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Mesocosm studies to assess acidity removal from acidic mine lakes through controlled eutrophication
Ecological Engineering 10 (1998) 229 – 245
Mesocosm studies to assess acidity removal from acidic mine lakes through controlled eutrophication A. Fyson a,b,*, B. Nixdorf b, M. Kalin c, C.E.W. Steinberg a a
Institute of Freshwater Ecology and Inland Fisheries, Mueggelseedamm 310, Berlin D-12587, Germany b Lehrstuhl Gewaesserschutz, Brandenburg Technical Uni6ersity Cottbus, Cottbus D-03013, Germany c Boojum Research Limited, 468 Queen Street East, Toronto, Ontario M5A 1T7, Canada Received 21 July 1997; received in revised form 17 February 1998; accepted 19 February 1998
Abstract Flooded lignite pits (Tagebaurestseen) in Lusatia, Germany, are acidic (pH 2.5 – 4) with high concentrations of iron. Mesocosms (total volume 20 l) were set up with water and sediment from a Tagebaurestsee (Koschen, pH 3.1, Fe 20 mg l − 1, acidity (KB8.2) 2.45 mM) to assess the effects of phosphate and organic amendments under natural light and low temperature. Chemical and biological parameters were observed over a 9-month period. Phosphate rock addition (300 mg l − 1) resulted in sustained reduction in acidity (H + and Fe3 + ) in the water column and induced the growth of Chlamydomonas spp. (Chlorophyceae) near the water surface and Lepocinclis teres (Euglenophyceae) in a band above the sediment. Addition of potatoes to mesocosms resulted in the generation of near-anoxic conditions above the sediment, and phosphorus, ammonium and carbon (organic and inorganic) were released as the potatoes decomposed. A pH \6 was attained with 5.1 g (dry weight) of potatoes and pH \8 with 34 g (dry weight). In both mesocosms, more than 90% of total acidity was removed. Algal growth, mainly of small, coccoid Chlorophyceae, Ulothrix sp. (Chlorophyceae) and Eunotia exigua (Bacillariophyceae), was observed throughout the water column. © 1998 Elsevier Science B.V. All rights reserved.
Keywords: Acidity removal; Controlled eutrophication; Euglenophyceae; Lepocinclis; Lignite; Mesocosms; Phosphate rock; Potatoes
* Corresponding author. Tel.: + 49 30 64181605; fax: + 49 30 64181600; e-mail: [email protected] 0925-8574/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0925-8574(98)00007-X
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1. Introduction Tagebaurestseen are acidic lakes with high iron and sulphate concentrations that are formed as water rises in mined-out open-cast lignite pits. During mining, the groundwater table is lowered, resulting in oxidation of pyrite- and marcasite-containing materials. Intensive extraction of lignite has resulted in a unique landscape in the Lusatia region in north-eastern Germany. A total of about 400 Tagebaurestseen, which are of variable size, typically 20–45 m deep and dimictic, provide a lake surface area of several hundred square kilometers in the Lusatia region in the states of Saxony and Brandenburg. Remediation efforts include diversion of river water to these lakes to raise the pH and nutrient concentrations in some Tagebaurestseen, but surface water supply is very limited (Gruenewald, 1996, Luckner and Eichhorn, 1996). Additions of limestone (CaCO3) or lime (CaO) to release alkalinity is one option. Biologically based remedial measures for Tagebaurestseen are an attractive proposition both environmentally and economically. Biologically based approaches have been suggested, particularly in light of the undetermined supply of contaminants to the infilling lakes and the potentially long-term nature of the problem (Klapper and Schultze, 1995, Nixdorf et al., 1997a). Addition of phosphate to a lake acidified through acid rain resulted in an increase in pH comparable to that predicted from the assimilation of CO2 coupled to nitrate uptake by phytoplankton (Davison et al., 1995). Tagebaurestseen share the oligotrophic character of this lake (low phosphorus concentrations), and enhancement of primary productivity through nutrient additions to the water column to increase the pH has been suggested (Nixdorf and Wollmann, 1997, Nixdorf et al., 1997a,b), but the chemistry of these lakes presents additional challenges. The dissolved phosphate-P concentration is B 10 mg l − 1 and acidity is from both iron and hydrogen ions. At pH 3, a typical pH value for the water column, the maximum concentration of CO2, the dominant form of dissolved inorganic carbon (DIC), is approximately 0.1 mg l − 1 (Stumm and Morgan, 1996), which will severely limit primary productivity. Blouin (1989) studied productivity of lakes with pHB 4 and documented very low activity. However, localized high algal biomass has been found in the depth profile of the water column in Tagebaurestseen with a small number of species from diverse algal taxa (Nixdorf et al., 1997a,b). Enhancement of anaerobic sediment microbial activity provides another approach to amelioration of lake water quality. Through provision of organic carbon and phosphate, microbial activity in the sediment is stimulated, alkalinity is generated (acidity consumed) and nutrients and carbon (organic and inorganic) are et al., 1996). Organic wastes have been used to enhance sulphate reduction in acid mine drainage to increase the pH. This in turn leads to precipitation of metals out of the water column and in sediments to the formation of metal sulphides. Vile and Weider (1993) report these processes in wetlands and Kalin (1993), Fyson et al. (1996) and Kalin et al. (1997) for a test system for tailings seepages. Sediments in
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this test system were constructed with organic matter (hay, alfalfa pellets and potato waste). In bioreactors, the same processes are utilized when acid mine drainage is passed through various types of vessel containing organic matter (Dvorak et al., 1992, Rowley et al., 1997). Mesocosms containing sediment and water from Lake Koschen, a typical Lusatia Tagebaurestsee, were used to determine in detail the interactions between sediment and water column. Phosphate and carbon were added to the sediment to quantify the processes that would be induced if controlled eutrophication were considered as an approach for acidity removal from Tagebaurestseen. To stimulate sediment activity, the choice of material is based on several criteria. The material must be available in large quantities at low cost. The release of inorganic phosphate and carbon must be gradual to provide a long-term, but steady supply of nutrients to the biological processes. A sedimentary phosphorus rock, mined in North Carolina, was used as a phosphate source and potatoes served as sources of organic carbon in the sediment. This paper summarizes the effects of rate and type of amendment on the chemistry of Lake Koschen water over time.
2. Experimental
2.1. Water and sediment sampling Lake Koschen was selected after screening the water chemistry from a database compiled on Tagebaurestseen by the Brandenberg Technical University, Cottbus. Lake Koschen was represenative of the majority of the lakes and was accessible. This lake has an area of 6.1 km2, a maximum depth of 25 m and an estimated volume of 1.5× 108 m3. This lake has been infilling with groundwater since 1975. Since monitoring began, the pH has been in the range 2.9–3.2 and productivity has been low (Nixdorf et al., 1997a,b). Koschen may be considered to be in the middle range of acidity for Tagebaurestseen (KB8.2 = 1.7–2.5 mM or 170–250 mg l − 1 equivalents of CaCO3) and is classified as very acidic in the scheme of Nixdorf et al. (1997a). Minimally disturbed sediment cores were collected at a depth of 21 m with a sediment corer of 5.5 cm i.d. (Umwelt, Mondsee, Austria). Following retrieval, the cores were divided into surface, middle and bottom layers, which were clearly distinguishable by their colour and texture. Water from Lake Koschen was collected from the thermocline (\ 10 m). The water and sediment were stored at B 10°C until set-up of the mesocosms.
