been indicated as a contributor to
hypolimnetic oxygen depletion (Boylen et al. ). The
greater organic biomass reaching the sediment surface
enhances the BOD, lowering the hypolimnetic oxygen con-
centration during stratication with the lowest oxygen
concentrations observed in October and November just
before fall turnover. Oxygen depletion in Lake George
becomes a concern when dissolved oxygen (DO) concen-
trations fall below 4 mg/L because at 10 WC, which is
roughly the hypolimnetic temperature during early fall,
4 mg/L dissolved oxygen is equivalent to 35% saturation.
Fish sensitive to low dissolved oxygen will avoid such
areas (Arend et al. ) and oxygen concentrations 30%
saturation result in the release of sediment-bound phos-
phorus (Eichler & Boylen ). In addition, for lakes
classied AA-Special in New York State, including Lake
George, at no time shall the DO concentration be less
than 4.0 mg/L(6 CRR-NY 703.3). Minimum hypolimnetic
oxygen concentrations in the Caldwell Sub-basin were
4 mg/L during ve years between 1980 and 2010 and for
three of those years the minimum oxygen concentration
was 4 mg/L.
The goal of this research was to determine: (1) if thermal
regimes vary among the major streams entering Lake
George, and if so, identify the factors that may be primarily
responsible for the observed differences; (2) estimate the
intrusion depth of each stream based on density; and (3)
identify possible impacts of varying intrusion depths to the
hypolimnetic oxygen depletion in Lake George. A better
understanding of intrusion depth, entrainment, and mixing
during stratication has broader implications on nutrient
availability, primary production, and food web dynamics
that could be applied to all waterbodies.
METHODS
Study site
Lake George, located in the southeast portion of the Adiron-
dack Park (NY), is a large (110 km
2
, 2.1 km
3
), long
(51.5 km), oligotrophic lake with a dominantly forested
178 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
watershed (Figure 1;Boylen & Kuliopulos ;Shuster
et al. ). Lake George surface water is maintained by
the LaChute Hydro Company, Inc. at an elevation of
97.5 m. The highest point in the watershed, Black Mountain,
reaches 800 m. Mean annual precipitation (20002008) was
114.7 ±18.8 cm with an average precipitation from May to
October of 64.3 ±35.7 cm (Eichler et al. ). Precipitation
in 2007 and 2008 between May and October was below
average at 48.0 cm and 55.9 cm, respectively. The 141
streams surrounding Lake George (Sutherland et al. )
supply the lake with 57% of its annual hydrologic budget
with precipitation directly on the lake and groundwater dis-
charge in the lake accounting for 25% and 18%, respectively
(Shuster et al. ). The lake has a relatively small water-
shed with a 4.6:1.0 land to lake surface ratio and a
residence time of 5.5 and 6.8 years based on volume and
conserved solutes, respectively (Shuster et al. ).
The lake has chemically distinct South and North basins
that are separated by a shallow sill in the Narrows, 18 km
from the south end. The South Basin, which is the focus of
this study, consists of two sub-basins, Caldwell and Dome
Island. The headwater and most southern sub-basin, Cald-
well, has a maximum depth of 31 m and receives
discharge from East, West, and English Brook (Figure 1).
The Dome Island Sub-basin is the largest sub-basin with a
maximum depth of 60 m (Boylen & Kuliopulos ) and
receives discharge from Finkle, Indian, and Shelving Rock
Brook. These six brooks are a sub-set of the 10 largest tribu-
taries entering Lake George and drain 23% of the entire
watershed (Swinton et al. ). The level of development
ranges from the most impacted in East and West Brook
watersheds to no development in the Shelving Rock Brook
watershed. All the tributaries are relatively narrow with
tree canopy covering the majority of the stream length.
