CSIRO LAND and WATER

G.A. Harrington, A.L. Herczeg and P.G. Cook

CSIRO Land and WaterTechnical Report 53/99, December 1999 (Updated January 2001)

C S I RO

Groundwater Sustainability andWater Quality in the Ti-Tree Basin,Central Australia

C S I R O L A N D a n d W A T E R

Groundwater Sustainability and Water Quality in the

Ti–Tree Basin, Central Australia

G. A. Harrington, A. L. Herczeg, and P. G. Cook

Technical Report 53/99, December 1999 (Updated January 2001)

2

EXECUTIVE SUMMARY

Long-term mean annual recharge to the fresh groundwater resource of the Ti-Tree Basin incentral Australia is ~ 1.9 mm.a-1 or, in terms of volumes, approximately 1140 ML.a-1.Other parts of the basin receive an average of 0.2 mm.a-1, giving a total volumetric rechargerate to the whole of the basin of ~ 2120 ML.a-1. More than half of the total average annualrecharge occurs via infiltration of flood waters following heavy, sustained rainfall eventswhich only occur every 5 – 10 years.

Turnover time of the low salinity groundwater resource is about 3,000 years, which meansthat <0.3% of the highest quality resource is replaced every decade. Sustained annual useof >0.03% of the groundwater resource will result in mining of the resource. However,these figures do not account for the potential effects of returning more saline irrigationdrainage to the water table, nor lateral inflow of more-saline groundwater if there issignificant drawdown near the irrigation area. Both of these factors would result indeterioration of the quality of the resource, and therefore reduce the amount of time it couldbe used for irrigation.

High nitrate concentrations in groundwater are inevitable given the widespread distributionof it’s sources (i.e. termite mounds, leguminous vegetation). Treatment at the point ofdelivering to domestic households will be the only option to lower NO3

- concentrationsbelow the NHMRC/ARMCAMZ guideline value.

The sodium-bicarbonate-chloride type groundwaters have evolved over hundreds tothousands of years through a combination of processes including evapotranspiration,dissolution of carbonate and sulphate minerals, and slow silicate mineral weatheringreactions in the soil and saturated zones.

This report summarises the major results and findings of a comprehensive three and a halfyear study. Full details are available in the PhD dissertation of the senior author (seereferences).

3

BACKGROUND AND SCOPE

The Ti-Tree Basin is a 5,500 km2 sedimentary basin located approximately 200 km north ofAlice Springs in the arid zone of Australia (Fig. 1). Several Tertiary sandstone andcarbonate aquifers contain large volumes of high quality groundwater (<1,000 mg.L-1)which currently is being used for a town water supply (~ 120 ML.a-1) and severalhorticultural developments: Ti-Tree Farms, Australian Territory Farms and Territory GrapeFarms (~ 2,500 ML.a-1). The horticulture industry produces crops of table grapes,asparagus, mangoes, melons and cut flowers for south-east Asia. It also captures valuableniche markets in the eastern states of Australia during November and December becausetheir local produce is not harvested until several months later.

NT

QLD

NSW

SA

WA

VIC

TAS

133° E 134°30' E

22° S

➠N

0 100

kilometres

50SCALE

ALICE SPRINGS

TI-TREE

BARROW CREEK

To Darwin

Sandover

Hig hway

Plenty Hig hway

Tanami Road

STU

ART

HIG

HW

AY

LEGEND

Town

Major Road

Margin of Ti-Tree basin

23° S

FIG. 1 Location of the Ti-Tree Basin, Northern Territory, Australia.

4

McDonald (1988) produced a comprehensive report describing the distribution ofgroundwater salinity and yield, and an estimate of the approximate size of the freshgroundwater resource. The current study used a range of environmental tracer techniques toaddress the following issues related to the long term use of this resource: (i) sustainablegroundwater extraction rates, based on current and proposed demands, (ii) water qualitycharacteristics that may impact on irrigation and potability such as very high nitrateconcentrations.

Recharge and Sustainable Extraction

Knowledge of the amounts of recharge, storage and groundwater flow is critical to theability to allocate water on a sustainable basis. In dry regions, where annual recharge fluxesare very low, conventional hydrological techniques (e.g., water balance or hydrographanalysis) often yield unreliable results.