2.2. Mesocosm design and set-up The mesocosms were 2.6 m long polycarbonate vertical columns with an i.d. of 10.5 cm which were sealed at the base with plexiglass plates (Fig. 1). The
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mesocosms were designed to be sufficiently large (15 l water over 0.5 m sediment) to provide for the establishment of chemical gradients and to contain sufficient water to allow for regular sampling without major disruption. They were fitted with multiple sampling ports to provide long-term flexibility in sampling regimen. Most of the water column (12.5 l) was circulated by means of a peristaltic pump (Fig. 1). The pumping rate was 2 l h − 1 or approximately four exchanges every 24 h. Polystyrene packing chips (0.15 m depth) were placed at the bottom of the mesocosm and the three sediment layers added in sequence to yield a total sediment depth of approximately 0.5 m. The sediment portion of the mesocosms was installed below a platform to maintain darkness. The experiments were set up in a cool basement laboratory, receiving natural lighting through a roof window and fairly steady temperature throughout the year (6–9°C in winter, 14– 17°C in summer). Supplementary lighting was provided in the form of fluorescent light banks. Photosynthetically active radiation was 150–800 mE m − 2 s − 1 at the top of the mesocosms and B20 mE m − 2 s − 1 at the sediment surface.
2.3. Chemical analysis Water samples for chemical analysis (200–500 ml) were collected from the
Fig. 1. Mesocosm configuration. Total height is 2.6 m and the internal diameter is 10.5 cm.
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middle port of the circulating water (0.7 m below the surface) and from a port 0.12 m above the sediment surface in the non-circulating zone. The volume removed during sampling from each mesocosm was replaced with Koschen lake water. The results from one control mesocosm are for a shorter period as it initially developed a leak and had to be set up again. Chemical parameters were determined by standard methods (DEV, 1992) on filtered water (0.45 mm). Conductivity, pH and O2 were measured with WTW meters and probes. Iron (total and Fe(II)) was determined photometrically by the o-phenanthroline method with a Shimadzu UV-2101 spectrophotometer. Dissolved inorganic phosphate (DIP) was determined photometrically by the molybdenum blue method. Total dissolved phosphate (TP) was determined by the same method following digestion in sulphuric acid and hydrogen peroxide at 150°C and neutralization with NaOH. Dissolved organic carbon (DOC) was determined by infra-red detection of CO2 following catalytic digestion in a Shimadzu Total Organic Carbon Analyzer-5000. Dissolved silicon was determined by the molybdenum blue method after addition of tartaric acid (pH 1.4– 1.6). Nitrate, sulphate and chloride were determined by ion chromatography. Ammonium-N (NH4+ -N) was determined by flow solution (Perstop Analytical) with photometric determination of an indophenol complex. Total sediment phosphorus was determined according to the sequential procedure of Psenner et al. (1984). Titrations against NaOH were carried out for determination of acidity (KB4.3 and KB8.2). KB4.3 and KB8.2 are the respective amounts of NaOH (expressed in mM) required to raise the pH to 4.3 and 8.2. KB8.2 is equivalent to acidity expressed in mg l − 1 CaCO3 milliequivalents, more frequently used in North America. Underwater photon flux density of photosynthetically active radiation was measured with a LI 193 SB Spherical Quantum Sensor (LICOR).
2.4. Experimental treatments To determine the effects of phosphate addition, three mesocosms were set up: a control with no amendment, with phosphate rock alone (300 mg l − 1) and with additions of both phosphate rock (300 mg l − 1) and NO3− (6.7 mg l − 1 of NaNO3). The phosphate rock contains phosphorus in the form of hydroxyapatite (e.g. Ca10(PO4)6(OH)2). The formulation used (Code 31; Texasgulf, Raleigh, NC) contained 14.1 mg g − 1 P, 349 mg g − 1 Ca and 120 mg g − 1 CO3, and is a fine powder which allows for a long suspension time in the water column and exposure of a large proportion of the phosphate to the low pH of the water. Phosphate rock was added by sprinkling it on the water surface 7 days after mesocosm set-up. The quantity of phosphate rock added to the mesocosms (300 mg l − 1) was based on preliminary shake flask experiments to establish a rate that was sufficient to raise the pH to around 4.5, to increase DIC from around 0.1 mg l − 1 to 1 mg l − 1, and to remove iron from the water column. Nitrate (6.7 g NaNO3) was added to one mesocosm 34 and 180 days after set-up.
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Table 1 Chemistry of water added to mesocosms and mesocosm water chemistry 3 h after addition of phosphate rock Parameter
Temperature pH Conductivity Fe total Fe2+ DIP-P TPO4-P NO3-N SO2− 4 Cl− DIC DOC Acidity (KB4.3) Acidity (KB8.2) Ca2+ Mg2+ O2
Unit
°C mS cm−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mM mM mg l−1 mg l−1 mg l−1
Water added to mesocosms
20 3.1 1578 19.2 0.27 6.5 B10 0.36 690 40 0.30 ND 1.82 2.45 180 30 ND
Mesocosm water
Control
Phosphate rock NO3
Phosphate rock only
15.9 3.11 1594 15.3 B0.006 0.012 0.03 0.34 690 40 0.38 1.8 1.28 1.82 184 24.8 8.8
16.7 4.48 1423 0.06 0.05 9.8 15.2 0.31 710 39 0.95 2.4 0.00 1.00 292 31.1 6.8
16.7 4.38 1418 0.09 0.05 9.9 14.9 0.32 710 40 0.90 2.1 0.00 0.80 249 29.6 6.9
ND, parameter was not determined.
To determine the effects of decomposable organic matter addition, whole potatoes were dropped into the mesocosms at rates of either 30 g (5.1 g dry weight) or 200 g (34 g dry weight) per mesocosm. Potatoes were placed in nets together with a similar weight of coarse gravel to keep the potatoes in the sediment during gas production from decomposition. Whole potatoes are used because on addition they are half-buried in the sediment and therefore decomposition products are supplied where they are needed at the sediment surface.
3. Results
3.1. Phosphate rock mesocosms Water chemistry of the mesocosms at the onset of the experiment is presented in Table 1. It is evident that phosphate rock addition removed nearly all dissolved iron and resulted in a dramatic increase in dissolved phosphate within 3 h of addition. The concentration of DIC and calcium increased, due to the dissolution of calcium carbonate in the phosphate rock. As expected, the pH increased and the acidity decreased due to iron removal. No substantial changes were noted for the other
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parameters measured. Comparing the characteristics of the original lake water at the time of collection to the control mesocosms, 7 days after set-up, indicates that, in this time, there was little change in water chemistry. Concentrations of Fe(II) and DIP had decreased during this period. As biological activity is relevant to some of the time-related changes in the water chemistry, the key observations on the occurrence are summarized. In the phosphate rock mesocosms, Chlamydomonas spp. were evident at the top of the mesocosms within 1 week of phosphate rock addition. At 66 days after set-up, a narrow band of O2 supersaturation (up to 17 mg l − 1 or 200% saturation), corresponding to a ‘green band’, developed approximately 10–15 cm above the sediment surface. This band was found to contain cells (104 –105 ml − 1) of a flagellated euglenophyte, Lepocinclis teres. Fig. 2 summarizes water chemistry in the circulating water data for a control and the two phosphate rock mesocosms over the 9 month period of observations. The changes in the measured parameters are discussed below. Related parameters are discussed together.
