Radiocarbon Dating Of Groundwater

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R ADI OC A RB O N DATI N G O F GR OU N D WATER

You’re the expert; how would you interpret these results?

Hint:don’t don’ttry trytotouse usethe the absolute absolute dates. Hint: dates. Compare Comparethe thedating datingresults results on each well from year to year to see tendencies. on each well from year to year to see tendencies.

OUR INTERPRETATIONS OUR OWN OWN IN TERPRETATIONS ARE ARE ON ON THE THE LAST LASTPAGE. PAGE

THIS STUDY EXAMPLE IS A SIMULATION

BETA ANALYTIC INC.

Ground Water Radiocarbon Dating of Wells along a Transect in the Aquifer Region Abstract Our aquifer is an unconfined aquifer which is the sole source of drinking water for the local population. Models suggest a slow recharge rate and predict the aquifer would provide adequate supply for a maximum of 150 years at its present exploitation rate. The Phase I study used radiocarbon dating to show that the water was old, confirming model calculations of a long mean residence time. This was done by dating 20 wells along a west/east transect. However, the easternmost wells unexpectedly showed younger dates than the others, but still old. This was not in agreement with our models and led to a Phase II study which consisted of yearly monitoring. Results showed progressively declining radiocarbon dates in wells at the easternmost end of the transect. Starting in 1995, we reduced the pumping rates of 5 wells which showed declining dates. By 1998, the radiocarbon dates of those wells had increased to their 1993 levels suggesting new maximum sustainable yields for those wells. The progression of wells with declining dates over the 5 year period allows us to postulate a flow direction of southeast to northwest for the region.

Theory Hydrological modeling suggests that our aquifer is an unconfined, stratified aquifer believed to have a very long residence time. By radiocarbon dating our wells, the accuracy of our models can be examined. If the ages are old, the slow recharge rate will be confirmed. By radiocarbon dating wells each year, changes can be observed and addressed. In doing this, we can address the subject of vulnerability since vulnerability of a well to contamination from surface water can be inferred using radiocarbon dating. Radiocarbon dating uses the amount of naturally occurring carbon-14 in the water, in association with its half life (5730 years) to derive the age. This age gives the approximate time that has past since the water entered the aquifer. In our stratified aquifer, overlying waters will be younger than those below. Increased draw-down or coning can be observed by measuring the age of the water over time. The data provides empirical evidence of aquifer dynamics surrounding the exploitation of our well field.

Method One liter of water was collected at 20 wells over a two day period in June 1993. The water was collected in one liter polyethylene bottles containing 0.5 grams sodium hydroxide. The bottles were then sent to Beta Analytic Inc. for radiocarbon dating . Radiocarbon dating was performed using accelerator mass spectrometry (AMS) counting. First, the dissolved inorganic carbon (DIC) was precipitated as strontium carbonate (SrCO3) using 1.2 grams of strontium chloride (SrCl2) per liter of water. This was rinsed to neutrality and dried. The precipitate was then acidified using 0.5 N H3PO4 and the carbon collected as CO 2 gas. The collected gas was then mixed with hydrogen over a cobalt catalyst and heated to 600o C, reducing the carbon to graphite on the cobalt. The graphite was then placed in an AMS and measured for its radiocarbon age. This process was repeated at yearly intervals between 1993 and 1998.

BETA ANALYTIC INC. 1993: Radiocarbon Date the Water and Establish a Baseline for Monitoring 1993: was Determine Radiocarbon Ageover and aEstablish Baseline for Monitoring Water collected from 20 wells two day aperiod in June 1993. The water was sent to Beta Analytic Inc. for radiocarbon dating. The following results were Water was collected from 20 wells over a two day period in June 1993. The water was returned. sent to Beta Analytic Inc. for radiocarbon dating. The following results were returned.

