Journal of Hydrology, 126 (1991 ) 225-245 225 Elsevier Science Publishers B.V., Amsterdam [2] Geochemistry of ground waters from the Great Artesian Basin, Australia A.L. Herczeg , T. Torgersen b, A.R. Chivas c and M.A. Habermehl d ~Centre Jor Groundwater Studies and CSIRO Division ~1" Water Resources, Private Bag No. 2, Glen Osmond, S.A. 5064, Australia UDepartment of Marine Sciences, University of Connecticut, A veo' Point, Grown, CT 06340, USA ~Research School o["Earth Seiences, The Australian National University, G.P.O. Box 4, Canberra, A.C.T. 2601, Australia dDivision o[" Continental Geology, Bureau o[ Mineral Resources, Geology and Geophysics, G.P.O. Box 378, Canberra, A.C.T. 2601, Australia (Rece ived 8 Dec embe r 1989; accepted after revision 11 November 199 0) ABSTRACT Herczeg, A.L., Torgersen, T., Chivas, A.R. and Habermehl, M.A., 1991. Geochemistry of ground waters from the Great A rtesian Basin, Australia. J. Hydr ol., 126: 225-245. Ground waters from the confined Lower Cretaceous-Jurassic aquifer of the Great Artesian Basin, Australia (GAB) are characterised by Na-HCO 3 type water s through out the eastern and central parts of the basin and Na-SQ-C1 type waters in the western part. Sodium and bicarbonate increase in concen- tration from the no rtheastern margins to the southwestern discharge areas along the regional groundwater flowlines. A mass-balance and equilibrium model for major dissolved species and stable carbon isotopes of dissolved inorganic carbon s how that the chemica l trends observe d are caused by mass transfe r reactions involving cation exchange of Na for Ca-Mg, carbonate dissolution and reactions between Na and kaolini te to form Na-smectite. The stable carbon isotopic composition of dissolved inorganic carbon (DIC) increa ses f rom value s o f about - 15 %o near t he basin marg ins to - 6 %o in the interior of the basin. An inverse relationship exists between 6~3C and DIC indicating addition of enriched carbon as the ground waters move basinwards. A t2C- ~3C mass balance indicates that the trend toward heavier 6~3C values in the interior of the basin results from bacterial reduction of carbon dioxide to produce methane rather t han di ssolution of, and equilibration with, carbona te mi nerals. The GAB aquifer system is apparently open to CO ~ which is the produc t of in situ anaerobic fermentation producing CO 2 enriched in t3C. The chemical evolution of the major dissolved spec ies and carb on iso tope distribu tion in the eastern and central par ts of the GAB c an then be envisaged as an initial pCO 2 up to sev era l orders o f magnitude abo ve atmospheric le vel is acquired within the recharge area by plant respiration and oxidation of organic matter in the soil zone. Initial y, silica te and carbonate minerals may dissolve , at least in some parts of the basin, resulting in increased a lkalinity, higher Ca and Mg concentrati ons, and a 6~3Cconce ntrat ion of the DIC of around - 12%0. Processes suc h as cation exchange of Na for Ca an d Mg in add ition to the removal of some Na by reverse wea ther ing which produ ces a Na smectite dominate in the interior of the GAB. The chemistry of ground waters derived from the western recharge areas is controlled by evaporite dissolution as indicated by very high CI/Br ratios.
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8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
E l s e v i e r S c i e n c e P u b l i s h e r s B . V . , A m s t e r d a m
[2]
Geochemistry of ground waters from the Great
Artesian Basin, Australia
A . L . H e r c z e g , T . T o r g e r s e n b , A . R . C h i v a s c a n d M . A . H a b e r m e h l d
~Centre Jor Groundwater Studie s and C SI RO Division ~1" W ater Resources, Private Bag No. 2,
Glen Osmond, S.A. 5064, Austral ia
UDepartment of M arine Sciences, University of Connecticut, A veo ' Point, Gro wn, C T 06340, US A
~Research Schoo l o[" Earth Seiences, The Austral ian Nati onal University , G.P.O . Bo x 4, C anberra,
A.C .T . 2601, Austra l ia
dDivision o[" Co ntine ntal Geology, Bureau o [ M ine ral Resources, Ge ology and Geophysics,
G.P.O. B ox 378, Canberra, A.C .T. 2601, Austral ia
(Received 8 December 1989; accepted after revision 11 November 1990)
ABSTRACT
Herczeg, A.L., Torgersen, T., Chivas, A.R. and Habermehl, M.A., 1991. Geochemistry of ground waters
from the Great Artesian Basin, Australia. J. Hydrol., 126: 225-245.
Ground waters from the confined Lower Cretaceous-Jurassic aquifer of the Great Artesian Basin,
Australia (GAB) are characterised by Na-H CO 3 type waters throughout the eastern and central parts o fthe basin and Na-SQ-C1 type waters in the western part. Sodium and bicarbonate increase in concen-
tration from the northeastern margins to the southwestern discharge areas along the regional groundwater
flowlines. A mass-balance and equilibrium model for major dissolved species and stable carbon isotopes
of dissolved inorganic carbon show that the chemical trends observed are caused by mass transfer reactions
involving cation exchange of Na for Ca-Mg, carbonate dissolution and reactions between Na and kaolinite
to form Na-smectite. The stable carbon isotopic composition of dissolved inorganic carbon (DIC)
increases from values of about - 15%o near the basin margins to - 6%o in the interior of the basin. An
inverse relationship exists between 6~3C and DIC indicating addition of enriched carbon as the ground
waters move basinwards. A t2C-~3C mass balance indicates that the trend toward heavier 6~3C values
in the interior of the basin results from bacterial reduction of carbon dioxide to produce methane
rather than dissolution of, and equilibration with, carbona te minerals. The GAB aquifer system is
apparently open to CO~ which is the product of in situ anaerobic fermentation producing CO 2 enrichedin t3C.
The chemical evolution of the major dissolved species and carbon isotope distribution in the eastern and
central par ts of the GAB can then be envisaged as an initial pCO 2 up to several orders of magnitude above
atmospheric level is acquired within the recharge area by plant respiration and oxidat ion of organic matter
in the soil zone. Initially, silicate and carbonate minerals may dissolve, at least in some parts of the basin,
resulting in increased alkalinity, higher Ca and Mg concentrations, and a 6~3C concentrat ion of the DIC
of around - 12%0. Processes such as cation exchange of Na for Ca and Mg in addition to the removal of
some Na by reverse weathering which produces a Na smectite dominate in the interior of the GAB. The
chemistry of ground waters derived from the western recharge areas is controlled by evaporite dissolution
as indicated by very high CI/Br ratios.
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
Ge oc h e m ic a l p r oc e s se s oc c u r r in g w i t h in gr ou n d w at e r s an d r e ac t ion s w i t h
aq u i f e r m in e r a l s h ave a p r o f ou n d e f f e c t on w at e r q u a l i t y . Th e r e f or e , t h e
e va lu a t ion o f t h e im p or t an c e o f su c h p r oc e s se s i s e s se n tia l i f gr ou n d w at e rr e s ou r c e s ar e t o b e p r o p e r ly d e v e l o p e d f o r h u m a n c o n s u m p t i o n a n d p a s to r a l
or m in in g ac t iv i t ie s . T h i s p ap e r in ve s t iga te s t h e p os s ib l e p h ys i c a l an d c h e m ic a l
p r oc e s se s a f f e c t in g t h e m aj or e l e m e n t an d s t ab le c ar b on i so t op ic c om p os i t ion
o f g r o u n d w a t e r s f r o m t h e G r e a t A r t e s ia n B a s i n ( G A B ) o f A u s tr a l ia .
