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Use of isotopic data to estimate water residence times of the Finger Lakes, New York
Journal of
Hydrology ELSEVIER
Journal of Hydrology 164 (1995) 1-18
[2]
Use of isotopic data to estimate water residence times of the Finger Lakes, New York Robert L. Michel*, T h o m a s F. Kraemer U.S. Geological Survey, 431 National Center, Reston, VA 22092, USA
Received 7 May 1994; revision accepted 15 July 1994
Abstract
Water retention times in the Finger Lakes, a group of 11 lakes in central New York with similar hydrologic and climatic characteristics, were estimated by use of a tritium-balance model. During July 1991, samples were collected from the 11 lakes and selected tributary streams and were analyzed for tritium, deuterium, and oxygen-18. Additional samples from some of the rites were collected in 1990, 1992 and 1993. Tritium concentration in lake water ranged from 24.6 Tritium Units (TU) (Otisco Lake) to 43.2 TU (Seneca Lake).The parameters in the model used to obtain water retention time (WRT) included relative humidity, evaporation rate, tritium concentrations of inflowing water and lake water, and W R T of the lake. A historical record of tritium concentrations in precipitation and runoff was obtained from rainfall data at Ottawa, Canada, analyses of local wines produced during 1977-1991, and streamflow samples collected in 1990-1991. The model was simulated in yearly steps for 1953-1991, and the W R T was varied to reproduce tritium concentrations measured in each lake in 1991. Water retention times obtained from model simulations ranged from 1 year for Otisco Lake to 12 years for Seneca Lake, and with the exception of Seneca Lake and Skaneateles Lake, were in agreement with earlier estimates obtained from runoff estimates and chloride balances. The sensitivity of the model to parameter changes was tested to determine possible reasons for the differences calculated for WRT's for Seneca Lake and Skaneateles Lake. The shorter W R T obtained from tritium data for Lake Seneca (12 years as compared to 18 years) can be explained by a yearly addition of less than 3 % by lake volume of ground water to the lake, the exact percentage depending on tritium concentration in the ground water.
* Corresponding author at: U.S. Geological Survey, MS 434, 345 Middlefield Road, Menlo Park, CA 94025-9991, USA. 0022-1694/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(94)02586-X
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R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
1. Introduction
The Finger Lakes region of central New York (Fig. 1) contains 11 lakes ranging in surface area from 2.6 km2 (Canadice Lake) to 175 km2 (Seneca Lake). The lakes are, in order of descending volume, Seneca Lake, Cayuga Lake, Canandaigua Lake, Skaneateles Lake, Keuka Lake, Owasco Lake, Conesus Lake, Hemlock Lake, Otisco
77030'
43030 '
76°30 ~
1 LAKE
ONTARIO
42°~
CONESUS SENECA LAKE CA YUGA LAKE
42°30 ' I--
Study Area
~ / ~
0
10
I
r
0
10
210
r
210
I
30
3p, I
40 KILOMETERS
Fig. 1. The Finger Lakes region.
40 MILES
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
3
Lake, Canadice Lake, and Honeoye Lake. The drainage areas for the lakes range from 32 km 2 for Canadice Lake to 2106 km 2 for Cayuga Lake (Schaffner and Oglesby, 1978). Seneca Lake has the greatest average depth (89 m) and the greatest volume (1.55 × 101° m3); Honeoye Lake is the shallowest (5 m) and has the smallest volume (3.5 x 107 m3). The region originally consisted of forest, much of which was cleared during the 18th and 19th centuries, as land was converted to agriculture. Some marginal agricultural land has returned to forest during the last century but the region is still an important agricultural area and is the center of the New York wineproducing district. The lakes provide drinking water for the major metropolitan areas in northwestern New York, and several lakes are used for recreation. Thus, knowledge of the hydrology of the Finger Lakes is important for management of these water resources. The purpose of this paper is to describe a relatively inexpensive method to estimate WRT's for groups of lakes where climatic and tritium deposition factors are the same. The lake beds were created by glacial scouring of a seabed deposited during the Devonian Period (Oglesby, 1978). As a result, all the lake basins trend north-south and dip toward the north. Outflow for all lakes is to the north, and perennial streams flowing into the lakes are generally to the south. Marshes are present at the northern and southern ends of some lakes. A few perennial streams drain to the lakes, but the maximum inflow occurs in early spring and is composed of snowmelt. The late winter and spring inflow accounts for approximately 75% of the inflow into these lakes in a typical year (Oglesby and Allee, 1969). In Cayuga Lake, the Seneca River flows in at the north end and most of the water leaves at the outflow before mixing with the lake water. The Seneca River is not considered in calculations of the water budget for Cayuga Lake. A small amount of water flows into Seneca Lake from Keuka Lake. The climate in the region consists of cold, snowy winters; mild dry summers; and an annual precipitation of 90-120 cm. Total precipitation tends to be highest in the east and lowest in the west. The nine smaller lakes freeze over during most winters, but Seneca Lake and Cayuga Lake only freeze over during abnormally cold winters. All lakes except Honeoye Lake are stratified in summer but no lake maintains a thermocline during the winter. Mixing throughout the water column occurs in each lake by December, and, on a yearly basis, the lakes are well mixed. 1.1. Previous estimates o f water residence times o f the Finger Lakes
The Water Residence Times of the Finger Lakes have been calculated from inflow and outflow data (Oglesby et al., 1973, 1975; Schaffner and Oglesby, 1978). The best outflow data are available for Owasco Lake, and an estimate for runoff per square kilometer of watershed was obtained for this lake. Schaffner and Oglesby (1978) assumed that this figure was representative for the other lakes, and they calculated WRT's relative to the size of the drainage area and the volume of the lake. The results indicated that WRT's range from less than 1 year to almost two decades (Table 1). WRT's were estimated using the runoff data collected by the U.S. Geological Survey (Table 1). The runoff for the drainage area of each lake was estimated from a contour map of runoff for the state of New York (Zembrzuski and Gannon, 1986).
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R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
Table 1 Tritium concentrations, water residence times, and evaporation constants for the Finger Lakes Lake
Average
Water rentention time, (years)
tritium concentration (TU)
Tritium model
(year-1) Schaffner and Oglesby (1978)
Canadice Canandaigua Cayuga
28.7 33.9 34.0
2 8.5 8.5
4.5 7.4 9.5
Conesus Hemlock Honeoye Keuka Otisco Owasco Seneca Skaneateles
29.9 29.9 29.4 31.6 26.7 26.2 38.5 34.0
2.5 2.5 1.5 6 1 1.5 12 8.5
1.4 2 <1 6 2 2 18 18
ak e
ke a
USGS Runoff data 4 10 10
0.038 0.017 0.011
2 2.5 1 8 1.5 3 23 14
0.057 0.048 0.13 0.021 0.064 0.022 0.007 0.015
= (evaporation rate) (surface area)/(volume).
The contour value that best represented conditions in each drainage basin was multiplied by the drainage-basin area to supply the total volume o f runoff out of each lake. Loss through evaporation was then included, and the fraction o f lake volume lost each year was calculated by adding these two components and dividing by the volume of the lake. The W R T was assumed to be the inverse of this number. Both of the above methods require the assumption that ground-water inflow or outflow is not an important c o m p o n e n t of the hydrologic budget of the lake. Eifler et al. (1989) applied the rate of decrease in salt concentrations in Cayuga Lake to estimate W R T for that lake. Salt mining and processing had resulted in the introduction of chloride above natural concentrations into Cayuga Lake. Dumping of salt into the lake ceased in 1970, and the chloride concentrations have been decreasing since then. By observing the rate at which chloride concentrations decreased and measuring the salt load brought in by streamflow, Effler et al. (1989) estimated that the residence time of chloride and water in the lake was approximately 10 years.
1.2. Use of isotope data to estimate water residence times Another method of obtaining estimates of W R T is by the use of tritium, a radioisotope of hydrogen (half-life = 12.43 years). During the period o f atmospheric weapons testing in the 1950s and 1960s, tritium concentrations in precipitation and surface waters in the northern hemisphere increased above natural concentrations by more than two orders o f magnitude. Since the peak production of tritium during the early 1960s, tritium concentrations in most surface waters have been decreasing because of radioactive decay, mixing with older water masses, and dilution by recent precipitation with lower tritium concentrations. The rate of this decrease is related to
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
the WRT of water in the system (Michel, 1992). Tritium has been used to determine the WRT of water in lakes and inland seas (Simpson, 1970; Michel and Suess, 1978; Gonfiantini et al., 1979; Carmi et al., 1984; Herczeg and Imboden, 1988) as well as the mixing of lake surface waters into deeper layers (Imboden et al., 1977). In this paper, we use tritium to determine residence times for the 11 Finger Lakes and compare them to time scales derived from the previously mentioned methods. We also present results of stable-isotope analyses of lake water as corroboration of some elements of our concepts of lake hydrology.
2. Water sampling and analyses 2.1. Methods
During July 1991, water samples were collected for determination of tritium, stable isotope ratios, and major ions to investigate the hydrology of the Finger Lakes in New York. Water samples were collected from boats at the 11 Finger Lakes by use of a centrifugal pump. Samples were collected from at least two depths from a mid-lake location in all lakes. For Seneca Lake and Cayuga Lake, several sampling locations were chosen and samples were collected from multiple depths. Water was collected from tributaries to all lakes except Canadice Lake, where no inflowing stream was observed during summer 1991. Before the 1991 sampling program, six samples from tributaries were collected during summer 1990. Additional sets of samples from tributaries were collected during April 1992 and April 1993. The flow of spring runoff in the Finger Lakes region is generally greatest in April. During 1993, samples were also collected from Cayuga Lake and the outflow from Keuka Lake. Samples for tritium determination were collected in 500 ml or 1 1glass bottles, sealed, and returned to the U.S. Geological Survey Tritium Laboratory in Reston, VA, for analyses. Samples were analyzed by electrolytic enrichment and liquid scintillation counting. Stable-isotope data were analyzed at the U.S. Geological Survey Stable Isotope Laboratory in Reston, VA. 2.2. Results
Samples collected from streams in July 1991 had tritium concentrations ranging from 16 TU to 31 TU. A histogram (Fig. 2) of all stream samples collected during July 1991 indicates that most values fall in the range of 22 to 28 TU. If the highest and lowest concentrations are dropped, the mean concentration measured was 24.5 TU and the standard deviation was 1.8 TU. Five of the six samples collected from tributaries in 1990 had tritium concentrations between 28 and 35.9 TU, with a mean of 32.7 TU. One sample had a concentration of 11.9 TU, which is much lower than any other tritium concentration measured in the area in subsequent years. Measurements of samples collected during the spring of 1992 and 1993 resulted in mean concentrations of 25.3 and 21.1 TU, respectively (Fig. 2). The tritium concentrations of the lake-water samples are given in Table 2. The mean
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20
,.-1 n.
