resource

BMR Journal of Australian Geology & Geophysics, II, 253-259

©

Commonwealth of Australia 1989

The Tatiara proclaimed region, southeastern South Australia: hydrogeological investigations and groundwater management F . Stadter l & A.J. Love 2 Following the proclamation of the Tatiara proclaimed region in 1984, hydrogeological investigations have been undertaken to assess the groundwater availability of the region. Groundwater supplies are obtained from an extensive unconfined aquifer. Hydrogeolog• ical conditions vary significantly through the region , with changes in aquifer lithology, depth of water table and groundwater quality. Preliminary groundwater assessments indicated that groundwater use for irrigation purposes in some areas was close to or possibly exceeded acceptable levels of groundwater extraction. Monitoring of groundwater quality has shown increased groundwater salinity

Introduction In 1984 a significant area of the upper southeast of South Australia was proclaimed under the South Australia Water Resources Act, to manage groundwater withdrawals in the region. The area was termed the Tatiara proclaimed region (Fig. I). Management of the groundwater resource was considered necessary because • there was a relatively high density of irrigation in parts of the region • preliminary assessments of groundwater availability indi• cated that extraction had approached or possibly exceeded replenishment in parts of the region • there was evidence of groundwater quality deterioration in the western part of the region • a rapid expansion in irrigation was evident (from 3840 ha in 1978 to 12 260 ha in 1984) (Stadter & Love, 1987).

...

or) §(

t: .J

Molineaux Sand

aeolian

(m)

Hydrogeology

12

Elevated above water table

Padthaway Formation

lagoonal / lacustrine

19

QUATERNARY AQUIFER SYSTEM

Bridgewater Formation

shallow marine/ coastal beach/ aeolian

88

Coomandook Formation

shallow marine

13

~I

Parilla/ Loxton Sand

fluviatile / strand-plain

47

xl

Murray Group Limestone

shallow marine

Ettrick Formation

shallow marine

Buccleuch Beds

shallow marine

~i 1- '

EXTENSION TO TATIARA PROCLAIMED REGION

Environment

Unditferenvariable tiated Quaternary

1-.

.......J

Geological unit

Maxi• mum thickness

Elevated above water table

:i

,,(

Table 1. Summary of geology and hydrogeology for the Tatiara proclaimed region.

2

'! I

/""'~ il'6i.

in a number of observation wells, particularly in the western part of the region. It is considered that these increases are mainly due to the localised leaching of salts left in the soil profile following irrigation. An assessment of groundwater resources has been made by determining the water balance of several areas within the region , total groundwater availability being the sum of the recharge to the aquifer and the lateral groundwater inflow. In order to maintain the maximum possible groundwater throughflow in these areas, groundwater use should not exceed localised recharge.

idOOj

108

Unconfined aquifer, semi-con• fined in southwest, high transmissivity and permeabilities, absent in east, flood irrigation common

Elevated above water table

TERTIARY SYSTEM

AQUIFER

Unconfined, hydraulically continuous with Quaternary aquifer, dual permeability, point recharge through runa• way holes, spray irrigation common

50

50 kilometres

TATIARA PROCLAIMED REGION (July 1984)

31

Also include upper clays of the Renmark Group and Buc• cleuch Beds, low vertical hy• draulic conductivity

40

Noracoorte.

Figure 1. Locality map of the Tatiara proclaimed region in the southwest margin of the Murray Basin.

The gross value of irrigated agricultural production from the region is approximately $25 million per year (Stadter & Love, 1987). Lucerne seed and pasture are the main irrigated crops, particularly in the west of the region, where the marginal quality of groundwater precludes the irriga• tion of more salt-sensitive crops. Elsewhere, the irrigated crops include small seeds, vegetables, oil seeds and cereals. South Australian Department of Mines and Energy, PO Box 93, Naracoorte, SA 5271 2 South Australian Department of Mines and Energy, PO Box 151, Eastwood, SA 5063

I

Renmark Group fluviatile Kanmantoo Group with in• trusions of Or• dovician granite

LOWER TERTIARY CON• FINING BED

CONFINED AQUIFER

Low permeability argillaceous upper section, higher permea• bility basal section, not used for irrigation.

87

HYDRAULIC BASEMENT

High-yielding wells have allowed flood irrigation in the west, while spray irrigation (particularly centre pivots) dominates the central region because of lower well yields. The initial groundwater management policy for the entire region was to limit additional irrigation development until a detailed assessment of sustainable groundwater use was undertaken.