The use of chemical and isotopic tracer techniques has advantages in that they integrateover long time scales and over large areas. We use chloride (recharge rates), stable isotopesof water (recharge mechanisms) and radiocarbon (recharge rates and groundwater residencetimes) to evaluate the long term water balance for the Ti-Tree Basin. Furthermore we havedeveloped a more reliable method for determining radiocarbon ages of groundwater in aridzone, sedimentary aquifers. Replenishment of groundwater in arid zones occurs via acombination of diffuse, widespread recharge through the soil plus rapid, more localisedinfiltration after flood events. We developed a range of chemical and isotopic methods toinvestigate the relative importance of these two processes.

Water Quality

The origin and evolution of fresh groundwater in arid and semi-arid zones is still not wellunderstood. Many practical implications such as salinity and chemical composition ofgroundwater are important for potability and irrigated crop yields. Furthermore, highsodium adsorption ratios (SAR, Eq. 1) in irrigation water can have long term detrimentaleffects on soil structure.

Nitrate concentrations greater than the Australian Drinking Water Guidelines value of 45mg.L-1 are of concern and it is still not clear whether high concentrations affect grape yieldsin the horticultural developments.

We have used mass balance calculations, as well as isotope tracer techniques to evaluatethe relative contributions of major ions from (a) rainfall accession, and (b) weathering andexchange reactions in the soil zone and the aquifer. Sources of nitrate were investigated

SAR =Na+

0.5(Ca2 + + Mg2+)(Eq. 1)

5

using measurements of 15N/14N ratios in groundwater and possible sources includingtermites, vegetation and rock minerals.

APPROACH

Extensive groundwater sampling was carried out between October 1996 and September1998 at 59 sites within the Ti-Tree Basin, including observation bores, grape farmproduction bores and nested piezometers. Samples were obtained from a broad areal extentof the basin, with a bias towards the main areas of interest (i.e., low salinity groundwaternear existing horticulture developments). These samples were analysed for a whole suite ofchemical and isotopic constituents, including major ions, carbon isotopes (14C and 13C/12C),stable hydrogen (2H/1H) and oxygen isotopes (18O/16O) in water molecules, nitrogenisotopes (15N/14N) and strontium isotopes (87Sr/86Sr).

Four shallow soil profiles were augered at various locations within the basin and analysedfor moisture content, chloride concentration and the stable isotopes of water (above).Thirty four chip samples from previous drilling exercises were also collected from the AliceSprings core library for element abundance (XRF), mineralogical (XRD), stable carbonisotope (13C/12C) and strontium isotope (87Sr/86Sr) analyses.

RESULTS

Groundwater Sustainability

Stable isotopes

The stable isotopic compositions of Ti-Tree Basin groundwaters are shown in Fig. 2 alongwith the general trend in rainfall compositions (solid black line), the mean annual rainfallcomposition and mean monthly compositions of various rainfall amounts for Alice Springs.

There are several key observations from this plot:

1) Groundwater data plot to the right of the rainfall trend on a slope of 4. This is typicalof evaporation of water during the recharge process (either from a surface water bodyor soil water).

2) The Alice Springs rainfall data plot on a slope of 6.9 and heavier rainfall amounts areincreasingly more depleted in heavy isotopes (2H and 18O). This is commonly knownas the “amount effect” caused by the preferential rainout of heavy isotopes of water atthe beginning of rain events.

6

3) The best fit line through the groundwater data, when extrapolated back to the AliceSprings rainfall trend, intersects at a value of about –71 ‰ and –11 ‰ for δ2H andδ18O respectively. This value is much lower than the long-term, mean annual rainfallcomposition of –33 ‰ and –6.8 ‰ respectively.

These results suggests that significant recharge to the groundwater only occurs followingheavy rainfall events (>100 mm/month) and that there is evaporation prior to recharge.

FIG. 2 δ2H-δ18O plot showing the Local Meteoric Water Line (LMWL) for Alice Springsand the composition of 59 groundwater samples from the Ti-Tree Basin. Also shown arethe long-term, amount-weighted mean (LTAWM) compositions of different monthlyrainfall amounts for Alice Springs.

Chloride

Chloride concentrations in groundwater (Fig. 3) range from 70 mg.L-1 up to 1,270 mg.L-1.Low concentrations are generally associated with areas of flood-out from the AllungraCreek (in the centre of the basin) and the Woodforde River (near the western margin).

