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Isotopic, Geochemical and Biological Tracing of the Source of an Impacted Karst Spring, Weldon Spring, Missouri
Environmental Forensics (2001) 2, 99±103 doi:10.1006/enfo.2000.0034, available online at http://www.idealibrary.com on
Isotopic, Geochemical and Biological Tracing of the Source of an Impacted Karst Spring, Weldon Spring, Missouri Robert E. Criss*, Samantha A. Fernandes{ and William E. Winston{ Earth & Planetary Sciences, Washington University, 1169, 1 Brookings Drive, St. Louis, MO 63130, U.S.A. (Received 15 October 2000, Revised manuscript accepted 17 December 2000) Weldon Spring is consistently enriched in 18O relative to other karst springs in east-central Missouri and western Illinois, suggesting an evaporated source component. Regional potentiometric head maps of the shallow aquifer suggest that Prairie Lake, an arti®cial lake built between 1954 and 1982, could represent this component. Isotopic, biological and chemical tracing of the spring conclusively verify the hypothesis that this lake has impacted Weldon Spring. Mixing calculations indicate that Weldon Spring is now comprised of approximately 80% lake water and 20% groundwater. Recent measurements indicate that the discharge rate of the spring is now approximately 10 times the rate prior to the construction of the lake, con®rming the augmentation of ¯ow by a new source. Analysis of the isotopic trends indicates that the subsurface travel time is short, and suggests that the conduits connecting the lake and the spring may be # 2001 AEHS progressively enlarging. Keywords: oxygen isotopes; hydrology; surface water-groundwater interactions
Introduction
Methods
Oxygen and hydrogen isotopes provide intrinsic, conservative ®ngerprints to study the origin, sources, and ¯owpaths of water. Dierent isotopic ratios arise among water bodies because of geographic and temporal variations in meteoric input and subsequent evolutionary dierences; consequently, these isotopes are of great value in tracing the sources of springs. As an example, the isotopic maps of Rose, Davisson and Criss (1996) and Rose and Davisson (1996) demonstrate that many ®rst magnitude springs in the southern Cascades of California originate from alpine snowmelt on mountains such as Mt Lassen, and travel down slope through permeable volcanic rocks for distances of 50 km or more. On an even larger scale, isotopes have been used to de®ne enormous ¯owpaths in groundwater systems in carbonate aquifers in Nevada (e.g. Winograd and Friedman, 1972; Davisson et al., 1999). Stable isotope data on springs and rivers in eastcentral Missouri and southwestern Illinois (Frederickson, Criss and Stueber, 1997; Frederickson and Criss, 1999; Coplen and Kendall, 2000) disclosed that Weldon Spring is signi®cantly higher in 18O than all the others. A search was initiated to determine the source of this distinctive water. In this paper, we present isotopic data that identi®es an arti®cial freshwater lake as the dominant source of water for this spring. We show that chemical and hydrological data are consistent with this identi®ed source, and provide the conclusive observation that freshwater diatoms in the lake are identical to those that emanate from the spring ori®ce.
Field measurements of temperature (8C), pH, turbidity (NTU; nephelometric turbidity units), speci®c conductivity (mS), dissolved oxygen (mg/L) and bicarbonate (mg/L) were made using portable equipment. Standard techniques were used for preparing water samples for oxygen and hydrogen isotope analyses on an isotope ratio mass spectrometer (Epstein and Mayeda, 1953; Coleman et al., 1982); the results are reported in the usual manner as per mil deviations from SMOW (Standard Mean Ocean Water). Compositional data were obtained by ICPMS (magnetic sector inductively coupled plasma mass spectrometry) and analysed using a computer program (EQ3NR) for geochemical aqueous speciation and solubility (Wolery, 1983).