15
lO o m Z
5
o
NNIN 17
@/ 19
21
23
25
27
29
31
TRITIUMCONCENTRATION,IN TRITIUMUNFI'S
Fig. 2. Tritium concentrations in stream water flowing into the Finger Lakes, 1991-1993.
tritium concentration and standard deviation for the 6 samples from Cayuga Lake in 1991 was 34.0 + 1.8 TU and for the 16 samples from Seneca Lake was 38.6 + 2.5 TU. Tritium concentrations in all the lake waters were equal to or greater than the mean concentration for runoff measured in 1991. Stable-isotope data for all lakes are reported in Table 3 in permil relative to VSMOW (Vienna Standard Mean Ocean Water) and normalized on scales such that the oxygen and hydrogen isotopic values of SLAP (Standard Light Antarctic Precipitation) are -55.5 %0 and -428 %0, respectively (Coplen, 1988). The 2or precision of the oxygen- and deuterium-isotope results is 0.2 %0 and 2 %0, respectively.
3. U s e o f tritium data to estimate water residence times
Tritium concentrations in water from most lakes are higher than that of inflowing stream water and precipitation due to the memory of higher tritium concentrations found in precipitation and runoff during the years following extensive atmospheric testing of fusion weapons. Tritium data can be used to estimate the WRT of the 11 Finger Lakes if the sources and sinks of tritium for each lake is available. 3.1. Tritium input to the Finger Lakes
The tritium concentrations measured in lake water during 1991 are related to the
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18 Table 2 Locations, depths, and tritium concentrations (3H) of water samples collected from the Finger Lakes, July, 1991 Lake
Latitude Longitude
Depth (m)
3H Concentration (TU)
Canadice
42°35~N 77°34'W 42°30~N 77° 17~W 42°30~N 76°44'W 42°30~N 76°32'W 42°33'N 76°36'W
0 17 0 33 100
29.3 25.9 35.1 33.5 34.2
67
34.1
7 33 100 92
37.4 31.6 34.2 32.4
0 8 8 0 17 0 17 0 33 0
29.3 29.5 31.9 31.0 33.0 30.2 32.6 31.2 32.0 24.6
15
28.8
0 25 0 67 0 33 67 100 133 0 67 133 158 67 100 33 100 0
26.7 26.1 37.3 37.0 41.1 35.9 35.7 44.0 39.4 43.2 35.9 39.9 37.6 36.2 37.7 36.5 39.4 37.8
0 0 33
32.1 35.5 34.5
Canandaigua Cayuga
Conesus
Hemlock Honeoye Keuka Otisco Owasco Seneca
42°32'N 76°32'W 42°47tN 77°43~W 42°30~N 77°361W 42°44'N 77°31'W 42°30~N 77°09'W 42°46'N 76° 17'W 42°53'N 76°301W 42°30'N 76°54'W 42°37'N 76°56'W
42°37'N 76°54'W
Skanea~ks
42o46~N 76o57tW 42o39'N 76o53'W 42o32/W 76o54'W 42o47PN 76o23'W
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R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
Table 3 Stable-isotope ratios of water from the Finger Lakes Lake
6D (~)
6180 (~)
Canadice Canandaigua Cayuga Conesus Hemlock Honeoye Keuka Otisco Owasco Seneca Skaneateles
-64.0 - 58.5 -60.5 -62.0 -64.0 -62.0 -55.0 -64.0 -65.5 -53.0 -58.5
-9.15 - 8.15 -8.60 -8.65 -9.35 -8.60 -7.45 -9.35 -9.70 -7.30 -8.20
Isotope ratios are an average value obtained from the measurement of several samples for Cayuga Lake and Seneca Lake. residence times of waters in these lakes and the history of tritium input in the lakes. Measurements of the tritium concentrations o f the inflowing waters is available only for 1990-1993. Prior to that period, no data on tritium concentrations in precipitation or stream water are available. The bulk o f the stream water flowing into the lakes is composed o f precipitation, primarily as melting snow, from that year. Thus, the tritium concentration in precipitation is a reliable estimate of the tritium concentration in the inflowing water during that same time period. The only other source of inflowing water would be ground water. Tritium concentration in ground water can range from zero to a concentration greater than that in present precipitation, depending on flow paths and the age o f the ground water. Because o f the small size of the drainage basins o f the lakes, ground water is not considered to be a significant factor in the hydrology o f the Finger Lakes. To determine W R T , we initially assume that the ground-water contribution to the lakes is negligible compared to that of precipitation or runoff. G r o u n d - w a t e r contributions to inflow and the effect of ground water on the W R T ' s determined from tritium data will be discussed later. N o long-term records of tritium concentrations in precipitation are available for any locations in the Finger Lakes region. The closest stations where tritium concentrations in precipitation have been monitored are Ottawa, C a n a d a (1953-present); Coshocton, O H (1966-1971); Chicago, I L (1960-1979); Bedford, M A (1958-1962); and Boston, M A (1963-1986). Yearly weighted-average tritium concentrations in precipitation for three o f these sites are shown in Fig. 3. The Bedford and Coshocton data sets are omitted because o f the short time that the stations were in operation and lack o f information on precipitation amounts. The Finger Lakes region is a b o u t 1100 k m east of Chicago, 420 km southwest o f Ottawa, and 600 k m west of Boston. The only continuous series o f tritium measurements that covers the period of fusion-bomb testing (1953-present) is for the Ottawa station. Measurements o f
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18 '
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• OTTAWA, CANADA
I.-
+ BOSTON, MASSACHUSETTS
103
102 Z
III 0 z
0 0 1950
1960
1970
1980
1990
YEAR Fig. 3. Tritium concentration measured in precipitation for Ottawa, Canada, Chicago, IL, and Boston, MA, and tritium concentration measured in water from Glenora wines, Seneca Lake area, New York.
tritium concentration in precipitation at Boston and Chicago show the same trends in tritium concentration as measurements of precipitation at Ottawa, although the tritium concentration in Boston precipitation is lower than tritium concentration at the other two sites. The International Atomic Energy Agency (IAEA) has developed a correlation between the Ottawa data set and other tritium data sets throughout the world for 1953-1978. An explanation of the development and use of this correlation is available in IAEA Technical Bulletin #206 (IAEA, 1981). Applying the equation of the form Csta = aCott + b
(1)
the IAEA derived correlation coefficients for tritium concentrations in precipitation for Chicago and for Boston. Values obtained for a and b were 0.95 and -33.58, respectively, for Chicago (r2 = 0.99) and 0.54 and 45.22 for Boston (r 2 = 0.90). The correlations indicate that the concentrations at Boston will be substantially lower than concentrations at the other two stations during the period of fusion-weapons testing. Tritium concentrations in precipitation are affected by several factors, including distance from the coast and latitude (Schell and Sauzay, 1974). The lower tritium concentration in precipitation at Boston is caused by mixing with oceanic water vapor, which has a lower tritium concentration. The stations at Ottawa and Chicago are affected little by oceanic water vapor because the predominant weather fronts for
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
10
these areas move northwest to southeast. The Finger Lakes region is several hundred miles inland from any ocean, and the weather fronts that traverse the area tend to have long continental tracks. Thus, the tritium concentration in precipitation at the Finger Lakes will reflect the continental effects and be higher than the tritium concentration in precipitation at coastal locations. Although water vapor from the Great Lakes may slightly reduce tritium concentration in precipitation that reaches the Finger Lakes, this reduction should be relatively small. Furthermore, the tritium concentration in the surface waters of the Great Lakes is higher than that in the Atlantic Ocean. Further evidence that continental effects are the predominant influence on tritium concentration of precipitation in the Finger Lakes region is provided by analysis of tritium concentration in wines produced from grapes grown near Seneca Lake. Tritium concentrations in wines have been used to derive estimates for the tritium concentration in precipitation for areas where the grapes are grown (Roether, 1967). Water in wine is derived totally from grapes that are collected in 1 year, and no other water is added. The water in the grapes will be derived from water taken up by the roots in the same year. Grape vines require well-drained soil, so very little water should remain in the soil from one growing season to the next. Because grape roots can extend to depths of 5-6 m, they could tap ground water in some cases. Chardonnay wines were furnished for tritium analysis by the Glenora Wine Cellars in Dundee, New York, for the years 1977-1984, 1986, and 1991. These wines are made from grapes grown on the western side of Seneca Lake in well-drained fields where no irrigation waters are applied (D. Munksgard, oral communication, 1991). The results of these analyses are shown on Fig. 3. The tritium concentration of the wines is similar to the tritium concentration in Ottawa precipitation for most years when both concentrations are available. The tritium concentrations in Boston precipitation are less than the tritium concentrations in the wines for all years. Thus, tritium concentration in precipitation at Ottawa was used to reconstruct the tritium concentration in precipitation in the Finger Lake basins for 1953-1977. The data set obtained from the measurement of tritium concentrations in wines is used to provide estimates for tritium concentrations in precipitation for 1977-1986, and tritium concentrations from measurements of streamwater given in this paper provide data for 1990-1991. 3.2. Water- and tritium-balance model The water volume contained in a lake over any time period changes as a function of inputs and outputs. The inputs consist of direct precipitation and inflow from stream water and ground water, and the outputs consist of evaporation and outflow as ground water or stream water. During the past four decades, the volumes of the Finger Lakes have been nearly constant, so the inputs and outputs have balanced. The water balance is given by dV --=O= dt
I-O=
P+ R+G-E-S
(2)
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
11
where V is the volume of the lake. The inflow term (/) consists of precipitation (P), streamflow into the lake (R), and ground-water inflow into the lake. The outflow (O) consists of evaporation (E), surface outflow through streams (S), and ground-water outflow. The net flow of ground water (G) for the lake can be positive (inflow) or negative (outflow). For the balance of constituents other than water, other factors such as loss or gain through chemical reactions must be taken into account. For tritium, corrections are required for changes in concentration produced by molecular exchange with the atmosphere and for loss through decay. The tritium concentration measured in lake water in 1991 was affected by the tritium concentrations of water flowing into and out of the lake, by molecular exchange, and by decay. In the initial model for the tritium balance of the Finger Lakes, we assumed that inflow of water equals loss of water through outflow and evaporation and that ground water flux is small compared to the other inputs and outputs (i.e./- O ~>G). Thus, the tritium balance in the lake during 1 year is given by dCl V dt = C i ( R + P) - CI(AV + S ) - ECe