254

F. STADTER & A.l. LOVE

Hydrogeological investigations have included the establish• ment of groundwater monitoring networks for .both p~t~n­ tiometric and salinity information, extensive dnllmg programs, aquifer testing and quantification of groundwater recharge.

Geology The study area is located on the southwestern margin of the Murray Basin, and contains sediments typical of both the Murray Basin and the Coastal Plain. The re¥ional geology has been described by Ludbrook (1961), Firman (1973), and Brown & Stephenson (1986). The Padthaway Ridge separates the Otway Basin to the south from the Murray Basin to the north and northeast. The Murray Basin is a relatively shallow intracratonic basin where sedimentation occurred during the Cainozoic, Cretaceous and Permian, while the Otway Basin has a thicker sedimentary sequence where deposition occurred on a passive continental margin associated with the break-up of the Australo-Antarctic palaeocontinent (Wopfner & Douglas, 1971). The sedimentary sequence in the Tatiara region generally increases in thickness away from the Padthaway Ridge towards the northeast and east (Stadter & Love, 1987). The lithologies in the central and eastern part of the region are typical of the Murray Basin sequence (Table I). In the west the Murray Group limestones were truncated by a late 'Pleistocene transgression and the sequence is more typical of the coastal plain, with the deposition. of the Coomandook, Bridgewater and Padthaway FormatIOns.

Groundwater systems Two major groundwater systems occur in the region, a confined aquifer and an unconfined aquifer (Table I, Fig. 2) (Shepherd, 1978; Stadter, 1984).

Confined aquifer The confined aquifer system consists of a series of thin interbedded limestone and sand aquifers within the Ren• mark Group and Buccleuch Beds, separated by thin clay confining beds (Stadter & Love, 1987). Groundwater in• flow is from the southeast across the Victorian border, with a general increase in salinity from around 1300 mg/L m the east up to about 3000 mg/L in the west. Leakage either into or out of the confined system is consid• ered to be minimal, because of the small difference in potentiometric head between the unconfined and confined aquifers and the relatively low vertical hydraulic conductiv• ity of the overlying Ettrick Formation confining bed. How• ever, there is a greater potential for leakage between the various sub-aquifers within the confined system, because the confining beds are thinner and their vertical hydraulic conductivity is higher. Minor inflow to the system may occur via direct recharge along the flanks of outcropping basement highs. Little use has been made of these groundwater resources because of the general availability of adequate supplies from the over• lying unconfined aquifer.

/ HOR/Z'o/VrAL. (

iii

Undifferentiated Quaternary and Tertiary sediments

Lower Tertiary Confining Bed

~ ~

Quoternary unconfined oquifer with semi-confining bed

Renmark Group aquifer system

~

Tertiary unconfined aquifer

Hydraulic basement

'+ .. I

,

(

(

o

Diffuse recharge

Direction of groundwater flow

Evaporative discharge

Major pumping centre

I

.sCALF /0

Point recharge

Figure 2. Conceptual hydrogeological model of the Tatiara proclaimed region, showing salient hydrogeological features.

TATIARA PROCLAIMED REGION Leakage either into or out of the confined system is consid• ered to be minimal, because of the small difference in potentiometric head between the unconfined and confined aquifers and the relatively low vertical hydraulic conductiv• ity of the overlying Ettrick Formation confining bed. How• ever, there is a greater potential for leakage between the various sub-aquifers within the confined system, because the confining beds are thinner and their vertical hydraulic conductivity is higher. Minor inflow to the system may occur via direct recharge along the flanks of outcropping basement highs. Little use has been made of these groundwater resources because of the general availability of adequate supplies from the over• lying unconfined aquifer.

Unconfined aquifer The unconfined aquifer is a multilithological system within units of the Tertiary Murray Group limestones and the Quaternary Padthaway, Bridgewater and Coomandook Formations. Tertiary and Quaternary aquifers merge in the central west of the area, where they are hydraulically interconnected.