Hydrologic Setting Weldon Spring is located in the town bearing its name in St Charles County, eastern Missouri (Figure 1). A historical plaque at the ori®ce indicates that the spring, named for town founder John Weldon who settled the area in 1789, has a ¯ow rate of 0.04 cubic feet per second (cfs). The spring is hosted in the Mississippianage Burlington Limestone and issues from a small concrete box that pools the water. The ¯ow rate of the spring seems to have increased dramatically over the last few decades, so the historical plaque is no longer accurate. For example, Vineyard and Feder (1982: 192) report a single ¯ow measurement of 0.08 cfs for November 1964. Discharge measurements by the USGS in 1986 and 1988 range from 0.15 to 0.35 cfs (John Shoemacker, pers. comm., 2000). Our visual estimates and ¯ow meter measurements on several occasions since 1995 range from 0.4 to 1.3 cfs. In short, the ¯ow appears to have increased steadily
*E-mail: [email protected] {E-mail: [email protected] {E-mail: [email protected]
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Figure 1. Map of the Weldon Spring region, showing contours of the potentiometric surface (dashed where approximate) for the shallow aquifer. Weldon Spring (elevation 165 m, or 540 ft) is directly down gradient of Prairie Lake (elevation 170 m or 565 ft). Inset shows area of detail, located approximately 40 km west of St Louis. Compiled from maps by Mugel (1996) and Kleeschulte and Emmett (1986).
over the last few decades, with the current rate probably being 10-fold greater than the historical rate. The potentiometric map (Figure 1) of the shallow aquifer in the Weldon Spring area shows that the spring, with an outlet elevation of 165 m (540 ft), is directly down-gradient of Prairie Lake, at an elevation of 170 m (565 ft). Prairie Lake is a 9-ha arti®cial lake located approximately 0.6 km southwest of the Weldon Spring ori®ce (Figure 1). Though we could not document the exact construction date, the lake is shown as a new feature on the 1982 photorevision of the 1954 Weldon Springs 7.50 topographic quadrangle, so it was impounded between 1954 and 1982. This lake appears to be a major source of the spring, as shown by the chemical, isotopic and biological data discussed below.
Isotopic Data and Physical Characteristics Measurements during 1995±2000 have disclosed that Weldon Spring has high d18O values (ÿ3.99 + 1.5-) that are atypical of karst springs in east-central Missouri and western Illinois (Figure 2). The average d18O values for several dozen other springs in this region all range from ÿ6.4 to ÿ8.0, close to the ÿ7.0- value for average meteoric precipitation at St Louis (Frederickson, Criss and Stueber, 1997; Frederickson, 1998; Frederickson and Criss, 1999). For example, Rockwoods Spring, the best-studied
spring in our database, has an average value of ÿ7.42 + 0.75-. Rockwoods Spring is used for comparison in this report (Figure 2) as it is well studied and is located only 17 km to the south of Weldon Spring. Prior to the fall of 1999, the d18O values in Weldon Spring, though anomalously high, exhibited variations that were generally parallel to those of Rockwoods Spring (Figure 2). For example, both springs exhibited relatively low d18O values during the winter of 1997± 1998, when El NinÄo conditions produced large amounts of isotopically-light meteoric precipitation (Criss, 1999). However, subsequent to fall 1999, the trend lines of the two springs are nearly antithetical (Figure 2). The 18O enrichment in Weldon Spring suggests the involvement of an important source component that has undergone signi®cant evaporation (e.g. Criss, 1999). Moreover, the large variations in the d18O values (ÿ6.87 to ÿ1.39-), temperature (6.6±18.98C) and speci®c conductivity (150±393 mS) in the spring also suggest a surface water source, which we hypothesized to be a lake. The consistently high turbidity (19±790 NTU) of Weldon Spring is extremely unusual for karst springs in this region, which are almost always 5 10 NTU except during intense rainfall events. Interestingly, the highest d18O values, the highest and lowest temperatures, the highest turbidity, and the highest ¯ow rate measured for Weldon Spring
Tracing the isotopic, geochemical and biological source of Weldon Spring, Missouri 101
now characterize Weldon Spring (see below). Similarly, the electrical conductivities of all the other lakes and one of the ponds are all signi®cantly higher than that of Weldon Spring, so these cannot represent the dilute endmember that is required by the electrical conductivity data. Moreover, a hydrologic connection between these other surface water bodies and Weldon Spring is very unlikely given the topographic and potentiometric surfaces (Figure 1).