During the last glaciation, the Laurentian Ice sheet
stripped away most of the soil cover from higher elevations
and deposited a greater proportion of sand in the southern
end of the watershed with ner clays deposited in the
North Basin. Upland deposits of sands and gravels are
sporadic and infrequent with varying thickness overlaying
fractured bedrock. Lower elevations have more consistent
sandy tills overlaying fractured bedrock which near lake
Figure 1 |(Left) The Lake George Basin along with streams and the monitored sub-watersheds outlined in black. In-lake sampling locations, Tea and Dome Islands, identied by stars; Tea
Island is the most southern location. Proler location indicated by circle. (Right) The ve major sub-basins of Lake George and their land catchment are identied accompanied
with a side-view of the deepest points along the length of Lake George based on Boylen & Kuliopulos (1981).
179 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
level is overlain by varved silts and clays. Kamic terraces
composed of sandy to gravelly sediments are common in
tributary valleys. Lake George bedrock is a mix of domi-
nantly granitic gneisses, charnockitic gneisses, garnet-
biotite-quartz-plagioclase gneisses, quartzites, metaanthro-
sites and metagabbros with smaller quantities of marble,
calcsilicates, and amphibolites (Shuster et al. ).
The effects of repeated glacial encroachment and retreat
in the Lake George watershed resulted in varying soil types
(Table 1) and thus sub-watersheds exhibit different hydraulic
conductivities. The greater deposition of sand in the south
end resulted in East and West Brook exhibiting the highest
inltration rates while Shelving Rock, the only watershed
monitored on the east side of the lake, is characterized by
steeper slopes and shallow/exposed bedrock limiting inl-
tration rates. The remaining watersheds (English, Finkle,
and Indian) have soils primarily composed of type B and
C soils which exhibit intermediate inltration rates.
Sampling
The six major tributaries of this study were monitored for
temperature and chemical composition between 2007 and
2010. Sampling occurred monthly between December and
March with the sampling rate increasing to 2-week intervals
between April and November. Temperature measurements
were taken during baseow conditions from March to
December using a Raytek Mini Temp IR thermometer,
which was routinely checked against National Institute of
Standards and Technology (NIST) certied thermometers.
Water temperature measurements were taken while either
in the stream or on the stream bank with the angle primarily
vertical. West, English, and Finkle Brook measurements
were taken near the mouth of the streams while East, Shel-
ving Rock, and Indian Brook measurements were taken
within 0.5 km of the lake. Shelving Rock and Indian
Brook were dominantly shaded by riparian vegetation
between the sampling location and the lake; a wetland
was located downstream of the East Brook sampling
location. Raw temperature measurements are presented
along with relative water temperature difference, calculated
as the individual stream temperature minus the average
temperature of all streams sampled on a specic date.
The Offshore Chemical Monitoring Program sampled
mid-lake locations from May to November during 2007
and 2008. Prior to stratication, sampling occurred every 2
weeks with monthly sampling during summer and 2-week
sampling reinstated during fall. The two locations utilized
in the South Basin were Tea Island in the Caldwell Sub-
basin and Dome Island in the Dome Island Sub-basin
(Figure 1). Vertical temperature proles were recorded at
set intervals of 0, 1, 2, 3, 5, 10, 15, 20, 25, 30 m with
additional measurements taken within depth ranges exhibit-
ing large temperature changes. Measurements were
recorded using a YSI temperature probe (various models),
which was routinely checked against NIST thermometers.
Samples were collected for analytical chemistry in the epi-
limnion (010 m; hose-integrated) and hypolimnion (1 m
off the bottom; grab). Sodium, calcium, magnesium, and
potassium were analyzed by atomic absorption spectropho-
tometry using a Perkin Elmer AAnalyst 5000 (Creed et al.
); chloride and sulfate were analyzed by ion chromato-
graphy using a Lachat 8000 QuikChem (Pfaff ).
Alkalinity was measured by Titration Method 2320 B (Cles-
ceri et al. ). Standard quality assurance and quality
control protocols included blanks, duplicate samples,
spikes, and external check standards every ten samples.
Additional description of sampling methods for the stream
and lake monitoring can be found in Swinton et al. ().
