insights into karst groundwater- stream interactions using...
TRANSCRIPT
Insights into karst groundwater-stream interactions using dissolved
natural radon concentrations, Central Texas
Stephanie S. Wong and Joe C. Yelderman Jr.Department of Geology | Baylor University | One Bear Place #97354 | Waco TX 76798
Presented at the NGWA Groundwater Summit 2015Monday, 16 March, 2015 || San Antonio, Texas
Outline
• Background – Groundwater/Surface water interactions in karst– Setting– Natural radon (Rn-222)– Research question
• Methods• Results• Conclusions• Acknowledgements
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Groundwater / Surface Water Interactions in Karst Systems
streamflowwatercress
3
Northern Segment
Barton Springs Segment
San Antonio Segment
Setting: The Edwards BFZ Aquifer
SALADO
SAN ANTONIO
AUSTIN
4(I-35 CAC, 2011)
Setting: The Edwards BFZ Aquifer
5
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
201320101990198019701960
Popu
latio
n
(United States Census Bureau, 2015)
2013: 326,843
Bell County Population
What are the impacts of construction and
increasing resource demands on the aquifer?
(I-35 CAC, 2011)
Setting: The Northern Segment, Edwards BFZ Aquifer
A A’
SALADO
AUSTIN
AUSTIN CHALK
EAGLE FORD GROUP
BUDA LIMESTONESDEL RIO CLAY
EDWARDS LIMESTONES
WALNUT FORMATION
Outcrop portion
Down-dip portion
Bell
Milam
Williamson
Bastrop
Travis
Burnet
Hays
6
Study area: Salado Springs complex
Big BoilingSpring
Anderson Spring
Critchfield Spring
Doc Benedict Spring
Little Bubbly Spring
• Historical – stage stop• Present – local landmark
(tourism)• Habitat for Salado
salamander, E. chisolmensis• Local groundwater
management measure USGS stream gage
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Big Boilingsprings
Salado salamander
Big BoilingSpring
Anderson Spring
Critchfield Spring
Doc Benedict Spring
Little Bubbly Spring
USGS stream gage
8
Study area: Salado Springs complex• Historical – stage stop• Present – local landmark
(tourism)• Habitat for Salado
salamander, E. chisolmensis• Local groundwater
management measure
Radon‐222
• Naturally-occurring; trace amounts in soils and geology
• Radioactive alpha decay product of radium-226 (uranium-238 series)
• Rapid dispersal from groundwater to atmosphere
• Applications (over short time scales): apparent age estimation, infiltration rates, discharge location and magnitude, fracture aperture estimation, contamination studies
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222Rn Half-life = 3.8 days
No Rn-222
Increasing Rn-222Decreasing Rn-222
Radon‐222 conceptual model
(Ref.: Stellato et al., 2012; Neupane et al., 2014; Hoehn and Von Gunten, 1989) 10
Is this true at Salado Creek?
Equilibrium
Distance or time since infiltration
Rn-
222
Distance or time since discharge
Rn-
222
Methods: field sampling
11
• Sampling:– Zero headspace– Sampling depth (~6 in)
• Focused sampling:– Salado Creek – Salado Springs complex
• Monitoring samples:– Big Boiling springs/Salado
Creek
Methods: analysis
RAD7 with the RAD H2O accessory
12
• Analysis within 24 hours• Instrument: RAD7• Solid state alpha detector
– Conversion of alpha radiation to an electrical signal
Radon concentration: Salado Creek basin(May‐June 2014)
127
3
8529
5
35 5 39 2
60113
80
LK STILLHOUSEHOLLOW
I‐35
I‐195
13* Radon concentrations are in pCi/L
222 R
n (p
Ci/L
)
5
125
127
3
85
295
35
5 3 9 2
60
113
80
Big BoilingSpring
Anderson Spring
Critchfield Spring
Doc Benedict SpringLittle Bubbly
Spring
238 228
15893
179
180
MEAN RADON CONCENTRATION:
Groundwater = 190 pCi/LSurface water = 84 pCi/L
Radon concentration: Salado Springs(March 2014)
14
* Radon concentrations are in pCi/L
58
USGS Stream Gage
Above dam: 104
Below dam: 80
194
Salado Creek
Big Boiling Springs
Spring, 2014
160
62
113
Autumn, 2014
164
44
102
Radon concentration: seasonal patterns
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* Radon concentrations are in pCi/L
