What is Isotope Hydrology Application of geochemical methods involving analyses of environmental isotopes (naturally occurring stable isotopes and uns

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1 Isotope Hydrology and Source-Water Protection: an Investigation from Lafayette Parish, Louisiana Bruce K. Darling, Ph.D. Geochemist/Hydrogeologist LBG-Guyton Guyton Associates Lafayette, Louisiana

2 What is Isotope Hydrology Application of geochemical methods involving analyses of environmental isotopes (naturally occurring stable isotopes and unstable isotopes) and unstable isotopes derived from anthropogenic sources to resolve questions related to the origin, flow, mixing, i and age of ground waters and surface waters.

3 Isotopic Methods in Hydrology Widely used to differentiate between sources of water Delineate recharge areas and flow paths Estimate residence times of ground waters Differentiate between potential sources of contamination

4 Lafayette Utilities System was interested in determining risk of contamination of sands of Upper Chicot aquifer from landfills, storage tanks, pipelines, agricultural activities, etc.

5 Questions Asked by LUS Is there evidence of modern recharge in Lafayette Parish? If so, how variable are recharge signatures? What do the isotopic signatures indicate with respect to the potential for contamination of major water producing sands of the Chicot aquifer?

6 LBG-Guyton Guyton recommended Collection of ground water samples for analysis of two radionuclides: Carbon-14 ( 14 C) Tritium ( 3 H) And two stable isotopes: Oxygen-18 (δ O) Deuterium (δ 2 H)

7 13 wells Sampling Program Nine public supply wells One municipal well not used for public supply Three private supply wells Three upper Chicot, 10 lower Chicot Upper Chicot 65 to 270 ft Lower Chicot 452 to 567 ft Samples analyzed for Major cations and anions Tritium ( 3 H) Carbon-14 ( 14 C) and Carbon-13 (δ 13 C) δ 18 O and δ 2 H

8 Francois Coulee Drainage Network Source: A hydrogeological study of the Chicot aquifer in Lafayette Parish, Louisiana (Williams, 1996)

9 Location of Wells in Lafayette Parish ND-1 CC-4/CC-5 CC-3 DB-1 DAL-1 VC L-1/L-12 DL L-3/L-4 YV-3

10 Chicot Aquifer Recharge Areas

11 N-S Chicot Aquifer Cross Section Source: LaDOTD/USGS Water Resources Technical Report 66

12 E-W Chicot Aquifer Cross Section Source: LaDOTD/USGS Water Resources Technical Report 66

13 Tritium 3 H Naturally occurring isotope of hydrogen Produced in upper atmosphere by interaction of Nitrogen-14 ( 14 N) with flux of neutrons generated by cosmic radiation: 14 N + n 12 C + 3 H Also generated by reactions associated with nuclear power plants and detonations of thermonuclear bombs Half-life life years

14 Tritium Incorporated directly into water by oxidation: 3 HHO or 3 H 2 O Activity measured by low-level level counting by Tritium Laboratory, Rosenstiel School of Marine and Atmospheric Sciences, ces, University of Miami Effective tracer of ground waters with recharge dates of ~50 years or younger

15 Tritium 3 H Concentration measured in Tritium Units A Tritium Unit is: 3 One atom of Hper10 18 atoms of H

16 Thermonuclear Bombs - Principal Anthropogenic Sources of Tritium

17 Operation Ivy, Shot Mike Yield 10 Mt, Date: November 1, 1951

18 Dt Detonations ti of fat Atomic Bombs and de Especially ill Thermonuclear Bombs Generated Tritium far in Excess of Tritium from Natural Processes

19 Pre-bomb levels of 3 H in North Hemisphere were estimated to be 5 TU per year. Generation of 3 H during period of above-ground testing of thermonuclear bombs rose to more than 2,000 TU by Since 1963 the amount of 3 H has decreased Since 1963, the amount of H has decreased to nearly pre-bomb levels.

20 10,000 1, H Measured in Precipitation at Ottawa, Canada ( ) 3 H H (Tritium Un Aug-53 Aug-56 Aug-59 Aug-62 Aug-65 Aug-68 Aug-71 Aug-74 Aug-77 Aug-80 Aug-83 Aug-86 Aug-89 Aug-92 Aug-95 Aug-98 Month - Year

21 3 H Measured in Precipitation at Baton Rouge, LA and Waco, TX 10,000 1,000 (Tritium Uni its) 3 H 100 Baton Rouge Waco 10 1 Dec-61 Dec-63 Dec-65 Dec-67 Dec-69 Dec-71 Dec-73 Dec-75 Dec-77 Dec-79 Dec-81 Dec-83 Dec-85 Month - Year Source: IAEA s Global Network of Isotopes in Precipitation Database