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Figure 3. Calcite and dolomite in Weldon Spring (d) and Prairie Lake (s) are both undersaturated and show correlated trends. The Q/K term is the quotient of the measured activity product and the calculated equilibrium value at the actual temperature. Comparison with the USGS data for Weldon Spring for 1986 and 1988 (data bars indicating range and average, plotted at arbitrary date) suggests that the degree of undersaturation is increasing over time.
over the 5-year period of observation, were all recorded during the past year. We conducted a directed search and identi®ed Prairie Lake as having the right isotopic, chemical, physical and hydrologic characteristics to be the major source of the water in Weldon Spring. The characteristics of six other lakes and two ponds in the region that were tested are not appropriate to represent the source. For example, the highest turbidity (20 NTU) that has been measured in these bodies is nearly 10 times lower than the high values that
Thermodynamic calculations using the computer program EQ3NR indicate that Weldon Spring and Prairie Lake are signi®cantly undersaturated with respect to both calcite and dolomite, even though the spring is hosted in limestone (Figure 3). The analyses of Weldon Spring made by the USGS in 1986 and 1988 (John Shoemacker, pers. comm.) are closer to equilibrium than our more recent samples, possibly indicating that the spring is moving further away from equilibrium with carbonate minerals over time. Sample pairs collected from the lake and the spring during May±September of 2000 reveal highly correlated trends, supporting the proposed connection between Prairie Lake and Weldon Spring.
Diatom Tracing The yellow-brown color and unusually high turbidities of both Prairie Lake and Weldon Spring suggested the presence of diatoms, which was con®rmed by optical microscopy. Subsequent examination with a scanning electron microscope disclosed the presence of visually similar diatoms in both Weldon Spring and Prairie Lake (Figure 4). These diatoms (Melosira?) are centric and range from 5±10 mm in diameter. Freshwater diatoms are phytoplankton, hence their emanation from a karst spring ori®ce is highly unusual and unequivocally con®rms a surface water source. The fact that similar diatoms are abundant in Prairie Lake con®rms this particular lake as being the source of the spring. The diatoms in Weldon Spring appear to have been abraded or partially dissolved during their subsurface transport from Prairie Lake, and hence have a less well-de®ned external morphology.
Mixing Model When ®rst compared on 15 April 2000, Weldon Spring and Prairie Lake were found to have similar and high d18O values (ÿ2.5 v. ÿ1.9-), low speci®c conductivities (182 v. 107 mS), and unusually high turbidities (193 v. 235 NTU) associated with a yellow-brown color. Mixing calculations based on these results suggest that Prairie Lake now contributes 80±90% of the water in Weldon Spring, assuming that the complementary groundwater component has a composition similar to typical karst springs in the area. In order to further test this result, semi-monthly water samples were subsequently collected from Prairie Lake and Weldon Spring. A plot of the average electrical conductivity v. the d18O values shows that Weldon Spring is very similar to Prairie Lake, but their dierence in detail requires a subordinate component
102 R.E. Criss et al.
Figure 4. Centric diatoms (Melosira?) in Prairie Lake (a) appear to be the same species as those emerging from Weldon Spring (b). Both have a porous ``hub'' surrounded by radial ``spokes'', though the spring diatoms appear to be abraded or partially dissolved so these features are less distinct. A hydrologic connection between the lake and the spring is unequivocal.
that has a higher conductivity and a lower d18O value, which are normal characteristics for shallow groundwaters in the area. If it is assumed that this latter component has a composition similar to the long-term average of Rockwoods Spring (Figure 5), then the mixing line indicates that Weldon Spring now represents a mixture of Prairie Lake water plus the groundwater component in an approximately 4 : 1 ratio. It is useful to compare the temporal variations of the d18O values, electrical conductivity, turbidity and temperature of Weldon Spring and Prairie Lake over the last 6 months. Figure 6 clearly shows that the temporal variations in d18O (Figure 6(a)) at Weldon Spring are parallel to those in Prairie Lake, though not identical. As mentioned above, the d18O and conductivity (Figure 6(b)) dierences between the spring and 700 Rockwoods Spring (19962000)
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the lake are primarily due to the groundwater component. The dierence in turbidity (Figure 6(c)) may re¯ect the incorporation of clastic materials in the ¯owing groundwater; the dierence in temperature (Figure 6(d)) primarily re¯ects the normal attenuation of seasonal temperature variations in the shallow subsurface. The close parallelism of the time series trends for Weldon Springs and Prairie Lake indicates that the subsurface transit time is very short, probably 52 weeks, in turn suggesting the dominance of ¯ow through rather large conduits. Continued time series data will allow the subsurface transit time between Prairie Lake and Weldon Spring to be determined accurately, will disclose whether the conduits are enlarging over time, and will reduce the uncertainty of the relative proportions of groundwater and lake water in the spring. Additional work will also indicate whether the short residence time of water in Prairie Lake, which may now be less than 1 year because of the leak, is responsible for the low electrical conductivity and the low d18O values of Prairie Lake relative to other lakes in the area.