3.2. Acidity: pH and iron The pH of both phosphate rock mesocosms declined for approximately 100 days, after which stable values around 3.2 were obtained, somewhat higher than in the control mesocosm. This decrease is likely due to the release of reduced iron, which is subsequently reoxidized and hydrolysed in the circulating water column, releasing hydrogen ions. As nearly all the acidity in the Koschen lake water was attributable to protons (reflected in low pH) and dissolved Fe3 + , the initial increase in buffer capacity of the water following phosphate rock addition is expected (Fig. 2b,c). Acidity (KB4.3, Fig. 2b; KB8.2, Fig. 2c) increased over the period of observations but remained substantially lower than in the control. This increase was associated with the gradual reappearance of iron in the water column (TP, Fig. 2d; Fe(II), Fig. 2e). The presence of Fe(II) in the circulating water indicates that reductive dissolution of Fe(III) hydroxide precipitates is occurring. The pH at the sediment surface (pH 6) was too high for substantial dissolution of the hydroxyapatite or iron phosphate precipitates. Sediment pH values are not reported in detail as they remained in the range 6 – 7.5 throughout the experiments in all mesocosms. In the control mesocosm, there was a steady removal of total iron (Fig. 2d) during the period of observations, suggesting a net oxidation and precipitation of Fe(III) hydroxides.
3.3. Nutrients: phosphorus and nitrogen Concentrations of DIP (Fig. 2f) and TP (Fig. 2g) showed a steady decline over time with TP greater than DIP, suggesting the presence of organic phosphorus forms. The final phosphorus concentrations were much higher than in the control mesocosm where concentrations of both DIP and TP remained low and fairly constant. There was a dramatic decline in NH4+ -N concentrations (Fig. 2j) in the
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phosphate rock mesocosms associated with an observed increase in algal growth. NO3− -N (Fig. 2k) concentrations remained steady in the phosphate rock columns until the NH4+ -N was depleted, after which concentrations declined to below detection limits (0.03 mg l − 1). The two peaks of NO3− -N following NO3− addition are clearly evident from the figure. The subsequent slow disappearence of this NO3− -N indicates that, when present, NH4+ -N is the preferred nitrogen form to
Fig. 2. Chemistry of phosphate rock mesocosms. ", control; , phosphate rock + 1 mg l − 1 nitrate; , phosphate rock only.
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support biomass production. Other differences in the water chemistry of the two phosphate rock mesocosms are small.
3.4. Carbon: DIC and DOC Dissolved inorganic carbon (DIC) showed a rapid decline following addition of phosphate rock (Fig. 2h). Thereafter, values were variable but generally low (B 0.3 mg l − 1). Dissolved organic carbon (DOC) showed a steady rise in all mesocosms (Fig. 2i).
3.5. Potato mesocosms The chemistry of all measured parameters for the potato and control mesocosms over a period of 7 months are summarized in Figs. 3–5, in the same way as for the phosphate rock mesocosms. Data are shown for both the circulating water and 0.12 m above the sediment.
3.6. Acidity: pH and iron In the control mesocosm, pH, acidity and dissolved iron concentrations changed little during the period of observations. In contrast, with potatoes all these parameters changed dramatically (Fig. 3a–o). With 200 g potatoes, the pH rose rapidly between 19 and 35 days after set-up (Fig. 3b). Thereafter, pH values continued to rise at a slower rate in the middle of the circulating water column. This rise is reflected in the acidity (KB4.3, Fig. 3e; KB8.2, Fig. 3h) and iron concentrations (Fig. 3kFig. 4n). The sharp peak in KB8.2 and Fe(II) at the bottom of the 200 g potato mesocosm (Fig. 3h,n) from 19–35 days after potato addition is the result of reductive dissolution of Fe(III) hydroxides in the sediment. With the addition of 30 g potatoes, changes in chemistry were generally slower than with 200 g. A rise in pH was first noted after 16 days at the bottom of the water column and after 56 days in the circulating water, and remained fairly constant around pH 4 through the winter months when the temperature was relatively low (6 – 9°C) and then rose to \ 6 by the end of the observation period (March, 7 months after set-up) (Fig. 3c). A brief, sharp peak in Fe2 + at the bottom of the mesocosm (Fig. 3o) is indicative of reductive dissolution of Fe(III) hydroxides in the sediment associated with potato decomposition and establishment of reducing conditions. Most acidity and iron had been removed by day 88 and no re-acidification was detected during the remainder of the sampling period (Fig. 3i,l,o).
3.7. Nutrients: phosphorus and nitrogen Inorganic phosphorus concentration (DIP; Fig. 4a–c) and total dissolved phosphorus (TP; Fig. 4d – f) increased through release from breakdown of the potatoes. The phosphate content of the potatoes was 2.25 mg g − 1 dry weight, and most was
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Fig. 3. Chemistry of potato mesocosms: (a – c) pH; (d – i) titratable acidity; (j – o) iron. , 12 cm above sediment; , middle of circulating water column.
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in the readily available fraction (NH4Cl soluble in procedure of Psenner et al. (1984)), more than enough to account for the measured concentrations in the water. With 200 g of potatoes, phosphorus concentrations increased to 924 mg l − 1 (19 days), then declined dramatically (30 mg l − 1 at 45 days) and was 5 mg l − 1 at the end of the observation period. O2 concentrations were much lower than in the control associated with a rapid depletion of NO3− -N (Fig. 5j–l) attributable to denitrification and assimilation. The dramatic rise in NO3− -N concentrations in the circulating water of the potato mesocosms suggests that nitrification is occurring. Breakdown of potatoes was reflected in the release of NH4+ -N into the water column (Fig. 5h–i).
Fig. 4. Chemistry of potato mesocosms: (a – f) phosphorus; (g – l) carbon. , 12 cm above sediment; , middle of circulating water column.
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Fig. 5. Chemistry of potato mesocosms: (a – c) silicon; (d – f) O2; (g – i) ammonium-N; (j – l) nitrate-N; (m – o) sulphate; (p–r) conductivity. , 12 cm above sediment; , middle of circulating water column.
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In the mesocosm with 30 g of potatoes, DIP concentration increased to 1545 mg l − 1 at 83 days in the circulating water, indicating diffusion of phosphorus from the sediment and decomposing potato. By the end of the observation period, DIP had decreased to 48 mg l − 1. Associated with the pH rise, iron concentration, acidity and NO3− -N declined, the latter to below detection limits (0.006 mg l − 1). No H2S was detected in the water column. However, a band of black particles appeared in a 5 –10 cm zone above the sediment associated with an O2 concentration of B 1 mg l − 1. In the circulating water, dissolved O2 concentration remained near saturation throughout the observation period.
3.8. Carbon: DIC and DOC DIC in the control mesocosm remained low (generally B 0.5 mg l − 1). In contrast, high concentrations (up to 10 mg l − 1 with 30 g potatoes and 50 mg l − 1 with 200 g potatoes) were found in the amended mesocosms (Fig. 4g–i). DOC values (Fig. 4j – l) remained high in these mesocosms (around 5–10 mg l − 1) as the potatoes decomposed. There was an increase in DOC in the control mesocosm but concentrations remained lower than in the potato mesocosms.