T h e G A B i s l oc a t e d in t h e s e m i -ar id an d ar id p ar t s o f e a s t e r n A u s t r a l ia an d
u n d e r l i e s ab ou t 22 ( 1 .7 x 106 k m 2 ) o f t h e A u s t r a l ian c on t in e n t (F ig . 1 ). I t
i s a c on f in e d gr ou n d w at e r b as in c om p ose d o f aq u i f e r s in c on t in e n t a l
q u a r t z o s e s a n d s t o n e s w i t h c o n f i n i n g b e d s o f p a r t l y m a r i n e m u d s t o n e a n d
s i l ts t on e o f Tr ia s s ic , Ju r as si c an d C r e t ac e ou s age u p t o 30 00 m t h ic k ( F ig . 2 ) .R e c h ar ge o f gr ou n d w at e r oc c u r s m a in ly in t h e e as te r n m ar g in a l zon e w i t h
som e m in or in p u t f r om t h e w e s te r n m ar g in . P r e c ip i t a t ion in t h e r ec h ar ge ar e a
ave r age s 6 0 0 m m ye ar ~ an d i s lar ge ly s e ason a l w i t h r e lat ive ly d r y w in t e r s
~ ' ~ ~ ~ ~' ~'
- . . . . .
• t ' f
Fig . 1 . Locat ion o f the Great Ar te s ian Bas in , Aus tra l ia ( ins e t ) and inferred f low l ines bas ed on poten-
t iom etr ic s ur faces ( a f ter Haberm ehl , 1980 ) . Recharge occurs m os t ly on the eas tern and nor theas tern
m arg ins s upp ly ing water to the wes tward and s outhward f lowing reg im es . Som e fur ther recharge occurs
in the wes tern m arg ins . D i s charge v ia upw ard l eakage takes p lace throug hout the bas in through ar te s ian
s pr ings and by d i f fus e d i s charge in the s outhwes tern m arg ina l par t o f the bas in .
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
~ C o n fl n L qg b a d - - - - B o u n d e r b e t w e e n c o n fi n e d a q u if e r s P o t O M l O O ~ f r l curface Jurassic aqulf orJ~ Oand conr~mmg l~ds
I I B . . . . . , . . . . . T o p o ~ r op h . . . . . f o u r { re ) o b eY * M S L
Fig . 2 . Genera l i s ed cros s - s ec t ion o f the Great Ar te s ian Bas in s howing the Juras s ic aqui fer and the over ly ingconf in ing b ed and cre taceous aqui fer . The s ec t ion re fers lo the l ine A- A ' ( a f ter Hab erm ehl , 1980 ) .
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
f r o m t h e n o r t h e a s t e r n a n d e a s t e r n m a r g i n s t o w a r d s t h e in t e r io r o f th e b a s in .
W a t e r s o f p r e d o m i n a n t l y N a - C 1 t yp e o c c u r n e a r th e w e s t e r n m a r g i n a l z o n e
w h e r e r e c h a r g e f r o m t h e w e s t d i s so l v e s e v a p o r i t e m i n e r a l s ( H a b e r m e h l , 1 980).
H a b e r m e h l ( 1 98 0, 1 9 8 6 ) a n d C a l f a n d H a b e r m e h l ( 1 98 4) d e a l w i th s o m e
a s pe c ts o f t h e h y d r o c h e m i s t r y o f t h e G A B g r o u n d w a t e rs i n te r m s o f g e n e ra lp a t te r n s t h r o u g h o u t t h e b as in . A d i s c u ss i o n o f t h e e n v i r o n m e n t a l is o t o p e
d i s t r i b u t i o n i n th e g r o u n d w a t e r s b y A i r e y e t al . (1 9 79 ) i n d i c a te s t h a t t h e
w a t e r s a r e e n t ir e l y o f m e t e o r i c o r i g in .
N i n e t e e n c h e m i c a l a n d i s o to p ic a n a l ys e s o f g r o u n d w a t e r s a m p le s w e r e
o b t a i n e d f r o m e x is t in g f l o w i n g a r t e s i a n w a t e r w e l ls in t h e J u r a s s i c a q u i f e r
o f t h e G A B i n 1 9 8 5 . S a m p l e w e l l l o c a l it ie s fo r t h i s s t u d y a r e s h o w n i n F i g . 3.
W e u s e d a c h e m i c a l a n d i s o to p i c m a s s b a l a n c e a n d e q u i l i b r i u m a c t i v i ty
r e l a t io n s h i p s to d e t e r m i n e th e t yp e s o f m i n e r a l - w a t e r i n t e r a c t i o n s t h a t a r e
l ik e ly to t a k e p l a c e. I d e n t i fi c a t io n o f s o l id m i n e r a l p h a s e s a n d t h e i r c h e m i c a lc o m p o s i t io n s w e r e c a r r ie d o u t t o v e ri fy w h e t h e r t h e p r o p o s e d c h e m i c a l
r e a c t i o n s d e r i v e d f r o m t h e r m o d y n a m i c m o d e l l in g a r e f ea s ib l e. A ~3C i s o t o p e
a n d d i ss o l v ed i n o r g a n i c c a r b o n m a s s b a l a n c e p la c e s a d d i t i o n a l c o n s t r a i n t s o n
t h e p r o p o s e d c h e m i c a l r e a c t i o n s i n v o l v i n g p r o t o n t r a n s f e r r e a c t io n s w i t h i n
t h e g r o u n d w a t e r s y s t e m .
ANALYTICAL METHODS
W a t e r s a m p l es w e r e c o l l e c te d i n a c i d - w a s h e d N a l g e n e b o t tl e s f r o m a r t e s ia n
w e l ls t h a t w e r e fl o w i n g c o n t i n u o u s l y a n d h a d b e e n d o i n g s o f o r s e v e r a l y e a r s
o r d e c a d e s . A l l s a m p l e b o t t le s w e r e f lu s h e d w i t h s e v e r a l v o l u m e s o f w a t e r
b e f o r e c h e m i s t r y s a m p l e s w e r e t a k e n . S e p a r a t e 5 0 0 -m l s a m p l e s w e r e ta k e n f o r
c a t i o n s a n d a n i o n s a n d f il te r e d i m m e d i a t e l y t h r o u g h 0.4 5-/~ m M i ll i po r e ®
f il te r s a n d t h e c a t i o n a l i q u o t a c i d i fi e d t o p H < 1 . 5 . A l k a l i n i t y s a m p l e s w e r e
c o l l e c te d i n 1 00-m l W h e a t o n ® b o t tl e s t o w h i c h 1 m l o f s a t u r a t e d H g C 1 2
s o l u t io n w a s a d d e d , a n d a r u b b e r s e p t u m w a s s e c u r ed b y a n a l u m i n i u m
c r i m p e d s e a l . T h e p H w a s m e a s u r e d i n t h e f i e l d u s i n g a p o r t a b l e O r i o n p H
m e t e r c a l i b r a t e d w i t h t w o s t a n d a r d b u f fe r s o lu t i o n s ( p H v a l u e s o f 7 a n d 9 ).
M a j o r c a t i o n s w e re a n a l y s e d by a to m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y .
S u l p h a t e , b r o m i d e a n d f lu o r i d e w e r e d e t e rm i n e d o n a D i o n e x ® i o n c h r o m a -
t o g r a p h . C h l o r i d e w a s d e t e r m i n e d s e p a r a te l y b y p o t e n t i o m e t r i c ti t r a t io n w i th
s il v er n i t ra t e . A q u e o u s s i lic a w a s a n a l y s e d b y a t o m i c a b s o r p t i o n f o l lo w i n g t h e
m e t h o d o f K i s s (1 982 ). A l k a l in i t y w a s m e a s u r e d b y a p o t e n t i o m e t r i c G r a n
t i tr a t i o n i n t h e la b o r a t o r y . T o t a l d i s s o l v e d i n o r g a n i c c a r b o n ( D I C ) w a s
d e t e r m i n e d b y c a l c u l a ti o n f ro m p H a n d a l ka l in i ty d a t a u s in g th e p r o g r a m
P H R E E Q E ( P a r k h u r s t e t a l ., 1 980). F o r d e t e r m i n a t i o n o f 61 3C D Ic, B a C 12 a n d
N a O H w e r e a d d e d t o a b o u t 2 5 0 m l o f w a t e r t o in d u c e p re c i p it a t i o n o f
d i s s o l v e d C O 2 a s B a C O 3 . T h e p r e c i p i ta t e s w e r e a c i d i fi e d i n a v a c u u m l in e a ft e r
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
r e t u r n t o t h e l a b o r a t o r y a n d t h e e v o l v e d C O 2 d e t e r m i n e d f o r i t s i so t o p ic
c o m p o s i t i o n o n a m o d i f ie d N u c l i d e M S 1 2 m a s s s pe c t r o m e t e r . P r e c is io n f o r
i s o t o p e a n a l y s e s i s n o m i n a l l y _+ 0.1 % o.