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Figure 5. The average electrical conductivity and d18O value of Weldon Spring (d) since April 2000, lies on a mixing line between Prairie Lake (s) and a groundwater component that is similar to the long-term average value for Rockwoods Spring (r). Weldon Spring appears to be a mixture of approximately 80±90% lake water and 10±20% groundwater. The average composition of six other lakes (j) located 3±7 km to the west (Figure 1) does not constitute an appropriate source for the spring.
Weldon Spring is signi®cantly enriched in 18O relative to other karst springs in east-central Missouri, suggesting that an evaporated surface water body is an important source component. We have conclusively veri®ed the hypothesis that Prairie Lake represents this component using a remarkably congruent data set that includes isotopic, chemical and physical data on water samples, potentiometric contours, ¯ow rate measurements on the spring, and tracing of diatoms. Calculations indicate that Weldon Spring is now a mixture of approximately 80% lake and 20% groundwater components, in good agreement with discharge measurements that suggest that the ¯ow rate of the impacted spring is now approximately 10 times the historical rate. The available data suggest that the connectivity between Prairie Lake and Weldon Spring may be increasing over time.
Tracing the isotopic, geochemical and biological source of Weldon Spring, Missouri 103 0
Acknowledgements
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We thank GC Frederickson, AM Stueber, and ML Davisson for valuable discussion. H Levin, DT Kremser and Jane C Walker provided expertise and ideas related to the diatom study. Kelly Carbery collected many of the samples and Rachel Lindvall performed cation analysis with an ICPMS. We thank Pastor D Miller and his congregation for maintaining the spring site, preserving its history, and allowing us access. This study was supported by NSF hydrology grant EAR 9814621.
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References
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Figure 6. Temporal variations in the d18O values (a), electrical conductivities (b), turbidities (c) and temperatures (d) of Prairie Lake (s) and Weldon Spring (d), since detailed sampling began in April 2000. These features exhibit highly similar variations that prove their communication over a short time scale.
Coleman, M.L., Shepherd, T.J., Durham, J.J., Rouse, J.E. and Moore, G.R. 1982. Reduction of water with zinc for hydrogen isotope analysis. Anal. Chem. 54, 993±995. Coplen, T.B. and Kendall, C. 2000. Stable hydrogen and oxygen isotope ratios for selected sites of the U.S. Geological Survey's NASQAN and Benchmark surface-water networks. U.S. Geological Survey, Open File Report 00-160, 409 p. Criss, R.E. 1999. Principles of Stable Isotope Distribution. New York, Oxford University Press, 254 p. Davisson, M.L., Smith, D.K., Kenneally, J. and Rose, T.P. 1999. Isotope hydrology of southern Nevada groundwater: Stable isotopes and radiocarbon. Water Resour. Res. 35, 279±294. Epstein, S. and Mayeda, T. 1953. Variation of O18 content of waters from natural sources. Geochim. Cosmochim. Acta 4, 213±224. Frederickson, G.C. 1998. Relationship between the stable isotopes of precipitation and springs and rivers in east central Missouri and southwestern Illinois. M.Sc. Thesis. Washington University, St. Louis, Missourri, 237 p. Frederickson, G.C. and Criss, R.E. 1999. Isotope hydrology and time constants of the unimpounded Meramec River basin, Missouri. Chem. Geol. 157, 303±317. Frederickson, G.C., Criss, R.E. and Steuber, A.M. 1997. Oxygen isotope relations in karst springs, east-central Missouri and southwestern Illinois. Geol. Soc. America Abstracts with Prgms 29, A 330. Kleeschulte, M.J. and Emmett, L.F. 1986. Compilation and preliminary interpretation of hydrologic data for the Weldon Spring radioactive waste-disposal sites, St Charles County, Missouri. U.S. Geological Survey, Water-Resources Investigation Report 85±427, 71 p. Mugel, D.N. 1996. Geohydrology of the Weldon Spring Ordnance Works, St Charles County, Missouri. U.S. Geological Survey, Water-Resources Investigation Report 96±4171, 47 p. Rose, T.P. and Davisson, M.L. 1996. Radiocarbon in Hydrologic Systems Containing Dissolved Magmatic Carbon Dioxide. Science 273, 1367±1370. Rose, T.P., Davisson, M.L. and Criss, R.E. 1996. Isotope hydrology of voluminous cold springs in fractured rock from an active volcanic region, northeastern California. J. Hydrol. 179, 207±236. Vineyard, J.D. and Feder, G.L. 1982. Springs of Missouri. Missouri Geological Survey and Water REsources WR 29, 212 p. Winograd, I.J. and Friedman, I. 1972. Deuterium as a tracer of regional groundwater ¯ow, south Great Basin, Nevada and California. Geol. Soc. Am. Bull. 83, 3691±3708. Wolery, T.J. 1983. A Computer Program for Geochemical Aqueous Speciation-Solubility Calculations: User's Guide and Documentation UCRL-53414. Lawrence Livermore National Laboratory, Livermore, California.