3.9. Other parameters Silicon concentrations (Fig. 5a–c) showed a decline in the potato mesocosms, especially with 200 g potatoes. Whether this is associated with the growth of diatoms or with some precipitation process is not known. Sulphate reduction was clearly occurring from day 40 as measurable dissolved H2S (up to 2 mg l − 1) was present. However H2S could not be smelled at the surface of the column, indicating metabolism and the formation and precipitation of FeS. The sulphate concentration declined in both potato mesocosms but not in the control (Fig. 5m– o). In the 200 g potato mesocosm, black particles, presumably predominately FeS, formed in the water column following the decline in O2 concentration to B 1 mg l − 1 consumption of O2 and the depletion of dissolved iron. These black particles dropped to the bottom of the water column after approximately 2 weeks in suspension. This settling process occurred at the same time as the dissolved phosphate concentration was reduced to very low levels in the circulating water and corresponded with an increase in dissolved phosphorus above the sediment (Fig. 4b,c,e – f)). Electrical conductivity of the water showed a substantial decline in the potato mesocosms (to around 1200 mS cm − 1 with 30 g potato and 1050 mS cm − 1 with 200 g potato) whereas values in the control remained fairly constant, around 1600 mS cm − 1 (Fig. 5p – r). Copious algal growth of Eunotia exigua and chlorophytes (small cocci, Ulothrix spp. and at least two Chlamydomonas spp.) was observed in both potato mesocosms following the rise in pH to \ 6.
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4. Discussion This mesocosm study sought to establish the effects of chemical and organic amendments on the chemistry of Tagebaurestseen water in order to determine the potential of controlled eutrophication to assist in acidity removal from such water bodies. Overall, it is clear that addition of phosphate rock can result in the sustained removal of acidity. At the end of the period of observations acidity was about 25% less acidity than the control. The observed increase in algal growth together with the NH4+ -N and phosphorus depletion was not clearly associated with the changes in acidity suggesting that iron cycling is not closely related to algal growth. Populations of Chlamydomonas spp. growing near the water surface of the mesocosms may take up CO2 from the atmosphere to help overcome the DIC limitation in the water column. The occurrence of L. teres in the low light at the bottom of the phosphate rock mesocosms is interesting. This organism is most likely growing heterotrophically using organic carbon resulting from decomposition around the sediment surface. Photoheterotrophy has been found in other euglenophytes (Cook, 1965, Leedale, 1967, Lee, 1989). One other acidophylic species, Euglena mutabilis, has been shown to use this mode of nutrition (Olaveson and Stokes, 1989). L. teres has been found in a number of Lusatia Tagebaurestseen (pH 2.6–6) and may be dominant in conditions where algal biomass is high (unpublished observations). The study of Davison et al. (1995) indicated that the observed increase in pH in the studied lake could be accounted for by assimilation. This will only result in an increase in pH if NO3− is used as nitrogen source (Eq. (1); Steinberg et al., 1998). With NH4+ -N and with CO2 (autotrophic assimilation) or acetate (photoheterotrophic assimilation), a decrease in pH can be expected (Eqs. (2) and (3)). 106CO2 +16NO3− +PO34 − +141H2O (CH2O)106(NH3)16H3PO4 +19OH − + 138O2
(1)
106CO2 +16NH4+ +PO34 − +141H2O (CH2O)106(NH3)16H3PO4 +130O2 + 13H +
(2)
53CH3COO − +16NH4+ +PO34 − (CH2O)106(NH3)16H3PO4 + 12H +
(3)
In the phosphate rock mesocosms, no evidence was found that the L. teres population associated with the band of O2 supersaturation above the sediment had any measurable effect on pH. If these algae were utilizing NO3− -N in the mesocosms, the resulting pH change at a pH of 3.2 would be less than 0.1 pH unit based on the measured NO3− -N concentrations. Addition of NO3− to a phosphate rock-treated mesocosm had no clear effects on chemistry and was not expressed in algal growth. Nitrate concentrations remained high until NH4+ -N was depleted. Although reducing conditions are likely present in the sediment, denitrification did not lead to extensive net reduction of NO3− .
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Potatoes were used as a decomposable organic carbon source since such material has been shown to be effective for the generation of anaerobic conditions above the sediment (Fyson et al., 1996, Kalin et al., 1997). The potatoes contained approximately 65% starch which was readily decomposed in anoxic conditions through fermentation and the subsequent anaerobic bacterial respiration of the volatile fatty acids produced. The potatoes also contained sufficient phosphorus (2.25 mg g − 1 dry weight P) to have a substantial effect on the chemistry and biology of the mesocosm system. This phosphorus content was similar to that reported for potatoes in the literature (Scherz and Senser, 1994). Input of phosphorus to the lake from decomposition of potatoes added at a rate of 30 g per mesocosm or 3.6 kg m − 2 would be 8 1 g m − 2 (based on a potato phosphorus content of 2.25 mg g − 1 dry weight). This would give a mean phosphorus concentration of 765 mg l − 1 in the mesocosms or 52 mg l − 1 for the whole lake (based on 25 m depth) if all remained in solution and was mixed throughout the water column. An additional input through sediment release in anoxic conditions may occur but as the sediment surface contains only 0.8 mg g dry weight or approxiately 6 g m − 2, this input would be less than from the potatoes. This study has shown that amendment addition can result in proliferation of algae. The phosphorus concentration resulting from potato addition is unlikely to promote excessive eutrophication. Potatoes added to mesocosms decomposed with a clear sequence of associated changes in the ecosystem: Consumption of O2. Release of nutrients and their diffusion up the water column. Increase in pH and removal of acidity. Precipitation of black particles. Reoxygenation of water. Overall, titratable acidity was removed and primary productivity was stimulated. Potatoes added reduced carbon and nutrients to the mesocosm ecosystem. With 200 g potatoes per mesocosm, potato decomposition generated anoxic, reducing conditions throughout the mesocosm. With 30 g potatoes per mesocosm, anoxic, reducing conditions were confined to the non-circulating water above the sediment. However, nutrients were supplied to the circulating water above and acidity was removed. Potatoes generated substantial changes in water chemistry within 3 weeks and removed most of the acidity within 7 weeks. A substantial improvement in water chemistry was maintained through the period of observations. Addition of phosphate alone may be inadequate to remove all acidity from the water column and generate a near-neutral pH as was achieved in a less acidic, acid rain affected lake (Davison et al., 1995). Organic carbon in the form of potatoes or other decomposable organic materials is likely necessary to initiate sediment activities which result both in elevated nutrient conditions and removal of acidity in the water column. Once such conditions are established, primary productivity may maintain acidity removal through continued support of alkalinity generating processes in the sediment and deep hypolimnion. Remediation of deep, acidic lakes has only been carried out on a large scale in acid rain affected lakes (Blomquist et al., 1993, Davison et al., 1995). However, the
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acidity of these lakes is lower and the pH higher (\ 4) than in the Tagebaurestseen. Use of macrophytes as a source of organic carbon in Tagebaurestseen is limited because of the large size and consequent lack of shallow water in these lakes. In these circumstances, phytoplankton can have an important role as a fuel for sediment alkalinity generating processes and more directly through alkalinity generation from assimilation linked to uptake of NO3+ -N as a nitrogen source. This study demonstrates that organic wastes remove acidity in the short term in asssociation with generation of reducing conditions around the sediment surface and so phosphorus will not be sequestered to the sediment. Whether the resulting increased algal growth in the water column can result in self-sustaining acidity removal is the subject of ongoing studies.
Acknowledgements The authors would like to thank Mike Kuhne, Joerg Koebcke and Dr Dieter Lessmann of BTU, Cottbus, and Hartwig Krumbeck, Dr Michael Hupfer and the Chemistry Laboratory of IGB, Berlin, for chemical analysis.