R E S U L T S
R e s u l t s o f c h e m i c a l a n d 6 ~3C a n a l y se s o f g r o u n d w a t e r s f r o m t h e J u r a ss i c
a q u i fe r s in t h e G r e a t A r t e s i a n B a s in a r e p r e s e n t e d i n T a b l e 1. T h e d a t a a r e
g r o u p e d in t o t h r e e t ra n s e c t s ( F ig . 3) w h i c h r e p r e s e n t d o w n g r a d i e n t e v o l u t i o n
f r o m t h e i r re s p e ct iv e r e c h a r g e a r e a s . T h e w a t e r s o n o u r L i n e E t r a n s e c t ( w e ll s
7 8 -9 2 ) a r e d o m i n a n t l y N a - H C O 3 t y p e w i t h t o t a l d i s s o l v e d s o l i d s ( T D S )
c o n t e n t s r a n g i n g f r o m 3 6 2 p p m i n t h e n o r t h e a s t e r n z o n e t o g r e a t e r t h a n
1 6 00 p p m n e a r t h e d i s c h a r g e a r e a s i n th e w e s t e r n a n d s o u t h w e s t e r n p o r t i o n s
o f t h e b a s i n . W e l ls 7 3- 7 7 , w h i c h lie o n a n e a s t - w e s t l i n e t o w a r d s t h e e a s t e r ns id e o f t h e b a s in ( L i n e A ) , t e n d t o h a v e h i g h e r T D S c o n t e n t s t h a n t h o s e
e m a n a t i n g f r o m t h e n o r t h e a s t e r n p a r t o f t h e b a si n . T h e y a r e c h a ra c t e r i s e d b y
h i g h e r N a , H C O 3 a n d C 1 c o n c e n t r a t i o n s t h a n t h e g r o u n d w a t e r s f r o m L i n e E
f u r t h e r t o t h e n o r t h . W a t e r s f r o m t h e w e s t e r n t r a n s e c t , L i n e W ( we lls 9 3- 9 8 )
a r e o f t h e N a -C 1 t yp e a n d h a v e m u c h h i g h e r T D S c o n t e n t s (u p t o 5 835 p p m )
t h a n t h e e a s t e r n t r a n s e c t s .
F i g u r e 4 s h o w s t h e t r e n d s o f c h e m i c a l p a r a m e t e r s a n d ~3C a l o n g t h e t h r e e
t r a n s e c t s d e fi n e d as L i n e s A , E a n d W ( s h o w n i n F ig . 3) f r o m n e a r t h e i n f e r r e d
r e c h a r g e a r e a s t o w a r d s t h e d i s c h a r g e a r e a in t h e s o u t h w e s t e r n p a r t o f t h eb a s i n . T h e r e is a g e n e r a l i n c r e as e i n d i s s o lv e d s o l id s c o n c e n t r a t i o n a l o n g a ll
t r a n s e c t s t o w a r d s t h e i n te r io r o f t h e G A B , w h i c h is a c o m m o n f e a tu r e o f m a n y
l a rg e g r o u n d w a t e r b a s in s . S i m i l a r ly N a c o n t e n t a n d a l k a l i n i ty s h o w s i m i l a r
i n cr e as e s t o t h o s e o f t h e T D S c o n t e n t a n d t h e y d o m i n a t e t h e T D S t re n d .
C h l o r i d e i n c r e a s e s g r a d u a l l y f o r t h e m o s t p a r t ( e x c e p t f o r t h e w e s t e r n
t r a n s e c t ) a n d i n c re a s e s b y a b o u t a f a c t o r o f t h r e e a l o n g L i n e E . T h e r e i s a q u i te
d i s t in c t s e p a r a t i o n o f c h l o r i d e c o n c e n t r a t i o n f o r th e t h r e e t ra n s e c t s a n d r a n g e
o v e r t w o o r d e r s o f m a g n i t u d e . P o t a s s i u m m i m i c s c h l o r id e t o s o m e e x t e n t a s
i t s h o w s a s l i g h t d e c r e a s e a l o n g L i n e E a n d t h e n s o m e in c r e a s e f o r th e l a s t f o u r
w e l ls . C a l c i u m a n d m a g n e s i u m s h o w a m a r k e d d e c r ea s e to a b o u t 30 0 k m t h e n
r e m a i n a t v e r y l o w c o n c e n t r a t i o n s e x c e p t f o r L i n e A w h i c h s h o w s a g e n e r a l
i n c r e a s e , a lb e i t a t a l o w l e v e l. A l k a l i n i t y , D I C a n d 6 ~3C s h o w s i m i l a r p a t t e r n s
f o r L i n e E a s t h e y i n c r e a se s t e a d i ly a lo n g t h e t ra n s e c t . A l k a l in i t y a n d D I C
c o n c e n t r a t i o n s a r e m a r k e d l y h i g h e r fo r L i n e A t h a n t h e o t h e r t r a n se c ts .
X - r a y d i f f r a c t i o n a n a l y s i s o f a q u i f e r s o l i d m a t e r i a l w a s c a r r i e d o u t t o
d e t e r m i n e w h i c h m i n e r a l s w i t h i n t h e b a s i n a r e l ik e ly to i n f l u e n c e t h e w a t e r
c h e m i s t r y . F i f t e e n d e e p d r i l l h o l e ( p e t r o l e u m e x p l o r a t i o n w e l l ) c o r e s a n d
c u t t in g s o f s a n d s t o n e s a n d c o n f in i n g m u d s t o n e s s h o w a u n i f o r m d i s t r ib u t i o n
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
1 0 , L , 1 , i , i , L , i , 0 . 0 0 ° , i , i , i , i , i , L ,2 0 0 4 0 0 6 0 0 8 0 0 I 0 0 0 1 2 0 0 1 4 0 0 2 o 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0
i s tance k m ) > D i s t a n c e k i n ) )
F ig . 5 . ( a ) - ( d ) P l o t s o f t h e m o l e r a t i o s o f N a / C I , (C a + M g ) / C 1 , C I / B r , a n d S r / C a a l o n g t h e t h r e e t ra n s ec t s
represented in Fig. 3.
w a t e r f r o m t h e U p p e r C r e t a c e o u s a q u i f e r s m u s t b e c o n s i d e r e d t o b e s l i g h t .
S a l i n e w a t e rs f r o m t h e m a r i n e C r e t a c e o u s m u d s t o n e s m a y l ea k in t o t h e
J u r a s s i c a q u i fe r , p a r t ic u l a r l y n e a r t h e d i s c h a r g e a r e a s b u t i t w o u l d n o t a c c o u n t
f o r t h e o v e r a l l c h e m i c a l t r e n d s d e s c r i b e d a b o v e .
T h e G A B w a t e rs a r e p r e ss u r is e d in t h e in t e r io r o f th e b a s i n ( a t m o r e t h a n2 00 b a r ) a n d t h e y a r e o f m e t e o r i c o r i g i n a s d e t e r m i n e d f r o m s t a b le i s o t o p i c
c o m p o s i t i o n o f h y d r o g e n i s o t o p e s in t h e w a t e r m o l e c u l e s (A i r e y e t a l. , 1 979 ).
