Barbara Jachimko

University of Zielona Góra

Institute of Environmental Protection

Zielona Góra, Poland

b.jachimko@iis.uz.zgora.pl

On the influence of meromixis on the chemical composition of acid pit lake

Abstract: The aim of the research was to evaluate the genesis of meromixis and the chemical composition development of the biggest meromictic pit lake no 54 located within “antropogenic lake district” in SW Poland. On the base of 27 years of monitoring, it was stated, that the acidification of the lake has been still becoming larger, although the pH value increased from 2.5 in the mixolimnion to above 4 in momimolimnion. The two stages pattern of the chemical composition development was formulated: the first period - intensive precipitation of iron salts, esp. K-jarosite in miksolimnion (0-13 m depth) and dissolution plus ferrous reduction in monimolimnion; the second period: due to increase of pH value about 4,5 in monimolimnion, oversaturation this layer due to alum salts, especially alunite. In the second period two buffering zones are being developed: miksolimnion with iron salts precipitation, and monimolimnion with the precipitation of aluminium salts.

Key words: acid pit lake, meromixis, jarosite

1. Introduction

Meromictic lakes – that are lakes which do not mix completely each layer. This phenomena is not common in natural lakes, but in mine pit lakes often happens. Three main reasons of that are: steep slopes, relatively great depth and exposure to highly mineralized inflows. There are the following mechanism of meromixis in pit lakes development: a) formation of a less mineralized mixolimnion by inflow of less mineralized water at the lake surface, b) enrichment of iron and sulfate due to the transport from mixolimnion to monimolimnion of secondary minerals, c) influence of sea water d) accumulation of substances in the monimolimnion due to biological decay, e) evaporation f) influence of ground water of high TDS ((Boehrer, Schultze 2000 and references there in). The deepest part of meromictic lake water body – monimolimnion – has often very specific chemical composition: is strong anoxia, rich of hydrogen sulfide and products of microbial decay. Meromixis has a strong influence on the neutralization method of lake. It is still open question how to recultivate the lakes in order not to destroy the meromixis, how it will be stable after treatment and how the results are stable. The aim of the research was to evaluate the genesis of meromixis and the influence on chemical composition of the biggest meromictic pit lake no 54 within “antropogenic lake district” in SW Poland.

2. The area of study

“Antropogenic lake district” is located along the polish-german border between Trzebiel and Łęknica. It comprises about 100 reservoirs of different size and chemical properties. The reservoirs were generated due to lignite mining which started in the second half of the 19-th centaury and was lasting till seventies of the 20-th centaury. The research presented below, were focused on the biggest lake, numbered 54 (Jędrczak 1992) which comes from 1973 y (there are no exact data about the rate and duration of filling up – fig. 1). The reservoir is surrounded by strongly eroded heap of

Fig. 1 The area of study

excavation. The surrounding area is covered by forest. Morphometry of the reservoir is presented in Table 1. From the previous study it is known, that the lake is extremely acid with high concentration of iron and sulphur. The reservoir is classified to the meromictic type with relatively deep monimolimnion (more than 10 m) (Solski, Jędrczak 1991a). The chemical composition of its waters was presented earlier by (Solski, Jędrczak 1990; Solski, Jędrczak, 1991b; Jędrczak, Jachimko, Najbar 1998).

Table 1 Morphometry of the reservoir no 54

Water level m above see level	Surface area m^2	Max length, m	Max width, m	extension coefficient	Max depta, m	Shore line lengh, m	Shoreline development
132,0	202.000,0	896	468	1,91	21,5	2625	1.65

The reservoir has no connections with surface waters. It results in the fact, that the chemical composition of its water depends mainly on the composition of ground water recharge and reactions within water body.

3. Methods

Water sampling

Water samples were taken in the deepest place of the lake from the surface, 1-, 3-, 5-, 7-, 9-, 11-, 13-, 15-, 17-, 19-, 21m depths. A two liter plastic Ruttner sampler was used.

Analitycal techniques

pH, salinity, redox potential and oxygen concentration were measured on line using WTW equipment. Cations concentration was measured by atomic absorption/emission spectrometry (Spectrometer Varian type AAS 10) with exception of iron II, which was analysed photometrically using the 1,10-phenanthroline method. Anion analysis were made by ion chromatography or titration methods.

Modelling

The hydrogeochemical simulator PHREEQC (Parkhurst and Apello, 1999) was used to calculate saturation indexes using constants included in the program.