References Blomquist, P., Bell, R.T., Olofsson, H., Stensdotter, U., Verde, K., 1993. Pelagic ecosystem responses to nutrient additions in acidified and limed lakes in Sweden. Ambio 22, 283 – 289. Blouin, A.C., 1989. Patterns of plankton species, pH and associated water chemistry in Nova Scotia lakes. Water Air Soil Pollut. 46, 343– 358. Cook, J.R., 1965. Influence of light on acetate utilization in green Euglena. Plant Cell Physiol. 6, 301. Davison, W., 1985. Conceptual models for transport at a redox boundary. In: Stumm, W. (Ed.), Chemical Processes in Lakes, Wiley, Chichester. Davison, W., George, D.G., Edwards, N.J.A., 1995. Controlled reversal of lake acidification by treatment with phosphate fertilizer. Nature 377, 504 – 507. DEV (Deutsche Einheitsverfahren zur Wasser, Abwasser und Schlammuntersuchung), 1992. Schlammuntersuchung. Verlag Chemie, Weinheim, D11, E5, D19. Dvorak, D.H., Hedin, R.S., M Edenborn, H., McIntire, P.E., 1992. Treatment of metal contaminated water using bacterial sulphate reduction: results from pilot scale reactors. Biotechnol. Bioeng. 40, 609–616. Fyson, A., Kalin, M., Smith, M.P., 1996. Microbially-mediated metal removal from acid mine drainage. In: Moo-Young, M., Anderson, W.A., Chakrabarty, A.M. (Eds.), Environmental Biotechnology: Principles and Practice. Kluwer, Dordrecht. Gruenewald, U., 1996. Bericht zur Erarbeitung von Grobaussagen zur Gewaessergu¨ueteentwicklung von Tagebauseen der Lausitz. LMBV Senftenberg and BTU, Cottbus, 64 pp. Kalin, M., 1993. Treatment of acidic seepage using microbiology. Final report, MEND Proj.3.11.1. CANMET, File: 015SQ.23440-2-9217. Kalin, M., Fyson, A., Smith, M.P., 1997. Microbial acid reduction in sediments — concepts and application. International Conference on the Abatement of Acidic Rock Drainage, Vancouver, 31 May–6 June 1997. Klapper, H., Schultze, M., 1995. Geogenically acidified mining lakes — living conditions and possibilities of restoration. Int. Rev. Ges. Hydrobiol. 80, 639 – 653. Lee, R.E., 1989. Phycology, 2nd ed. Cambridge University Press, Cambridge.
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Leedale, G.F., 1967. Euglenoid Flagellates. Prentice-Hall, Eaglewood Cliffs, NJ. Luckner, L., Eichhorn, D., 1996. Flutungskonzept fuer die Tagebaurestloecher. Wasserwirtsch. Wassertech. 4, 18–28. Mills, A.L., Bell, P.E., Herlihy, A.T., 1989. Microbes, sediments and acidified water: The importance of biological buffering. In: Rao, S.S. (Ed.), Acid Stress and Aquatic Microbial Interactions. CRC Press, Boca Raton, pp. 1–19. Nixdorf, B., Wollmann, K., 1997. Ecological potential for planktonic developments and food web interactions in extremely acidic mining lakes. In: Geller, W., Klapper, H., Schulze, M. (Eds.), Proceedings of the Workshop Abatement of Geogenic Acidification in Mining Lakes, Magdeburg, 4–6 September 1995. Springer-Verlag, Berlin, in press. Nixdorf, B., Lessmann, D., Gruenewald, U., Uhlmann, W., 1997a. Limnology of extremely acidic mining lakes in Lusatia (eastern Germany) and their fate between acidity and eutrophication. Mine. Proceedings of 4th International Conference on the Abatement of Acidic Rock Drainage, Vancouver, 31 May–6 June 1997, pp. 1745– 1760. Nixdorf, B., Mischke, U., Lessmann, D., 1997b. Chrysophytes and Chlamydomonads: pioneers of colonization in extremely acidic mining lakes (pH B3) in Lusatia (Germany). Hydrobiologia (in press). Olaveson, M.M., Stokes, P.M., 1989. Responses of the acidophilic alga Euglena mutabilis (Euglenophyceae) to carbon enrichment at pH 3. J. Phycol. 25, 529 – 539. Psenner, R., Pucsko, R., Sagar, M., 1984. Die Fraktioierung organischer und anorganischer Phosphorverbindung von Sedimenten-Versuch einer Definition oekologisch wichtiger Fraktionen. Arch. Hydrobiol. 70 (Suppl.), 111–155. Rowley, M.V., Warkentin, D.D., Sicotte, V., 1997. Site demonstration of the biosulphide process at the former Britannia Mine. Proceedings of 4th International Conference on the Abatement of Acidic Rock Drainage, Vancouver, 31 May– 6 June 1997, pp. 1531 – 1548. Scherz, H., F. Senser, 1994. Die Zusammensetzung der Lebensmittel: Na¨hrwert-Tabellen, 5th ed., CRC Press, Boca Raton. Steinberg, C.E.W., Adrian, R., Behrendt, H., Behrendt, H., Hupfer, M., Kasprzak, P., Meinelt, T., Obermann, A., Brueggemann, R., 1998. Fundamentals of Applied Limnology (in German). In: Lecher and Luhr (Eds.), Handbook of Water Management. Parey-Verlag, in press. Stumm, W. and J.J. Morgan, 1996. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, 3rd edn. Wiley, New York, 1022 pp. Vile, M.A., Weider, R.K., 1993. Alkalinity generation by Fe3 + reduction in wetlands constructed for acid mine drainage treatment. Water Air Soil Pollut. 69, 425 – 441.
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Mesocosm studies to assess acidity removal from acidic mine lakes through controlled eutrophication A. Fyson a,b,*, B. Nixdorf b, M. Kalin c, C.E.W. Steinberg a a
Institute of Freshwater Ecology and Inland Fisheries, Mueggelseedamm 310, Berlin D-12587, Germany b Lehrstuhl Gewaesserschutz, Brandenburg Technical Uni6ersity Cottbus, Cottbus D-03013, Germany c Boojum Research Limited, 468 Queen Street East, Toronto, Ontario M5A 1T7, Canada Received 21 July 1997; received in revised form 17 February 1998; accepted 19 February 1998
Abstract Flooded lignite pits (Tagebaurestseen) in Lusatia, Germany, are acidic (pH 2.5 – 4) with high concentrations of iron. Mesocosms (total volume 20 l) were set up with water and sediment from a Tagebaurestsee (Koschen, pH 3.1, Fe 20 mg l − 1, acidity (KB8.2) 2.45 mM) to assess the effects of phosphate and organic amendments under natural light and low temperature. Chemical and biological parameters were observed over a 9-month period. Phosphate rock addition (300 mg l − 1) resulted in sustained reduction in acidity (H + and Fe3 + ) in the water column and induced the growth of Chlamydomonas spp. (Chlorophyceae) near the water surface and Lepocinclis teres (Euglenophyceae) in a band above the sediment. Addition of potatoes to mesocosms resulted in the generation of near-anoxic conditions above the sediment, and phosphorus, ammonium and carbon (organic and inorganic) were released as the potatoes decomposed. A pH \6 was attained with 5.1 g (dry weight) of potatoes and pH \8 with 34 g (dry weight). In both mesocosms, more than 90% of total acidity was removed. Algal growth, mainly of small, coccoid Chlorophyceae, Ulothrix sp. (Chlorophyceae) and Eunotia exigua (Bacillariophyceae), was observed throughout the water column. © 1998 Elsevier Science B.V. All rights reserved.