Isotopic, Geochemical and Biological Tracing of the Source of an Impacted Karst Spring, Weldon Spring, Missouri Robert E. Criss*, Samantha A. Fernandes{ and William E. Winston{ Earth & Planetary Sciences, Washington University, 1169, 1 Brookings Drive, St. Louis, MO 63130, U.S.A. (Received 15 October 2000, Revised manuscript accepted 17 December 2000) Weldon Spring is consistently enriched in 18O relative to other karst springs in east-central Missouri and western Illinois, suggesting an evaporated source component. Regional potentiometric head maps of the shallow aquifer suggest that Prairie Lake, an arti®cial lake built between 1954 and 1982, could represent this component. Isotopic, biological and chemical tracing of the spring conclusively verify the hypothesis that this lake has impacted Weldon Spring. Mixing calculations indicate that Weldon Spring is now comprised of approximately 80% lake water and 20% groundwater. Recent measurements indicate that the discharge rate of the spring is now approximately 10 times the rate prior to the construction of the lake, con®rming the augmentation of ¯ow by a new source. Analysis of the isotopic trends indicates that the subsurface travel time is short, and suggests that the conduits connecting the lake and the spring may be # 2001 AEHS progressively enlarging. Keywords: oxygen isotopes; hydrology; surface water-groundwater interactions
Introduction
Methods
Oxygen and hydrogen isotopes provide intrinsic, conservative ®ngerprints to study the origin, sources, and ¯owpaths of water. Dierent isotopic ratios arise among water bodies because of geographic and temporal variations in meteoric input and subsequent evolutionary dierences; consequently, these isotopes are of great value in tracing the sources of springs. As an example, the isotopic maps of Rose, Davisson and Criss (1996) and Rose and Davisson (1996) demonstrate that many ®rst magnitude springs in the southern Cascades of California originate from alpine snowmelt on mountains such as Mt Lassen, and travel down slope through permeable volcanic rocks for distances of 50 km or more. On an even larger scale, isotopes have been used to de®ne enormous ¯owpaths in groundwater systems in carbonate aquifers in Nevada (e.g. Winograd and Friedman, 1972; Davisson et al., 1999). Stable isotope data on springs and rivers in eastcentral Missouri and southwestern Illinois (Frederickson, Criss and Stueber, 1997; Frederickson and Criss, 1999; Coplen and Kendall, 2000) disclosed that Weldon Spring is signi®cantly higher in 18O than all the others. A search was initiated to determine the source of this distinctive water. In this paper, we present isotopic data that identi®es an arti®cial freshwater lake as the dominant source of water for this spring. We show that chemical and hydrological data are consistent with this identi®ed source, and provide the conclusive observation that freshwater diatoms in the lake are identical to those that emanate from the spring ori®ce.