(3)
where A is the tritium decay constant (0.0557 year-l), Ci is the tritium concentration in precipitation, CI is the average tritium concentration of the lake water, and Ce is the change in tritium concentration resulting from molecular exchange and evaporating lake water. The term Ce was defined by Imboden et al. (1977) as Ce =
(aCl - hCa) c~k(1 -- h)
(4)
In Eq. 4, h is the relative humidity, Ca is the tritium concentration in the water vapor over the lake, c~ is the equilibrium fractionation factor for tritium between liquid and vapor phases (0.9) and otk is the kinetic fractionation factor (1.1). The change produced by evaporation and molecular exchange can be positive or negative, depending on the relative tritium concentrations of lake water and precipitation. Eq. 3 can be divided by volume and rearranged to give dCl dt
:
kiCi
-
•C1
-
-
koCl - keCe
(5)
where k i is the fraction of the lake volume that enters the lake by precipitation and surface runoff and/Co and ke are the fractions that leave the lake as outflow and by evaporation, respectively. For a steady-state system (i.e. no increase or decrease of volume with time), the inverse of k i is the same as the WRT (1/ki = WRT) determined by Schaffner and Oglesby (1978). Because we assume that the lake volume is constant, ki = ko + ke
(6)
In addition, we assume that the tritium concentrations in the lake are uniform and that tritium concentrations in water vapor are the same as concentrations in precipitation. Values for Ci were obtained from the Ottawa tritium data and supplemented by measurements made in this study as discussed above. The average
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R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
evaporation rate used was 66 cm year -1 calculated for Cayuga Lake by Ogiesby (1978). The value of ke calculated for each lake was determined using this value. The value for Ce can be obtained if the relative humidity is known. As the relative humidity increases and approaches 100%, Ce becomes very sensitive to small changes in the value of h. Relative humidities at Binghamton, New York, published by the U.S. Department of Commerce in monthly climatological data reports are in the range of 68-86% with an average of 76%. Thus, a value of 76% is used for h in calculating Ce for all lakes. Eq. 5 can be used to calculate tritium concentration in lake water for each of the Finger Lakes in 1991 for different values o f k i by calculating the tritium concentration of lake water in 1-year steps during 1953-1991 applying the data given previously (see Przewlocki and Yurtsever, 1974). The only unknowns are ki and k o, which are related by Eq. 6, and the tritium concentration of the lake water before 1953. Though no data are available on pre-bomb tritium concentrations in the Finger Lakes region, data from other areas indicate that concentrations were probably less than l0 TU (Thatcher, 1962; Leventhal and Libby, 1970). Changing initial tritium concentrations from 0-25 TU results in a change of less than 0.1 TU in 1991 concentrations, so the uncertainty in the tritium concentrations in the pre-1953 period does not significantly affect the results. Values for ki (and ko) can be varied for each lake until the tritium concentration estimated in model simulation for the lake water in July 1991 matches the average tritium concentration measured for the lake in July 1991. Fig. 4 shows the results of the model calculations for the tritium concentration that would be present in Keuka Lake water in July 1991 for different values of WRT (1/ki). If the value of k i drops to less than 0.021 year -l (WRT = 47.6 years), the amount of water entering the lake is less than that lost by evaporation, indicating that Keuka Lake is a terminal lake. This solution is not possible, so calculations are only carried out for k i > 0.021 year -1 . Changing WRT from 1 to 47.6 years produces a variation in the tritium concentration in Keuka Lake water in July 1991 of 27-46 TU, with the highest tritium concentration occurring for a WRT of about 30 years. The average tritium concentration measured in Keuka Lake in 1991 is 31.5 TU and is represented by the dotted line in Fig. 4. The WRT curve crosses 31.5 TU at 6 years, indicating the tritium concentration measured in July 1991 could only be produced by a WRT equal to 6 years. Fig. 5 shows the change in tritium concentrations in Keuka Lake water for 1953-1993, based on an assumed WRT of 6 years and the model and data previously discussed. Only two data points are available for Keuka Lake during this period, 1991 and 1993, but both were matched in the model curve simulation. 3.3. W a t e r residence times
WRT's were obtained from the tritium model for all Finger Lakes, and these values are given in Table 1. Included in Table 1 are average tritium concentrations in each lake in July 1991, ke (fraction lost to evaporation per year), and the WRT's obtained from Schaffner and Oglesby (1978) and U.S. Geological Survey runoff data. For all Finger Lakes except Seneca Lake and Skaneateles Lake, the three methods result in similar WRT's. The WRT's derived for these two lakes from tritium data is much lower than the estimates derived from either runoff method. In Fig. 6, WRT's
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18 -
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RIEu.IM.
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Fig. 4. Tritium concentration in Keuka Lake water in 1991 as a function of WRT. Actual concentration measured in 1991 (31.6 TU) is represented by the dotted line. (Graph is terminated at 47.6 years when ki = ke).
derived from tritium data collected in this study are plotted along with WRT's derived by Schaffner and Oglesby 0978). The WRT's derived from tritium data for Seneca Lake and Skaneateles Lake are far from the l:l correspondence line. If the WRT's derived from tritium for these two lakes are deleted, a linear regression of the WRT's derived from tritium vs. those derived by Schaffner and Oglesby (1978), yields a slope of 1.01 (r2 = 0.9). In the regression the intercept is assumed to be zero, because there should be no offset between the two data sets (Taylor, 1982). Thus, for most lakes, tritium data provide the same value of WRT as that derived from measurements of surface-water flow. This result indicates that the simple model applied is adequate for determining the tritium and water balances of nine of the Finger Lakes, and that ground-water inflow is not an important factor in the hydrology of these lakes. The sensitivity of the tritium model to changes in parameters and assumptions was tested for Seneca Lake to determine possible causes for the lower WRT obtained by this method. No reasonable changes in the values of humidity or evaporation rate will shift the WRT of Seneca Lake by 7 years. It would require a rise in average relative humidity to over 90% to produce a WRT of 18 years. Two possible mechanisms that would shift the WRT of Seneca Lake from ll to 18 years to match the estimates derived from runoff data are: (1) a change in the tritium concentration in precipitation and runoff, and (2) inflow of ground water. Because the tritium concentration curve for precipitation and runoff is based on a
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R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
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z"
0 I,,~ n" IZ LU 0 Z O O
102
i
10
=1, 1950
1960
1970
1980
1990
YEAR
Fig. 5. Predicted tritium concentration of water in Keuka Lake during the period of atmospheric fusionweapons testing. Measured concentrationsfor 1991 and 1993 indicated on graph. correlation with data from Ottawa, it is possible that the actual concentrations during the bomb transient were either higher or lower in the Finger Lakes region. A 20% decrease in tritium concentrations used for input in the model would increase the W R T for Seneca Lake to 19 years. However, for the period 1977-1991, the input data were based on tritium measurements of wines or runoff waters, which should accurately reflect the tritium concentrations in the incoming water. A decrease in the assumed tritium concentrations would also increase the W R T for the other Finger Lakes, so it is unlikely that the lower W R T in Seneca Lake is caused by an incorrect tritium input function. The ground-water terms were not included in this model as ground-water flow is not considered to be an important hydrologic factor for these lakes. If ground water is introduced into a lake, its effect on the calculated W R T will depend on the tritium concentration and inflow rate o f the ground water. In a lake with a long W R T such as Seneca Lake, even a small yearly inflow o f ground water can have a significant effect. If Eq. 5 is modified to allow for the introduction o f 1% by lake volume o f tritium-free ground water (Cc = 0) or 3% by lake volume of ground water with ambient tritium concentrations (Cc = Ci) into Seneca Lake each year, the calculated value of 1/ki would be raised to 18 years. In contrast, the actual W R T o f the lake representing inflow from all sources would be shorter than a W R T obtained only by consideration o f surface flow. Salinity data (Wing and Ahrnsbrak, 1992) indicate that a small amount of ground
15
R.L. Michel, T.F. Kraemer / Journal o f Hydrology 164 (1995) 1-18 '
'
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WATER RETENTION TIME, IN YEARS DETERMINED FROM TRITIUM MODEL Fig. 6. Comparison of WRT's determined by Schaffner and Oglesby (1978) and WRT's determined from tritium data in this study. (Error bars shown for WRT are based on analytical uncertainty of tritium data.)
water does flow into Seneca Lake. Therefore, flow measurements of surface water alone would result in an estimate of WRT that is too high for Seneca Lake. Not enough information is presently available to determine the reason for the discrepancy for Skaneateles Lake. For the smaller lakes with shorter WRT's, ground-water inflow is too slow relative to surface flow to have a substantial effect on tritium concentrations and does not affect estimated WRT's of these lakes significantly.
