MINERALOGY
AND WATER CHEMISTRY OF THE LAKE ACIGÖL, DENIZLI, TURKEY
Halim Mutlu, "Selahattin Kadir, and 'Aydoğan Akbulut For correspondence: Osmangazi University. Department of Geological Engineering. Bademlik 26030 Eskisehir, Turkey;
[email protected] "General Directorate of Mineral Research and Exploration of Turkey (MTA) 06520 Ankara. Turkey ABSTRACT: Lake Acigöl of Turkey is a perennial lake with a brine composiıion ofNa-CI-S04• The lake is fed mainly by two distinct water sources. The first is groundwater of Mg-HC03 type and springs of Na-S04 type. The difference in composition of inflow waters is atıributed to different ways of water cycling. Ephemeral mudflats around Lake Acigöl are corrıposed of gypsum, calcite, dolomite, huntite, together with clastic minerals. Brine-soaked mudflat exist locally and contain efflorescence of halite, bloedite, thenardite and/or mirabilite precipitati.ng from the artesian groundwater. Dominance of aragonite and high TOC values recorded in the gel-like lake sediments are the indicator of bacterial activity in the lake. Fluid-mineral equilibria calculations performed on the waters of the Lake Acigöl basin successfully predicted precipitation of minerals detected in the recent sediments.
INTRODUCTION There are severallakes in southwestem Anatolla evolving in graben basins of Miocene to Pliocene age. Of these, Lake Acigöllocated east of the city of Denizli develops in a NE-SW trending graben system (Fig. 1). The basin with a maximum length and width of about 45 km and 14 km, respectively, has an area of about 157 km2 of which only 55-60 km2 are covered by the lake waters. Tota1ly, 400,000 tons of sodium su1fate are produced annualIy from the lake (Otuzbir Kimya Co. Ltd., personal communication 1998) which represents 85% of Turkey's production. The mean annual precipitation is 397 mm and annual evaporation is 754 mm (DSı 1996). Acigöl attains its maximum (163 cm) and minimum (less than 1 m) depth during December-January and August-September periods, respectively. The water depth increases from north to south. The basement of the area comprises the highlands in the south and is represented by Jurassic-Cretaceous Yandag cherty limestone and Marmaris ophiolites of Cretaceous age which consist of dunite, harzburgite, and gabbroic rocks together with limestone-chert and radiolarite blocks. The flat areas around the lake are covered by Oligocene Çambasi fonnation composed of alternations of conglomerate, sandstone, and mudstone. The Pliocene Çameli formation consists of lacustrine sedirnents of conglomerate, sandstone, claystone, marl, dolomite and limestones and builds the high elevations to north and south of the basin (Fig. 1) (Bilgin et al. 1990). Quaternary a1luvium deposits (tluvial and lacustrine sediments) are the youngest sediments around the lake. The Acigöl basin is surrounded in the south and north by two major faUıts extending in NE-SW direction. The one in the south is very distinctive in the field while the presence of the northem one was determined only in air photos of the area. The aim of this paper is to present a preliminary deseription of the sediments and mineralogy of the Lake Acigöl basin. The purpose of the investigation is also to determine the processes affecting the chemical composition and mineral equilibrium of the brine collected in the lake.