050
100150200
1 2 3
Rn-
222
050
100150200
1 2 3
Rn-
222
Salado Creek
Big Boiling Springs
Radon concentration: seasonal patterns
Season Modeled GW flow (cfs)
Observed GW flow (cfs)
Spring 2014 3.23 1.64
Autumn 2014 1.79 1.1
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(Lee and Hollyday, 1991)
Decreasing Rn-222
Distance or time since discharge
Rn-
222
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1. Clear difference between groundwater and surface water Rn‐222 concentrations at downtown Salado
Increasing Rn-222
Distance or time since infiltration
Rn-
222
Equilibrium
0 pCi/L
84 pCi/L190 pCi/L
• Complimented by other field measurements, Rn-222 concentration allow identification, confirmation, and monitoring of groundwater discharge sites
Cascade
2. Rn‐222 concentrations at Salado Springs support stream gauge site
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Big BoilingSpring
Anderson Spring
Critchfield Spring
Doc Benedict SpringLittle Bubbly
Spring
USGS stream gage
• No major groundwater addition after Salado Springs
• Additional geochemical and dye trace data indicate spring connectivity
• Stream gauge location is suitable for aquifer monitoring
3. The use of Rn‐222 as a natural tracer is relevant in a critical habitat setting
Big BoilingSpring
Anderson Spring
Critchfield Spring
Doc Benedict Spring
Little Bubbly Spring
• Short half-life – useful in karst system
• Relatively quick analysis turn-around time
• Natural tracers are useful in critical habitat settings
USGS stream gage
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Salado salamander
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Sources• Cecil, L.D. and J.R. Green, 2000, Radon-222. In Environmental Tracers in Subsurface Hydrology. P. Cook, and
A.L. Herczeg (eds.), pp. 175-194.• Cothern, C.R. and J.E. Smith, Jr., 1987, Environmental Radon, Environmental Science Research Volume 35.
Plenum Press, New York. 363 p.• Durrance, E.M., 1986, Radioactivity in Geology. Ellis Horwood Limited, Chichester, 441 p.• Durridge Company, 2014, RAD7 Radon Detector User Manual. Durridge Company Inc., Billerica, Massachusetts.• Ellins, K.K., A. Roman-Mas, and R. Lee, 1990, Using 222Rn to examine groundwater/surface discharge interaction
in the Rio Grande de Manati, Puerto Rico. Journal of Hydrology, 114(1-4):319-341.• Hoehn E. and H.R. Von Gunten, 1989, Radon in groundwater: a tool to assess infiltration from surface waters into
aquifers. Water Resources Research, 25(8):1795-1803.• Interstate 35 Corridor Advisory Committee (I-35 CAC), 2011, I-35 Corridor Advisory Committee Plan.
<http://ftp.dot.state.tx.us/pub/txdot-info/my35/advisory_plan.pdf> Last accessed 11 March, 2015.• Lee, R.W. and E.F. Hollyday, 1991, Use of radon measurements in Carters Creek, Maury County, Tennessee, to
determine location and magnitude of groundwater seepage. In Field Studies of Radon in Rocks, Soils, and Water. L.C.S. Gundersen and R.B. Wanty (eds.), pp.237-242. C.K. Smoley, Boca Raton, Florida.
• Neupane, R.P., J.D. White, P.M. Allen, and S.I. Dworkin, 2014, A snapshot comparison of environmental tracers for estimating groundwater contribution to recession flow in a mountain stream. Hydrological Processes, in press.
• Stellato, L., F. Terrasi, F. Marzaioli, M. Belli, U. Sansone, and F. Celico, 2013, Is 222Rn a suitable tracer of stream-groundwater interactions? A case study in central Italy. Applied Geochemistry, 32:108-117.
• Torgersen, T., J. Benoit, and D. Mackie, 1992, Lithological control of groundwater 222Rn concentrations in gractured rock media. In Isotopes of Nobel Gases as Tracers in Environmental Studies, pp.263-287.IAEA, Vienna.
• United States Census Bureau, 2015, State and County QuickFacts. <http://quickfacts.census.gov/qfd/states/48/48027.html> Last accessed 10 March, 2015.
• Photo credits: P. Diaz, S. Wong, J.C. Yelderman Jr.
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