4. Results

Composition of lake water

pH value has been changing in a different way in mikso and monimolimnion (fig. 2). In 1981y pH was nearly the same at all depths. The small increase of its value, by 0,2 unit, was noticed between 18 and 20m. From 1988y pH values have differed significantly from the upper and lower strata. Miksolimnion pH has been staying on the same level as at the beginning, but monimolimnion pH has been increasing systematically and reached in 2008 values about 4,8-4,9. Also chemocline has changed its location over time, from about 9-11 m initially to between 7 and 8 m below water level currently.

Fig. 2 pH profiles in the years 1988-2008

The iron concentration in the miksolimnion has been almost constant with depth, and there was also no tendency of change with time. The concentration of iron in monimolimnion has been increasing with depth over research period. The changes of iron concentration have two profiles. The first one (1981 and 1988y) presents continuously increase of iron amount from the surface to the bottom. In the second profile, there is a sharp boundary, about an order of magnitude, between concentration in the upper and lower part of water body. Two zones of iron concentration in the monimolimnion can be distinguished:

1. in the years 1981-88 concentration varied from about 100 to more than 800 mg/dm³;

2. since 1993y - from about 100 to more than 1500 mg/dm³ (fig. 3).

Fig. 3 Total iron profiles in the years 1988-2008

Sulphate concentration in miksolimnion was constant with depth, although decreased slightly with time (fig. 4). In monimolimnion sulphate content has been increasing with depth. Since 1993 lower concentration of sulphate in the layer 0-7 m, rapid increase between 7-9 m to the values about 3000 mg/dm³, and then constant increase to a max concentration about 4300 mg/dm³ has been observed.

Fig. 4 Sulphate profiles in the years 1988-2008

Two different profiles describe aluminium concentration. In 1981y aluminium concentration was the highest over research time and increased with depth. In the years that follows, the concentration of aluminium decreased with time and depth from about 35 to less than 10 mg/dm³ (fig. 5).

Fig. 5 Aluminium profiles in the years 1988-2008

In 1981y potassium concentration was low and varied from 3 in miksolimnion to 7 mg/dm³ close to the bottom (fig. 6). The potassium amount in miksolimnion hasn’t changed significantly with time. The maximum concentration, 5.8 mg/dm³, was stated in 2008y. In monimolimnion potassium concentration has been increasing gradually and exceed 30 mg/dm³ in 2008y.

Fig. 6 Potassium profiles in the years 1988-2008

Equilibrium with solids

Among the secondary minerals, which play the most important role in acid mine water chemistry the following ferrous salts are mentioned: goethite, jarosite K and schwertmannit (Evangelou 1985, Stumm, Morgan 1981, Lenk, Wisotzky 2007). Although all of them have positive SI values in the lake under research, due to potassium stratification, it seems, that the most important role in ferrous and sulphate transport from miksolimnion to monimolimnion played jarosite K precipitation (table 2).

Table 2 SI for chosen solids in 1981 and 2006y

Solid name	formula	data	log IS with depth, m below water level
			0	1	3	5	7	9	11	13	15	17	19	21
	Fe(OH)3a	1981 2007	-1,15 -1,32	-1,15 -1,34	-1,16 -1,29	-1,17 -1,11	-1,17 -1,18	-0,76 *)	-0,72	-0,72	-0,69	-0,67	-0,64	-0,64
goethite	FeOOH	1981 2007	4,50 4,30	4,50 4,18	4,50 3,99	4,40 4,11	3,97 4,01	3,39	3,39	3,67	3,17	2,85	2,59	3,16
jarosite K	KFe3(SO4)2(OH)6	1981 2007	2,81 2,83	2,66 2,61	2,61 2,28	2,61 2,57	1,7 2,44	3,39	3,67	0,75	-0,64	-1,44	-2,14	-0,97
alunite	KAl3(SO4)2(OH)6	2007							3,33	3,92	2,06	2,68	5,36	6,29
gibsite	Al(OH)3	2007							-0,82	-1,15	-1,59	-1,31	-0,05	0,43

*) lack of value means very low level

Although at the beginning, the SI for ferrous salts were also positive in monimolimnion, the amount of ferrous salt was too small, to affect on water properties. Iron and sulphate have been cumulated in deeper part of water body, and the reduction of ferrous to ferric influenced on pH values. Over the years that followed, the pH in monimolimnion reached 4 or more and created the conditions for precipitation of alum compounds, such as alunite or gibbsite. SI for this minerals prior to monimolimnion pH increase so low, that it excluded the possibility of precipitation.