Keywords: Acidity removal; Controlled eutrophication; Euglenophyceae; Lepocinclis; Lignite; Mesocosms; Phosphate rock; Potatoes
* Corresponding author. Tel.: + 49 30 64181605; fax: + 49 30 64181600; e-mail: [email protected] 0925-8574/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0925-8574(98)00007-X
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1. Introduction Tagebaurestseen are acidic lakes with high iron and sulphate concentrations that are formed as water rises in mined-out open-cast lignite pits. During mining, the groundwater table is lowered, resulting in oxidation of pyrite- and marcasite-containing materials. Intensive extraction of lignite has resulted in a unique landscape in the Lusatia region in north-eastern Germany. A total of about 400 Tagebaurestseen, which are of variable size, typically 20–45 m deep and dimictic, provide a lake surface area of several hundred square kilometers in the Lusatia region in the states of Saxony and Brandenburg. Remediation efforts include diversion of river water to these lakes to raise the pH and nutrient concentrations in some Tagebaurestseen, but surface water supply is very limited (Gruenewald, 1996, Luckner and Eichhorn, 1996). Additions of limestone (CaCO3) or lime (CaO) to release alkalinity is one option. Biologically based remedial measures for Tagebaurestseen are an attractive proposition both environmentally and economically. Biologically based approaches have been suggested, particularly in light of the undetermined supply of contaminants to the infilling lakes and the potentially long-term nature of the problem (Klapper and Schultze, 1995, Nixdorf et al., 1997a). Addition of phosphate to a lake acidified through acid rain resulted in an increase in pH comparable to that predicted from the assimilation of CO2 coupled to nitrate uptake by phytoplankton (Davison et al., 1995). Tagebaurestseen share the oligotrophic character of this lake (low phosphorus concentrations), and enhancement of primary productivity through nutrient additions to the water column to increase the pH has been suggested (Nixdorf and Wollmann, 1997, Nixdorf et al., 1997a,b), but the chemistry of these lakes presents additional challenges. The dissolved phosphate-P concentration is B 10 mg l − 1 and acidity is from both iron and hydrogen ions. At pH 3, a typical pH value for the water column, the maximum concentration of CO2, the dominant form of dissolved inorganic carbon (DIC), is approximately 0.1 mg l − 1 (Stumm and Morgan, 1996), which will severely limit primary productivity. Blouin (1989) studied productivity of lakes with pHB 4 and documented very low activity. However, localized high algal biomass has been found in the depth profile of the water column in Tagebaurestseen with a small number of species from diverse algal taxa (Nixdorf et al., 1997a,b). Enhancement of anaerobic sediment microbial activity provides another approach to amelioration of lake water quality. Through provision of organic carbon and phosphate, microbial activity in the sediment is stimulated, alkalinity is generated (acidity consumed) and nutrients and carbon (organic and inorganic) are et al., 1996). Organic wastes have been used to enhance sulphate reduction in acid mine drainage to increase the pH. This in turn leads to precipitation of metals out of the water column and in sediments to the formation of metal sulphides. Vile and Weider (1993) report these processes in wetlands and Kalin (1993), Fyson et al. (1996) and Kalin et al. (1997) for a test system for tailings seepages. Sediments in
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this test system were constructed with organic matter (hay, alfalfa pellets and potato waste). In bioreactors, the same processes are utilized when acid mine drainage is passed through various types of vessel containing organic matter (Dvorak et al., 1992, Rowley et al., 1997). Mesocosms containing sediment and water from Lake Koschen, a typical Lusatia Tagebaurestsee, were used to determine in detail the interactions between sediment and water column. Phosphate and carbon were added to the sediment to quantify the processes that would be induced if controlled eutrophication were considered as an approach for acidity removal from Tagebaurestseen. To stimulate sediment activity, the choice of material is based on several criteria. The material must be available in large quantities at low cost. The release of inorganic phosphate and carbon must be gradual to provide a long-term, but steady supply of nutrients to the biological processes. A sedimentary phosphorus rock, mined in North Carolina, was used as a phosphate source and potatoes served as sources of organic carbon in the sediment. This paper summarizes the effects of rate and type of amendment on the chemistry of Lake Koschen water over time.
2. Experimental
2.1. Water and sediment sampling Lake Koschen was selected after screening the water chemistry from a database compiled on Tagebaurestseen by the Brandenberg Technical University, Cottbus. Lake Koschen was represenative of the majority of the lakes and was accessible. This lake has an area of 6.1 km2, a maximum depth of 25 m and an estimated volume of 1.5× 108 m3. This lake has been infilling with groundwater since 1975. Since monitoring began, the pH has been in the range 2.9–3.2 and productivity has been low (Nixdorf et al., 1997a,b). Koschen may be considered to be in the middle range of acidity for Tagebaurestseen (KB8.2 = 1.7–2.5 mM or 170–250 mg l − 1 equivalents of CaCO3) and is classified as very acidic in the scheme of Nixdorf et al. (1997a). Minimally disturbed sediment cores were collected at a depth of 21 m with a sediment corer of 5.5 cm i.d. (Umwelt, Mondsee, Austria). Following retrieval, the cores were divided into surface, middle and bottom layers, which were clearly distinguishable by their colour and texture. Water from Lake Koschen was collected from the thermocline (\ 10 m). The water and sediment were stored at B 10°C until set-up of the mesocosms.
2.2. Mesocosm design and set-up The mesocosms were 2.6 m long polycarbonate vertical columns with an i.d. of 10.5 cm which were sealed at the base with plexiglass plates (Fig. 1). The
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mesocosms were designed to be sufficiently large (15 l water over 0.5 m sediment) to provide for the establishment of chemical gradients and to contain sufficient water to allow for regular sampling without major disruption. They were fitted with multiple sampling ports to provide long-term flexibility in sampling regimen. Most of the water column (12.5 l) was circulated by means of a peristaltic pump (Fig. 1). The pumping rate was 2 l h − 1 or approximately four exchanges every 24 h. Polystyrene packing chips (0.15 m depth) were placed at the bottom of the mesocosm and the three sediment layers added in sequence to yield a total sediment depth of approximately 0.5 m. The sediment portion of the mesocosms was installed below a platform to maintain darkness. The experiments were set up in a cool basement laboratory, receiving natural lighting through a roof window and fairly steady temperature throughout the year (6–9°C in winter, 14– 17°C in summer). Supplementary lighting was provided in the form of fluorescent light banks. Photosynthetically active radiation was 150–800 mE m − 2 s − 1 at the top of the mesocosms and B20 mE m − 2 s − 1 at the sediment surface.
2.3. Chemical analysis Water samples for chemical analysis (200–500 ml) were collected from the
Fig. 1. Mesocosm configuration. Total height is 2.6 m and the internal diameter is 10.5 cm.
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middle port of the circulating water (0.7 m below the surface) and from a port 0.12 m above the sediment surface in the non-circulating zone. The volume removed during sampling from each mesocosm was replaced with Koschen lake water. The results from one control mesocosm are for a shorter period as it initially developed a leak and had to be set up again. Chemical parameters were determined by standard methods (DEV, 1992) on filtered water (0.45 mm). Conductivity, pH and O2 were measured with WTW meters and probes. Iron (total and Fe(II)) was determined photometrically by the o-phenanthroline method with a Shimadzu UV-2101 spectrophotometer. Dissolved inorganic phosphate (DIP) was determined photometrically by the molybdenum blue method. Total dissolved phosphate (TP) was determined by the same method following digestion in sulphuric acid and hydrogen peroxide at 150°C and neutralization with NaOH. Dissolved organic carbon (DOC) was determined by infra-red detection of CO2 following catalytic digestion in a Shimadzu Total Organic Carbon Analyzer-5000. Dissolved silicon was determined by the molybdenum blue method after addition of tartaric acid (pH 1.4– 1.6). Nitrate, sulphate and chloride were determined by ion chromatography. Ammonium-N (NH4+ -N) was determined by flow solution (Perstop Analytical) with photometric determination of an indophenol complex. Total sediment phosphorus was determined according to the sequential procedure of Psenner et al. (1984). Titrations against NaOH were carried out for determination of acidity (KB4.3 and KB8.2). KB4.3 and KB8.2 are the respective amounts of NaOH (expressed in mM) required to raise the pH to 4.3 and 8.2. KB8.2 is equivalent to acidity expressed in mg l − 1 CaCO3 milliequivalents, more frequently used in North America. Underwater photon flux density of photosynthetically active radiation was measured with a LI 193 SB Spherical Quantum Sensor (LICOR).
