applications of dissolved n2 and ar in groundwater · applications of n 2 – ar measurements in...

Applications of Dissolved N2 and Arin Groundwater

L. Niel Plummer, Eurybiades Busenberg, and Peggy K. Widman

U.S. Geological SurveyReston, VA

Ø O2 unstable

Ø N2 can be affectedby NO3 reduction

Ø Terrigenic sourceof He

Ø Most reliable are Ar,Ne, Kr, Xe

Ø Ar and N2 solubilitiesfrom air are 3-5 orders of magnitude > than

that of remaining noble gases. Canbe measured by gaschromatography.

0 10 20 30 40

Temperature, oC

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Co

ncen

trat

ion,

ccS

TP/g

Air - Water Equilibrium

N2

O2

Ar

NeKr

HeXe

Applications of N2 – ArMeasurements in Groundwater

• Reconstruct recharge temperatures—ü KH calculation in dating with atmospheric gases

such as CFCs and SF6

ü Paleoclimatic reconstruction (check 14C age)• Reconstruct excess air—ü Function of recharge rate and hydrogeologyü Needed in dating with CFCs and esp. SF6.

• Trace water sources through aquifers• Recharge mechanism— Focused/diffuse recharge;

rapid recharge; (altitude of recharge)• Reconstruct initial nitrate in agricultural studies

BACKGROUND: N2-Ar

• Heaton, Vogel, Talma (early 1980s)Paleorecharge temperatures, excess air,

denitrification.• Don Fisher, USGS (1970s)

Developed GC method, field sampling device.• Current state at USGS

Dual column GC method, new sample collection, QA/QC procedures, software. Extensive use in agricultural studies, dating with CFCs and SF6.

Bruce B. Hanshaw (the late), at Philip flowing well, Philip, South Dakota,68oC, Madison aquifer, circa 1979; Bruce had a good sense of humor.

Fisher dissolved gas sample container

Sample Collection

• 160 cc glass bottle with septum stopper• Filled without headspace under water• Needle used to release fluid as stopper inserted• Needle removed.• Bottle weight measured empty and full.

CTR III / TCD

Inner Column Outer Column

•Measures Ar, N2, and O2+Ar; O2 determined by difference•Thermal Conductivity Detector

CTR III/ TCD

CTR I / FID

Nafion DrierNafion Drier

Gas flow

ControlGas flow

Control

Schematic of analytical system

• Headspace equilibrated >24 hrs• Water temp. measured in bottle immediately after injection• All measurements made on same initial vol. of gas• Two columns (each a double column; two detectors.• Calibrated with known gas standards.•QA/QC daily with water samples equilibrated in water baths at 9, 16, and 24 oC in same sample container.

• Separates all the gases•Used only for CH4

and CO2

• CO2 converted to CH4 for measurement with FID

Temperature of Standard Water - N2/Ar Calculated Temperature

March 2004-July 2004

-1.00

-0.50

0.00

0.50

1.00

3/1/20

04

3/2/20

04

3/3/20

04

3/5/20

04

3/8/20

04

3/12/2

004

3/15/2

004

3/22/2

004

3/23/2

004

3/24/2

004

3/26/2

004

3/29/2

004

3/30/2

004

4/19/2

004

4/20/2

004

4/21/2

004

4/28/2

004

4/29/2

004

5/3/20

04

5/5/20

04

5/6/20

04

5/7/20

04

5/19/2

004

5/20/2

004

5/21/2

004

5/25/2

004

5/27/2

004

6/1/20

04

6/4/20

04

6/14/2

004

6/21/2

004

6/22/2

004

6/24/2

004

6/28/2

004

7/6/20

04

7/7/20

04

7/9/20

04

7/12/2

004

7/14/2

004

7/15/2

004

7/20/2

004

7/22/2

004

7/22/2

004

Date of Analysis

Dev

iatio

n fr

om

cal

cula

ted

N2/

Ar

tem

per

atu

re f

rom

sta

nd

ard

wat

er

Blue = 9oCYellow = 16oCRed = 24oC

0.5 oC

-0.5 oC

Water StandardsBlue = 9 oCYellow = 16 oCRed = 24 oC

http://water.usgs.gov/lab/cfc

See extensive bibliography

Why Measure N2 – Ar, when suites of noble gases can be measured?