-100

-80

-60

-40

-20

0

-14 -12 -10 -8 -6 -4 -2

δ18O (‰, SMOW)

Weighted-Mean RainfallGroundwater Samples

Mean Rainfall

0-50 mm/month

>200 mm/month

150-200 mm/month

100-150 mm/month

50-100 mm/month

LMWL: δ2H = 6.9δ18O + 4.5

Groundwaters: δ2H = 4.0δ18O - 27.3

7

7620 000 mN

7580 000 mN

7460 000 mN

420

000

mN

380

000

mN

340

000

mN

300

000

mN

➠N

0

Kilometres

50

MT STIRLING

MT SOLITARY

MTFINNIS

MT DUNKIN

MT AIRY

"ANNINGIE"

"PINE HILL"

"BUSHYPARK"

CENTRAL MT STUART

MT TOPS

MT OCTY

"TI-TREE"

MT SKINNER

"ATARTINGA"

"STIRLING"

MT GLASIER

MT BOOTHBY

MT LUCY

"ALCOOTA"

NANCY HILL "MTSKINNER"

7500 000 mN

7540 000 mN

"AILERON"

Allunga Creek

MTBLEECHMORE

Muller

Cre

ek

Woo

dfor

deR

iver

Hanson

Rive

r

MountainMargin of Ti-Tree BasinMajor water courseStation boundaryBore hole location

0 - 250 mg/L

251 - 500 mg/L

>501 mg/L

LEGEND

Harrington map (Chloride), 9/99/GIGA/GER.

32

57

22

33

59

9 8 1311

32

10

4 512

NP3NP2

NP1

2324

18 34 58

37

16

35

39 14

36

31

29

25

2627

4544 42

21 19

20

381715

167

37

28

30

4746

4340

41

FIG. 3 Spatial distribution of chloride (Cl-) concentration in Ti-Tree Basin groundwaters.The shaded blue area represents the extent of a flood-out from the Allungra Creek inFebruary 1974 (after McDonald, 1988).

Because there are no known chloride bearing minerals in the basin, we have assumed theonly input of Cl to the groundwater is by rainfall plus dry fallout from the atmosphere.Therefore, the following mass balance approach was applied.

P.Clp = R.Clgw (Eq. 2)

where P = mean annual rainfallClp = Cl concentration in rainfall (+ dry fallout)R = mean annual rechargeClgw = Cl concentration in groundwater

8

Therefore, if we have reasonable estimates of P and ClP, we can estimate R from anymeasured groundwater Cl concentration. Low Cl concentrations correspond to relativelyhigh recharge rates, and vice versa. Mean annual rainfall over the last 45 years at AileronStation (10 km south of the basin) is 290 mm/yr. Cl concentrations in rainfall have beenmeasured near Alice Springs (Hutton, 1964; Keywood et al, 1997) and we adopted a valueof 0.5 mg.L-1 as the long term average ClP for the Ti-Tree area. Recharge rates estimatedusing this approach range from 0.1 – 2 mm.a-1 (mean = 0.8 mm.a-1), with the higherrecharge rates recorded in the flood-out areas.

Radiocarbon

Radiocarbon (14C) concentrations range from 4%mc (% modern carbon) to 88%mc, butexhibit no consistent trends with either depth below the water table or location down thehydraulic gradient. Groundwater ages can be estimated by assuming an initial 14Cconcentration of 85%mc, using a 14C half-life (t1/2) of 5,730 years and the measured 14Cconcentrations:

t=(1/λ).ln(C0/C) (Eq. 3)

where: λ = ln2/(t1/2)C0 = initial 14C concentration, which needs to be determined for individual samplesC = measured 14C concentration

Addition of carbon that is devoid of 14C to the groundwater can occur by dissolution ofcarbonate minerals or oxidation of very old organic matter. We corrected for this using anew scheme, specific to arid zone sandstone/carbonate aquifers. It involves a combinationof chemical mass balance and strontium isotope mass balance to partition the relativecontributions of soil CO2, carbonate and silicate weathering processes to the 14C and 12Cdissolved in groundwater. The calculated ages (Fig. 4) range from modern to 20,000 years,although the majority are < 8,000 years.

14C ages are indicative of groundwater residence time, however estimates of groundwaterrecharge rates are far more useful for the purpose of management. Using an approachsimilar to that of Vogel (1967), estimates of recharge rates can be determined from thegroundwater ages providing the depth below water table is known:

R = H.λ .θ. lnH

H − z−

C0

C

(Eq. 4)

where H = total thickness of the aquiferθ = mean aquifer porosityz = sample depth below the water table.

9

FIG. 4 Frequency distribution of corrected 14C ages of Ti Tree Basin groundwaters.Seventy five percent of the samples are less than 8,000 years old, and more than half areless than 6,000 years old.