Depth to water table varies considerably throughout the region. To the east, the water table occurs at depths of about 45 m beneath sand dunes, while in the west the coastal plain is characterised by a shallow water table generally less than 5 m below ground level. Groundwater movement is in a general west to northwest direction through the region. There is considerable variation in hydraulic gradient caused by changes in aquifer perme• ability, variation in aquifer thickness and the effects of local recharge. Aquifer sediments have both a high primary permeability and an even higher secondary permeability in areas of solution activity. This dual permeability is reflected in the variation of transmissivity from 500 to over 14000 m2/ day. A comparison of the pre-development water table contours and the present day water table contours (Fig. 4) indicates that there has been no appreciable regional decline of the water table, and hence no appreciable change in ground• water storage as a result of irrigation practices. The main inflow into the system is through recharge from local rainfall and lateral throughflow. Lateral throughflow across the Victorian border has been estimated to be 14 000 ML/ year (Stadter & Love, 1987) . Recharge occurs via two mechanisms, diffuse and point recharge. Diffuse re• charge has been estimated from chloride profiles and changes

The variation in lithology of the unconfined aquifer results in inhomogeneous and anisotropic hydraulic conditions. An isopach plan of the unconfined aquifer is presented in Figure 3. i ~' Ad"'Id'

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Figure 3. Isopach plan of unconfined aquifer.

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255

10

kilometres

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256

F. STADTER & A.J. LOVE

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AR CHIBALD

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10

kilometres

Figure 4. Water table contours, October 1985 and pre-1970.

in groundwater storage to be about 10- 20 mm/yr in the eastern part of the region and up to 50 mm / yr in the west. Point recharge occurs via surface discharge into numerous runaway holes in the Bordertown and Mundulla areas. Point recharge to the unconfined aquifer is estimated at about 2300 ML/yr. The distribution of groundwater quality in the unconfined aquifer is complex. Salinity increases from about 1500 mg/L in the eastern part of the area to more than 9000 mg/L in the west (Fig. 5). In the west, rapid lateral changes in groundwater quality occur. These changes are caused by the heterogeneity of the aquifer system, local increases in recharge coincident with the eastern flanks of basement highs, and evapotran• spiration from the shallow water table. In the central region, lateral changes in groundwater quality are attrib• uted to direct recharge through runoff into numerous run• away holes. Groundwater samples collected at various depths during the drilling programs show that salinity stratification within the aquifer also occurs (Fig. 6) . This is particularly evident in the southwest (Fig. 6a,d), where a clay horizon separates marginal irrigation quality water from overlying saline water. Groundwater salinity monitoring has been undertaken since 1978 for changes in groundwater quality resulting from

irrigation practices. The results indicate annual increases in groundwater salinity of up to 320 mg/ L in a number of irrigation wells (Fig. 7) . However, a number of wells indicate a trend of decreasing salinity. These observed trends in groundwater quality can be partly attributed to; -downward leakage (particularly in the southwest) - inappropriate well-completion methods --partially penetrating wells, where upconing could result caused by a greater vertical flow component - lateral migration of different quality groundwater to• wards the extraction wells. We consider, however, that the dominant cause of salinity increase to date is the downward leaching of salts remain• ing in the soil profile following irrigation . This leaching is enhanced in areas of higher recharge and where the water table occurs at relatively shallow depth, as in the west. We ca lculate that for the irrigation of a lucerne seed crop using groundwater with a salinity of 3000 mg/ L, the salinity of the return flow to the aquifer could be 5500- 8200 mg/L (using irrigation efficiencies of 50% and 70% respectively, and assuming that there was no salt uptake by the irrigated crop and that the salts were com• pletely flushed from the soil profile) .

TATIARA PROCLAIMED REGION

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20

I kilometres

Figure S. Water quality distribution of unconfined aquifer.

Although the data are limited, it appears that the larger annual increases in groundwater salinity occur in areas of higher groundwater salinity. This supports the cause of salinity increase discussed above, as irrigation with a higher salinity groundwater would result in a greater salt load being returned to the aquifer. With this continual leaching of salts into the aquifer and given the relatively slow movement of groundwater (- 50 m!yr), it is likely that the magnitude of the annual salinity increases will become more pronounced.

Groundwater resource assessment

to the hydraulic continuity of the unconfined aquifer across the region (the groundwater outflow from one area must balance the inflow into the area immediately down• gradient). The results of this assessment indicate that to minimise the effects of any long term groundwater quality deterioration, particularly for irrigators located down• gradient of areas with a high level of irrigation develop• ment, the maximum possible groundwater throughflow should be maintained. The management philosophy that has therefore been adopted for the next five years is to maintain groundwater allocations at the level of local re• charge within each area. Further investigations are to be undertaken to examine in detail the cause of the groundwater quality deterioration in the western part of the region, together with a further attempt to establish a usable groundwater computer model for the region for determination of longer term manage• ment policies.