• Relatively high concentrations; Waters can be analyzed by relatively simple GC methods.

• In some cases, most of the information obtained from noble gases can also be obtained from N2-Ar

• Reconstruct amounts of denitrification. Initial NO3; Agricultural studies; fertilizers fate.

• Cost effective. Large numbers of samples can be collected and analyzed; potential for mapping flow in aquifers.

Limitations of N2 – Ar data

• Only 2 independent measurements, but as many as 4 unknowns (Recharge temperature; Excess air; Altitude of recharge; Excess N2).

• Gas fractionation? Need additional gases to even detect. Most applications of N2 – Ar assume complete dissolution of excess air. This is an approximation, but considering the relatively high concentrations of N2 and Ar in groundwater, appears to be a reasonable assumption.

Recharge Temperature - Excess Air

N2(meas)= N2(equil)+N2(Ex air)+N2(denit)

Ar(meas)=Ar(equil)+Ar(Ex air)

N2 (mg/L)

Ar

(mg

/L)

Air-W

ater e

quilib

rium

Excess Air, cc/L

RechargeTemperatureIn Degrees C

Samples from wells in fractured rock in VirginiaRT 9 – 14 oC3-10 cc/L excess air

Aerobic watersN2(denit) = 0

Altitude of rechargeassumed

Lewis Mountain Spring

Sampling from a TypicalSpring Box and Weir

Springs and wells from Blue RidgeMountains of Virginia; ShenandoahNational Park

0

4

8

12

Exc

ess

Air

(cm

3 ST

P k

g-1),

Hel

ium

Springs, 1996Springs, 1997Wells, 1996Wells, 1997

A.

1:1

Line

0 4 8 12Excess Air (cm3STP kg-1), Neon

0

4

8

12

Exc

ess

Air

(cm

3 ST

P k

g-1),

N2-

Ar


B.

1:1

Line

Largest deviations probably gas leaksin sample container.

Comparison of excess aircalculated from mass spectrometric He and Newith excess air from GC N2 -Ar

Springs and wells, Blue Ridge, VA

No evidence of gas fractionation

0.4

0.6

0.8

1.0

Dis

solv

ed A

r (m

g/L)

SpringsApril, 1996August, 1997

02

4

8

Excess Air (cm

3 L-1 )

5

0

10

15

20

Recha

rge

Tem

p. (

o C)

A.

12 16 20 24 28Dissolved N2 (mg/L)

0.4

0.6

0.8

1.0

Dis

solv

ed A

r (m

g/L

)

WellsApril, 1996August, 1997

02

4

8

Excess Air (cm

3 L-1 )

5

0

10

15

20

Recha

rge

Tem

p. (

o C)

B.

Springs and Wells– Blue Ridge Mts., VA

• Springs—residuum and colluvium; 0-2 cc/L excess air [sand aquifers]

• Wells-- fractured rock; 0-8 cc/L excessair [river floods; arroyos; fractured rock]

• Wide range of RT for springs;• Narrow range of RT for wells• Mean annual Temp about 7.8 oC

Springs and wells Blue Ridge, VA

• Most springs have recharge temperatures near the water temperature (shallow)

•Seasonal dependence of RT (shallow recharge)

• A few springs warmed in circulation (deeper)

• Discharge from most wells warmed in ground (deeper)

• 3 samples cooled following a summer thunderstorm

0 4 8 12 16 20Water Temperature (oC)

0

4

8

12

16

20N

2/A

r R

ech

arg

e T

emp

erat

ure

(oC

)


1:1 L

ine

CoolingAfterRecharge

WarmingAfterRecharge

Mean AnnualTemperature

Böhlke and Denver (1995)

•CFC ages and NO3: 40-yr record of NO3 recharge rate.•Records increase in fertilizer application from the 1970’s.•20-35 % of applied fertilizer reached the aquifer.•Mean residence time of 20 yrs for gw discharge to local streams.