Field measurements of temperature (8C), pH, turbidity (NTU; nephelometric turbidity units), speci®c conductivity (mS), dissolved oxygen (mg/L) and bicarbonate (mg/L) were made using portable equipment. Standard techniques were used for preparing water samples for oxygen and hydrogen isotope analyses on an isotope ratio mass spectrometer (Epstein and Mayeda, 1953; Coleman et al., 1982); the results are reported in the usual manner as per mil deviations from SMOW (Standard Mean Ocean Water). Compositional data were obtained by ICPMS (magnetic sector inductively coupled plasma mass spectrometry) and analysed using a computer program (EQ3NR) for geochemical aqueous speciation and solubility (Wolery, 1983).
Hydrologic Setting Weldon Spring is located in the town bearing its name in St Charles County, eastern Missouri (Figure 1). A historical plaque at the ori®ce indicates that the spring, named for town founder John Weldon who settled the area in 1789, has a ¯ow rate of 0.04 cubic feet per second (cfs). The spring is hosted in the Mississippianage Burlington Limestone and issues from a small concrete box that pools the water. The ¯ow rate of the spring seems to have increased dramatically over the last few decades, so the historical plaque is no longer accurate. For example, Vineyard and Feder (1982: 192) report a single ¯ow measurement of 0.08 cfs for November 1964. Discharge measurements by the USGS in 1986 and 1988 range from 0.15 to 0.35 cfs (John Shoemacker, pers. comm., 2000). Our visual estimates and ¯ow meter measurements on several occasions since 1995 range from 0.4 to 1.3 cfs. In short, the ¯ow appears to have increased steadily
*E-mail: [email protected] {E-mail: [email protected] {E-mail: [email protected]
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Figure 1. Map of the Weldon Spring region, showing contours of the potentiometric surface (dashed where approximate) for the shallow aquifer. Weldon Spring (elevation 165 m, or 540 ft) is directly down gradient of Prairie Lake (elevation 170 m or 565 ft). Inset shows area of detail, located approximately 40 km west of St Louis. Compiled from maps by Mugel (1996) and Kleeschulte and Emmett (1986).
over the last few decades, with the current rate probably being 10-fold greater than the historical rate. The potentiometric map (Figure 1) of the shallow aquifer in the Weldon Spring area shows that the spring, with an outlet elevation of 165 m (540 ft), is directly down-gradient of Prairie Lake, at an elevation of 170 m (565 ft). Prairie Lake is a 9-ha arti®cial lake located approximately 0.6 km southwest of the Weldon Spring ori®ce (Figure 1). Though we could not document the exact construction date, the lake is shown as a new feature on the 1982 photorevision of the 1954 Weldon Springs 7.50 topographic quadrangle, so it was impounded between 1954 and 1982. This lake appears to be a major source of the spring, as shown by the chemical, isotopic and biological data discussed below.
Isotopic Data and Physical Characteristics Measurements during 1995±2000 have disclosed that Weldon Spring has high d18O values (ÿ3.99 + 1.5-) that are atypical of karst springs in east-central Missouri and western Illinois (Figure 2). The average d18O values for several dozen other springs in this region all range from ÿ6.4 to ÿ8.0, close to the ÿ7.0- value for average meteoric precipitation at St Louis (Frederickson, Criss and Stueber, 1997; Frederickson, 1998; Frederickson and Criss, 1999). For example, Rockwoods Spring, the best-studied
spring in our database, has an average value of ÿ7.42 + 0.75-. Rockwoods Spring is used for comparison in this report (Figure 2) as it is well studied and is located only 17 km to the south of Weldon Spring. Prior to the fall of 1999, the d18O values in Weldon Spring, though anomalously high, exhibited variations that were generally parallel to those of Rockwoods Spring (Figure 2). For example, both springs exhibited relatively low d18O values during the winter of 1997± 1998, when El NinÄo conditions produced large amounts of isotopically-light meteoric precipitation (Criss, 1999). However, subsequent to fall 1999, the trend lines of the two springs are nearly antithetical (Figure 2). The 18O enrichment in Weldon Spring suggests the involvement of an important source component that has undergone signi®cant evaporation (e.g. Criss, 1999). Moreover, the large variations in the d18O values (ÿ6.87 to ÿ1.39-), temperature (6.6±18.98C) and speci®c conductivity (150±393 mS) in the spring also suggest a surface water source, which we hypothesized to be a lake. The consistently high turbidity (19±790 NTU) of Weldon Spring is extremely unusual for karst springs in this region, which are almost always 5 10 NTU except during intense rainfall events. Interestingly, the highest d18O values, the highest and lowest temperatures, the highest turbidity, and the highest ¯ow rate measured for Weldon Spring
Tracing the isotopic, geochemical and biological source of Weldon Spring, Missouri 101
now characterize Weldon Spring (see below). Similarly, the electrical conductivities of all the other lakes and one of the ponds are all signi®cantly higher than that of Weldon Spring, so these cannot represent the dilute endmember that is required by the electrical conductivity data. Moreover, a hydrologic connection between these other surface water bodies and Weldon Spring is very unlikely given the topographic and potentiometric surfaces (Figure 1).