MATERIALS AND METHODS Fieldwork was undertaken at Lake Acigöl basin in July of 1996 and August of 1997. Sediment samples were collected along two transects with a sampling interval of 250 m. The first transect (i) extends for about 7.5 km and lies between the Hatipler site and NE of the Akpinar village. The second transect (il) with a length of about 19 km extends between SW of the Akpinar village and south of the Çardak town (Fig. 1). Since a solar evaporation pond of one of the sodium su1fate producing facilities crosses the transect II, the central part of this transect could not be sampled. Tota1ly, 23 and 49 sediment samples were collected from the transects i and II, respectively. Sediments of the mud tlats were sampled from a depth of about 50 cm by a hand-auger, whereas lake sediments were collected with a grabber from a boat in addition, 12 water samples were collected from the basin. of these, 2 are from springs and 3 from ponds at different localities of the Acigöl basin. The other 7 water samples were taken from different parts of the lake (Fig. 1). Finally, a total of 30 samples, selected from the two transects was analyzed for their total organic carbon (TOC) content Mineralogical analyses of the samples were performed by Xray diffraction (XRD) at the laboratories of the General Directorate of Mineral Research and Exploration of Turkey (MTA). A Rigaku-Geigertlex model X-ray diffractometer with a Cu target was used in the analyses. All samples were air dried and powdered to 100 mesh size prior to analysis. Water samples were collected into 500-ml polyethylene containers and filtered prior to analyses. Chemical analyses of waters were also conducted at the laboratories of MT A using standard methods reported in Fishman and Friedman (1989). Na and K concentrations were determined with tlame photometry. The titration method was used for Ca, Mg, CL, and carbonates analyses. SO 4 concentraıions were determined with ion chromatography. Total organic carbon analyses were carried out at the Research Laboratories of Turkish Petroleum Corp. (1p AO) on air dried
Carbonates and Evaporites, v. 14, no. 2, 1999, p. 191-199.
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Figure 2. Generalized mineral distribution in the Acigöl Lake basin (Dots represent the boundary between dry mudflat and brine-soaked mudflat). 193
MINERALOGY
AND WATER CHEMISTRY
Table 1. Mineralogical variation along the transect i. Site No
G
Q
F
KS-t KS-2 KS-3 KS-4 KS-5 KS-6 KS-? KS-6 KS-9 KS-l0 KS-ll KS-12 KS-13 TK-1O TK-9 TK-6 TK-? TK-6 TK-5 TK-4 TK-3 TK-2 TK-l
+
+++
+
+
+++
+
+
+++
+
OF THE LAKE ACIGÖL, DENIZLI,
-
M
Ar
Hnt
HI
Tn
TOC (%)
Ch
Cc
001
++
+
+++
+
++
+
+++
+
-
++
+
+++
+
0.12
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0.15
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0.12
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0.24
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0.40 1.33
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0.12 0.25
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1.25 1.51
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1.09
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0.93
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G.Gypsum, Q.Quartz, F.Feldspar, M.Mıca, Ch.Chlorite, Cc.Calcite, DoWolomite, Ar.Aragonite, HntHuntite, HI:Halite, Tn:Thenardite. +: Relative abundane of mineral; tr.:trace. TOC: Total organic carbon (%). - . not analyzed.
Table 2. Mineralogical variation along the transeet II. Srte No
G
Q
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+ ++ +
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AK-14 AK-13 AK-12 AK-ll AK-ID AK-9 AK-a AK-7 AK-6 AK-5 AK-4 AK-3 AK-2 AK-l
F
M
Ch
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+
-
Cc
001
Ar
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Hnl
HI
Tn
BI
TOC (%)
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-
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+ + + +
041
++
++ ++
0.31
++ ++ ++
+++ ++ ++
++ + +
++ + +++
0.59
++
+ ++
+++ ++
0.55
K ++
0.80
+
R
E
A
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+ +
+
Ir.
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-
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0.40
0.40
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++ ++
+ + +
+ +
045
+ ++ ++
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++ ++
0.33
+
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0.44
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B
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+ +
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-
-
042
0.39
-
++ + ++ +++ ++ + + ++
0.83 + +
1.34 0.83
-
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0.81
+ + ++ +
1.25
+
+
1.55
-
G.Gypsum, Q.Quartz, F.Feldspar. M.Mıca. Ch.Chlorite. Cc.Calcrte. Docüoıomne, Ar.Aragonıle, HntHunlıle. HLHalrte. Tn:Thenardite. BI:Bloedrte. +: Relalive abundane of mineral; tr.frace. TOC: Total organic carbon (%). - : not analyzed.