5. Discussion of the results

Based on the results presented above, its is difficult unambiguously describe the origin of meromixis of the lake 54. The main factor it seems to be enrichment of iron and sulfate due to the transport from mixolimnion to monimolimnion of secondary minerals. It is proved, that the chemistry of lake (especially upper layer – monimolimnion) has been changed due to meromixis and secondary caused its stabilization. Two periods of lake chemical development can be distinguished:

1. “the iron period” – till 1988y, with intensive processes of ferrous precipitation in miksolimnion (termocline was then located between 9 and 11m below water level).

Sedimentation of jarosite K and may be also goethite, caused transport of ferrous, potassium and sulphate to the deeper part of water body. On the boundary oxic and anoxic zones dissolution of minerals took place and caused liberation of ions. It is known that dissolution of jarosite is incongruent process, but the mechanism of it is has been not sufficiently described. We can although suppose, that the main reason of the pH change in monimolimnion was ferrous reduction. This processes affected on pH values by consuming protons, but didn’t reduce acidity of water (Schoepke 2001). The potassium stratification confirms the main role of jarosite K precipitation in miksolimnion.

2. “the iron-alum period” – from 1988 up to now, with two buffering zones, based on ferrous precipitation in miksolimnion and alum in monimolimnion. The chemistry of miksolimnion was similar to that for “iron period”. Chemical composition of miksolimnion didn’t show any significant changes and was similar to initial composition. The only difference is alum concentration, which decreased by about 20 mg/dm³, which means almost 50% of the initial value. Due to the fact, that monimolimnion acted as a sink for ferrous precipitates, the amounts of dissolution products has been gradually increasing. The concentration of almost all (excluding alum) analyzed ions increased significantly. The pH values increased from about 3 to 4,5-4,9. It created the possibility of alum salts precipitation, which process lowered alum concentration in monimolimnion and in a lesser extent, affected on sulphate and potassium concentrations.

6. Conclusions

1.Meromixis influenced greatly on the composition of water body in the reservoir 54.

2.Intensive processes of ferrous salts precipitation in oxic zone and dissolution of the precipitations with ferrous reduction in monimolimnion, can cause the increase of pH value of monimolimnion.

3.The increase of pH the monimolimnion created alum-buffering zone.

4.Monimolimnion acted as a sink for the products of ferrous salts dissolution, which affected on water density and stabilized meromixis.

8. References

Boehrer B, Schulze M., 2000 On the relevance of meromixis in mine pit lakes. IMWA/02000-/pp 200-213.

Jędrczak A., Skład chemiczny wód pojezierza antropogenicznego w Łuku Mużakowskim. Wyd. WSInż. w Zielonej Górze, Seria Monografie1992;, Nr 5, ss 135 (in polish)

Solski A., Jędrczak A.,; Meromixis in acidotrophic reservoirs of the „anthropogenic lake district”. Pol. Arch. Hydrob., 1991a No 38/3/4, 327-346

Solski A., Jędrczak A., Ionic composition of waters of the „anthropogenic lake district”. Pol. Arch. Hydrob., 1990;No 37/3, 361-382

Solski A., Jędrczak A.,; Forms of iron in waters of acidotrophic- meromictic reservoirs of the „anthropogenic lake district”. Pol. Arch. Hydrob., 1991b No 38/3/4, 315-326.

Jędrczak A., JachimkoB, Najbar N.: Zmianyfizyczno-chemicznych cech największego zbiornika na pojezierzu antropogenicznym w okresie kilkunastu lat.. Zeszyty Naukowe Politechniki Zielonogórskiej 1998(Inżynieria Środowiska 7) nr 116, p.5-18 (in polish)

Parkhurst D., Appelo C.A.J.: User’s guide to PHREEQC (Version2)- a computer program for speciacion, batch-reaction, one-dimensional transport, and inverse geochemical calculations. Denver, 1999

Evangelou, V.P. Pyrite oxidation and its control; 1995, New York

Stumm, W., Morgan, J.J. Aquatic chemistry – an introduction emphasing chemical equilibria in natural waters, 2nd ed.; 1981 New York

Lenk S., Wisotzky F.: Chemische Beschaffenheit und mmodellierte Genese von Grundwassern in Braunkohlenabraumkippen des Tagebaues Inden Grundwasser (2007) 12:301-313

Schöpke, R.; Koch, R.; Querfelli I.; Striemann M.; Preuß V.; Regel R.;. Anwendung des Neutralisationspotentials bei der Bilanzierung von Saeure-Base-Reaktionen im Umfeld des Braunkohlebergbaues. Grundwasser 1/2001 23-29