2.4. Experimental treatments To determine the effects of phosphate addition, three mesocosms were set up: a control with no amendment, with phosphate rock alone (300 mg l − 1) and with additions of both phosphate rock (300 mg l − 1) and NO3− (6.7 mg l − 1 of NaNO3). The phosphate rock contains phosphorus in the form of hydroxyapatite (e.g. Ca10(PO4)6(OH)2). The formulation used (Code 31; Texasgulf, Raleigh, NC) contained 14.1 mg g − 1 P, 349 mg g − 1 Ca and 120 mg g − 1 CO3, and is a fine powder which allows for a long suspension time in the water column and exposure of a large proportion of the phosphate to the low pH of the water. Phosphate rock was added by sprinkling it on the water surface 7 days after mesocosm set-up. The quantity of phosphate rock added to the mesocosms (300 mg l − 1) was based on preliminary shake flask experiments to establish a rate that was sufficient to raise the pH to around 4.5, to increase DIC from around 0.1 mg l − 1 to 1 mg l − 1, and to remove iron from the water column. Nitrate (6.7 g NaNO3) was added to one mesocosm 34 and 180 days after set-up.
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Table 1 Chemistry of water added to mesocosms and mesocosm water chemistry 3 h after addition of phosphate rock Parameter
Temperature pH Conductivity Fe total Fe2+ DIP-P TPO4-P NO3-N SO2− 4 Cl− DIC DOC Acidity (KB4.3) Acidity (KB8.2) Ca2+ Mg2+ O2
Unit
°C mS cm−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mg l−1 mM mM mg l−1 mg l−1 mg l−1
Water added to mesocosms
20 3.1 1578 19.2 0.27 6.5 B10 0.36 690 40 0.30 ND 1.82 2.45 180 30 ND
Mesocosm water
Control
Phosphate rock NO3
Phosphate rock only
15.9 3.11 1594 15.3 B0.006 0.012 0.03 0.34 690 40 0.38 1.8 1.28 1.82 184 24.8 8.8
16.7 4.48 1423 0.06 0.05 9.8 15.2 0.31 710 39 0.95 2.4 0.00 1.00 292 31.1 6.8
16.7 4.38 1418 0.09 0.05 9.9 14.9 0.32 710 40 0.90 2.1 0.00 0.80 249 29.6 6.9
ND, parameter was not determined.
To determine the effects of decomposable organic matter addition, whole potatoes were dropped into the mesocosms at rates of either 30 g (5.1 g dry weight) or 200 g (34 g dry weight) per mesocosm. Potatoes were placed in nets together with a similar weight of coarse gravel to keep the potatoes in the sediment during gas production from decomposition. Whole potatoes are used because on addition they are half-buried in the sediment and therefore decomposition products are supplied where they are needed at the sediment surface.
3. Results
3.1. Phosphate rock mesocosms Water chemistry of the mesocosms at the onset of the experiment is presented in Table 1. It is evident that phosphate rock addition removed nearly all dissolved iron and resulted in a dramatic increase in dissolved phosphate within 3 h of addition. The concentration of DIC and calcium increased, due to the dissolution of calcium carbonate in the phosphate rock. As expected, the pH increased and the acidity decreased due to iron removal. No substantial changes were noted for the other
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parameters measured. Comparing the characteristics of the original lake water at the time of collection to the control mesocosms, 7 days after set-up, indicates that, in this time, there was little change in water chemistry. Concentrations of Fe(II) and DIP had decreased during this period. As biological activity is relevant to some of the time-related changes in the water chemistry, the key observations on the occurrence are summarized. In the phosphate rock mesocosms, Chlamydomonas spp. were evident at the top of the mesocosms within 1 week of phosphate rock addition. At 66 days after set-up, a narrow band of O2 supersaturation (up to 17 mg l − 1 or 200% saturation), corresponding to a ‘green band’, developed approximately 10–15 cm above the sediment surface. This band was found to contain cells (104 –105 ml − 1) of a flagellated euglenophyte, Lepocinclis teres. Fig. 2 summarizes water chemistry in the circulating water data for a control and the two phosphate rock mesocosms over the 9 month period of observations. The changes in the measured parameters are discussed below. Related parameters are discussed together.
3.2. Acidity: pH and iron The pH of both phosphate rock mesocosms declined for approximately 100 days, after which stable values around 3.2 were obtained, somewhat higher than in the control mesocosm. This decrease is likely due to the release of reduced iron, which is subsequently reoxidized and hydrolysed in the circulating water column, releasing hydrogen ions. As nearly all the acidity in the Koschen lake water was attributable to protons (reflected in low pH) and dissolved Fe3 + , the initial increase in buffer capacity of the water following phosphate rock addition is expected (Fig. 2b,c). Acidity (KB4.3, Fig. 2b; KB8.2, Fig. 2c) increased over the period of observations but remained substantially lower than in the control. This increase was associated with the gradual reappearance of iron in the water column (TP, Fig. 2d; Fe(II), Fig. 2e). The presence of Fe(II) in the circulating water indicates that reductive dissolution of Fe(III) hydroxide precipitates is occurring. The pH at the sediment surface (pH 6) was too high for substantial dissolution of the hydroxyapatite or iron phosphate precipitates. Sediment pH values are not reported in detail as they remained in the range 6 – 7.5 throughout the experiments in all mesocosms. In the control mesocosm, there was a steady removal of total iron (Fig. 2d) during the period of observations, suggesting a net oxidation and precipitation of Fe(III) hydroxides.
3.3. Nutrients: phosphorus and nitrogen Concentrations of DIP (Fig. 2f) and TP (Fig. 2g) showed a steady decline over time with TP greater than DIP, suggesting the presence of organic phosphorus forms. The final phosphorus concentrations were much higher than in the control mesocosm where concentrations of both DIP and TP remained low and fairly constant. There was a dramatic decline in NH4+ -N concentrations (Fig. 2j) in the
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phosphate rock mesocosms associated with an observed increase in algal growth. NO3− -N (Fig. 2k) concentrations remained steady in the phosphate rock columns until the NH4+ -N was depleted, after which concentrations declined to below detection limits (0.03 mg l − 1). The two peaks of NO3− -N following NO3− addition are clearly evident from the figure. The subsequent slow disappearence of this NO3− -N indicates that, when present, NH4+ -N is the preferred nitrogen form to
Fig. 2. Chemistry of phosphate rock mesocosms. ", control; , phosphate rock + 1 mg l − 1 nitrate; , phosphate rock only.
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support biomass production. Other differences in the water chemistry of the two phosphate rock mesocosms are small.