U sin g 36C1a n d t o t a l c h l o r i d e c o n c e n t r a t i o n s , B e n t le y e t al . (1 9 86 ) c o n c l u d e d
t h a t i o n f i l t r a t io n d o e s n o t o c c u r in t h e G A B b u t t h a t t h e r e is a c o n t r i b u t i o n
o f 'd e a d ' c h l o r i d e in t h e e a s t e r n f lo w r e g im e s , p o s s i b ly f r o m a n u n d e r l y i n g
a q u i fe r th a t c o n t a i n s c o n n a t e w a te r . W e c o n c l u d e t h a t a c o m b i n a t i o n o f
e v a p o t r a n s p i r a t i o n w i th i n t h e r e c h a rg e a r e a s a n d m i n e r a l d i s s o l u t io n p r o v i d e
t h e m o s t p la u s ib l e m e c h a n i s m f o r th e c h e m i c a l t r e n d s o b s e r v e d a l o n g L i n e s
E a n d W . E v a p o r i t e d i s s o l u t i o n a n d e n t r a i n m e n t o f m o r e s a l i ne g r o u n d w a t e r s
a r e n o t i m p o r t a n t c o n t r i b u t o r s e x c e p t i n t h e w e s t e r n a n d s o u t h w e s t e r n
m a r g i n s o f th e b a s in .
P l o t t i n g t h e i o n /C 1 r a t io s a s a f u n c t i o n o f d i s t a n c e a l o n g e a c h t r a n s e c t
a l lo w s u s t o d e l i n e a t e c h e m i c a l p r o c e s s e s t h a t a r e n o t r e l a t e d t o e v a p o t r a n s -
p i r a t i o n v a r i a ti o n s d u r i n g r e c h a rg e b e c a u s e C1 is c o n s e r v e d d u r i n g g r o u n d -
w a t e r e v o l u t io n t h r o u g h o u t m o s t o f th e G A B . S o d i u m / c h l o r i d e r a ti o s t e n d t o
b e v a r i a b le t h r o u g h o u t t h e b a s i n ( F ig . 5 ( a )) b u t g e n e r a l ly in c r e a s e b y a f a c t o r
o f a b o u t t h r e e . S o d i u m / c h l o r i d e m o l e r a t i o s a r e a lw a y s g r e a te r t h a n t w o
( e x ce p t f o r L i n e W ) , d e m o n s t r a t i n g t h a t h a l i te d i s s o l u t i o n o r m i x i n g w i t h
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
G R E A T A R T E S I A N B A S IN G E O C H E M I S T R Y 2 3 5
1
A
¢ o 0
_ .
*2 ' i i i i5 10 15 20 25
A l k a l i n i t y
F i g . 6 . S a t u r a t i o n i n d e x o f G A B g r o u n d w a t e r s w i t h r e sp e c t t o c a lc i te ( g iv e n a s t h e ra t i o o f t h e io n i c a c t i v i t y
p r o d u c t d i v i d e d b y t h e e q u i l i b r i u m s a t u r a t i o n v a l u e ) a s a f u n c t i o n o f a l k a l i n i t y . A v a l u e o f z e r o r e p r e s e n t s
s a t u r a t i o n w h i l e v a l u e s le s s t h a n 0 r e p r e s e n t u n d e r s a t u r a t i o n .
t r a p p e d s e a w a t e r b r in e s d o e s n o t c o n t r i b u t e t o t h e i n c r e as e in s o d i u m o b s e r v e d
h e r e . T h e r a t i o o f ( C a + M g ) /C 1 d e c r e a se s m a r k e d l y fo r t h e f ir s t 30 0 k m a l o n g
t r a n s e c t E ( F ig . 5 ( b )) b u t t h e n r e m a i n s a t v e r y l o w le v e ls , t h e r e f o r e C a a n d M g
a r e r e m o v e d f r o m s o l u t io n d u r i n g t h e f i r st o n e - t h i r d p o r t i o n o f th e t r a n se c t .
C h l o r i d e / b r o m i d e ra t io s r a n g e o v e r a fa c t o r o f t w o f o r L i n e s E a n d A
( F ig . 5 ( c)); h o w e v e r , a ll b u t o n e a r e w i t h i n t h e r a n g e o f v a l u e s t h a t c o u l d b e
d e r i v e d f r o m r e c h a r g e o f s e a w a t e r - d e r i v e d c y c li c s a lt . T h e s i m i l a r i ty o f C 1 / B r
r a t io s f o r L i n e s A a n d E t o t h o s e f o u n d i n s e a w a t e r ( 5 5 0 -7 0 0) d e m o n s t r a t e
t h a t n o e v a p o r i t e d i s s o l u t i o n h a s ta k e n p la c e w h i c h w o u l d g e n e r a t e C 1 /B rm o l e r a ti o s m u c h g r e a t e r t h a n a p p r o x i m a t e l y 5 0 0- 70 0. T h e h i g h C 1 / B r r a ti o
m e a s u r e d a t w e l l 83 in d i c a t e s t h a t s o m e m i x i n g o f w a t e r s h a s o c c u r r e d
b e t w e e n w e ll s 8 2 a n d 8 3, p o s s ib l y b y u p w a r d l e a k a g e o f C l - r i c h w a t e r d e r i v e d
f r o m t h e L o w e r P a l a e o z o i c - P r o t e r o z o i c G e o r g i n a B a s in u n d e r l y i n g t h e G A B .
T h e e a s t e r n m a r g i n o f t h e G e o r g i n a B a s i n is l o c a t e d in t h e a re a , a n d a f au l t
s t r u c t u r e is p r e s e n t i n t h e G A B s e q u e n c e i n t h e r eg i o n . C h l o r i d e / b r o m i d e
r a ti o s o f g r o u n d w a t e r s f r o m t h e w e s t e rn m a r g i n a r e m u c h g r e a t e r th a n 1 000
d e m o n s t r a t i n g t h a t t h e y ar e d e r iv e d f r o m e v a p o r i te d i s so l u t io n .
S t r o n t i u m / c a l c i u m r a t io s ( F ig . 5 ( d )) s h o w a g e n e r a l i n c re a s e in L i n e s E a n dW w h i c h c o u l d b e c a u s e d b y l o ss o f C a v i a c a t io n e x c h a n g e . P r e c i p i ta t i o n o f
C a C O 3 is n o t p o s s ib l e b e c a u s e a ll b u t o n e o f t h e w a t e r s f r o m L i n e E a r e
u n d e r s a t u r a t e d w i t h r e s p e c t t o C a C O ~ ( F i g . 6 ) .
Mineral-solution interactions
T h e t y pe s o f m i n e r a l - s o l u t i o n r e a c t i o n s th a t m a y c o n t r o l t h e c o m p o s i t i o n
a n d t r e n d o f h y d r o c h e m i c a l s pe c i es i n o u r t r a n s e c ts A a n d E a r e c a t i o n
e x c h a n g e , d i s s o l u t i o n o f c a lc i te o r f e ld s p a r s o r r e a c t i o n s i n v o l v i n g c la y
m i n e r a ls . I t is i m p o r t a n t t o n o t e t h a t t h e t h r e e t r a n s e c t s d e s c r i b e d a b o v e a ll
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
s h o w d i ff e re n t c h a r a c te r i s ti c s a n d t h e r e f o r e t h e t y pe s o f c h e m i c a l r e a c t io n s
n e e d n o t o p e r a t e t o t h e s a m e e x t e n t in a ll p a r t s o f t h e b a s i n a n d t h e r e f o r e w i ll
b e d i s c u s s e d s e p a r a t e l y .
W e a s s u m e t h a t c h l o r id e io n s b e h a v e c o n s e r v a t iv e l y t h r o u g h o u t L i n e E ,
w i t h p e r h a p s t h e e x c e p t i o n o f so m e a d d i t i o n o f C l - r ic h w a t e r b e t w e e n w e l ls8 2 a n d 83 (s e e a b o v e ) . N o r m a l i s i n g a ll o t h e r c h e m i c a l s p ec i es t o c h l o r i d e a l l o w s
u s to e v a l u a t e t h e t y p e s o f c h e m i c a l p r o c e s s e s a ff e c t in g o t h e r d i s s o l v e d s p e c i e s
w i t h i n t h e G A B t h a t a r e n o t r e l a t e d t o p r o c e s s e s s u c h a s e v a p o t r a n s p i r a t i o n .