3.4. Supplementalfinding from stable isotope analyses Stable isotope ratios in precipitation at Ottawa exhibit a strong seasonal signal (IAEA, 1981), with the lightest average monthly isotopic value during winter (6D = -155%o, 61So = -17.7%0) and the heaviest average-monthly isotopic values during the summer months (6D = -48%0, 6~SO = -7.5%0). Because spring snowmelt provides about 75% of the water flowing into the lakes during an average year, the winter isotopic ratio will be important in determining the isotopic ratios found in these lakes. In eighteen samples of spring runoff collected in the Finger Lakes during April, 1993, the 6I) ranged from -79.4 %0to -98.0 %0and averaged -89 %0. The 6180 ratios in these samples ranged from -11.64 %0 to -14.02 %0 and averaged -12.9 %0. These isotopic ratios are similar to isotopic ratios in precipitation at Ottawa in March
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
16
and April. The stable isotope data for runoff during spring 1993 fall close to the meteoric water line (Craig and Gordon, 1965); a regression of 6D vs. 61So gives a line with a slope of 7.6 and an intercept of 10 (r 2 = 0.98). Samples were also collected for stable isotope analyses during July 1991 and October 1993 to determine hydrogen and oxygen isotope ratios in streamflow during periods of low flow. About 25% of the inflow to the lake is during these low-flow periods. Isotopic ratios of water were heavier during the summer and fall than during the spring averaging -63 %0 for 6D and -9.3 %0 for 6180. The stable isotope ratios during periods of low flow are close to the meteoric water line defined by the spring time samples, although there is some indication of a decrease in the ratio of 6D to 6180. Isotopic ratios of hydrogen and oxygen for all lake waters are heavier than the ratios measured for spring runoff (Fig. 7). The heavier ratios are due to evaporation, which enriches residual water in the heavier isotopes. Evaporation can also change the ratio of 6D to 6180. Kinetic fractionation processes can be more important than equilibrium fractionation processes during evaporation and can result in a reduction in the slope of 6D vs. 6180. The lake waters show the effect of evaporation; a linear regression of 6D vs. 6180 for the stable isotope data from lake water (Table 3) yields a slope of 5 (r2 = 0.95) and an intercept o f - 17.5. Stable-isotope ratios provide another tracer for studying the hydrologic balance of lakes (Gat, 1981; Gilath and Gonfiantini, 1983; Krabbenhoft et al., 1990). For the -50
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UMM ""UNOV %
oK/SLOPE -90
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,
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6180, IN PERMIL Fig. 7. Stable-isotope ratios of hydrogen and oxygen for lake water and spring and summer runoff in the Finger Lakes region. (The water line shown is based on measurements of samples collected during the spring runoff of 1992.)
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
17
Finger Lakes, isotopic ratios in a lake are assumed to be constant from year to year, so
dtl = 0 = ki~i - koSl - ke~e (7) dt where ki, ke, and/Co are previously defined, 61 is the isotopic ratio of the water in the lake, 8i is the isotopic ratio of the inflowing water, and 6e is the isotopic ratio of evaporating lake water. Both 61 and 8i can be measured, so
1 ki
-~-
W R T = ke + k°
(8)
The stable-isotope data indicate that lakes with the largest W R T ' s tend to have the heavier isotopic concentrations. This relation would be expected because a larger W R T would result in longer exposure of lake water to evaporation. To solve Eq. 7 for ki requires a calculation of a value for Be. This would require information on the isotopic ratios o f hydrogen and oxygen in atmospheric water vapor which was not measured in this study. Thus, the present stable-isotope data set cannot be used to furnish an independent check on the calculated value of ki.
4. Summary A model was used to determine the water retention times o f the 11 Finger Lakes by use of tritium data. Estimates o f W R T simulated with the model were similar to estimates calculated from runoff methods, with the exception o f Seneca Lake and Skaneateles Lake. F o r Seneca Lake, the most plausible explanation for the difference in W R T obtained by the runoff data and tritium model is the introduction of small amounts of ground water into the lake ( 1 - 3 % by volume of lake water). The contribution o f ground water would indicate that the W R T obtained only by runoff data is too high. The method described in the paper is a relatively inexpensive method to estimate W R T for groups o f lakes within a limited area where climatic and tritiumdeposition factors are the same.
Acknowledgments We wish to thank David Munksgard and the Glenora Wine Cellars o f Dundee, New York, for providing vintage wines for tritium analyses. The paper was improved by reviews by Dave Krabbenhoft and Bill Evans (USGS). Tritium analyses were performed by J. Jaeschke.
References Carmi, I., Gat, J.R. and Stiller, M., 1984.Tritium in the Dead Sea. Earth Planet. Sci. Lett., 71: 377-389. Craig, H. and Gordon, L.I., 1965. Deuterium and oxygen-18 variations in the oceans and marine
18
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
atmosphere, In: E. Tongiorgi (Editor), Stable Isotopes in Oceanographic Studies and Paleotemperatures. Lischi and Figli, Pisa, pp. 9-130. Coplen, T.B., 1988. Normalization of oxygen and hydrogen isotope data. Chemical Geol., 72: 293-297. Effler, S.W., Auer, M.T. and Johnson, N.A., 1989. Modeling CI concentrations in Cayuga Lake, U.S.A. Water, Air and Soil Poll., 44: 347-362. Gat, J.R., 1981. Lakes. In: J.R. Gat and R. Gonfiantini (Editors), Stable Isotope Hydrology, Deuterium and Oxygen-18 in the Water Cycle. IAEA, Vienna, pp. 202-222. Gilath, C. and Gonfiantini, R., 1983. Lake dynamics. In: Guidebook on Nuclear Techniques in Hydrology. IAEA, Vienna, pp. 129-161. Gonfiantini, R., Zuppi, G.M., Eccles, D.H. and Ferro,W., 1979.Isotope investigation of Lake Malawi. In: Isotopes in Lake Studies. IAEA, Vienna, pp. 195-207. Herczeg, A.L. and Imboden, D.M., 1988. Tritium hydrologic studies in four closed-basin lakes in the Great Basin, U.S.A. Limnol. Oceanogr., 33: 157-173. Imboden, D.M., Weiss, R.F., Craig, H., Michel, R.L. and Goldman,C.R., 1977. Lake Tahoe geochemical study. 1. Lake chemistry and tritium mixing study. Limnol. Oceanogr., 22: 1039-1051. International Atomic Energy Agency (IAEA), 1981. Statistical Treatment of Environmental Isotope Data in Precipitation. Tech. Report Series No. 206, IAEA, Vienna, 255 pp. Krabbenhoft, D.P., Bowser, C.J., Anderson, M.P. and Valley, J.W., 1990. Estimating groundwater exchange with lakes. 1. The stable isotope mass balance method. Water Resour. Res., 26: 2445-2453. l.,¢venthal, J.S. and Libby, W.F., 1970. Tritium fallout in the Pacific United States. J. Geophys. Res., 75: 7628-7633. Michel, R.L., 1992. Residence times in river basins as determined by analysis of long-term tritium records. J. Hydrol., 130: 367-378. Michel, R.L. and Suess, H.E., 1978. Tritium in the Caspian Sea. Earth Planet. Sci. Lett., 39: 309-312. Oglesby, R.T., 1978. Limnology of Lake Cayuga. In: J.A. Bloomfield (Editor), Lakes of New York State. Volume 1: Ecology of the Finger Lakes. Academic Press, New York, pp. 2-120. Oglesby, R.T. and Allee, D.J., 1969. Hydrology and Flushing Characteristics. In: Ecology of Cayuga Lake and the proposed Bell Station (nuclear powered). Publ. No. 27, Water Resources and Marine Science Center, Cornell University, Ithaca, NY, pp 42-72. Oglesby, R.T., Schaffner, W.R. and Mills, E.L., 1975. Nitrogen, Phosphorus and Eutrophication in the Finger Lakes. In: Publ. No. 94, Water Resources and Marine Science Center, Cornell University, Ithaca, NY. Oglesby, R.T., Hamilton, L.S., Mills, E.L. and Willing, P., 1973. Owasco Lake and its Watershed. In: Publ. No. 70, Water Resources and Marine Science Center, Comell University, Ithaca, NY. Przewlocki, K. and Yurtsever, Y., 1974. Some conceptual mathmatical models and digital simulation approach in the use of tracers in hydrological systems. In: Symposium on Isotope Techniques in Groundwater Hydrology, Vienna. IAEA, Vienna, pp. 425-450. Roether, W., 1967. Estimating the tritium to ground water from wine samples: Groundwater and direct run-off contribution to central European surface waters. In: Isotopes in Hydrology. IAEA, Vienna, pp. 73-91. Schaffner, W.R. and Oglesby, R.T., 1978. Limnology of eight Finger Lakes. In: J.A. Bloomfield (Editor), Lakes of New York State. Volume 1: Ecology of the Finger Lakes. Academic Press, New York, pp. 313-470. Schell, W.R. and Sauzay, G., 1974. World distribution of environmental tritium. In: Physical Behavior of Radioactive Contaminates in the Atmosphere. IAEA, Vienna, pp. 375-400. Simpson, H.J., 1970. Tritium in Crater Lake, Oregon. J. Geophys. Res., 75: 5195-5207. Taylor, J.R., 1982. An Introduction to Error Analysis. University Science Books, Mill Valley, CA. Thatcher, L.L., 1962. The distribution of tritium fallout in precipitation over North America, Bull. Int. Assoc. Sci. Hydrol., 7: 48-58. Wing, M.R. and Ahrnsbrak, W.F., 1992. Evidence for deep saline springs in Seneca Lake. EOS Trans. Am. Geophys. Union, 73:211. Zembrzuski, T.J. and Gannon, W.B., 1986. New York surface-water resources. In: D.W. Moody, E.B. Chase and D.A. Aronson (Editors), National Water Summary, 1985 - - Hydrological Events and Surface Water Resources, U. S. Geological Survey Water-Supply Paper No. 2300, 349 pp.