Water density was calculated for both the lake and
streams using temperature and salinity (comprising
sodium, chloride, calcium, magnesium, potassium, sulfate,
and bicarbonate). Although both epilimnetic and hypolim-
netic lake samples were taken and their calculated
salinities were within 5% of each other, only the epilimnetic
value was used in the calculation as it was more
Table 1 |Percent of soil types in the monitored Lake George sub-watersheds
Sub-watershed A B C D
East 25.4 48.9 24.0 1.7
West 23.8 49.9 24.0 2.3
English 6.1 58.5 33.4 2.0
Finkle 4.8 41.3 48.3 5.6
Shelving Rock 2.1 37.5 0.2 60.3
Indian 3.9 50.6 42.8 2.7
Data obtained from the Lake George Association and Warren County Soil and Water Con-
servation District. Type A: <10% clay with >90% sand and gravel; B: 1020% clay with 50
90% sand; C: 2040% clay with <50% sand; D: >40% clay with <50% sand (USDA 2009).
180 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
representative of the water column (010 m) than a grab
sample 1 m off the bottom, which could be inuenced by
underow. While bicarbonate was not measured directly
during the 20072008 stream study, it is estimated as being
equivalent to calcium on a weight basis. Based on prelimi-
nary West and Finkle Brook 2015 data from the Jefferson
Project, calcium and alkalinity (reported as calcium carbon-
ate) exhibited a strong correlation (r
2
>0.93, N¼6) during
baseow conditions with calcium comprising 3760% of
the calcium carbonate. Therefore, bicarbonate was esti-
mated as an equivalent mass of calcium. Alkalinity
measurements were taken for the lake sampling locations
and were included in the salinity calculations.
McCutcheon et al. ()dened water density as:
ρs(kg m3)¼ρoþAS þBS3=2þCS2
ρ
o
,A, and Bare based on temperature, Cis a constant,
and Sis salinity (g/kg).
Stream intrusion depth was based on density calculations
for each stream and the corresponding sub-basin, Caldwell or
Dome Island. Isopleths were constructed using SigmaPlot
and because density was interpolated between sampling
dates and within individual proles some static instabilities
may be present. Isopleths contours are depicted using the
oceanographic sigma or density anomaly measurement
which is the water density difference from 1,000 kg/m
3
.
Previous stream projects
To verify the temperature trends observed in this stream study,
previous stream studies that included West or East Brook and
any of the other four streams included in the 20072010 project
were examined for comparison. Fuhs ()sampled a total of
18 streams from July 1970 to July 1971; temperature data
from West, English, Finkle, and Indian Brook are included
here for comparison. The Nationwide Urban Runoff Program
(NURP) focused on streams having different levels of water-
shed development at the south end of the lake from July 1980
to June 1982, and therefore only includes West and English
Brook (Sutherland et al. ). Sutherland (unpublished data)
sampled from August 2002 through November 2005 on
West, East, English, Finkle, and Indian Brook.
High-resolution prole data
A YSI 6950 vertical proler equipped with EXO 2 sonde
capable of recording temperature and chloride proles at
1-m resolution every 90 minutes was deployed near the
deepest location (53 m) in the Dome Island Sub-basin
during the fall of 2014 as part of the Jefferson Project.
The water temperature probe had a resolution of 0.001 WC
with an accuracy of 0.01 WC between 5 and 35 WC. The
chloride probe had a resolution of 0.01 mg/L with accu-
racy ±15% of reading or 5 mg/L between 0 and
1,000 mg/L. Air temperature was measured using the Vai-
sala Weather Transmitter WXT520 attached to the
vertical proler platform. The air temperature measure-
ments had a resolution of 0.1 WC with an accuracy
ranging from 0.2 at 20 WC to 0.4 at 40 WC. The Jefferson
Project is a collaboration among Rensselaer Polytechnic
Institute, IBM Research, and the FUND for Lake George
with the goal of combining multiple high-resolution data
from weather, stream, and in-lake sensors to create meteor-
ological, hydrologic, hydrodynamic, and food web models
to better understand the effects of anthropogenic develop-
ment and climate change on the health and function of
Lake George.
Statistical analysis
Statistical analyses were conducted in SPSS or SigmaPlot.