C h l o r i d e c o n c e n t r a t i o n s i n r a in f a ll i n r e g i o n s , w h i c h a r e m o r e t h a n 1 00 k m
f r o m t h e c o a s t, f a l l i n t o a n a r r o w r a n g e o f le ss t h a n 1 t o 5 p p m ( B l a c k b u r n a n d
M c L e o d , 1 9 83). T h e s e c o n c e n t r a t i o n s a re a n o r d e r o f m a g n i t u d e o r m o r e
l o w e r t h a n C 1 - c o n c e n t r a t io n s i n e v e n t h e m o r e d i lu t e re g i o n a l g r o u n d w a t e rs
w i t h in t h e G A B ( T a b l e 1). E l e v a t e d c h lo r i d e c o n c e n t r a t i o n s i n t h e g r o u n d
w a t e rs a r e p r o b a b l y c a u s e d b y e v a p o t r a n s p i r a t i o n d u r i n g r e c h a r g e a n d t h ise v a p o t r a n s p i r a t i o n a ffe c ts a ll i o n s e q u a ll y . C y c li c w e t t i n g a n d d r y i n g in t h e
u n s a t u r a t e d z o n e ( e .g . D r e v e r a n d S m i t h , 1 9 78) w i t h i n t h e r e c h a r g e a r e a s i s
n o t i m p o r t a n t b e c a u s e th e r e c h a r g e w a te r s d o n o t e v a p o r a t e t o d r y n e s s w i t h in
t h e s o il z o n e . T h i s p ro c e s s c a n o n l y f r a c t i o n a t e s o d i u m a n d c h l o r i d e i f h a l i t e
f o r m s in t h e so i l z o n e , a n d t h is d o e s n o t o c c u r i n th e G A B r e c h a r g e a r e a s .
T h e t y p e s o f re a c t i o n s p r o p o s e d f o r L i n e E m u s t s i m u l t a n e o u s l y s a ti sf y t h e
o b s e r v e d in c r e a s e s i n N a , a l k a li n i ty a n d D I C a n d a t t h e s a m e t i m e t h e d e c r ea s e
in C a a n d M g . F u r t h e r m o r e , t h e r e la t i ve ly c o n s e r v a t iv e b e h a v i o u r o f a q u e o u s
s il ic a m u s t b e a c c o u n t e d f o r i n a n y r e a c t i o n s i n v o l v i n g s i li c a te m i n e r a l s .T h e r e le a s e o f N a , t o t h e m o r e d i l u t e w a te r s n e a r e r t h e r e c h a r g e a r e a s m a y
b e a c c o m p l i s h e d b y t h e f o l l o w i n g : (i) d i s s o l u t i o n o f N a b e a r i n g m i n e r a l s
( e. g. p l a g i o c l a s e o r o r t h o c l a s e ) ; (ii) c a t i o n e x c h a n g e w h i c h r e l e as e s N a * f r o m
c l ay m i n e r a l s ; (iii) c o n v e r s i o n o f a N a - s m e c t i t e t o k a o l i n i te . I n c o n g r u e n t
d i s s o l u t i o n o f a l b it e re l ea s e s N a a n d H C O 3 i n t o s o l u t i o n in a n a p p r o x i m a t e l y
1 : 1 r a t io a c c o r d i n g t o t h e e q u a t i o n
2 N a A 1 S i 3 O s + l l H 2 0 + 2 C O 2 ~ A 1 2 S i2 O s( OH ) 4 + 2 N a
a l b i t e k a o l i n i t e+ 2 H C O 3 + 4 H 4 S I O 4 ( 1 )
o r
2 . 3 3 N a A 1 S i 3 0 8 + 8 . 6 4 H 2 0 + 2 C O 2 ~ N a 03 3A 1 2 3 3S i 36 7O t0 (O H ) 2
a l b i t e N a - m o n t m o r i l l o n i t e
+ 2 N a + 2 H C O 3 + 3 .3 2 H 4 S I O 4 ( 2)
w h i c h a l s o r e le a se s o n e e q u i v a l e n t o f a l k a li n i ty f o r e a c h e q u i v a l e n t o f N a b u t
a l so r e l e as e s s u b s t a n t i a l a m o u n t s o f d is s o l v e d s il ic a f o r e a c h m o l e o f N a
p r o d u c e d . S i l i c a d o e s n o t r e a c h e l ev a t e d c o n c e n t r a t i o n s a n y w h e r e in t h e G A B
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
which produces Ca, Mg and HC O 3. However, Ca a nd M g decrease mar ked ly
with increasing TDS c ontent and in fact never reach very high concent rati ons
except in waters from Line E near the eastern margin.
Ma ss -ba l a n c e c a l c u l a t io n s
We tested a model based on that proposed by Blake (1989) for the
generation of waters high in Na HC O 3 in the Ot way basin of southeast
Australia. The model hypothesises a three-stage process involving: (1) release
of Ca, Mg and HCO~ via carbonate dissolution; (2) cation exchange of Ca (and
TABLE 3
Thermodynamic data used in calculating mineral-solution equilibria in Fig. 7. H and Grepresent the enthalpy and free energy of formation for each mineral phase in units of
cal tool L. Log K is the equilibrium constant derived from free energy data at 25°C
Mineral H G log K Ref.(cal mol 1) (cal mol 1)
Albite -939.7 -886.3 2. 59 Helgeson t al., 1978Calcite -288.8 -270.1 -8.48 Parkhurst et al., 1980Na-beidellite - 1368.2 - 1277.8 - 7.9 Nesbitt, 1977Kaolinite - 982.2 - 905.6 Helgeson et al ., 1978Gibbsite -309.1 -276.2 10.38 Helgeson t al., 1978
Quartz -217.65 -204.65 -4.01 Helgeson t al., 1978Amorphous s il ica -214.57 -202.89 - Helgeson et al., 1978
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
F ig . 7. A c t i v i t y - a c t iv i t y d i a g r a m d e p i c t i n g t h e s ta b i l it y fi el d o f N a m i n e r a l s . T h e r m o d y n a m i c d a t a f o r
c a l c u l a t i o n o f m i n e r a l s t a b i l i t y f ie l d s a r e g i v e n i n T a b l e 3 . T h e s t a b i l i t y fi e ld s a r e g i v e n a t 2 5 ° C w h i l e
a c t iv i ti e s fo r d i s s o l v e d sp e c i e s a r e a t i n s i tu g r o u n d w a t e r t e m p e r a t u r e s ( 3 0 - 9 5 ° C ) . W i t h i n t h i s t e m p e r a t u r e
r a n g e t h e m i n e r a l - w a t e r s t a b i li ty f ie l d s d o n o t c h a n g e s i g n if ic a n t ly f ro m t h o s e d e n o t e d h e r e . T h e w a t e r s
a p p e a r t o b e a t o r n e a r e q u i l i b r i u m w i t h r e s p e c t t o k a o l in i t e , a n d a N a - s m e c t i te . T h e y a r e u n d e r s a t u r a t e d
w i th r e s p e c t to a m o r p h o u s s i li ca a n d c a l c it e a n d s u p e r s a t u r a t e d w i th r e s p e c t to q u a r t z .
M g ) f o r N a o n c l a y m i n e r a l s ite s ; (3) b a c k r e a c t i o n o f N a w i t h k a o l i n i t e to
p r o d u c e a N a b e a r i n g s m e c t i t e . T h i s w o u l d r e s u l t i n a n i n c r e a se i n a lk a l in i t y
t h r o u g h t h e d i s s o l u t io n o f c a r b o n a t e s , e x c h a n g e o f N a f o r C a t o s u p p l y N a ,
a n d e q u i l i b r iu m c o n t r o l s o n N a a n d S i O 2 b y r e a c t io n o f N a w i t h k a o l in i t e t o
f o r m a N a - b e a r i n g s m e c t i t e .