Hydrology ELSEVIER
Journal of Hydrology 164 (1995) 1-18
[2]
Use of isotopic data to estimate water residence times of the Finger Lakes, New York Robert L. Michel*, T h o m a s F. Kraemer U.S. Geological Survey, 431 National Center, Reston, VA 22092, USA
Received 7 May 1994; revision accepted 15 July 1994
Abstract
Water retention times in the Finger Lakes, a group of 11 lakes in central New York with similar hydrologic and climatic characteristics, were estimated by use of a tritium-balance model. During July 1991, samples were collected from the 11 lakes and selected tributary streams and were analyzed for tritium, deuterium, and oxygen-18. Additional samples from some of the rites were collected in 1990, 1992 and 1993. Tritium concentration in lake water ranged from 24.6 Tritium Units (TU) (Otisco Lake) to 43.2 TU (Seneca Lake).The parameters in the model used to obtain water retention time (WRT) included relative humidity, evaporation rate, tritium concentrations of inflowing water and lake water, and W R T of the lake. A historical record of tritium concentrations in precipitation and runoff was obtained from rainfall data at Ottawa, Canada, analyses of local wines produced during 1977-1991, and streamflow samples collected in 1990-1991. The model was simulated in yearly steps for 1953-1991, and the W R T was varied to reproduce tritium concentrations measured in each lake in 1991. Water retention times obtained from model simulations ranged from 1 year for Otisco Lake to 12 years for Seneca Lake, and with the exception of Seneca Lake and Skaneateles Lake, were in agreement with earlier estimates obtained from runoff estimates and chloride balances. The sensitivity of the model to parameter changes was tested to determine possible reasons for the differences calculated for WRT's for Seneca Lake and Skaneateles Lake. The shorter W R T obtained from tritium data for Lake Seneca (12 years as compared to 18 years) can be explained by a yearly addition of less than 3 % by lake volume of ground water to the lake, the exact percentage depending on tritium concentration in the ground water.
* Corresponding author at: U.S. Geological Survey, MS 434, 345 Middlefield Road, Menlo Park, CA 94025-9991, USA. 0022-1694/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(94)02586-X
2
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
1. Introduction
The Finger Lakes region of central New York (Fig. 1) contains 11 lakes ranging in surface area from 2.6 km2 (Canadice Lake) to 175 km2 (Seneca Lake). The lakes are, in order of descending volume, Seneca Lake, Cayuga Lake, Canandaigua Lake, Skaneateles Lake, Keuka Lake, Owasco Lake, Conesus Lake, Hemlock Lake, Otisco
77030'
43030 '
76°30 ~
1 LAKE
ONTARIO
42°~
CONESUS SENECA LAKE CA YUGA LAKE
42°30 ' I--
Study Area
~ / ~
0
10
I
r
0
10
210
r
210
I
30
3p, I
40 KILOMETERS
Fig. 1. The Finger Lakes region.
40 MILES
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
3
Lake, Canadice Lake, and Honeoye Lake. The drainage areas for the lakes range from 32 km 2 for Canadice Lake to 2106 km 2 for Cayuga Lake (Schaffner and Oglesby, 1978). Seneca Lake has the greatest average depth (89 m) and the greatest volume (1.55 × 101° m3); Honeoye Lake is the shallowest (5 m) and has the smallest volume (3.5 x 107 m3). The region originally consisted of forest, much of which was cleared during the 18th and 19th centuries, as land was converted to agriculture. Some marginal agricultural land has returned to forest during the last century but the region is still an important agricultural area and is the center of the New York wineproducing district. The lakes provide drinking water for the major metropolitan areas in northwestern New York, and several lakes are used for recreation. Thus, knowledge of the hydrology of the Finger Lakes is important for management of these water resources. The purpose of this paper is to describe a relatively inexpensive method to estimate WRT's for groups of lakes where climatic and tritium deposition factors are the same. The lake beds were created by glacial scouring of a seabed deposited during the Devonian Period (Oglesby, 1978). As a result, all the lake basins trend north-south and dip toward the north. Outflow for all lakes is to the north, and perennial streams flowing into the lakes are generally to the south. Marshes are present at the northern and southern ends of some lakes. A few perennial streams drain to the lakes, but the maximum inflow occurs in early spring and is composed of snowmelt. The late winter and spring inflow accounts for approximately 75% of the inflow into these lakes in a typical year (Oglesby and Allee, 1969). In Cayuga Lake, the Seneca River flows in at the north end and most of the water leaves at the outflow before mixing with the lake water. The Seneca River is not considered in calculations of the water budget for Cayuga Lake. A small amount of water flows into Seneca Lake from Keuka Lake. The climate in the region consists of cold, snowy winters; mild dry summers; and an annual precipitation of 90-120 cm. Total precipitation tends to be highest in the east and lowest in the west. The nine smaller lakes freeze over during most winters, but Seneca Lake and Cayuga Lake only freeze over during abnormally cold winters. All lakes except Honeoye Lake are stratified in summer but no lake maintains a thermocline during the winter. Mixing throughout the water column occurs in each lake by December, and, on a yearly basis, the lakes are well mixed. 1.1. Previous estimates o f water residence times o f the Finger Lakes
The Water Residence Times of the Finger Lakes have been calculated from inflow and outflow data (Oglesby et al., 1973, 1975; Schaffner and Oglesby, 1978). The best outflow data are available for Owasco Lake, and an estimate for runoff per square kilometer of watershed was obtained for this lake. Schaffner and Oglesby (1978) assumed that this figure was representative for the other lakes, and they calculated WRT's relative to the size of the drainage area and the volume of the lake. The results indicated that WRT's range from less than 1 year to almost two decades (Table 1). WRT's were estimated using the runoff data collected by the U.S. Geological Survey (Table 1). The runoff for the drainage area of each lake was estimated from a contour map of runoff for the state of New York (Zembrzuski and Gannon, 1986).
4
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
Table 1 Tritium concentrations, water residence times, and evaporation constants for the Finger Lakes Lake
Average
Water rentention time, (years)
tritium concentration (TU)
Tritium model
(year-1) Schaffner and Oglesby (1978)
Canadice Canandaigua Cayuga
28.7 33.9 34.0
2 8.5 8.5
4.5 7.4 9.5
Conesus Hemlock Honeoye Keuka Otisco Owasco Seneca Skaneateles
29.9 29.9 29.4 31.6 26.7 26.2 38.5 34.0
2.5 2.5 1.5 6 1 1.5 12 8.5
1.4 2 <1 6 2 2 18 18
ak e
ke a
USGS Runoff data 4 10 10
0.038 0.017 0.011
2 2.5 1 8 1.5 3 23 14
0.057 0.048 0.13 0.021 0.064 0.022 0.007 0.015
= (evaporation rate) (surface area)/(volume).
The contour value that best represented conditions in each drainage basin was multiplied by the drainage-basin area to supply the total volume o f runoff out of each lake. Loss through evaporation was then included, and the fraction o f lake volume lost each year was calculated by adding these two components and dividing by the volume of the lake. The W R T was assumed to be the inverse of this number. Both of the above methods require the assumption that ground-water inflow or outflow is not an important c o m p o n e n t of the hydrologic budget of the lake. Eifler et al. (1989) applied the rate of decrease in salt concentrations in Cayuga Lake to estimate W R T for that lake. Salt mining and processing had resulted in the introduction of chloride above natural concentrations into Cayuga Lake. Dumping of salt into the lake ceased in 1970, and the chloride concentrations have been decreasing since then. By observing the rate at which chloride concentrations decreased and measuring the salt load brought in by streamflow, Effler et al. (1989) estimated that the residence time of chloride and water in the lake was approximately 10 years.
1.2. Use of isotope data to estimate water residence times Another method of obtaining estimates of W R T is by the use of tritium, a radioisotope of hydrogen (half-life = 12.43 years). During the period o f atmospheric weapons testing in the 1950s and 1960s, tritium concentrations in precipitation and surface waters in the northern hemisphere increased above natural concentrations by more than two orders o f magnitude. Since the peak production of tritium during the early 1960s, tritium concentrations in most surface waters have been decreasing because of radioactive decay, mixing with older water masses, and dilution by recent precipitation with lower tritium concentrations. The rate of this decrease is related to
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
the WRT of water in the system (Michel, 1992). Tritium has been used to determine the WRT of water in lakes and inland seas (Simpson, 1970; Michel and Suess, 1978; Gonfiantini et al., 1979; Carmi et al., 1984; Herczeg and Imboden, 1988) as well as the mixing of lake surface waters into deeper layers (Imboden et al., 1977). In this paper, we use tritium to determine residence times for the 11 Finger Lakes and compare them to time scales derived from the previously mentioned methods. We also present results of stable-isotope analyses of lake water as corroboration of some elements of our concepts of lake hydrology.