Dep th Rad io carb on W ell # Depth Radiocarbon Date Date Well # (meters ) Date (meters) 6/15/93 1 92 38000 6/15/98 2 1 38000 6/15/93 86 92 37500 6/15/98 3 2 37500 6/15/93 62 86 34980 6/15/98 4 3 34980 6/15/93 77 62 35500 6/15/98 5 4 35500 6/15/93 80 77 36770 6/15/98 5 80 36770 6/15/93 6 61 34890 6/15/98 6 61 34890 6/15/93 7 57 33500 6/15/98 8 7 33500 6/15/93 48 57 32100 6/15/98 9 8 32100 6/15/93 77 48 35750 6/15/98 10 9 35750 6/15/93 68 77 35300 6/15/98 10 68 35300 6/16/93 11 82 36900 6/16/98 11 82 36900 6/16/93 12 65 35400 6/16/98 13 12 35400 6/16/93 71 65 35590 6/16/98 14 13 35590 6/16/93 88 71 37900 6/16/98 15 14 37900 6/16/93 65 88 35390 6/16/98 15 65 35390 6/16/93 16 88 37810 6/16/98 16 88 37810 6/16/93 17 75 35690 6/16/98 17 75 35690 6/16/93 18 81 36850 6/16/98 18 81 36850 6/16/93 19 75 18000 6/16/98 20 19 18000 6/16/93 66 75 19500 6/16/98 20 66 19500 Date

Question No. 1

Question No. 1

These 18 wells These 18 wells are all are all showing showing old water old radiocarbon radiocarbon dates. dates Why? Why?

Radiocarbon Ages of 20 wells In June 1993 Question No. 2

Question No. 2

40000

Radiocarbon Date

The water dates in these 2 wells The water datesare in younger thanare the these 2 wells younger than the others. others. Why? Why?

45000

35000 30000 25000 20000 15000 10000 5000 0 1

3

5

7

9

11

13

15

17

19

Well Number

2

BETA ANALYTIC INC. 1993 to 1995: Well Field Monitoring 1993 to 1995: Well Fieldfrom Monitoring Water was collected the same 20 wells and radiocarbon dated each year over Water the same wells and radiocarbon dated each over adynamic 3 year period. a 3 was yearcollected period. from Changes in 20 radiocarbon dates were used to year interpret changes, and their causes, within aquifer.dynamic changes, and their causes, within the Changes in radiocarbon dates were used the to interpret aquifer. Question No. 3 Radiocarbon Dates

Well #

Depth (meters)

Jun- 93

Jun- 94

Jun- 95

1

92

38000

37890

38270

2

86

37500

37450

37580

3

62

34980

34890

35010

4

77

35500

35600

35450

5

80

36770

36660

36710

6

61

34890

34810

34920

7

57

33500

33390

33590

8

48

32100

32050

32150

9

77

35750

35720

35850

10

68

35300

35210

35420

11

82

36900

36810

36790

12

65

35400

35380

35410

13

71

35590

35640

35510

14

88

37900

37910

37870

15

65

35390

35360

35280

16

88

37810

37780

29000

17

75

35690

35640

24000

18

81

36850

36880

18000

19

75

18000

12000

7000

20

66

19500

11700

8200

No significant No significant change was change was observed inthese these observed in 15 wells wells over 15 over3 3 years. years. Why? Why? Question No. 4 Question No. 4

Over a 3 year Over a 3 year period, period, 5 wells 5 wells showed showed decreasing decreasing radiocarbon ages. radiocarbon dates. Why? Why?

15Wells WellsShowed ShowedNoNo Change 15 Change Over3 3Years Years Over W ell1

40000

WWell 24 ell

30000

ell WWell 46

30000

WWell 35 ell

ell WWell 57

20000 20000

ell WWell 68 W ell9 W ell7 W ell1 0 W ell8 W ell1 1 W ell9 W ell1 2 W ell1 0 W ell1 3

10000

10000 0 Jun-93 0

Jun-93 YearJun-94 of Dating

WWell 1114 ell

Jun-95

Jun-95

WWell ell 1125 W ell1 3 W ell1 4 W ell1 5

Year of Measurement

15 wells showed consistent radiocarbon ages over 3 years. 15 wells showed consistent radiocarbon dates over 3 years

50000 40000 Radiocarbon Date

W ell2 W ell1 W ell3

Radiocarbon Date

40000 50000 Radiocarbon Radiocarbon Age Date

5 Wells Wells Showed 5 ShowedChanges Changes Over 3 Years Over 3 Years 40000 W ell1 6

30000

W ell1W7ell1 6

30000

W ell1W8ell1 7

20000 20000

W ell1W9ell1 8 W ell20 W ell1 9

W ell20

10000

10000 0 Jun-93 0

Jun-95

Jun-93Year ofJun-94 Jun-95 Dating Year of Measurement

WellsWells 19 and continued to decrease from 1920 and 20 continued to decrease their previous dates. Wells 16, 17 and from theirlower previous lower values. Wells 18 started declining dates in 1995. 16, 17showing and 18 started showing declining dates in 1995.