T h e r e a c t io n b e t w e e n s o d i u m a n d k a o l in i t e m a y t a k e t h e g e n e r a l fo r m3 A 1 2 S i2 0 5( O H )4 + N a + + 4 H 4 S i O 4
k a o l i n i t e3Na0.33A12.33SiB67Ot0(OH)2 + H + + 1 1 . 5 H 2 0 ( 4 )
N a - s m e c t i t e
T h e s t o i c h i o m e t r y is s u c h t h a t o n e e q u i v a l e n t o f N a is c o n s u m e d f o r e a c h
e q u i v a le n t o f a l ka l in i ty c o n s u m e d . T h e a m o u n t o f H 4 S iO 4 c o n s u m e d d e p e n d s
s o m e w h a t o n t h e a m o u n t o f S i i n t h e re s pe c t i v e c la ys a n d n e e d n o t b e in t h e
p r o p o r t i o n g i v e n a b o v e b u t i s s o m e w h e r e b e t w e e n 1 a n d 4 m o l e s d e p e n d i n g
o n t h e s t o i c h i o m e t r y o f t h e c la y m i n e r a l s .
I n t h e a b o v e s c e n a r i o , n o e x t e r n a l s o u r c e o f C O2 is re q u i r e d t o d i s s o lv e
C a C O 3 b e c a u s e t h e p r o t o n s n e c e ss a ry fo r c a r b o n a t e d i s s o l u t i o n a re d e r iv e d
f r o m e q n . ( 4) a b o v e , p r o c e e d i n g t o t h e r i g h t . A c t iv i t ie s o f i o n s c a l c u l a te d f r o m
t h e c o m p u t e r p r o g r a m P H R E E Q E ( P a r k h u r s t e t a l. , 1 980) a r e p l o t t e d o n a
m i n e r a l s t a b i l i t y d i a g r a m f o r t h e N a - H - S i O 2 s y s te m in F ig . 7 u s i n g t h e r m o -
d y n a m i c d a t a i n T a b l e 3 . M o s t o f th e w a t e r s a r e e q u i l i b r a t e d w i t h r e s p e ct t o
k a o l i n i t e a n d a N a - m i x e d l a ye r c l a y r e p r e s e n t e d h e r e b y N a - b e i d e l li te . T h e N a
c o n c e n t r a t i o n i n t h e s e w a t e r s m a y t h e r e f o r e b e c o n t r o l l e d b y e q u i l i b r i u m
b e t w e e n k a o l in i t e a n d a N a - s m e c t i te . F u r t h e r m o r e , N a - s m e c t i te s a re p r e s e n t
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
t h a t o n l y e qn . (4 ) p r o c e e d s t o r e m o v e N a f r o m s o l u t i o n b y c o n v e r s i o n o f
k a o l i n i t e t o N a - s m e c t i t e .
F r o m t h e r e s u l t s p r e s e n t e d a b o v e , t h e r e i s n o a p r i o r i r e q u i r e m e n t f o r a n
e x t e r n a l s o u r c e o f C O 2 to d i s s o l v e s o l i d c a r b o n a t e o t h e r t h a n t h a t p r e s e n t in
t h e r e c h a r g i n g w a te r . I f n o a d d i t i o n a l C O o w e r e to b e a c q u i r e d b y t h e g r o u n dw a t e r d u r i n g i t s t r a v e r se a c r o s s t h e b a s i n , t h e n t h e p H w o u l d r i se t o v a l u e s in
e xc es s o f 8 o r 9 b e c a u s e o f c o n s u m p t i o n o f C O o . R e m o v a l o f H C O 3 v ia
c o n v e r s i o n o f k a o l in i t e to a c a t i o n - b e a r i n g c l a y s u c h a s s o d i u m - s m e c t i t e
i n v o lv e s tr a n s fe r o f h y d r o g e n i o n f r o m c la ys t o s o l u t io n w h i c h c o n s e r v e s D I C
i n t h e w a t e r s b u t d e c r e a s e s a l k a l i n i t y. A n e t i n c r e a s e in a l k a l i n i t y c a n t h e r e f o r e
o n l y b e a c c o m p l i s h e d b y c a r b o n a t e d i s s o lu t i o n b u t m a i n t a i n i n g h i g h p C O 2
v a l u e s m u s t i n v o l v e a n e x t e r n a l s o u r c e o f C O ~. E v i d e n c e f o r a n e x t e r n a l
s o u r c e o f C O 2 is p r e s e n t e d i n th e f o l l o w i n g s e c t io n .
Carbonate chemistry and ;-~C
T h e c o m p o n e n t s o f t h e c a r b o n a t e s y s t e m a n d t h e ' 3C / '2 C r a t io o f t o ta l
d i s s o l v e d C O 2 a r e i n f l u e n c e d b y s e v er a l p ro c e s s e s w i th i n g r o u n d w a t e r
s y s t e m s . T h o s e p r o c e s s e s w h i c h a r e l i k e l y t o b e m o s t i m p o r t a n t w i t h i n t h e
G A B i n c l u d e C O2 r e s p i r a t io n b y p l a n t m a t t e r w i t h i n t h e s oi l z o n e o f t h e
r e c h a r g e a r e a s , d i s s o l u t i o n o r p r e c i p i t a t i o n o f c a r b o n a t e m i n e r a l s s u c h a s
c a lc i t e o r d o l o m i t e , a d d i t i o n o f C O 2 fr o m d e g r a d a t i o n o f o r g a n i c m a t t e r
w i t h in t h e a qu i fe r , a n d m e t h a n e p r o d u c t i o n . A n y o t h e r p r o c e s s t h a t i n v o lv e st h e t r a n s f e r o f p r o t o n s , s u c h a s t h e h y d r o l y s i s r e a c t i o n s i n v o l v i n g c la y
m i n e r a l s , o r t h e t r a n s f e r o f e l e c t ro n s b y b a c t e r i a ll y m e d i a t e d r e d o x r e a c t i o n s
( e . g . , s u l p h a t e r e d u c t i o n ) w i l l i n d i r e c t l y a f f e c t t h e c a r b o n a t e s y s t e m b e c a u s e
a d d i t i o n o r r e m o v a l o f h y d r o g e n i o n s w il l p e r t u r b t h e C O 2 s y s t e m a n d
t h e r e f o r e i n d i r e c t l y p r o m o t e c a l c i t e d i s s o l u t i o n o r p r e c i p i t a t i o n . A l l
c o m p o n e n t s o f t h e c a r b o n a t e s ys te m , t a k e n t o g e t h e r w i t h t h e s ta b le c a r b o n
i s o t o p e d i s t r ib u t i o n , c a n p r o v i d e a c o n s t r a i n t o n t h e p o s s i b l e c h e m i c a l
p ro c e s se s o c c u r r i n g w i th i n t h e g r o u n d w a t e r s y s te m a n d d e t e r m i n e t h e r o le o f
C O 2 i n t h e s e r e a c t i o n s .T h e r e i s a g e n e r a l i n c r e a s e i n t h e a l k a l i n i t y , D I C a n d 6 ' 3 C D , c v a l u e s w i t h
d i s t a n c e a l o n g L i n e E ( F ig s . 4 ( g )- 4 ( i) ) i n d i c a t i n g a d d i t i o n o f is o t o p i c a ll y
e n r i c h e d D I C a l o n g L i n e E . C o n s i d e r th e c as e w h e r e C O2 d e r iv e d f r o m
o x i d a t i o n o f o r g a n i c m a t t e r o r p l a n t r o o t r e s p i r a t io n i n t h e r e c h a r g e a r e a s
r e a c ts w i t h C a C O 3 a c c o r d i n g t o e q n . ( 3a ) ( se e a b o v e ) t o p r o d u c e t w o e q u i v -
a l e n t s o f a l k a li n i ty f o r e a c h n e t m o l e o f D I C p r o d u c e d . I f e q n . ( 3a ) p r o c e e d s
i n s t r ic t s t o i c h i o m e t r i c p r o p o r t i o n s , t h e ~ '3C H co ~ c a n b e g i v e n b y t h e e x p r e s s i o n :
~ ~t~( ~1 3C ( f i C c 0 2 - ]- C a C 0 3) ( 5 )HCO; = 2
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
G R E A T A R T E S I A N B A S I N G E O C H E M I S T R Y 241
-4
aa . - 6
J
- 1 0
0 -12 • • • ~ ~ A
- 1 4 •
n
- 1 6 . 0 0 .1 0 . 2 0 , 3
1 / D I C
F ig . 8 . 0 ~C t ~ l ~ a s a f u n c t i o n o f t h e in v e r s e o f D 1 C f o r g r o u n d w a t e r s f r o m t h e G A B J u r a s s i c a q u i f e r. T h e
t r e n d o f d a t a r e p r e s e n te d b y t h e d i a g o n a l l i ne i n d ic a t e s a d d i t io n o f a ~ C e n r i c h e d s o u r c e o f D I C w i th a
~ l~ C gl C, c o m p o s i t i o n o f a b o u t 0 %0 .