2. Water sampling and analyses 2.1. Methods
During July 1991, water samples were collected for determination of tritium, stable isotope ratios, and major ions to investigate the hydrology of the Finger Lakes in New York. Water samples were collected from boats at the 11 Finger Lakes by use of a centrifugal pump. Samples were collected from at least two depths from a mid-lake location in all lakes. For Seneca Lake and Cayuga Lake, several sampling locations were chosen and samples were collected from multiple depths. Water was collected from tributaries to all lakes except Canadice Lake, where no inflowing stream was observed during summer 1991. Before the 1991 sampling program, six samples from tributaries were collected during summer 1990. Additional sets of samples from tributaries were collected during April 1992 and April 1993. The flow of spring runoff in the Finger Lakes region is generally greatest in April. During 1993, samples were also collected from Cayuga Lake and the outflow from Keuka Lake. Samples for tritium determination were collected in 500 ml or 1 1glass bottles, sealed, and returned to the U.S. Geological Survey Tritium Laboratory in Reston, VA, for analyses. Samples were analyzed by electrolytic enrichment and liquid scintillation counting. Stable-isotope data were analyzed at the U.S. Geological Survey Stable Isotope Laboratory in Reston, VA. 2.2. Results
Samples collected from streams in July 1991 had tritium concentrations ranging from 16 TU to 31 TU. A histogram (Fig. 2) of all stream samples collected during July 1991 indicates that most values fall in the range of 22 to 28 TU. If the highest and lowest concentrations are dropped, the mean concentration measured was 24.5 TU and the standard deviation was 1.8 TU. Five of the six samples collected from tributaries in 1990 had tritium concentrations between 28 and 35.9 TU, with a mean of 32.7 TU. One sample had a concentration of 11.9 TU, which is much lower than any other tritium concentration measured in the area in subsequent years. Measurements of samples collected during the spring of 1992 and 1993 resulted in mean concentrations of 25.3 and 21.1 TU, respectively (Fig. 2). The tritium concentrations of the lake-water samples are given in Table 2. The mean
6
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
20
,.-1 n.
15
lO o m Z
5
o
NNIN 17
@/ 19
21
23
25
27
29
31
TRITIUMCONCENTRATION,IN TRITIUMUNFI'S
Fig. 2. Tritium concentrations in stream water flowing into the Finger Lakes, 1991-1993.
tritium concentration and standard deviation for the 6 samples from Cayuga Lake in 1991 was 34.0 + 1.8 TU and for the 16 samples from Seneca Lake was 38.6 + 2.5 TU. Tritium concentrations in all the lake waters were equal to or greater than the mean concentration for runoff measured in 1991. Stable-isotope data for all lakes are reported in Table 3 in permil relative to VSMOW (Vienna Standard Mean Ocean Water) and normalized on scales such that the oxygen and hydrogen isotopic values of SLAP (Standard Light Antarctic Precipitation) are -55.5 %0 and -428 %0, respectively (Coplen, 1988). The 2or precision of the oxygen- and deuterium-isotope results is 0.2 %0 and 2 %0, respectively.
3. U s e o f tritium data to estimate water residence times
Tritium concentrations in water from most lakes are higher than that of inflowing stream water and precipitation due to the memory of higher tritium concentrations found in precipitation and runoff during the years following extensive atmospheric testing of fusion weapons. Tritium data can be used to estimate the WRT of the 11 Finger Lakes if the sources and sinks of tritium for each lake is available. 3.1. Tritium input to the Finger Lakes
The tritium concentrations measured in lake water during 1991 are related to the
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18 Table 2 Locations, depths, and tritium concentrations (3H) of water samples collected from the Finger Lakes, July, 1991 Lake
Latitude Longitude
Depth (m)
3H Concentration (TU)
Canadice
42°35~N 77°34'W 42°30~N 77° 17~W 42°30~N 76°44'W 42°30~N 76°32'W 42°33'N 76°36'W
0 17 0 33 100
29.3 25.9 35.1 33.5 34.2
67
34.1
7 33 100 92
37.4 31.6 34.2 32.4
0 8 8 0 17 0 17 0 33 0
29.3 29.5 31.9 31.0 33.0 30.2 32.6 31.2 32.0 24.6
15
28.8
0 25 0 67 0 33 67 100 133 0 67 133 158 67 100 33 100 0
26.7 26.1 37.3 37.0 41.1 35.9 35.7 44.0 39.4 43.2 35.9 39.9 37.6 36.2 37.7 36.5 39.4 37.8
0 0 33
32.1 35.5 34.5
Canandaigua Cayuga
Conesus
Hemlock Honeoye Keuka Otisco Owasco Seneca
42°32'N 76°32'W 42°47tN 77°43~W 42°30~N 77°361W 42°44'N 77°31'W 42°30~N 77°09'W 42°46'N 76° 17'W 42°53'N 76°301W 42°30'N 76°54'W 42°37'N 76°56'W
42°37'N 76°54'W
Skanea~ks
42o46~N 76o57tW 42o39'N 76o53'W 42o32/W 76o54'W 42o47PN 76o23'W
8
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
Table 3 Stable-isotope ratios of water from the Finger Lakes Lake
6D (~)
6180 (~)
Canadice Canandaigua Cayuga Conesus Hemlock Honeoye Keuka Otisco Owasco Seneca Skaneateles
-64.0 - 58.5 -60.5 -62.0 -64.0 -62.0 -55.0 -64.0 -65.5 -53.0 -58.5
-9.15 - 8.15 -8.60 -8.65 -9.35 -8.60 -7.45 -9.35 -9.70 -7.30 -8.20
Isotope ratios are an average value obtained from the measurement of several samples for Cayuga Lake and Seneca Lake. residence times of waters in these lakes and the history of tritium input in the lakes. Measurements of the tritium concentrations o f the inflowing waters is available only for 1990-1993. Prior to that period, no data on tritium concentrations in precipitation or stream water are available. The bulk o f the stream water flowing into the lakes is composed o f precipitation, primarily as melting snow, from that year. Thus, the tritium concentration in precipitation is a reliable estimate of the tritium concentration in the inflowing water during that same time period. The only other source of inflowing water would be ground water. Tritium concentration in ground water can range from zero to a concentration greater than that in present precipitation, depending on flow paths and the age o f the ground water. Because o f the small size of the drainage basins o f the lakes, ground water is not considered to be a significant factor in the hydrology o f the Finger Lakes. To determine W R T , we initially assume that the ground-water contribution to the lakes is negligible compared to that of precipitation or runoff. G r o u n d - w a t e r contributions to inflow and the effect of ground water on the W R T ' s determined from tritium data will be discussed later. N o long-term records of tritium concentrations in precipitation are available for any locations in the Finger Lakes region. The closest stations where tritium concentrations in precipitation have been monitored are Ottawa, C a n a d a (1953-present); Coshocton, O H (1966-1971); Chicago, I L (1960-1979); Bedford, M A (1958-1962); and Boston, M A (1963-1986). Yearly weighted-average tritium concentrations in precipitation for three o f these sites are shown in Fig. 3. The Bedford and Coshocton data sets are omitted because o f the short time that the stations were in operation and lack o f information on precipitation amounts. The Finger Lakes region is a b o u t 1100 k m east of Chicago, 420 km southwest o f Ottawa, and 600 k m west of Boston. The only continuous series o f tritium measurements that covers the period of fusion-bomb testing (1953-present) is for the Ottawa station. Measurements o f
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18 '
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103
102 Z
III 0 z
0 0 1950
1960
1970
1980
1990
YEAR Fig. 3. Tritium concentration measured in precipitation for Ottawa, Canada, Chicago, IL, and Boston, MA, and tritium concentration measured in water from Glenora wines, Seneca Lake area, New York.
tritium concentration in precipitation at Boston and Chicago show the same trends in tritium concentration as measurements of precipitation at Ottawa, although the tritium concentration in Boston precipitation is lower than tritium concentration at the other two sites. The International Atomic Energy Agency (IAEA) has developed a correlation between the Ottawa data set and other tritium data sets throughout the world for 1953-1978. An explanation of the development and use of this correlation is available in IAEA Technical Bulletin #206 (IAEA, 1981). Applying the equation of the form Csta = aCott + b
(1)
the IAEA derived correlation coefficients for tritium concentrations in precipitation for Chicago and for Boston. Values obtained for a and b were 0.95 and -33.58, respectively, for Chicago (r2 = 0.99) and 0.54 and 45.22 for Boston (r 2 = 0.90). The correlations indicate that the concentrations at Boston will be substantially lower than concentrations at the other two stations during the period of fusion-weapons testing. Tritium concentrations in precipitation are affected by several factors, including distance from the coast and latitude (Schell and Sauzay, 1974). The lower tritium concentration in precipitation at Boston is caused by mixing with oceanic water vapor, which has a lower tritium concentration. The stations at Ottawa and Chicago are affected little by oceanic water vapor because the predominant weather fronts for
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
10
these areas move northwest to southeast. The Finger Lakes region is several hundred miles inland from any ocean, and the weather fronts that traverse the area tend to have long continental tracks. Thus, the tritium concentration in precipitation at the Finger Lakes will reflect the continental effects and be higher than the tritium concentration in precipitation at coastal locations. Although water vapor from the Great Lakes may slightly reduce tritium concentration in precipitation that reaches the Finger Lakes, this reduction should be relatively small. Furthermore, the tritium concentration in the surface waters of the Great Lakes is higher than that in the Atlantic Ocean. Further evidence that continental effects are the predominant influence on tritium concentration of precipitation in the Finger Lakes region is provided by analysis of tritium concentration in wines produced from grapes grown near Seneca Lake. Tritium concentrations in wines have been used to derive estimates for the tritium concentration in precipitation for areas where the grapes are grown (Roether, 1967). Water in wine is derived totally from grapes that are collected in 1 year, and no other water is added. The water in the grapes will be derived from water taken up by the roots in the same year. Grape vines require well-drained soil, so very little water should remain in the soil from one growing season to the next. Because grape roots can extend to depths of 5-6 m, they could tap ground water in some cases. Chardonnay wines were furnished for tritium analysis by the Glenora Wine Cellars in Dundee, New York, for the years 1977-1984, 1986, and 1991. These wines are made from grapes grown on the western side of Seneca Lake in well-drained fields where no irrigation waters are applied (D. Munksgard, oral communication, 1991). The results of these analyses are shown on Fig. 3. The tritium concentration of the wines is similar to the tritium concentration in Ottawa precipitation for most years when both concentrations are available. The tritium concentrations in Boston precipitation are less than the tritium concentrations in the wines for all years. Thus, tritium concentration in precipitation at Ottawa was used to reconstruct the tritium concentration in precipitation in the Finger Lake basins for 1953-1977. The data set obtained from the measurement of tritium concentrations in wines is used to provide estimates for tritium concentrations in precipitation for 1977-1986, and tritium concentrations from measurements of streamwater given in this paper provide data for 1990-1991. 3.2. Water- and tritium-balance model The water volume contained in a lake over any time period changes as a function of inputs and outputs. The inputs consist of direct precipitation and inflow from stream water and ground water, and the outputs consist of evaporation and outflow as ground water or stream water. During the past four decades, the volumes of the Finger Lakes have been nearly constant, so the inputs and outputs have balanced. The water balance is given by dV --=O= dt