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Year Figure 2. Temporal variations in the d18O values of Rockwoods Spring (r) and Weldon Spring (d). Rockwoods Spring is a typical karst spring in east-central Missouri, and closely approximates a damped running average of meteoric precipitation in the region (Frederickson and Criss, 1999). In contrast, Weldon Spring is comparatively enriched in 18O, suggesting an evaporated source such as a lake. Prior to the Autumn of 1999 (- - -), the trends for Weldon and Rockwoods Springs were approximately parallel, though the d18O values for Weldon Spring are higher and more variable. 1.0 Equilbrium
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Figure 3. Calcite and dolomite in Weldon Spring (d) and Prairie Lake (s) are both undersaturated and show correlated trends. The Q/K term is the quotient of the measured activity product and the calculated equilibrium value at the actual temperature. Comparison with the USGS data for Weldon Spring for 1986 and 1988 (data bars indicating range and average, plotted at arbitrary date) suggests that the degree of undersaturation is increasing over time.
over the 5-year period of observation, were all recorded during the past year. We conducted a directed search and identi®ed Prairie Lake as having the right isotopic, chemical, physical and hydrologic characteristics to be the major source of the water in Weldon Spring. The characteristics of six other lakes and two ponds in the region that were tested are not appropriate to represent the source. For example, the highest turbidity (20 NTU) that has been measured in these bodies is nearly 10 times lower than the high values that
Thermodynamic calculations using the computer program EQ3NR indicate that Weldon Spring and Prairie Lake are signi®cantly undersaturated with respect to both calcite and dolomite, even though the spring is hosted in limestone (Figure 3). The analyses of Weldon Spring made by the USGS in 1986 and 1988 (John Shoemacker, pers. comm.) are closer to equilibrium than our more recent samples, possibly indicating that the spring is moving further away from equilibrium with carbonate minerals over time. Sample pairs collected from the lake and the spring during May±September of 2000 reveal highly correlated trends, supporting the proposed connection between Prairie Lake and Weldon Spring.
Diatom Tracing The yellow-brown color and unusually high turbidities of both Prairie Lake and Weldon Spring suggested the presence of diatoms, which was con®rmed by optical microscopy. Subsequent examination with a scanning electron microscope disclosed the presence of visually similar diatoms in both Weldon Spring and Prairie Lake (Figure 4). These diatoms (Melosira?) are centric and range from 5±10 mm in diameter. Freshwater diatoms are phytoplankton, hence their emanation from a karst spring ori®ce is highly unusual and unequivocally con®rms a surface water source. The fact that similar diatoms are abundant in Prairie Lake con®rms this particular lake as being the source of the spring. The diatoms in Weldon Spring appear to have been abraded or partially dissolved during their subsurface transport from Prairie Lake, and hence have a less well-de®ned external morphology.
Mixing Model When ®rst compared on 15 April 2000, Weldon Spring and Prairie Lake were found to have similar and high d18O values (ÿ2.5 v. ÿ1.9-), low speci®c conductivities (182 v. 107 mS), and unusually high turbidities (193 v. 235 NTU) associated with a yellow-brown color. Mixing calculations based on these results suggest that Prairie Lake now contributes 80±90% of the water in Weldon Spring, assuming that the complementary groundwater component has a composition similar to typical karst springs in the area. In order to further test this result, semi-monthly water samples were subsequently collected from Prairie Lake and Weldon Spring. A plot of the average electrical conductivity v. the d18O values shows that Weldon Spring is very similar to Prairie Lake, but their dierence in detail requires a subordinate component
102 R.E. Criss et al.
Figure 4. Centric diatoms (Melosira?) in Prairie Lake (a) appear to be the same species as those emerging from Weldon Spring (b). Both have a porous ``hub'' surrounded by radial ``spokes'', though the spring diatoms appear to be abraded or partially dissolved so these features are less distinct. A hydrologic connection between the lake and the spring is unequivocal.