If the 6~3C concentrat ion of CO~ derived from plant respiration and
Jegradation in the soil zone in semi-arid regions within the recharge areas of
the GAB is estimated to be between - 20 and - 24%0 (Salamons and Mook,
1986) and the 613C concentration of CaCO3 within the aquifer is between 0
and -6%o, which is the range for non-marine carbonates (Salamons and
Mook, 1986), then the resultant bicarbonate produced in a closed system
according to eqn. (5) would be approximately -12%o. Therefore, if only
carbonate dissolution via addition of CO 2 was involved in adding DIC to thewater, then the 613C concentrat ion would evolve to about - 12 + 2%0 given
enough time or a sufficient amount of material and water residence time. Such
a scenario could adequately explain the GAB 613C data for the first 600 km o f
the Line E transect and the data for Line W (Fig. 4(i)). However, this process
does not explain the enriched 6~3C values measured for the last 600 km o f Line
E nor the enriched values in Line A.
On a plot of 6~3C values vs. the inverse of DIC concen tra tion (Fig. 8) we
observe two trends for the data. One group of points appear to be independent
of DIC concentrati on at a 613C value of about - 12 to - 14%o. Another group
of points define a linear trend with increasing ~3C values as I/DIC decreases.
The latter linear trend indicates mixing of DIC derived from two end-
members; one derived from dissolution of carbonate minerals and the other
end-member having an isotopically enriched source with a 6~3C composit ion
of around - 2 to 0%0.
Processes which could provide DIC enriched in '-~C include:
(1) Exchange of carbon atoms between dissolved CO, and calcite which
may ul timately result in a 6~3C concen tra tion of dissolved HCO 3 which is
close to that of the solid material, that is, about 0%o. However, isotopic
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
e x c h a n g e w o u l d i n c re a se 6 t3C c o n c e n t r a t i o n o f t h e D I C w i t h o u t a c o n -
c o m i t a n t i n c r e a s e i n D I C c o n c e n t r a t i o n , t h e r e f o r e t h i s p r o c e s s c a n n o t b e
i m p o r t a n t i n d e t e r m i n i n g th e o b s e r v e d t r e n d s .
( 2) P r e f e r e n t i a l m e t a b o l i s m o f ~3C e n r i c h e d c a r b o n b y b a c t e r i a d u r i n g
o x i d a t i o n o f o r g a n i c m a t t e r i n a g r o u n d w a t e r s y s te m ( e .g . C h a p p e l l e e t a l. ,19 88). T h e m a g n i t u d e o f th i s e n r i c h m e n t is o f t h e o r d e r o f - 1 0 t o - 1 5 % o
f o r C O2 r e la t iv e t o o r g a n i c m a t t e r w h i c h w o u l d r e s u lt i n a 6 ~3C e n d - m e m b e r
c o m p o s i t i o n o f a b o u t - 1 2% o; t o o l i g h t t o ex p la i n th e G A B d a t a .
(3 ) P r o d u c t i o n o f m e t h a n e v ia C O 2 r e d u c t io n :
C O , + 4 H ~ --* C H a + 2 H 2 0 ( 6 a )
o r b y a c e t a t e f e r m e n t a t i o n :
2 C H 2 0 ~ C H 4 + C O 2 ( 6 b)
B o t h o f t h e se p ro c e s se s in c r ea s e th e D I C c o n c e n t r a t i o n a n d e n r i c h t h e D I C
i n 13C ( G a m e s e t a l ., 1 9 78 ; W h i t i c a r e t a l. , 1 9 86 ). C a r b o n d i o x i d e r e d u c t i o n
a l s o r e s u l t s i n a n e t i n c r e a s e i n D I C b e c a u s e t h e m e t h a n e p r o d u c t i o n s t e p
a lw a y s a c c o m p a n i e s C O2 p r o d u c t i o n v ia o t h e r a n a e r o b i c o x i d a t i o n p r o ce s se s
( K la s s , 1 9 84 ). T h e a m o u n t o f C O 2 c o n s u m e d t o g i v e m e t h a n e is le ss t h a n t h a t
p r o d u c e d b y f e r m e n t a t i v e o x i d a t i o n r e s u l t i n g in a n e t b u i l d - u p o f C O 2 . In
a q u i fe r s w h e r e t h e o r g a n i c c a r b o n c o n t e n t i s l o w , t h e r e d u c t i o n s t e p te n d s t o
d o m i n a t e t h e p r o d u c t i o n o f m e t h a n e ( e .g . G r o s s m a n e t al . , 19 89 ).
D I C w i th 6 L3C c o m p o s i t i o n s o f m o r e t h a n 0%0 h a v e b e e n r e c o r d e d ing r o u n d w a t e r s y t e m s ( e. g. B a r k e r a n d F r i t z , 1 9 81 ; G r o s s m a n e t a l ., 1 9 89 )
w h i c h in d i c a te s t h e i m p o r t a n c e o f m e t h a n o g e n e s i s in t h e D I C m a s s - b a la n c e
o f m a n y c o n f i n e d a n d p h r e a ti c g r o u n d w a t e r s ys te m s . F re e h y d r o g e n , m e t h a n e
a n d o t h e r l ig h t h y d r o c a r b o n s a r e p r e se n t in m o s t p a r ts o f t h e G A B
( H a b e r m e h l , 1 986 ), t h e r e f o r e t h e s e r e a c t i o n s m u s t b e t a k i n g p l a c e . T h e l in e a r
r e l a t i o n s h i p s e e n in F i g . 8 th e n , c a n b e e x p l a in e d b y a d d i t i o n o f C O 2 w i t h a n
i s o to p i c c o m p o s i t i o n w h i c h i s c o n s i s te n t w i th a c o m b i n a t i o n o f a n a e r o b i c
f e r m e n t a t i o n a n d r e d u c t i o n o f C O2 t o p r o d u c e m e t h a n e .
T h e r e l a t i o n s h i p b e t w e e n a l k a l in i t y a n d D I C f o r th e t h r e e t r a n s e c t s ( Fi g . 9)
c a n i n d i c a t e th e e x t e n t o f o p e n - o r c l o s e d - s y s te m b e h a v i o u r w i t h i n t h e a q u i f e r
w i t h r e s p e ct t o C O 2 . E a c h o f th e t r a n s e c t s s h o w l i n e a r r e la t i o n s h i p s , a l t h o u g h
w i t h d i ff e r e n t g r a d i e n t s a n d f o r L i n e s A a n d E a t le a s t, m o r e t h a n 1 m o l e D I C
is p r o d u c e d p e r e q u i v a l e n t o f a l ka l in i ty p r o d u c e d . I f c a r b o n a t e d i s s o l u t io n
w a s t h e o n l y s o u r c e o f D I C t o t h e g r o u n d w a t e rs , t h e n t h e ra t i o o f
A A I k : A D I C w o u l d b e a b o u t 2 : 1. H o w e v e r i f s o m e a l k a l in i t y is r e m o v e d v i a
p r o d u c t i o n o f o n e p r o t o n d u r i n g t h e c o n v e r s i o n o f k a o l in i t e t o N a - s m e c t i t e
( e q n . (4 )) t h e n t h e r a t i o w o u l d a p p r o a c h 1 : 1. T h i s v a l u e c a n o n l y b e
a c h i ev e d i f t h e r e w a s n o n e t p r o d u c t i o n o f N a .
