USING GROUNDWATER AGE AND OTHER ISOTOPIC SIGNATURES TO DELINEATE GROUNDWATER FLOW AND STRATIFICATION

Transcription

1 USING GROUNDWATER AGE AND OTHER ISOTOPIC SIGNATURES TO DELINEATE GROUNDWATER FLOW AND STRATIFICATION Jean E. Moran, Lawrence Livermore National Laboratory, U.S.A., and G. Bryant Hudson, Lawrence Livermore National Laboratory, U.S.A.,

2 The Question How will water quality in deep aquifers used for public water supply be affected by vertical transport The Tools Groundwater Age (Tritium-Helium method) Oxygen isotopes (recharge water source) Ultra low level VOC analysis (part per trillion detection limits)

3 Atmospheric nuclear testing introduced a large amount of tritium into the atmosphere in the early 1960 s H at recharge (pci/l) Portland Santa Maria Ottawa Recharge Year Water recharged before 1950 does not contain tritium now

4 For every tritium decay, an atom of 3 He is produced Tritium atoms Helium-3 atoms Time (years) p n n Beta decay p p n 3 H 3 He

5 The 3 He from 3 H decay starts to accumulate once the water has become groundwater 0 years 12 years 24 years Age (years) = 18 x ln( He / 3 H )

6 Results from H- 3 He age measurements in Orange Co. CA Groundwater age in years

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10 Mean Groundwater Age

11 VOC Detections

12 MTBE Concentrations

13 Groundwater age and Fraction Pre-Modern Increase with Depth (80%) (100%) (0%) (0%) (0%) (0%) (0%) (36%) (100%) (28%)

14 Kaiser Pond < 1 yr. 21L05 < 1 yr. 21L03 PT-7 PT-8 PT-6 5 yrs. 6 yrs.4yrs. PT yrs. PT-4 PT yrs. 8.8 yrs. M-7: 5.5 yrs. M-3: 19.7 yrs M-2: 30.8 yrs. M-9: 19 yrs. Above the Hayward Fault, young groundwater is observed throughout the alluvial section Below the fault, groundwater is stratified

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16 In the Llagas Basin of Santa Clara County, groundwater stratification is related to water quality Wren Leavesley Gilman Luchessa Recharge to the shallow aquifer is widely distributed, with frequent occurrence of groundwater ages of < 7 years

17 Aquifer Interbedded Aquitard Elevation (m) x (m) 5000 y (m) y (m) A geostatistical representation of heterogeneous basin materials supports a fully confined region in the southern part of the basin where tritium-dead groundwater is observed in deep wells Elevation (m) y (m) Aquifers Interbedded x (m) Aquitards Basement

18 CSUS campus: CSUS MW-3, 3A CSUS EXT DWR MW-3, 3A DWR MW-2, 2A American River N DWR MW-1, 1A 0 m 300

19 CSUS Wellfield 0 50 Unconfined interval Shallow Monitoring Wells 100 Extraction Well Feet Confining layer Confined interval Deep Monitoring Wells = unconfined interval = confined interval = confining layer = sandy interval Feet

20 MtBE and δ 18 O in shallow wells match observations in river water

21 The slurry wall is not effective in area 2

22 Recharge rates can be determined from ages in nested monitor wells In this example, the average vertical recharge rate is 5.7 ft/yr; higher in the upper intervals

23 Fate and Transport of Nitrate Below Dairies Dairy #1 (Kings County) Irrigation Canal (Sierra Snowmelt) Dairy #2 (Merced County) Dairy Field Sites Groundwater flow Lagoons Dairy Dairy #1 (Kings County) Multi-level level well + CPT, DP CPT, DP only 1 mile Multi-level 2 monitoring wells 3-4 levels in perched aquifer Continuous core recovered Cone Penetrometer (CPT) survey Physical sediment properties Direct Push (DP) survey Synoptic water & soil sampling

24 Depth (ft) Groundwater age High NO 3 Little excess N 2 Moderate O 2 Age 0-21 yr Low NO 3 Excess N 2 Low O 2 Age 0-36 yr Unsaturated Low O 2 OXIC ANOXIC Perched Aquifer Groundwater age gives an estimate of the rate of denitrification 4 8 Deep Aquifer No NO 3 - No excess N 2 Age = >50 yr Depth (ft) Depth (m) H/ 3 He Apparent Age (years) Multi-level wells: Excess N 2 by MIMS, Age by 3 H- 3 He