3

BETA ANALYTIC INC. 19931993 to 1998: Monitoring and Remediation to 1998: Monitoring and Remediation Between andand 1998, 10 of1020ofwells showed changing radiocarbon dates. Ofdates. these 10,Of the Between1993 1993 1998, 20 wells showed changing radiocarbon pumping rate was decreased onwas 5 of decreased them after 1995 attempt to re-stabilize wells. to these 10, the pumping rate on 5inofanthem after 1995 in anthe attempt re-stabilize the wells. Question No. 5 Well #

Depth (meters)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Jun-94

92 86 62 77 80 61 57 48 77 68 82 65 71 88 65 88 75 81

Jun-93 38000 37500 34980 35500 36770 34890 33500 32100 35750 35300 36900 35400 35590 37900 35390 37810 35690 36850

Radiocarbon Dates Jun-95 Jun-96

37890 37450 34890 35600 36660 34810 33390 32050 35720 35210 36810 35380 35640 37910 35360 37780 35640 36880

38270 37580 35010 35450 36710 34920 33590 32150 35850 35420 36790 35410 35510 37870 35280 29000 24000 18000

38100 37550 35100 35700 36690 34860 33610 32210 35690 35350 36950 35290 35610 37900 35340 18500 16000 10000

37910 37290 34950 35470 36710 35000 33490 32010 35850 35430 36740 35510 24000 27980 22900 26660 27980 19340

37980 37650 35110 35610 36680 34990 33460 32180 35670 35380 24000 22000 17500 18000 14000 36670 35700 36750

19

75

18000

12000

7000

17750

29770

35200

20

66

19500

11700

8200

19400

31660

35430

Jun-97

Jun-98

Between and Between1993 1993 and 1998, 20 1998,10 10of of the the 20 wells showed wells showed changing declining radiocarbon radiocarbon dates. dates. Why? Why?

Question No. 6

An engineer decided

Why? Why?

Increasing Radiocarbon Dates with Decreasing Increasing Radiocarbon Pump Rate After Dates 1995 With Decreasing Pump Rate After 1995

40000 50000 Radiocarbon RadiocarbonDate Date

An engineer decided that that pumping thethe pumping shouldshould be be reduced in wells reduced in wells 16 to 2016 to 20 1995 after theafter 1995 the indication indication possible of decliningofages, and possible draw-down of draw-down of younger younger waters from waters from above. The above. The radiocarbon radiocarbon dates ages returned to their became older. initial values by 1998.

40000

30000

W ell1 6

30000

W ell1 7 W ell1 6 W ell1 8

20000 20000

W ell1 7 W ell1 9 W ell1 8 W ell20 W ell1 9

10000

W ell20

10000

0

0

1993

1995

1997

1993 1994Year 1995 1996 1997 1998 of Dating Year of Measurement

4

BETA ANALYTIC INC. Question No. 4: Why do 5 of 20 wells in the same Question 4:decreasing Why do 5 ages of 20while wellsthe in the same aquifer region No. show other 15 aquifer region show decreasing ages while the do not? Question No. 1: 1: Why Why do do 18 18of of 20 20wells wellsininthe the same other 15 do not? Question No. Younger waters from above are being drawn down, indicating aquifer region show old radiocarbon dates? same aquifer region show old radiocarbon dates? Younger waters from above are being drawn down, OUR ANSWERS OUR ANSWERS

eventual contamination from surface water. This could be indicating eventual contamination from surface water. due to over exploitation due to population growth, This could be due to over exploitation due to 1) The recharge rate is very slow or 2) perhaps the construction of too manyconstruction new wells upstream, newnew upstream 1) The recharge rate is very slow or 2) perhaps the population growth, of too many wells aquifer is unexpectedly confined. exploitation measures, recent geological phenomena, or upstream, new upstream exploitation measures, aquifer is confined. cracked casings. recent geological phenomena, or cracked casings.