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G R E A T A R T E S I A N B A S I N G E O C H E M I S T R Y 243
30
• Un e E ( s l ope = 0 .67)A
O Line A s l o p e = 0.78
O 20 t = L i ne W ( s l ope = 1 .0) o 0
oo~ 0
O
_~m 10 •
&
O 1'0 2'0 30
DIC (mmo l / kg )
F i g . 9 , A l k a l i n i t y v s . D I C f o r al l t h r e e t r a n s e c t s s h o w l in e a r r e l a t i o n s h i p s w i th g r a d i e n t s r a n g i n g f r o m l
t o 0. 6 7. L i n e s A a n d E h a v e m o r e D I C a d d e d p e r e q u i v a l e n t o f a l k a li n it y a d d e d i n d i c a t i n g a s o u r c e o f
a d d i t i o n a l C O , t h a t i s n o t a c c o m p a n i e d b y g e n e r a t i o n o f a l k al in i ty .
B e c a u s e a n a e r o b i c o x i d a t i o n o f c a r b o n c o u p l e d w i t h m e t h a n o g e n e s i s
p r o d u c e s C O 2 w i th l i tt le o r n o c o n c o m i t a n t i n c re a s e in a l k a l i n i ty , w e s h o u l d
s ee m o r e D I C t h a n a l k a li n i ty p r o d u c e d a s t h e w a t e rs e v o l v e v ia t h is p ro c e s s .
T h e a d d e d C O2 i n d u c e s th e d i s s o l u t io n o f c a r b o n a t e s w h i c h i n t u r n i n c r ea s es
a l k a li n i ty a n d D I C i n a 2 : 1 r a t i o . W e d o o b s e r v e t h a t a b o u t 5 0 % a n d 3 0 %
m o r e m o l e s o f D I C a r e p r o d u c e d t h a n e q u i v a l e n t o f a l ka l in i ty fo r L i n e s E a n d
A r e s p e c t iv e l y ( F ig . 9 ). T h e r e f o r e , a d d i t i o n o f C O ~ v i a f e r m e n t a t i v e b a c t e r i a l l y
m e d i a t e d r e a c t i o n s is m o r e i m p o r t a n t t h a n c a r b o n a t e d i s s o l u t i o n t h r o u g h o u t
m o s t o f t h e G A B . T h i s is p a r t ic u l a r l y n o t i c e a b l e t o w a r d t h e b a s i n w a r d w e llsw h e r e in s i t u m i c r o b i o l o g i ca l ly m e d i a t e d f e r m e n t a t i o n r e a c t io n s b e c o m e m o r e
d o m i n a n t in p r o d u c i n g th e a d d e d D I C t o th e G A B w a te rs .
C O N C L U S I O N S
T h e c h e m i c a l e v o l u t i o n o f t h e m a j o r d i s so l v e d sp ec ie s a n d c a r b o n i s o t o pe
d i s t r i b u t i o n i n t h e e a s te r n a n d c e n t r a l p a r t s o f t h e G A B c a n t h e n b e e n v i s a g e d
a s fo l lo w s . A n i n i t ia l le v e l o f p C O 2 u p t o t h r e e o r d e r s o f m a g n i t u d e a b o v e
a t m o s p h e r i c le v e l s o f p C O 2 is a c q u i r e d b y w a t e rs n e a r t h e r e c h a r g e a r e a o w i n g
t o p l a n t r e s p i r a t i o n a n d o x i d a t i o n o f o r g a n i c m a t t e r i n t h e s o il zo n e . I n i t i a ll y ,
s il ic a te a n d c a r b o n a t e m i n e r a l s m a y d i s so l v e , a t l e as t in s o m e p a r t s o f t h e
b a s i n , re s u l t in g i n i n c r e a s e d a lk a l in i t y , N a , C a a n d M g c o n c e n t r a t i o n s a n d
5~3C DIc v a l u e s a r o u n d - 1 2 % o . A s t h e w a t e r s p r o g r e s s b a s i n w a r d s , c a t i o n
e x c h a n g e o f N a f o r C a a n d M g a n d s u b s e q u e n t r e a c t io n i n v o l v in g N a a n d
k a o l i n i t e t o p r o d u c e N a - s m e c t i t e e x e r t s a n i m p o r t a n t c o n t r o l o n t h e d i s -
t r i b u t i o n o f d is s o l v e d s pe c ie s f o r a l a rg e p a r t o f t h e G r e a t A r t e s i a n B a s i n . C l a y
m i n e r a l r e a c t io n s b u f f er a q u e o u s s il i ca c o n c e n t r a t i o n s b e l o w a m o r p h o u s S iO ~
s a t u r a t i o n . C a t i o n e x c h a n g e a n d d i s s o lu t i o n o f c a r b o n a t e m i n e r a l s d o n o t
8/13/2019 Geochemistry of Ground Waters From the Great Artesian Basin, Australia
a p p e a r t o b e i m p o r t a n t a s t h e g r o u n d w a t e r s a p p r o a c h t h e d i s c h a r g e a re a s a s
t h e r e i s n e t N a r e m o v a l f r o m t h e a q u e o u s p h a s e .
A d i s s o l v e d i n o r g a n i c c a r b o n i s o t o p e m a s s b a l a n c e i n d i c a t e s t h a t t h e
g r o u n d w a t e r s y s t em is o p e n t o C O 2 a n d t h a t a d d i t io n o f C O 2 is a c c o m p l i s h e d
b y f e r m e n t a t i o n p r o c e s s es o c c u r r i n g in s it u. S o m e o f t h e a d d e d C O , is ab y - p r o d u c t o f m e t h a n o g e n e s i s in d i c a t e d b y a t3C e n r ic h e d e n d - m e m b e r f o r th e
D I C . T h e a d d i t i o n o f C O2 d r iv e s t h e c a r b o n a t e d i s s o lu t io n r e a c t i o n , t h e r eb y
e x e rt in g a n i m p o r t a n t c o n t r o l o n t h e e v o l u t i o n o f t h e N a - H C O 3 w a t e rs w i t h in
t he G A B .
A C K N O W L E D G E M E N T S
W e t h a n k D . J . G r e g g , M . M a l i k i d e s a n d E . K i s s f o r a s s i s t a n c e w i t h f i e l d
s a m p l in g a n d l a b o r a t o r y a n a l y s is o f w a t e r s . R e v i e w s o f e a r l ie r v e r s io n s o f th i sp a p e r b y E . M a z o r , A . V e n g o s h a n d t h r ee a n o n y m o u s r ef er e es a r e a p pr e c i-
a te d . M . A . H a b e r m e h l p u b l i s h e s w i th t h e p e rm i s si o n o f th e D i r e c t o r , B u r e a u
o f M i n e r a l R e s o u r c e s , G e o l o g y a n d G e o p h y s ic s .
R E F E R E N C E S
A i r e y , P . L , C a l f , G . E . , C a m b e l l , B . L ., H a r t l e y , P . E . , R o m a n , D . a n d H a b e r m e h l , M . A . , 1 9 79 .
A s p e c t s o f th e i s o t o p e h y d r o l o g y o f t h e G r e a t A r t e s i a n B a sin . I n : I s o t o p e H y d r o l o g y ,
V o l . 1 , I A E A , V i e n n a , p p . 2 0 5 - 2 1 9 .
B a r k e r , J. F . a n d F r it z , P ., 1 9 8 1 . T h e o c c u r r e n c e a n d o r i g i n o f m e t h a n e i n so m e g r o u n d w a t e r
f low s ys tem s . C an . J . E a r t h S c i ., 18 : 1802-181 6 .
B e n t l e y , H . W . , P h il li ps , F . W . , D a v i s , S . N . , H a b e r m e h l , M . A . , A i r e y , P . L . , C a l f , G . E . , E l m o r e ,
D . , G o v e , H . E . a n d T o r g e r s e n , T . , 1 9 8 6 . C h l o r i n e - 3 6 d a t i n g o f v e r y o l d g r o u n d w a t e r 1 . T h e
G r e a t A r t e s i a n B a s i n . W a t e r R e s o u r . R e s . , 2 2 : 1 9 9 1 - 2 0 0 1 .
B l a c k b u r n , G . a n d M c L e o d , S . , 1 9 8 3 . S a l i n it y o f a t m o s p h e r i c p r e c i p i t a t io n i n t h e M u r r a y -
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