We attempted to model the groundwater system by the use of a groundwater computer-modelling package. However, an accurate determination of the water balance by this method proved difficult, because of the regional nature of the assessment, the variability in aquifer properties, lack of accurate data in some areas, and a time constraint for management decision making.

Conclusions

We then divided the region into 7 areas, and determined a separate water balance for each. Consideration was given

One of the major problems facing water resource manage• ment of the Murray Basin is degradation of groundwater

258 0)

F. STADTER & A.J. LOVE

STR 110

SALINITY

b)

(mg/L)

5000

WLL 105

SALINITY (mg/L)

10000

•

•

•

5000

0

• •

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•• •

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•

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d)

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5000

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40

Padthaway Formation with Keppoch Cloy at bose Bridgewater Formation

Coomandook Formation

~ ~

r+:+l ~

Buccleuch Beds

Renmark Group

Basement

Ettrick Formation

Figure 6. Water quality profiles, Hundreds of Stirling and Willalooka (for location of individual wells see Fig. 5).

quality as a result of changes in land use and irrigation practices in the last 100 years. The clearing of native vegetation has caused a decrease in evapotranspiration and a dramatic increase in diffuse recharge. This has resulted in rising groundwater levels and a deterioration of ground• water quality owing to remobilisation of salts trapped in the soil profile. Rising groundwater levels and salinisation of the land surface are rapidly becoming common in the southern part of the basin, where changes in groundwater storage have resulted in hydrogeological disequilibrium. No long term changes in groundwater levels have yet been observed in the Tatiara proclaimed region. The area is considered to be in hydrogeological equilibrium as present groundwater recharge rates are balanced by losses through irrigation practices. The main concern in the region is localised deterioration of groundwater quality in the unconfined aquifer as a result of leaching of salts left in the soil profile after irrigation. Considering the marginal quality of the groundwater and the observed annual increases in salinity, this trend is likely to have a deleterious effect on irrigation practices in the future. Management of groundwater withdrawal may, at best, only limit groundwater salinity increase in this type of environment.

References Brown, eM., & Stephenson, A.E., 1986 - Murray Basin subsur• face stratigraphic data base. Bureau of Mineral Resources. Australia. Report 262. Firman, J.B., 1973 - Regional stratigraphy of surficial deposits in the Murray Basin and Gambier Embayment. Geological Survey of South Australia. Report of Investigation, 39. Ludbrook, N.H ., 1961 - Stratigraphy of the Murray Basin in South Australia. Geological Survey of South Australia. Bulletin 36. Shepherd, R.G., 1978 - Underground resources of South Aus• tralia. South Australian Department of Mines and Energy. Bulletin 48. Stadter, F., & Love, A.J., 1987 - Tatiara proclaimed region . Groundwater assessment. South Australian Department of Mines and Energy. Report Book 87/87. Stadter, M.H., 1984 - Keith- Willalooka- Bordertown irrigation area investigation - Progress Report No.2. South Australian De• partment of Mines and Energy. Report Book 84/29. Williams, A.F., 1979 - Hydrogeological investigations in the Wi 1lalooka and Keith irrigation areas. Progress Report No. I for South East Water Resources Investigation Committee. South Australian Department of Mines and Energy. Report Book 79/84. Wopfner, H., & Douglas, J.G., (editors), 1971 - The Otway Basin of southeastern Australia. Geological Surveys of South Australia and Victoria. Special Bulletin.

TATIARA PROCLAIMED REGION

259

b)

STR 107

4500 r'- - - - - - - - - - - - - - - - - - -

8500

r '- - - - - - - - - - -- - - - -- - - -

4000

8000

a)

STR 105

Salinity increase

= 121 mg/L per annum

Salinity increase = 321 mg/L per annum

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81

82

83

84

85

86

87

Salinity increase = 43 mg/L per annum

3000 . • ---------------------

1000

L___~__~____L____L_ _~____L____L_ _~____L____L__~

1978

e)

79

BO

PET 101

BI

82

83

84

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86

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1988

Salinity increase = 134 mg/L per annum

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, 1978

d)

79

80

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81

82

83

84

85

86

87

Salinity increase = 32 mg/L per annum

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500

f)

1978

79

80

CAN 101

1988

81

82

83

84

85

86

87

-

1988

Salinity decrease = 14mg/L per annum

5OOOr'---------------------

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1988

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1988

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Figure 7. Salinity monitoring results, Hundreds of Stirling, Willalooka, Wirrega, Pendleton and Cannawigara (for location of individual wells see Fig. 5).