Normality and equal variance were conducted on base-
ow stream data to determine if parametric or non-
parametric analyses were appropriate. Signicance of
main factors was determined using analysis of variance
(ANOVA) with the Holm-Sidak method used for pairwise
comparisons when data were normally distributed. Data
that were non-normally distributed required the non-para-
metric counterparts: KruskalWallis and Dunnsmethods
to determine signicant differences between main factors
and pairwise comparisons, respectively. Correlations were
conducted using the Spearman rank order when data dis-
tribution was non-normal.
Seasonal components were based on solstice and equi-
nox dates: spring (March 21June 20), summer (June 21
September 22), and fall (September 23December 20).
181 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
RESULTS
Comparing baseow temperatures among the six streams
identied two main ndings: (1) East and West Brook temp-
eratures were cooler than other streams during summer
months (Figure 2) and (2) a seasonal trend of the relative
water temperature difference for East and West Brook indi-
cates a greater proportion of discharge originates from
groundwater than other streams in the study (Figure 3).
Comparing baseow stream temperatures by season indi-
cated only summer temperatures were signicantly
(KruskalWallis, p<0.05) different among streams with
pairwise comparisons showing East and West Brook being
signicantly (Dunns method, p<0.05) cooler than English,
Finkle, and Indian Brook (Figure 2). Shelving Rock Brook
temperatures while not signicantly different from other
streams were consistently cooler than all other streams in
spring and fall.
To reduce inter-annual variability and thus enhance stat-
istical rigor, the relative water temperature differences were
compared among streams. Comparing the relative stream
temperatures strengthened the differences observed during
summer and resulted in new signicant comparisons
during the spring and fall. Along with East and West
Brook, signicant summer water temperature differences
now included Shelving Rock being signicantly cooler
than English, Finkle, and Indian Brook. During the spring,
East, West, and Shelving Rock Brook were signicantly
(Dunns method, p<0.05) cooler than Finkle and/or
Indian Brook. During fall, Shelving Rock Brook was signi-
cantly (Holm-Sidak, p<0.05) cooler than West, English,
and Finkle Brook.
The relative water temperatures of East and West Brook
illustrate the seasonal effect on stream temperatures with
cooler temperatures between late April and early October,
peak differences reached 4.0 WC in July. By early October,
East and West Brook temperatures transitioned to generally
warmer temperatures (Figure 3). The transition between
warmer and cooler relative temperatures in the spring and
fall occurred between 5 and 10 WC for both the water and
air (Figures 3 and 4). When water and air temperatures
were below 5 WC, East and West Brook generally were
warmer than other major streams and when the tempera-
tures were above 10 WC, their temperatures generally were
cooler than other major streams. Stream temperature data
from the early 1970s (Fuhs ), 1980s (Sutherland et al.
), and 2000s (Sutherland unpublished) veried the sea-
sonal trend (Figure 4). The transition range from 5 to
10 WC is near that of groundwater for the Lake George
area, which is 8.3 WC year-round (USEPA ). Ground-
water temperature remains relatively constant year-round
and approximates the average annual air temperature for a
region (Brunke & Gonser ;Hayashi & Rosenberry
Figure 2 |Baseow stream temperatures recorded in six major streams to Lake George
between 2007 and 2010 separated by season. Larger symbols represent the
seasonal median (summer and spring) or mean (fall ) for each stream based on
normality and equal variance. Different letters represent signicant differ-
ences between streams during summer (Dunns method). Streams are listed
based upon location from south to north.
Figure 3 |Relative temperature difference for East and West Brook between 2007 and
2010 exhibit a strong seasonal component. Between mid-April and mid-
October, East and West temperatures were generally cooler than the average
temperature of all monitored streams entering Lake George. Daily air temp-
erature data were obtained for the NOAA Glens Falls Airport (KGFL). Air
temperature is the daily average from 2007 to 2010 smoothed using a 3-day
average.
182 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
). Between 2007 and 2010 the average annual air temp-
erature ranged from 7.2 to 8.8 WC based on data from the
National Oceanic and Atmospheric Administration station
at Glens Falls Airport (KGFL).