This approach provides average recharge rates over the interval between where thegroundwater sample entered the saturated zone and the bore location. Conversely, rechargerates estimated using the steady-state chloride mass-balance approach (Eq. 2) apply only tothe location in the landscape where the sample first entered the saturated zone. Thus, acombined (14C and Cl) approach provides an indication of the spatial variability ofrecharge. Recharge estimates for both the chloride mass-balance and 14C methods arepresented in Fig. 5. The lines projected up gradient from bore locations represent theinterval over which the 14C model applies, and colours are indicative of the recharge rate(see legend). The coloured circles at the end of the projected lines represent the rechargerates at that location, as determined from Cl mass balance (see legend).

0

1

2

3

4

5

6

7

8

9

10F

requ

ency

<0

0-2

2-4

4-6

6-8

8-10

10-1

2

12-1

4

14-1

6

16-1

8

18-2

0

20-2

2

22-2

4

24-2

6

>26

Age (thousand years)

Corrected 14C agesTi Tree basin

10

7580 000 mN

420

000

mN

340

000

mN

300

000

mN

MT SOLITARY

MT FINNIS

MT DUNKIN

MT AIRY

"PINE HILL"

CENTRAL MT STUART

MT OCTY

"TI-TREE"

MT SKINNER

"ATARTINGA"

MT GLASIER

MT BOOTHBY

NANCY HILL "MTSKINNER"

7500 000 mN

7540 000 mN

"AILERON"

Woo

dfor

deR

iver

Hanson

Rive

r

MountainMargin of Ti-TreeBasin

LEGEND

57

9

2

10

412

NP3NP2

NP1

37

16

39 14

31

29

25

2627

45

412120

15

Major water courseFloodplainStation boundaryBore hole location

R (Vogel Model)(mm/a)

0 - 0.50.5 - 1.51.5 - 2.5>2.5

R (CMB)(mm/a)

0 - 0.50.5 - 1.01.0 - 1.51.5 - 2.0

37

30

47

38

33

3

56723

43

0

Kilometres

3010 2038

0 00

0 m

N

34

241846

FIG. 5 Groundwater recharge rates calculated using the 14C model and chloride massbalance (CMB) approach.

The mean recharge rate determined for the basin using the 14C model (3.5 mm.a-1) issignificantly higher than that determined using the chloride mass balance (0.8 mm.a-1).This is most likely due to two samples (bore #s 14 and 38) having much higher 14C-derivedrecharge rates (27 and 50 mm.a-1 respectively) than the remaining samples. There is anapparent discrepancy between estimates of recharge rates for several bores because the 14Cmodel provides spatially averaged recharge rates, whereas the Cl mass balance applies onlyto a specific location. For example, bore #39 has a spatially-averaged recharge rate (14Cmodel) of < 0.5 mm.a-1, while the recharge rate at the location it entered the saturated zone(Cl mass balance) is 1.8 mm.a-1, reflecting the contribution of flood-out recharge betweenthis point and the sampling location.

Highest apparent recharge rates occur in the centre of the basin near the flood-out of theAllungra Creek, as well as several areas near the Woodforde River in the western part ofthe basin. The mean 14C derived recharge rate for bores in the centre of the basin (aroundthe Allungra Creek flood-out ) is 8.8 mm.a-1. However, this value is biased towards thehigh end due to two very high values (see above). To establish a water balance for thegroundwater with TDS < 1,000 mg.L-1 (i.e., the area encompassing the Allungra Creekflood-out) we used the median recharge rate of 1.9 mm.a-1. The estimated total volume ofaquifer containing groundwater with TDS < 1,000 mg.L-1 is 3.3 x 1010 m3 (McDonald,1988). The average annual recharge to this area (600 km2) is 1.1 x 106 m3. The averageturnover time can be calculated by dividing the total storage by recharge flux:

11

R

Vt

θ= (Eq. 5)

where θ is porosity (equal to ~ 10% in the TTB), which yields a turnover time of 2,900years. That is, if this part of the basin were to be completely emptied, it would take about3,000 years to fill to the present level.

Water Quality

Relatively low salinity groundwater (< 1,000 mg.L-1) underlies predominantly flood-outareas, and high salinity groundwater (up to 4,200 mg.L-1) underlies the flat, Mulga andSpinifex covered plains. There is a trend in chemical composition from sodium-bicarbonate-chloride type waters to sodium-chloride dominated at the higher salinities.Concentrations of potassium and silica are relatively high compared with other arid zonegroundwater systems throughout the world.