I-O=
P+ R+G-E-S
(2)
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
11
where V is the volume of the lake. The inflow term (/) consists of precipitation (P), streamflow into the lake (R), and ground-water inflow into the lake. The outflow (O) consists of evaporation (E), surface outflow through streams (S), and ground-water outflow. The net flow of ground water (G) for the lake can be positive (inflow) or negative (outflow). For the balance of constituents other than water, other factors such as loss or gain through chemical reactions must be taken into account. For tritium, corrections are required for changes in concentration produced by molecular exchange with the atmosphere and for loss through decay. The tritium concentration measured in lake water in 1991 was affected by the tritium concentrations of water flowing into and out of the lake, by molecular exchange, and by decay. In the initial model for the tritium balance of the Finger Lakes, we assumed that inflow of water equals loss of water through outflow and evaporation and that ground water flux is small compared to the other inputs and outputs (i.e./- O ~>G). Thus, the tritium balance in the lake during 1 year is given by dCl V dt = C i ( R + P) - CI(AV + S ) - ECe
(3)
where A is the tritium decay constant (0.0557 year-l), Ci is the tritium concentration in precipitation, CI is the average tritium concentration of the lake water, and Ce is the change in tritium concentration resulting from molecular exchange and evaporating lake water. The term Ce was defined by Imboden et al. (1977) as Ce =
(aCl - hCa) c~k(1 -- h)
(4)
In Eq. 4, h is the relative humidity, Ca is the tritium concentration in the water vapor over the lake, c~ is the equilibrium fractionation factor for tritium between liquid and vapor phases (0.9) and otk is the kinetic fractionation factor (1.1). The change produced by evaporation and molecular exchange can be positive or negative, depending on the relative tritium concentrations of lake water and precipitation. Eq. 3 can be divided by volume and rearranged to give dCl dt
:
kiCi
-
•C1
-
-
koCl - keCe
(5)
where k i is the fraction of the lake volume that enters the lake by precipitation and surface runoff and/Co and ke are the fractions that leave the lake as outflow and by evaporation, respectively. For a steady-state system (i.e. no increase or decrease of volume with time), the inverse of k i is the same as the WRT (1/ki = WRT) determined by Schaffner and Oglesby (1978). Because we assume that the lake volume is constant, ki = ko + ke
(6)
In addition, we assume that the tritium concentrations in the lake are uniform and that tritium concentrations in water vapor are the same as concentrations in precipitation. Values for Ci were obtained from the Ottawa tritium data and supplemented by measurements made in this study as discussed above. The average
12
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
evaporation rate used was 66 cm year -1 calculated for Cayuga Lake by Ogiesby (1978). The value of ke calculated for each lake was determined using this value. The value for Ce can be obtained if the relative humidity is known. As the relative humidity increases and approaches 100%, Ce becomes very sensitive to small changes in the value of h. Relative humidities at Binghamton, New York, published by the U.S. Department of Commerce in monthly climatological data reports are in the range of 68-86% with an average of 76%. Thus, a value of 76% is used for h in calculating Ce for all lakes. Eq. 5 can be used to calculate tritium concentration in lake water for each of the Finger Lakes in 1991 for different values o f k i by calculating the tritium concentration of lake water in 1-year steps during 1953-1991 applying the data given previously (see Przewlocki and Yurtsever, 1974). The only unknowns are ki and k o, which are related by Eq. 6, and the tritium concentration of the lake water before 1953. Though no data are available on pre-bomb tritium concentrations in the Finger Lakes region, data from other areas indicate that concentrations were probably less than l0 TU (Thatcher, 1962; Leventhal and Libby, 1970). Changing initial tritium concentrations from 0-25 TU results in a change of less than 0.1 TU in 1991 concentrations, so the uncertainty in the tritium concentrations in the pre-1953 period does not significantly affect the results. Values for ki (and ko) can be varied for each lake until the tritium concentration estimated in model simulation for the lake water in July 1991 matches the average tritium concentration measured for the lake in July 1991. Fig. 4 shows the results of the model calculations for the tritium concentration that would be present in Keuka Lake water in July 1991 for different values of WRT (1/ki). If the value of k i drops to less than 0.021 year -l (WRT = 47.6 years), the amount of water entering the lake is less than that lost by evaporation, indicating that Keuka Lake is a terminal lake. This solution is not possible, so calculations are only carried out for k i > 0.021 year -1 . Changing WRT from 1 to 47.6 years produces a variation in the tritium concentration in Keuka Lake water in July 1991 of 27-46 TU, with the highest tritium concentration occurring for a WRT of about 30 years. The average tritium concentration measured in Keuka Lake in 1991 is 31.5 TU and is represented by the dotted line in Fig. 4. The WRT curve crosses 31.5 TU at 6 years, indicating the tritium concentration measured in July 1991 could only be produced by a WRT equal to 6 years. Fig. 5 shows the change in tritium concentrations in Keuka Lake water for 1953-1993, based on an assumed WRT of 6 years and the model and data previously discussed. Only two data points are available for Keuka Lake during this period, 1991 and 1993, but both were matched in the model curve simulation. 3.3. W a t e r residence times
WRT's were obtained from the tritium model for all Finger Lakes, and these values are given in Table 1. Included in Table 1 are average tritium concentrations in each lake in July 1991, ke (fraction lost to evaporation per year), and the WRT's obtained from Schaffner and Oglesby (1978) and U.S. Geological Survey runoff data. For all Finger Lakes except Seneca Lake and Skaneateles Lake, the three methods result in similar WRT's. The WRT's derived for these two lakes from tritium data is much lower than the estimates derived from either runoff method. In Fig. 6, WRT's
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18 -
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Fig. 4. Tritium concentration in Keuka Lake water in 1991 as a function of WRT. Actual concentration measured in 1991 (31.6 TU) is represented by the dotted line. (Graph is terminated at 47.6 years when ki = ke).
derived from tritium data collected in this study are plotted along with WRT's derived by Schaffner and Oglesby 0978). The WRT's derived from tritium data for Seneca Lake and Skaneateles Lake are far from the l:l correspondence line. If the WRT's derived from tritium for these two lakes are deleted, a linear regression of the WRT's derived from tritium vs. those derived by Schaffner and Oglesby (1978), yields a slope of 1.01 (r2 = 0.9). In the regression the intercept is assumed to be zero, because there should be no offset between the two data sets (Taylor, 1982). Thus, for most lakes, tritium data provide the same value of WRT as that derived from measurements of surface-water flow. This result indicates that the simple model applied is adequate for determining the tritium and water balances of nine of the Finger Lakes, and that ground-water inflow is not an important factor in the hydrology of these lakes. The sensitivity of the tritium model to changes in parameters and assumptions was tested for Seneca Lake to determine possible causes for the lower WRT obtained by this method. No reasonable changes in the values of humidity or evaporation rate will shift the WRT of Seneca Lake by 7 years. It would require a rise in average relative humidity to over 90% to produce a WRT of 18 years. Two possible mechanisms that would shift the WRT of Seneca Lake from ll to 18 years to match the estimates derived from runoff data are: (1) a change in the tritium concentration in precipitation and runoff, and (2) inflow of ground water. Because the tritium concentration curve for precipitation and runoff is based on a
14
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
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1960
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1980
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YEAR
Fig. 5. Predicted tritium concentration of water in Keuka Lake during the period of atmospheric fusionweapons testing. Measured concentrationsfor 1991 and 1993 indicated on graph. correlation with data from Ottawa, it is possible that the actual concentrations during the bomb transient were either higher or lower in the Finger Lakes region. A 20% decrease in tritium concentrations used for input in the model would increase the W R T for Seneca Lake to 19 years. However, for the period 1977-1991, the input data were based on tritium measurements of wines or runoff waters, which should accurately reflect the tritium concentrations in the incoming water. A decrease in the assumed tritium concentrations would also increase the W R T for the other Finger Lakes, so it is unlikely that the lower W R T in Seneca Lake is caused by an incorrect tritium input function. The ground-water terms were not included in this model as ground-water flow is not considered to be an important hydrologic factor for these lakes. If ground water is introduced into a lake, its effect on the calculated W R T will depend on the tritium concentration and inflow rate o f the ground water. In a lake with a long W R T such as Seneca Lake, even a small yearly inflow o f ground water can have a significant effect. If Eq. 5 is modified to allow for the introduction o f 1% by lake volume o f tritium-free ground water (Cc = 0) or 3% by lake volume of ground water with ambient tritium concentrations (Cc = Ci) into Seneca Lake each year, the calculated value of 1/ki would be raised to 18 years. In contrast, the actual W R T o f the lake representing inflow from all sources would be shorter than a W R T obtained only by consideration o f surface flow. Salinity data (Wing and Ahrnsbrak, 1992) indicate that a small amount of ground
15
R.L. Michel, T.F. Kraemer / Journal o f Hydrology 164 (1995) 1-18 '
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WATER RETENTION TIME, IN YEARS DETERMINED FROM TRITIUM MODEL Fig. 6. Comparison of WRT's determined by Schaffner and Oglesby (1978) and WRT's determined from tritium data in this study. (Error bars shown for WRT are based on analytical uncertainty of tritium data.)
water does flow into Seneca Lake. Therefore, flow measurements of surface water alone would result in an estimate of WRT that is too high for Seneca Lake. Not enough information is presently available to determine the reason for the discrepancy for Skaneateles Lake. For the smaller lakes with shorter WRT's, ground-water inflow is too slow relative to surface flow to have a substantial effect on tritium concentrations and does not affect estimated WRT's of these lakes significantly.