Delmarva PeninsulaAtlantic Coastal Plain

What to do about denitrification?• Aerobic samples; No denitrificationü Assume altitude of recharge and solve N2 and Ar for

T and Excess air• Anaerobic samplesü If Ne data available, solve N2, Ar, Ne for T, Excess

air and Excess N2 (at assumed recharge altitude)ü If no Ne data, determine an average T or excess air

from other aerobic samples in the data set.• Can Probably ignore denitrification in most

paleowaters

Denitrification

Excess Air

Average Recharge Temperature 14.3 ± 2.0

Average Excess Air3.0 ± 1.1 cc STP/L

All samples aerobic (3-8 mg/L O2) except Sample A(in duplicate)

A

Sample A:16 mg/L of N2 fromdenitrificationbrings recharge temperature to 13.2 oC and excessair to 4.3 ccSTP/L

Denitrification

Paleoclimatic Reconstruction

Fontes-Garnier Model 14C Ages

Middle Potomac Aquifer

VIRGINIA

Chesapeake Bay ImpactCrater (Eocene, some 40 million yrs ago)

Atla

ntic

Oce

an

10 ka

20 ka30 ka

Nitrogen-Argon Recharge Temperatures (Coastal Plain)

HoloceneHolocene

0 5,000 10,000 15,000 20,000 25,000Radiocarbon Age, Yrs

4

8

12

16

20N

2 -

Ar

Rec

har

ge

Tem

per

atu

re o C

Local Shallow g.w. temp.

9 oC

PleistocenePleistocene

• Anaerobic, but initial NO3

low in paleowaters

• Aeschbach-Hertig et al found 9 oCcooling in NGTs in Aquiaaquifer just north of here in the Maryland Coastal Plain

Altitude of recharge dependence• Assume altitude of recharge—ü ± 500 m à ± 2.3 oC; ± 100 mà analytical

uncertainty (± 0.5 oC)ü Estimate from hydrologic conceptualization

• Reconstruct using local lapse rate and (for deep UZ) temp-depth profile—ü Calc. RT as function of altitude finding

intersection with atmospheric lapse (Zuber et al.)ü Calc. depth below land surface and RT using

local thermal profiles.

Tracing Sources of Recharge to Ground-water Systems

• Stable isotopes

• Water comp.

• 14C data

• Dissolved gases

• RT, Ex Air

• Alt/depth of RC

• Focused/diffuse

recharge

Recharge Temperatures across top of aquifer

Altitude of recharge assumed:5000 feet (SW sources, NW, AboArroyo, NE zones)6500 feet (NMF, EMF, Tijeras)8000 feet (West Central zone)

•Mean Annual temp at Alb.13.6 oC

•Large areas of the basin have recharge temperatures well above this.

•Warm recharge in NMF and NW zones found above cold RC of West Central zone

•Paleowaters•15 to >30ka 14C yrs•Depleted stable isotopes•Recharge altitude assumed: 8,000 feet •Both cold and warm recharge occurred. Cold=focused recharge or high alt. recharge.Warm=diffuse recharge through deep UZ

± 1500 feet alt. = ± 2.3 oC

West Central Zone Waters

Deep UZTo about 900 feet

Direct infiltrationFrom rivers, arroyos

0 10 20 30 40Temperature, oC

Rel

ativ

e A

ltitu

de, m

eter

s

Land Surface at Albuquerque

AlbuquerqueAlt. 1500 m, MAT 13.6 oC

Temperature profile5.5 oC/100m

2,500

2,000

1,500

1,000

500

0

-500

Lapse rate cooling-0.55 oC/100m

Combining Altitude – Temperature andDepth – Temperature Profiles with N2-ArMeasurements


Rel

ativ

e A

ltitu

de, m

eter

s



A

C

A. Lapse rate cooling, -0.55 oC/100mC. Local geothermal warming, 5.5 oC/100m

Eastern Mountain Front ZoneMiddle Rio Grande Basin, NM2,500

2,000

1,500

1,000

500

0

-500

Water temperature plotted atdepth of water table belowthe land surface (EMF waters)


Rel

ativ

e A

ltitu

de,

met

ers



A

CD

2,500

2,000

1,500

1,000

500

0

-500

A. Lapse rate cooling, -0.55 oC/100mC. Local geothermal warming, 5.5 oC/100m

D. Apparent N2-Ar RT along geothermal gradient

Calculated apparent recharge temperaturefor waters at solubility equilibrium alongthe temperature-depth profile, but at an assumed recharge altitude (1500 m) (Line D).