The range in chemical compositions and salinities are determined by three main processes:accession of ions from rainfall plus dry fallout, water-rock interactions during the passageof water through the soil zones and evapotranspiration. Our mass balance calculationsshow that weathering of silicate and carbonate minerals plus dissolution of gypsum ismainly responsible for water compositions in the <1,000 mg.L-1 range. Evapotranspiration,cation exchange and precipitation of carbonate minerals is more important at the highersalinities.

Nitrate concentrations in groundwater range from 3 to 280 mg.L-1 (mean 72 mg.L-1) but donot show any spatial patterns. The δ15N(NO3

-) values ranges from +2 ‰ to +15 ‰, AIRwhich is consistent with sources of nitrate from termite mounds, and to a lesser extent fromN fixed by Mulga vegetation. The variations in δ15N are not consistent with the process ofdenitrification within the aquifer. The relatively high δ15N values are probably caused byloss of 15N depleted ammonia during transformation of organic N into NO3 by termiteactivity and oxidation.

12

OUTCOMES AND MANAGEMENT IMPLICATIONS

1) Long-term mean annual recharge to the fresh groundwater resource is ~ 1.9 mm.a-1 or~ 1140 ML.a-1. Other parts of the basin receive an average of 0.2 mm.a-1, giving atotal volumetric recharge to the basin of ~ 2120 ML.a-1. Recharge events only occurafter heavy, sustained rainfall events which occur every 5 – 10 years.

2) Turnover time of the low salinity water (<1,000 mg L-1) is about 3,000 years whichmeans that <0.3% of the highest quality resource is replaced every decade. Sustainedannual use of >0.03% of the groundwater resource will result in mining of theresource. However, these figures do not account for the potential effects of returningmore saline irrigation drainage to the water table, nor lateral inflow of more-salinegroundwater if there is significant drawdown near the irrigation area. Both of thesefactors would result in deterioration of the quality of the resource, and thereforefurther reduce its suitability for irrigation.

3) High nitrate concentrations are inevitable given the widespread distribution of it’ssources (i.e. termite mounds). Treatment at the point of delivering to domestichouseholds will be the only option to lower NO3

- concentrations below theNHMRC/ARMCAMZ guideline value.

4) The sodium-bicarbonate-chloride type groundwaters have evolved over hundreds tothousands of years through a combination of processes including evapotranspiration,dissolution of carbonate and sulphate minerals, and slow silicate mineral weatheringreactions in the soil and saturated zones.

13

REFERENCES

Harrington, G. A. (1999) Recharge mechanisms and chemical evolution in an aridgroundwater system, Central Australia. PhD Dissertation, Flinders University ofSouth Australia, 293p.

Harrington, G. A. and Herczeg, A. L. (1998) A geochemical model for groundwaters ofthe arid Ti-Tree Basin, Central Australia. In: G.B. Arehart and J.R. Hulston(Editors) Water Rock Interaction (Proc. 9th Int. Symp. Taupo, NZ), Balkema,Rotterdam, pp. 231-234.

Harrington, G.A., Herczeg, A. L. and Cook, P. G. (1998) Groundwater sustainability inthe Ti-Tree Basin, Central Australia: inferences from environmental isotopes andhydrochemistry. Proceedings of 1998 International Groundwater conference,International Association of Hydrogeologists, Melbourne, Australia, pp. 157-162.

Harrington, G. A. and Herczeg, A. L. (1999) Estimating groundwater 14C ages in the aridTi-Tree Basin, central Australia: Use of 87Sr/86Sr to constrain sources of inorganiccarbon. In Isotope techniques in Water Resources Management and Development(Proc. Symp. Vienna, May 1999), IAEA, Vienna, (In press).

Hutton, J. T. (1983) Soluble ions in rainwater collected near Alice Springs, N.T., and theirrelation to locally derived atmospheric dust. Trans. Royal Soc. S.A., vol. 107, part 2,pp. 138.

Keywood, M. D. (1995) Origins and sources of atmospheric precipitation from Australia:chlorine-36 and major-element chemistry. PhD thesis, Australian NationalUniversity, Canberra, 207pp. (unpublished).

McDonald, P. S. (1988) Groundwater studies, Ti-Tree Basin, 1984-1988. NorthernTerritory Power and Water Authority, Report No. 1/90.

Vogel, J. C. (1967) Investigations of groundwater flow with radiocarbon. In Isotopes inHydrology (Proc. Symp. Vienna, 1966), IAEA, Vienna, pp. 355-369.