3.4. Supplementalfinding from stable isotope analyses Stable isotope ratios in precipitation at Ottawa exhibit a strong seasonal signal (IAEA, 1981), with the lightest average monthly isotopic value during winter (6D = -155%o, 61So = -17.7%0) and the heaviest average-monthly isotopic values during the summer months (6D = -48%0, 6~SO = -7.5%0). Because spring snowmelt provides about 75% of the water flowing into the lakes during an average year, the winter isotopic ratio will be important in determining the isotopic ratios found in these lakes. In eighteen samples of spring runoff collected in the Finger Lakes during April, 1993, the 6I) ranged from -79.4 %0to -98.0 %0and averaged -89 %0. The 6180 ratios in these samples ranged from -11.64 %0 to -14.02 %0 and averaged -12.9 %0. These isotopic ratios are similar to isotopic ratios in precipitation at Ottawa in March
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
16
and April. The stable isotope data for runoff during spring 1993 fall close to the meteoric water line (Craig and Gordon, 1965); a regression of 6D vs. 61So gives a line with a slope of 7.6 and an intercept of 10 (r 2 = 0.98). Samples were also collected for stable isotope analyses during July 1991 and October 1993 to determine hydrogen and oxygen isotope ratios in streamflow during periods of low flow. About 25% of the inflow to the lake is during these low-flow periods. Isotopic ratios of water were heavier during the summer and fall than during the spring averaging -63 %0 for 6D and -9.3 %0 for 6180. The stable isotope ratios during periods of low flow are close to the meteoric water line defined by the spring time samples, although there is some indication of a decrease in the ratio of 6D to 6180. Isotopic ratios of hydrogen and oxygen for all lake waters are heavier than the ratios measured for spring runoff (Fig. 7). The heavier ratios are due to evaporation, which enriches residual water in the heavier isotopes. Evaporation can also change the ratio of 6D to 6180. Kinetic fractionation processes can be more important than equilibrium fractionation processes during evaporation and can result in a reduction in the slope of 6D vs. 6180. The lake waters show the effect of evaporation; a linear regression of 6D vs. 6180 for the stable isotope data from lake water (Table 3) yields a slope of 5 (r2 = 0.95) and an intercept o f - 17.5. Stable-isotope ratios provide another tracer for studying the hydrologic balance of lakes (Gat, 1981; Gilath and Gonfiantini, 1983; Krabbenhoft et al., 1990). For the -50
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R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
17
Finger Lakes, isotopic ratios in a lake are assumed to be constant from year to year, so
dtl = 0 = ki~i - koSl - ke~e (7) dt where ki, ke, and/Co are previously defined, 61 is the isotopic ratio of the water in the lake, 8i is the isotopic ratio of the inflowing water, and 6e is the isotopic ratio of evaporating lake water. Both 61 and 8i can be measured, so
1 ki
-~-
W R T = ke + k°
(8)
The stable-isotope data indicate that lakes with the largest W R T ' s tend to have the heavier isotopic concentrations. This relation would be expected because a larger W R T would result in longer exposure of lake water to evaporation. To solve Eq. 7 for ki requires a calculation of a value for Be. This would require information on the isotopic ratios o f hydrogen and oxygen in atmospheric water vapor which was not measured in this study. Thus, the present stable-isotope data set cannot be used to furnish an independent check on the calculated value of ki.
4. Summary A model was used to determine the water retention times o f the 11 Finger Lakes by use of tritium data. Estimates o f W R T simulated with the model were similar to estimates calculated from runoff methods, with the exception o f Seneca Lake and Skaneateles Lake. F o r Seneca Lake, the most plausible explanation for the difference in W R T obtained by the runoff data and tritium model is the introduction of small amounts of ground water into the lake ( 1 - 3 % by volume of lake water). The contribution o f ground water would indicate that the W R T obtained only by runoff data is too high. The method described in the paper is a relatively inexpensive method to estimate W R T for groups o f lakes within a limited area where climatic and tritiumdeposition factors are the same.
Acknowledgments We wish to thank David Munksgard and the Glenora Wine Cellars o f Dundee, New York, for providing vintage wines for tritium analyses. The paper was improved by reviews by Dave Krabbenhoft and Bill Evans (USGS). Tritium analyses were performed by J. Jaeschke.
References Carmi, I., Gat, J.R. and Stiller, M., 1984.Tritium in the Dead Sea. Earth Planet. Sci. Lett., 71: 377-389. Craig, H. and Gordon, L.I., 1965. Deuterium and oxygen-18 variations in the oceans and marine
18
R.L. Michel, T.F. Kraemer / Journal of Hydrology 164 (1995) 1-18
atmosphere, In: E. Tongiorgi (Editor), Stable Isotopes in Oceanographic Studies and Paleotemperatures. Lischi and Figli, Pisa, pp. 9-130. Coplen, T.B., 1988. Normalization of oxygen and hydrogen isotope data. Chemical Geol., 72: 293-297. Effler, S.W., Auer, M.T. and Johnson, N.A., 1989. Modeling CI concentrations in Cayuga Lake, U.S.A. Water, Air and Soil Poll., 44: 347-362. Gat, J.R., 1981. Lakes. In: J.R. Gat and R. Gonfiantini (Editors), Stable Isotope Hydrology, Deuterium and Oxygen-18 in the Water Cycle. IAEA, Vienna, pp. 202-222. Gilath, C. and Gonfiantini, R., 1983. Lake dynamics. In: Guidebook on Nuclear Techniques in Hydrology. IAEA, Vienna, pp. 129-161. Gonfiantini, R., Zuppi, G.M., Eccles, D.H. and Ferro,W., 1979.Isotope investigation of Lake Malawi. In: Isotopes in Lake Studies. IAEA, Vienna, pp. 195-207. Herczeg, A.L. and Imboden, D.M., 1988. Tritium hydrologic studies in four closed-basin lakes in the Great Basin, U.S.A. Limnol. Oceanogr., 33: 157-173. Imboden, D.M., Weiss, R.F., Craig, H., Michel, R.L. and Goldman,C.R., 1977. Lake Tahoe geochemical study. 1. Lake chemistry and tritium mixing study. Limnol. Oceanogr., 22: 1039-1051. International Atomic Energy Agency (IAEA), 1981. Statistical Treatment of Environmental Isotope Data in Precipitation. Tech. Report Series No. 206, IAEA, Vienna, 255 pp. Krabbenhoft, D.P., Bowser, C.J., Anderson, M.P. and Valley, J.W., 1990. Estimating groundwater exchange with lakes. 1. The stable isotope mass balance method. Water Resour. Res., 26: 2445-2453. l.,¢venthal, J.S. and Libby, W.F., 1970. Tritium fallout in the Pacific United States. J. Geophys. Res., 75: 7628-7633. Michel, R.L., 1992. Residence times in river basins as determined by analysis of long-term tritium records. J. Hydrol., 130: 367-378. Michel, R.L. and Suess, H.E., 1978. Tritium in the Caspian Sea. Earth Planet. Sci. Lett., 39: 309-312. Oglesby, R.T., 1978. Limnology of Lake Cayuga. In: J.A. Bloomfield (Editor), Lakes of New York State. Volume 1: Ecology of the Finger Lakes. Academic Press, New York, pp. 2-120. Oglesby, R.T. and Allee, D.J., 1969. Hydrology and Flushing Characteristics. In: Ecology of Cayuga Lake and the proposed Bell Station (nuclear powered). Publ. No. 27, Water Resources and Marine Science Center, Cornell University, Ithaca, NY, pp 42-72. Oglesby, R.T., Schaffner, W.R. and Mills, E.L., 1975. Nitrogen, Phosphorus and Eutrophication in the Finger Lakes. In: Publ. No. 94, Water Resources and Marine Science Center, Cornell University, Ithaca, NY. Oglesby, R.T., Hamilton, L.S., Mills, E.L. and Willing, P., 1973. Owasco Lake and its Watershed. In: Publ. No. 70, Water Resources and Marine Science Center, Comell University, Ithaca, NY. Przewlocki, K. and Yurtsever, Y., 1974. Some conceptual mathmatical models and digital simulation approach in the use of tracers in hydrological systems. In: Symposium on Isotope Techniques in Groundwater Hydrology, Vienna. IAEA, Vienna, pp. 425-450. Roether, W., 1967. Estimating the tritium to ground water from wine samples: Groundwater and direct run-off contribution to central European surface waters. In: Isotopes in Hydrology. IAEA, Vienna, pp. 73-91. Schaffner, W.R. and Oglesby, R.T., 1978. Limnology of eight Finger Lakes. In: J.A. Bloomfield (Editor), Lakes of New York State. Volume 1: Ecology of the Finger Lakes. Academic Press, New York, pp. 313-470. Schell, W.R. and Sauzay, G., 1974. World distribution of environmental tritium. In: Physical Behavior of Radioactive Contaminates in the Atmosphere. IAEA, Vienna, pp. 375-400. Simpson, H.J., 1970. Tritium in Crater Lake, Oregon. J. Geophys. Res., 75: 5195-5207. Taylor, J.R., 1982. An Introduction to Error Analysis. University Science Books, Mill Valley, CA. Thatcher, L.L., 1962. The distribution of tritium fallout in precipitation over North America, Bull. Int. Assoc. Sci. Hydrol., 7: 48-58. Wing, M.R. and Ahrnsbrak, W.F., 1992. Evidence for deep saline springs in Seneca Lake. EOS Trans. Am. Geophys. Union, 73:211. Zembrzuski, T.J. and Gannon, W.B., 1986. New York surface-water resources. In: D.W. Moody, E.B. Chase and D.A. Aronson (Editors), National Water Summary, 1985 - - Hydrological Events and Surface Water Resources, U. S. Geological Survey Water-Supply Paper No. 2300, 349 pp.