Rel

ativ

e A

ltit

ud

e, m

eter

s



A

B

CD

2,500

2,000

1,500

1,000

500

0

-500

A. Lapse rate cooling, -0.55 oC/100mB. Apparent N2-Ar RT along lapse rate

C. Local geothermal warming, 5.5 oC/100mD. Apparent N2-Ar RT along geothermal gradient

Calculated apparent recharge temperaturefor waters at solubility equilibrium alongthe local atmospheric lapse rate, but at an assumed recharge altitude (1500 m) (Line B).


Rel

ativ

e A

ltit

ud

e, m

eter

s



A

B

CD

2,500

2,000

1,500

1,000

500

0

-500



Calculate RT of ground-water samplesat 1500 m. Extrapolate to apparentRT (line B), then to Lapse rate to estimate altitude of recharge and RT


Rel

ativ

e A

ltit

ud

e, m

eter

s



A

B

CD

2,500

2,000

1,500

1,000

500

0

-500



Calculate RT of ground-water samplesat 1500 m. Extrapolate to apparentRT (line D), then to temperature profile to estimate depth of recharge and RT


Rel

ativ

e A

ltitu

de,

met

ers



A

B

CD

Eastern Mountain Front ZoneMiddle Rio Grande Basin, NM

2,500

2,000

1,500

1,000

500

0

-500

14C ages of 0-12ka



All calculations at referenceAltitude of 1500 m; use lapserate to find RT at other altitudes.


Rel

ativ

e A

ltitu

de, m

eter

s


Albuquerque todayAlt. 1500 MAT = 8.6(5 oC cooling)

A

B

CD

A. Lapse rate cooling, -0.55 oC/100mB. Apparent N2-Ar RT along lapse rateC. Local geothermal warming, 5.5 oC/100mD. Apparent N2-Ar RT along geothermal gradient

Eastern Mountain Front ZoneMiddle Rio Grande Basin, NM

2,500

2,000

1,500

1,000

500

0

-500

14C ages > 12ka

All calculations at referenceAltitude of 1500 m; use lapserate to find RT at other altitudes.

ØRT determined by GC from N2-Ar measurements in ground water typically ±0.5 oC on laboratory standards; best suited for aerobic or low nitrate waters where altitude of recharge is known.

ØLarge uncertainties in RT and Ex air can result if denitrification is significant.

ØPossible to combine a local depth-temperature profile with N2-Ar data to estimate depth of recharge below deep unsaturated zones in arid regions, but estimation of altitude of recharge from the local atmospheric lapse rate is poorly constrained.

ØMost waters recharged along the Eastern Mountain Front of the MRGB, NM were recharge at depths of up to 150 meters below the land surface.

ØThousands of measurements of N2-Ar in US groundwater have been made at the USGS over the past decade to estimate 1) recharge temperature for CFC/SF6 dating studies, 2) agricultural denitrification studies, 3) paleo-temperature reconstruction, 4) tracing sources of water in aquifers, and 5) interpreting recharge mechanism.

CONCLUSIONS

Ø Increasingly, geochemists and hydrologists are including measurements of concentrations of dissolved gases in hydrologic investigations. These include suites of noble gases (Ar, Ne, Kr, Xe, He) to study recharge conditions and deep groundwater circulation, gases that are indicators of microbial processes (O2, CH4, CO2, H2S, N2O, N2, etc.), and gases that can be used to interpret groundwater age (CFCs, SF6, 4He, 85Kr, 39Ar, 81Kr, etc.).

ØThousands of measurements of N2 and Ar in groundwater have been made at the USGS over the past decade to estimate 1) recharge temperature and excess air for CFC/SF6 dating studies, 2) estimate quantities of denitrification in agricultural studies, 3) reconstruct paleo-environmental conditions, 4) identify and trace sources of water in aquifers, 5) interpret recharge mechanisms, and 6) estimate altitude or depth of recharge.

CONCLUDING REMARKS

applications of dissolved n2 and ar in groundwater · applications of n 2 – ar measurements in...

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