Intrusion depth
Stream intrusion into a lake or reservoir is dictated by the
relative density difference between the stream and lake/
reservoir. In the Lake George watershed, temperatures
among streams can vary substantially on an individual day
resulting in colder water intruding deeper in the lake.
Prior to the establishment of the thermocline in 2007, intru-
sion depths of East, West, and English Brook ranged from
near surface to underow (Figure 5). Between the onset of
stratication and mid-August, East and West Brook entered
the lake as interow at the deeper portion of the
metalimnion while English inserted in the shallow portion
of the metalimnion. In early August, the thermocline
tended to break up into a double layer thermocline with
East and West Brook entering the deeper portion, and Eng-
lish entering the shallow portion. By late August, East and
West Brook intruded into the hypolimnion, while English
Brook inserted at the deeper portion of the metalimnion,
with all three streams entering the hypolimnion through
turnover.
In early May 2008, English Brook entered the lake near
the surface and East Brook as underow (Figure 5). Estab-
lishment of a strong thermocline was delayed allowing
East, West, and English Brook to intrude at 15 m in mid-
June compared to 10 m at the same time in 2007. East,
West, and English Brook generally inserted between 10
and 15 m until August with a tendency for East Brook to
intrude the deepest and English the shallowest. Through
Figure 4 |Stream temperatures indicate the seasonal trend of East and West being warmer in cold months and cooler in warm months as was evident in all earlier studies. The transition
between warmer and colder temperatures occurred in mid-spring and mid-fall between 5 and 10 WC, which is in the range of groundwater temperature for the region. Daily air
temperature data were obtained for the NOAA Glens Falls Airport (KGFL). Air temperature is the daily average for the years indicated in each graph smoothed using a 3-day
average.
183 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
September, English maintained the 10 to 15 m intrusion
depth with East and West entering between 15 and 25 m.
By October, all were intruding as underow.
Streams entering the Dome Island Sub-basin had a ten-
dency to insert either above or within the metalimnion until
late August or September during 2007 and 2008, respect-
ively, with the exception of Shelving Rock in early 2007
(Figure 5). Finkle and Indian Brook inserted at the lake sur-
face through May 2007 while Shelving Rock Brook tended
to enter as underow or interow. Between June and early
August, all streams inserted within the metalimnion, with
intrusion below the metalimnion beginning in mid-August.
By mid-September, stream inputs intruded as underow.
Similar patterns were observed during 2008.
Streams intrusion depth into Lake George was domi-
nantly inuenced by temperature with the dissolved ion
composition (salinity) having a minimal impact on most
streams. To illustrate this point, stream densities were com-
pared to the average lake density (02 m) of the receiving
sub-basin with density differences attributed to temperature
and salinity quantied (Figure 6). East and West Brook
show consistently denser stream discharge for both temp-
erature and salinity with peak differences reaching
2.0 kg/m
3
in mid-summer. Finkle and Indian Brook
show that warmer water temperatures in late spring
resulted in the overow observed in 2007 and 2008. The
lack of development in the Shelving Rock Brook watershed
resulted in the stream water comprising fewer dissolved
ions and thus the salinity component always resulted in
the stream discharge being less dense than the lake; how-
ever, the cooler water temperatures overwhelm the
impact. Finkle Brook was the stream most impacted by dis-
solved ions because of the high salt concentrations that are
believed to be a result of a previously uncovered road salt
Figure 5 |Lake density (kg/m
3
) isopleths for the Caldwell and Dome Island Sub-basins during 2007 and 2008 determined from temperature and salinity. The calculated intrusion depth for
each stream was based on the relative density difference between lake proles and stream discharge. The cooler water temperature in East and West Brook indicates intrusion
during summer is within metalimnion or hypolimnion, limiting its incorporation to the SML.
184 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
storage facility (Swinton et al. ). However, the maxi-
mum density difference measured in August 2007
attributed <0.3 kg/m
3
to salinity with the temperature
accounting for a density difference of 1.2 kg/m
3
;thegradi-
ent on the isopleths is 0.2 kg/m
3
.