194
TURKEY
MVlLU, KADIR, AND AKBULUT rate of transfonnation of aragonite to calcite. However, Lippman (1973) points out mat increasing amounts of sodium chloride in solution somewhat negate the effect of the magnesium ions. As stated by Kinsman and Holland (1969), with rising temperature (during the summer time, it is 50°C in Lake Acigöl), the required minimum level of Mg concentration decreases; the concentration of magnesium is then accompanied by a decrease in free CO;2, thus facilitating the aragonite precipitation. Dolomite content remains nearly the same along both transects (Tables 1 and 2). it is thought that dolomite is either directly precipitated from the water or sequentially crystallized from the alteration of calcite exposed to Mg-rich brines mat have been stripped of their Ca and SO4 content which alsa facilitates formatian of gypsum.
from 3 to 35 m which are well above the groundwater level during rainy seasons, Perennial springs issuing in the southeastern part of the lake and/or direct runoff are the other sources of inflow. High discharge rate of springs (averaging as 900 and 410 l/sec for KS-3 and KS-l, respectively) which well explains the eloseness of the water body to the southeastem margin (Fig. 1), is provided by the karstic occurrences within the Yandag limestone. Chemical composition of waters from the Lake Acigöl basin was given in Table 3. Data on chemistry of some groundwater wells in the eastern part of the basin were alsa included to Table 3. All the waters have a slightly basic character with pH values ranging from 7.1 to 8.8. Based on the classification ofEugster and Hardie (1978), spring waters (KS-l and KS-3) are NaS04' pond waters (KS-2, KS4, and KS-5) are Na-Cl-(COJ and groundwater (33280B, 33282, and 34201) are Mg-HC03 type waters with TDS (total dissolved solid) values between 0.566 and 1.604 gll. Brines, however, display aNa-CI-(S0.J nature having TDS values up to about 211 gll. it is noticed in HC03+C03-S04-CI and Na+K-Ca-Mg diagrams (Fig. 3) that composition ofbrine waters is defined by differenees in inflow water compositions, coupled with evaporative concentration and precipitation of calcite and gypsum. in addition, Mg/Ca ratio increasing from inflow to brine waters (from 0.84 to 10.1) alsa indieates the removal of ca by precipitation (Table 3). However, Mg/Ca ratio never attains to a level to precipitate magnesite. Regarding the mineral assemblage and water chemistry, Acigöl basin elosely resembles the Saline Valley, USA (Hardie 1968; Hardie and Eugster 1970).
Halite and thenardite are precipitated as efflorescence crusts in brine-soaked mudf1ats. Although not recognized at the time of present study, mirabilite crystallizes during the periods ofbrine cooling but dissolves upon warming. Bloedite (N~S04·MgS04.s:E~0)wasalsorecordedinonesample(S-5) from the transect II. It may be formed from thenardite and/or mirabilite under the influence of magnesium sulfate ions at temperatures above about 20°C (Kuehn 1952). in their study on Ca-Mg carbonates in some saline lakes of Central Europe, South Asia and Turkey, Muller et al. (1972) and lrion (1973) state mat Mg-calcite and aragonite are the primary minerals while dolomite, huntite and magnesite are the secondary minerals in Lake AcigöL. Except for magnesite, mineralogic assemblage suggested by them is consistent with our observations.
Higher concentration of springs, in comparison to other inflow waters, is attributed to somewhat deeper circulation of groundwater along the fault in the south. Groundwater is responsible for dissolution of Na- and S04-bearing minerals which were formerly deposited in the lake. The presence of gypsum and thenardite minerals recorded in the core samples from two explanatory wells (W-I and W-2) drilled at the
HYDROGEOCHEN.USTRY Based on data available (DSı 1996), groundwater discharge is the main inflow to the Acigöllake. Static level in groundwater wells drilled northeast and southwest parts of the basin ranges
Table 3. Chemical composition of waters (in ppm) from the Acigöl basin. TDS: Total dissolved solids. K Sample pHlab no. Springs