Question No. 2: 2:Why Why are arewaters watersfrom from2 2wells wells much Question Question No. No. 5: Why5:do 10 ofdo2010 wells decreasing Question No. Why of show 20 wells show younger than 18 others in the aquifer region? radiocarbon much younger than 18 others insame the same aquifer ages over a 5 year period? decreasing radiocarbon ages over a 5 year region? period? Upstream exploitation is causing unsustainable yields in

1) They could be part of a different aquifer, or 2) they downstream at theirispresent rates. Younger Upstreamwells exploitation causingpump unsustainable yields 1) They could be par t of a different aquifer, or 2) they could be the same aquifer showing signs of higher waters are being drawn down, suggesting imminent surf ace in downstream wells at their present pump rates. could be theofsame aquifer showing waters signs of(suggesting higher draw-down younger overlying Younger waters are being drawn down, suggesting water contamination. As time passes, the situation gets worse, draw-down of younger overlying waters (suggesting eventual contamination from surface waters), or 3) the causing imminent surface water contamination. As time more and more downstream wells to be affected. The eventual contamination from surface waters), or 3) the casings could be cracking. passes, the situation gets worse, causing more andbe dates suggest that wells 19 and 20 will be the first to casings could be cracking. more downstream wells to be The affected. Thefollow datesin contaminated from surface waters. r est will suggest that wells 19 and 20 will be the first to be time, unless pump rates are reduced. contaminated from surface waters. The rest will Question No. Why do 15 in the in same Question No.3: 3: Why dowells 15 wells the same follow in time, unless pump rates are reduced. aquifer region the same radiocarbon age age aquifer regionmaintain maintain the same radiocarbon

Question No. 6: Why do the radiocarbon ages of 5 wells 6: Why do the radiocarbon ages of get Question older whenNo. pumping is reduced? 5 wells get older when pumping is reduced?

over 3 3 years? years? over

1) The The aquifer in in this geographic region, or or 2) Reducing the pumping rate returned each cone of cone inf luence 1) aquiferisisstable stable this geographic region, Reducing the pumping rate returned each of 2) perhaps the aquifer is confined, or 3) exploitation is back to its 1993 level. The r adiocarbon dates showed that the perhaps the aquifer is confined, or 3) exploitation is not influence back to its 1993 level. The radiocarbon dates not exceeding maximum sustainable in region. this downstream maintain their priorcould pump not rates exceeding maximum sustainable yieldyield in this showedwells thatcould the not downstream wells region. without increased draw-down from above. maintain their prior pump rates without increased draw-down from above. Of particular particular iinterest nterest isisthe radiocarbondates datesover o ver the t he 55 year year monitoring monitoring period aa flow forfor thethe Of thefact factthat thatt he radiocarbon period1) 1)suggest suggest flowdirection direction aquifer and 2) o ffer a mechanism to quantify maximum sustainable yield. aquifer and 2) offer a mechanism to quantify maximum sustainable yield. The direction of of flow flow path The arrow arrow is is pointing pointing in in the t he probable probable direction based on expanding reduction in radiocarbon dates. pat h based on expandi ng reduction i n radiocarbon Contamination is likely to follow this path if pumping is not dates. Contamination is li kely to follow this pat h if reduced. pumping is not redu ced. Surface View ; Radiocarbon Ages vs. Location vs. Year;1993-1995

decreasing the pump rates on 16 wells 16 to 20, wells 16 ByAfter decreasing t he pump rates on wells to 20, radiocarbon to 18 returned to their 1993 values, and wells 19 and 20 ages returned to their 1993 values, defining maximum exceeded their 1993 values. This implies the previous pump sustainable yields at t he given exploitation rates. rates were exceeding maximum sustainable yield. Surface View ; Radiocarbon Age vs.Location vs. Yr; 1993 to 1998 Jun-93 Jun-94 Jun-95

Jun-93

Jun-96

Jun-95 1

3

5

7

9

11

13

15

17

Jun-97

19

Jun-98

Lo cation; W e ll N um b er 1

0-10000

10 000-20000

20 000-30000

2

3

30 000-40000

4

5

6

7

8

9 1

1

1

1

1

1

1

1

1

1

20

Location: Well Number 0-10000

10000-20000

20000-30000

30000-40000

5