Initial high-resolution prole data from the Dome Island
Sub-basin illustrated a cold-water, less saline underow
during a fall storm event in 2014. A storm system beginning
on November 24 and ending on November 26 had a cumu-
lative rainfall of 2.31 cm distributed between two events.
The rst event on November 24 resulted in bottom water
(53 m) decreasing temperature by 1WC and chloride by
2 mg/L (Figure 7). Decreases in both temperature and
chloride below 40 m are visually detectable shortly after pre-
cipitation fell on November 24 indicating that the underow
was entrained in water below 40 m. The second event on
November 26 did not exhibit the same magnitude of
change because the air temperature had dropped 20 WC
between the two events resulting in the second event being
a mixture of rain and snow.
Figure 6 |Relative density between stream discharge and the lake surface (02 m). The contribution of temperature and dissolved ions (salinity) are quantied. Values below the zero line
indicate the stream discharge is denser than the lake surface and values above indicate the stream discharge is less dense.
185 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
DISCUSSION
Summer baseow temperatures in East, West, and Shel-
ving Rock Brook clearly were cooler than other major
streams monitored, and based on historic West Brook
studies, this is not a new phenomenon. Therefore, the dis-
cussion will focus on: (1) how soil composition may
inuence the different seasonal thermal regimes
observed; (2) how deeper intrusion depths may incorpor-
ate organics and oxygen into the hypolimnion; and (3)
the implications of hypolimnetic intrusion on the
oxygen depletion regularly observed in the Caldwell
Sub-basin.
Seasonal thermal regimes
Three distinct seasonal temperature patterns exist in the six
major streams monitored around Lake George between 2007
and 2010 that can be explained in part by the soil composition
within each watershed. The most common thermal regime is
representative of English, Finkle, and Indian Brook. Median
summer baseow temperature among these streams was very
consistent ranging from 17.3 to 17.5 WC(Figure 2). However
the spring (average) and fall (median) temperatures differed
by 1.7 WC and 1.1 WC, respectively. These streams are located
along the west side of the lake with >83% of the soils compris-
ing type B and C (Table 1)havinganinltration rate between
Figure 7 |Temperature and chloride data collected using a YSI vertical proler near Shelving Rock during a November 2014 precipitation event verifying stream discharge entering the lake
as underow with entrainment dominantly below 40 m. Air temperature was measured using the weather station on the vertical proler.
186 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
0.05 and 0.30 in/hr (Guo ). Since soil type and topography
are similar, the inuence of groundwater should be fairly con-
sistent among the streams, as seen in the average summer
temperatures. Stream temperatures in the spring and fall
experience increased variation because the inuence of
groundwater and canopy cover diminish, enhancing the
effect of climate and stream morphology.
The cooler summer temperatures in East and West Brook
can be explained by the greater proportion of fast inltrating
soils; 75% of soils in these two watersheds comprise type A
and B soils. These soils consist primarily of sand and gravel
which result in inltration rates between 0.15 and 0.45 in/hr
(Guo ); the faster inltration rate means groundwater see-
page into the streams can occur more readily. The assumption
that a greater proportion of discharge from these streams orig-
inates from groundwater is supported by the cooler water
temperatures in warm months and warmer water tempera-
tures in cold months. Since groundwater in the region
remains 8.3 WC throughout the year, it acts as a temperature
buffer during summer and winter. The interaction between
stream water and groundwater is well documented (Castro
&Hornberger;Stanford & Ward ;Evans et al. ;
Malcolm et al. ) along with its ability to buffer stream
temperatures (Holmes ;Malcolm et al. ;Johnson
). One of the most compelling studies by Shepherd et al.
()combined three studies along the PacicNorthwest
coast of the United States varying in watershed size, discharge
rate, and temperature during different decades using different
methodologies and having different goals: all documented that
intra-gravel temperature was warmer in the winter and cooler
in the summer than the stream temperature with the tran-
sitions occurring around March and October.