25 Groundwater flow simulations incorporating realizations of site hydrogeology suggest that irrigation drives the local groundwater cycle Future work will include forward and backward particle tracking and direct model calibration using groundwater age

26 Conclusions Groundwater age measurements in tightly-spaced wells allow delineation of recharge areas and flow patterns, including bulk flow rate Groundwater age can indicate vertical transport of young groundwater to deep aquifers Distinctly stratified groundwater ages offer evidence for relatively continuous confinement Groundwater ages can be used to assess aquifer contamination vulnerability and to predict future water quality at deep-screened wells

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28 Assessing Groundwater Vulnerability Water Age VOCs detected? Vulnerability Post-modern Yes Vulnerable sources present Post-modern No Vulnerable no sources Pre-modern No Low vulnerability Pre-modern Yes Short-circuit Type(s) of VOCs detected, and age analysis including mixing, give additional time-of-travel information.

29 Nearly all the water produced from Sacramento wells is pre-modern

30 Young groundwater is observed only near the river

31 PCE was detected in about half of the drinking water wells

32 Coastal Basins 1 N=194 Central Valley Basins 2 N=338 No VOCs (54%) 48 (14%) MtBE 29 (15%) 87 (25%) PCE 18 (9%) 166 (49%) CHCl 3 68 (35%) 252 (74%) 1 Includes Santa Clara Valley, Basins of San Mateo County (San Mateo Plain, Westside Basin and Coastside Basin), Livermore-Amador Basin, and Niles Cone (Fremont, CA) and 2 Includes Bakersfield, Chico and surrounding northern Sacramento Valley, Sacramento, San Joaquin County urban areas (Stockton, Lodi, and Manteca); wells from areas outside of alluvial basins are not included on this table 3 Samples had <RL for all compounds listed in Table 1

33 The river recharge phenomenon occurs all along the Central Valley

34 Results are from Long-screened Production Wells Advantages You get a good, clean sample (both dissolved gases and low level VOCs) Characteristics are usually known It s the water people are using and drinking Disadvantages Vertical information is smeared Signal is diluted Flow field is altered It s the water people are drinking

35 Atmospheric nuclear testing introduced a large amount of tritium into the atmosphere in the early 1960 s H at recharge (pci/l) Portland Santa Maria Ottawa Recharge Year

36 American River water recharges a large part of the basin

37 The ratio of 18 O to 16 O in precipitation varies because lighter 16 O is favored during evaporation while 18 O is favored during condensation. The net effect is lighter water at colder, higher elev.

38 In California, Imported Water Serves as a Natural δ 18 O Tracer Ambient Groundwater In the Tri-Valley Area δ 18 O -7.7 Hetch Hetchy water Map Source: California Data Exchange Center

39 Groundwater Age Dating Isotopes Isotope Half Life (yrs) Limitations 85 Kr L needed 3 H 3 He Excess air and terrigenic He 32 Si 150 Complex chemistry/analysis 39 Ar L, special counting tech. 14 C 5730 Complex biogeochemistry 81 Kr 229,000 need 16,000 L 36 Cl 301,000 Water-Rock interaction, need AMS 129 I 15.6 x10 6 Need AMS CFCs cover roughly the same span as 3 H and 85 Kr Radiogenic 4 He covers roughly the same span as 14 C

40 % equilibrium saturation 250% 150% 50% 3 He from 3 H decay 4 He from U, Th decay 3He 4He Ne Ar Kr Xe Measurement of dissolved gases gives: Recharge temperature Amount of excess air 3 H- 3 He age 4 He accumulation age

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42 Comparing the decay-corrected tritium for the sample and the initial tritium at recharge exposes pre-1955 water H at recharge (pci/l) GW samples 0% pre % pre % pre % pre 1955 From Talbert Gap "Water Factory 21" Recharge Year

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44 Fremont wells have narrow age distributions; some Livermore wells have older water components H at recharge (pci/l) Portland Santa Maria Ottawa Fremont Livermore Recharge Year