that has a higher conductivity and a lower d18O value, which are normal characteristics for shallow groundwaters in the area. If it is assumed that this latter component has a composition similar to the long-term average of Rockwoods Spring (Figure 5), then the mixing line indicates that Weldon Spring now represents a mixture of Prairie Lake water plus the groundwater component in an approximately 4 : 1 ratio. It is useful to compare the temporal variations of the d18O values, electrical conductivity, turbidity and temperature of Weldon Spring and Prairie Lake over the last 6 months. Figure 6 clearly shows that the temporal variations in d18O (Figure 6(a)) at Weldon Spring are parallel to those in Prairie Lake, though not identical. As mentioned above, the d18O and conductivity (Figure 6(b)) dierences between the spring and 700 Rockwoods Spring (19962000)
600
E.C., µs
500
Conclusions
400 Other Lakes
300 200
Weldon Spring
100 0
the lake are primarily due to the groundwater component. The dierence in turbidity (Figure 6(c)) may re¯ect the incorporation of clastic materials in the ¯owing groundwater; the dierence in temperature (Figure 6(d)) primarily re¯ects the normal attenuation of seasonal temperature variations in the shallow subsurface. The close parallelism of the time series trends for Weldon Springs and Prairie Lake indicates that the subsurface transit time is very short, probably 52 weeks, in turn suggesting the dominance of ¯ow through rather large conduits. Continued time series data will allow the subsurface transit time between Prairie Lake and Weldon Spring to be determined accurately, will disclose whether the conduits are enlarging over time, and will reduce the uncertainty of the relative proportions of groundwater and lake water in the spring. Additional work will also indicate whether the short residence time of water in Prairie Lake, which may now be less than 1 year because of the leak, is responsible for the low electrical conductivity and the low d18O values of Prairie Lake relative to other lakes in the area.
Prairie Lake
8
7
6
5
4
3
2
1
0
d18O
Figure 5. The average electrical conductivity and d18O value of Weldon Spring (d) since April 2000, lies on a mixing line between Prairie Lake (s) and a groundwater component that is similar to the long-term average value for Rockwoods Spring (r). Weldon Spring appears to be a mixture of approximately 80±90% lake water and 10±20% groundwater. The average composition of six other lakes (j) located 3±7 km to the west (Figure 1) does not constitute an appropriate source for the spring.
Weldon Spring is signi®cantly enriched in 18O relative to other karst springs in east-central Missouri, suggesting that an evaporated surface water body is an important source component. We have conclusively veri®ed the hypothesis that Prairie Lake represents this component using a remarkably congruent data set that includes isotopic, chemical and physical data on water samples, potentiometric contours, ¯ow rate measurements on the spring, and tracing of diatoms. Calculations indicate that Weldon Spring is now a mixture of approximately 80% lake and 20% groundwater components, in good agreement with discharge measurements that suggest that the ¯ow rate of the impacted spring is now approximately 10 times the historical rate. The available data suggest that the connectivity between Prairie Lake and Weldon Spring may be increasing over time.
Tracing the isotopic, geochemical and biological source of Weldon Spring, Missouri 103 0
Acknowledgements
(a)
0.5
We thank GC Frederickson, AM Stueber, and ML Davisson for valuable discussion. H Levin, DT Kremser and Jane C Walker provided expertise and ideas related to the diatom study. Kelly Carbery collected many of the samples and Rachel Lindvall performed cation analysis with an ICPMS. We thank Pastor D Miller and his congregation for maintaining the spring site, preserving its history, and allowing us access. This study was supported by NSF hydrology grant EAR 9814621.
δ18O
1 1.5 2 2.5 3
200
(b)
References
180 EC µS
160 140 120 100 80 (c) 700
NTU
600 500 400 300 200 (d) 30
T°C
25 20 15 10 100
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Figure 6. Temporal variations in the d18O values (a), electrical conductivities (b), turbidities (c) and temperatures (d) of Prairie Lake (s) and Weldon Spring (d), since detailed sampling began in April 2000. These features exhibit highly similar variations that prove their communication over a short time scale.
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