Shelving Rock Brook, the only stream monitored on the
east side of the lake, is characterized as undeveloped forest
with steeper slopes and a greater proportion of shallow/
exposed bedrock relative to the other watersheds monitored.
Shelving Rock Brook temperatures generally were cooler
than the other streams with the exception of East and West
Brook during summer. Shelving Rock Brook temperatures
did not exhibit a seasonal component when analyzing relative
temperature differences, indicating groundwater inputs were
not the primary inuencing factor. While the heavily forested
watershed may aid in maintaining cooler water temperatures,
it is likely not a principal factor because the Indian Brook
watershed also is heavily forested but exhibited a different ther-
mal regime. The most probable explanation is the greater
proportion of shallow/exposed bedrock. Shallow streams
with bedrock bottoms can transfer up to 25% of the energy
absorbed by the streams to the bedrock resulting in a dampen-
ing of diurnal temperature (Brown ). Additionally, the
stream corridor is predominantly shaded providing insulation
by limiting energy absorption from solar radiation. Small
stream temperatures are difcult to predict because they
respond more rapidly to energy inputs and watershed charac-
teristics than do larger streams (Smith ), making it
necessary to have continuous temperature measurements
and energy ux calculations among the streams and their sub-
strates to fully understand the different thermal regimes
observed throughout the Lake George Basin.
Intrusions
As stream discharge enters a lake it will intrude according to its
density relative to the water column. If the stream discharge is
less dense than the lake, the discharge enters as overow and
becomes incorporated into the surface mixed layer (SML).
This was the case for Finkle and Indian Brooks through late
spring of 2007. If the stream discharge is denser than the
lake, as is often the case for East, West, and Shelving, the
ow will propagate down the slope of the lake. As the ow con-
tinues down the slope of the lake, a head begins to form at the
front of the ow; it is this region of the ow that mixes with the
lake water (Simpson ).Theextentofmixingandentrain-
ment of the head depends on the velocity, density difference,
and the degree of slope. As any of these parameters increase,
mixing between the head of the ow and the lake water also
increases (Simpson ). The ow will continue down the
slope until it reaches a depth of neutral buoyancy, at which
time it begins to propagate horizontally (Alavian et al. ).
Prior to stratication, the water column is well mixed and
while East and West Brook may intrude at varying depths, all
inputs will likely become mixed throughout the water column.
Once the thermocline develops, intrusion depth will dic-
tate the region of the water column where the discharge will
incorporate. If the discharge intrudes above the metalimnion,
the ow will become incorporated into the SML (Cortés et al.
). However, if the intrusion is within the metalimnion, it
likely will not become incorporated into the SML quickly,
187 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
due to the rate of vertical mixing within a thermocline being
on the order of heat diffusion (Quay et al. ;Fee et al.
). Therefore, stream discharge intruding into the meta-
limnion will dominantly remain in the metalimnion for an
extended period of time, possibly creating a nutrient-rich
layer for primary producers to utilize. If the discharge is
denser than the metalimnion, the ow continues to propagate
down the slope into the hypolimnion (Cortés et al. ). East
and West Brook intrusion depths during the summer and fall
imply the inputs would predominantly be maintained in the
metalimnion or hypolimnion with little incorporating into
the SML until fall turnover.
High-resolution proler data conrmed that underow
does occur during fall at the deepest depths of the Dome
Island Sub-basin. Initial data identied a colder, less saline
pulse of water entered the sub-basin during a storm event in
November 2014 (Figure 7). Streams entering the lake in this
area have watersheds that primarily are forested and are repre-
sentative of Shelving Rock Brook, which is located 1.5 km
away from the proler. Shelving Rock Brook chloride concen-
tration was 1mg/L (Swinton et al. )withNovember
baseow temperatures measured between 2007 and 2010 ran-
ging from 0.6 to 6.6 WC. The colder and less saline inputs
entered the lake as underow with entrainment affecting
water below 40 m; little temperature change was detected in
the top 40 m of the water column. The high-resolution data illus-
trate the power to identify complex hydrodynamic mixing
during storm events. An additional proler was deployed in
the Caldwell Sub-basin, near East and West Brook during the
2015 season, which should allow us to test the hypothesis that
cooler stream temperatures insert into the hypolimnion as
implied by the calculated intrusion depth. The high-resolution
data can additionally identify if intrusion depths vary through-
out the day and how temperature changes during summer
storm events inuence intrusion depth and mixing patterns,
assuming that the inputs during baseow conditions can be
detectable at these offshore locations. The inputs may simply
be too small to detect and, if that is the case, installation of
static CTD sensor strings near shore could benet future studies.