22 Decay Curve Based on 3 H in Precipitation at Baton Rouge, its) ritium Un 3 H (T Year

23 Results of Analyses for Tritium in samples of ground water from Lafayette Parish wells 3 H (TU) ND-1 CC-3 CC-4 CC-5 DB-1 DA-1 VC L-1 L-12 L-3 L-4 YV-3 DL-1 Well

24 Tritium Analyses Indicate Demonstrably bomb to post-bomb bomb values for wells with depths 150 ft or less Probably pre-bomb bomb values for wells greater than 150 ft Possibly vertical leakage of younger water into deeper sands as result of gradients imposed by pumping Youngest waters occur in northern and eastern areas of Lafayette Parish

25 Radiocarbon 14 C Naturally occurring isotope of carbon Formed in upper atmosphere by interaction of 14 with neutrons generated by cosmic radiation: 14 N + n 14 C + H 14 N Also generated by complex reactions associated with with nuclear power plants and detonations of thermonuclear bombs Half-life life ~5,540 years

26 Radiocarbon 14 C Becomes incorporated as a dissolved constituent of water as follows: 14 CO 2 + H 2 O H 14 2 CO 3 then H 2 14 CO 3 H + + H CO 14 CO - 3

27 A = A -λt o e Radiometric Decay Equation A = Activity of sample A o = Initial activity of sample λ = Decay constant (ln(2)/half-life life of isotope) t = Time elapsed since initiation of decay

28 14 C Decay Curve Carbon) t Modern C (Percen ,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 Years

29 14 C as a Function of Depth in Lafayette Parish Wells 4 C (PMC) C Fit 20 0 r 2 = Depth of Well (Ft)

30 Apparent and Adjusted 14 CAges 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 ND-1 CC-3 CC-4 CC-5 DB-1 DA-1 VC L-1 L-12 L-3 L-4 YV-3 DL-1 Apparent Adjusted

31 14 C Analyses Indicate Ages ranging from modern to between 4,000 and 6,000 years Youngest 14 C ages associated with bomb/post-bomb 3 H Oldest 14 C ages associated with deeper wells

32 Correlation between 3 H and 14 C in Lafayette Parish Ground Waters 3 H (T Tritium Un nits) TU = *PMC r 2 = C (Percent Modern Carbon)

33 Combined 3 Hand 14 C Analyses Indicate Reasonable correlation between 3 Hand 14 C Probable mixing between bomb/pre-bomb and post- bomb waters Youngest waters typically occur in shallow sands of northern and eastern areas of Lafayette Parish Oldest waters tapped by deeper supply wells Best strategy to minimize threat of contamination o is to drill municipal supply wells to sands of lower Chicot aquifer

34 Stable Isotopes of Oxygen and Hydrogen Oxygen-18 ( 18 O) Abundance reported as δ 18 O Standard Mean Ocean Water (SMOW) Deuterium ( 2 H) Abundance reported as δ 18 O SMOW Abundances vary according to fractionation processes associated with temperature of precipitation, amount of precipitation, i i evaporation, elevation, and ddi distance from source

35 Annual Mean δ 18 O in Precipitation over North American Continent Source: Global Network of Isotopes in Precipitation Database

36 For 18 O and 2 H Precipitation becomes increasingly depleted in both isotopes toward the higher latitudes in the northern hemisphere Weighted means vary from month to month During glacial epochs, precipitation was more depleted

37 Weighted Monthly δ 18 O in Precipitation over North American Continent Source: Global Network of Isotopes in Precipitation Database

38 Annual Mean δ 2 H in Precipitation over North American Continent Source: Global Network of Isotopes in Precipitation Database

39 Weighted Monthly δ 2 H in Precipitation over North American Continent Source: Global Network of Isotopes in Precipitation Database

40 Distribution of Stable Isotopes in Ground Waters of Southern Louisiana δd D o /oosmow δ 18 O ( o / oo SMOW) GMWL Laf - USGS Laf - LBG Acadia Allen Evangeline Jeff Davis St. Landry Vernon

41 δ 18 O and δ 2 H of Chicot Aquifer Waters Are consistent with normal abundances for the region Are not indicative of precipitation in colder and wetter climatic regime Do not manifest patterns indicative of evaporative enrichment

42 Conclusions Shallow ground waters are demonstrably modern Ages increase (often sharply) with depth François coulee area of northern Lafayette Parish appears to have greatest potential for recharge Safest strategy for LUS and other suppliers of public water is to complete wells in lower Chicot sand