Implications to hypolimnetic oxygen depletion
The depth of stream intrusion is of great importance when
studying or managing a lake because the nutrients, sediments,
gases, and pollutants incorporated in the discharge will be
available at specic levels of the lake during periods of strati-
cation. The primary concern with deeper intrusion depths in
the Caldwell Sub-basin of Lake George deals with seasonal
hypolimnetic oxygen depletion. In the Caldwell Sub-basin,
the hypolimnion regularly experiences oxygen depletion
below 4 mg/L during late summer/early fall (Boylen et al.
). This is the only sub-basin of the lake where oxygen
depletion this severe has been documented and may be
impacted by deep intrusion of nutrient-rich oxygenated
stream discharge. East and West Brook are two sub-watersheds
at the south end of Lake George with high residential and com-
mercial development associated with tourism. West Brook has
been known since the late 1960searly 1970s to contribute
elevated levels of nitrogen and chloride (which also affects
water density) to the lake that originate from the local waste
water treatment plant (Aulenbach & Tofflemire )andapre-
viously uncovered road salt storage facility. Since at least the
early 1980s, East Brook and an adjacent small sub-watershed
have contributed high levels of phosphorus to Lake George
through stormwater runoff (Sutherland et al. ). When
these nutrient-rich oxygenated inputs are incorporated into
the hypolimnion, they can have both positive and negative
effects on the dissolved oxygen levels. The addition of organic
material could negatively affect the oxygen levels by elevating
the microbial activity and thus promote hypolimnetic oxygen
depletion. On the other hand, the input of oxygen-rich stream
discharge could attenuate the seasonal hypolimnetic oxygen
depletion. An in-depth study on how stream discharge impacts
hypolimnetic oxygen concentration is required to determine
the extent and importance of supplying both organics and
oxygen to the hypolimnion during stratication.
CONCLUSION
Soil type variability within the Lake George watershed
resulted in distinct stream temperature regimes that inuenced
intrusion depth into the lake. Streams receiving a large pro-
portion of groundwater or more shallow/exposed bedrock
exhibited cooler summer temperatures dictating deeper intru-
sion depthsthat could isolate stream inputs to the hypolimnion
during periods of stratication. High-resolution proler data
conrmed underow intrusion during the fall of 2014 and
188 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
merit additional data collection to determine if the stream dis-
charge is able to penetrate the thermocline to incorporate into
the hypolimnion. If so, this deeper intrusion could have both
negative and positive impacts on the hypolimnetic oxygen
depletion that occursin that sub-basin of thelake. Theaddition
of high-resolution proler data throughout the lake as part of
the Jefferson Project will answer questions similar to these
and progress our understanding of the hydrodynamics in
Lake George as well as the eld of hydrodynamics.
ACKNOWLEDGEMENTS
This research was a combination of several studies with
separate funding sources. The 20072010 stream study was
funded by The Lake George Watershed Coalition. The
DFWI Offshore Chemical Monitoring Program has been
jointly funded by Rensselaer Polytechnic Institute and the
FUND for Lake George. The high-resolution proler data
from The Jefferson Project, a collaboration between
Rensselaer Polytechnic Institute, IBM Research, and the
FUND for Lake George, was made possible through
nancial contributions of its participants. The DFWI is
grateful for the steadfast support of the David M. &
Margaret A. Darrin and United Parcel Service endowments.
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First received 18 September 2015; accepted in revised form 22 December 2015. Available online 9 February 2016
190 M. W. Swinton et al. |Lake intrusion depth inuenced by groundwater Hydrology Research |48.1 |2017
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