3.4. Carbon: DIC and DOC Dissolved inorganic carbon (DIC) showed a rapid decline following addition of phosphate rock (Fig. 2h). Thereafter, values were variable but generally low (B 0.3 mg l − 1). Dissolved organic carbon (DOC) showed a steady rise in all mesocosms (Fig. 2i).
3.5. Potato mesocosms The chemistry of all measured parameters for the potato and control mesocosms over a period of 7 months are summarized in Figs. 3–5, in the same way as for the phosphate rock mesocosms. Data are shown for both the circulating water and 0.12 m above the sediment.
3.6. Acidity: pH and iron In the control mesocosm, pH, acidity and dissolved iron concentrations changed little during the period of observations. In contrast, with potatoes all these parameters changed dramatically (Fig. 3a–o). With 200 g potatoes, the pH rose rapidly between 19 and 35 days after set-up (Fig. 3b). Thereafter, pH values continued to rise at a slower rate in the middle of the circulating water column. This rise is reflected in the acidity (KB4.3, Fig. 3e; KB8.2, Fig. 3h) and iron concentrations (Fig. 3kFig. 4n). The sharp peak in KB8.2 and Fe(II) at the bottom of the 200 g potato mesocosm (Fig. 3h,n) from 19–35 days after potato addition is the result of reductive dissolution of Fe(III) hydroxides in the sediment. With the addition of 30 g potatoes, changes in chemistry were generally slower than with 200 g. A rise in pH was first noted after 16 days at the bottom of the water column and after 56 days in the circulating water, and remained fairly constant around pH 4 through the winter months when the temperature was relatively low (6 – 9°C) and then rose to \ 6 by the end of the observation period (March, 7 months after set-up) (Fig. 3c). A brief, sharp peak in Fe2 + at the bottom of the mesocosm (Fig. 3o) is indicative of reductive dissolution of Fe(III) hydroxides in the sediment associated with potato decomposition and establishment of reducing conditions. Most acidity and iron had been removed by day 88 and no re-acidification was detected during the remainder of the sampling period (Fig. 3i,l,o).
3.7. Nutrients: phosphorus and nitrogen Inorganic phosphorus concentration (DIP; Fig. 4a–c) and total dissolved phosphorus (TP; Fig. 4d – f) increased through release from breakdown of the potatoes. The phosphate content of the potatoes was 2.25 mg g − 1 dry weight, and most was
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Fig. 3. Chemistry of potato mesocosms: (a – c) pH; (d – i) titratable acidity; (j – o) iron. , 12 cm above sediment; , middle of circulating water column.
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in the readily available fraction (NH4Cl soluble in procedure of Psenner et al. (1984)), more than enough to account for the measured concentrations in the water. With 200 g of potatoes, phosphorus concentrations increased to 924 mg l − 1 (19 days), then declined dramatically (30 mg l − 1 at 45 days) and was 5 mg l − 1 at the end of the observation period. O2 concentrations were much lower than in the control associated with a rapid depletion of NO3− -N (Fig. 5j–l) attributable to denitrification and assimilation. The dramatic rise in NO3− -N concentrations in the circulating water of the potato mesocosms suggests that nitrification is occurring. Breakdown of potatoes was reflected in the release of NH4+ -N into the water column (Fig. 5h–i).
Fig. 4. Chemistry of potato mesocosms: (a – f) phosphorus; (g – l) carbon. , 12 cm above sediment; , middle of circulating water column.
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Fig. 5. Chemistry of potato mesocosms: (a – c) silicon; (d – f) O2; (g – i) ammonium-N; (j – l) nitrate-N; (m – o) sulphate; (p–r) conductivity. , 12 cm above sediment; , middle of circulating water column.
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In the mesocosm with 30 g of potatoes, DIP concentration increased to 1545 mg l − 1 at 83 days in the circulating water, indicating diffusion of phosphorus from the sediment and decomposing potato. By the end of the observation period, DIP had decreased to 48 mg l − 1. Associated with the pH rise, iron concentration, acidity and NO3− -N declined, the latter to below detection limits (0.006 mg l − 1). No H2S was detected in the water column. However, a band of black particles appeared in a 5 –10 cm zone above the sediment associated with an O2 concentration of B 1 mg l − 1. In the circulating water, dissolved O2 concentration remained near saturation throughout the observation period.
3.8. Carbon: DIC and DOC DIC in the control mesocosm remained low (generally B 0.5 mg l − 1). In contrast, high concentrations (up to 10 mg l − 1 with 30 g potatoes and 50 mg l − 1 with 200 g potatoes) were found in the amended mesocosms (Fig. 4g–i). DOC values (Fig. 4j – l) remained high in these mesocosms (around 5–10 mg l − 1) as the potatoes decomposed. There was an increase in DOC in the control mesocosm but concentrations remained lower than in the potato mesocosms.
3.9. Other parameters Silicon concentrations (Fig. 5a–c) showed a decline in the potato mesocosms, especially with 200 g potatoes. Whether this is associated with the growth of diatoms or with some precipitation process is not known. Sulphate reduction was clearly occurring from day 40 as measurable dissolved H2S (up to 2 mg l − 1) was present. However H2S could not be smelled at the surface of the column, indicating metabolism and the formation and precipitation of FeS. The sulphate concentration declined in both potato mesocosms but not in the control (Fig. 5m– o). In the 200 g potato mesocosm, black particles, presumably predominately FeS, formed in the water column following the decline in O2 concentration to B 1 mg l − 1 consumption of O2 and the depletion of dissolved iron. These black particles dropped to the bottom of the water column after approximately 2 weeks in suspension. This settling process occurred at the same time as the dissolved phosphate concentration was reduced to very low levels in the circulating water and corresponded with an increase in dissolved phosphorus above the sediment (Fig. 4b,c,e – f)). Electrical conductivity of the water showed a substantial decline in the potato mesocosms (to around 1200 mS cm − 1 with 30 g potato and 1050 mS cm − 1 with 200 g potato) whereas values in the control remained fairly constant, around 1600 mS cm − 1 (Fig. 5p – r). Copious algal growth of Eunotia exigua and chlorophytes (small cocci, Ulothrix spp. and at least two Chlamydomonas spp.) was observed in both potato mesocosms following the rise in pH to \ 6.
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4. Discussion This mesocosm study sought to establish the effects of chemical and organic amendments on the chemistry of Tagebaurestseen water in order to determine the potential of controlled eutrophication to assist in acidity removal from such water bodies. Overall, it is clear that addition of phosphate rock can result in the sustained removal of acidity. At the end of the period of observations acidity was about 25% less acidity than the control. The observed increase in algal growth together with the NH4+ -N and phosphorus depletion was not clearly associated with the changes in acidity suggesting that iron cycling is not closely related to algal growth. Populations of Chlamydomonas spp. growing near the water surface of the mesocosms may take up CO2 from the atmosphere to help overcome the DIC limitation in the water column. The occurrence of L. teres in the low light at the bottom of the phosphate rock mesocosms is interesting. This organism is most likely growing heterotrophically using organic carbon resulting from decomposition around the sediment surface. Photoheterotrophy has been found in other euglenophytes (Cook, 1965, Leedale, 1967, Lee, 1989). One other acidophylic species, Euglena mutabilis, has been shown to use this mode of nutrition (Olaveson and Stokes, 1989). L. teres has been found in a number of Lusatia Tagebaurestseen (pH 2.6–6) and may be dominant in conditions where algal biomass is high (unpublished observations). The study of Davison et al. (1995) indicated that the observed increase in pH in the studied lake could be accounted for by assimilation. This will only result in an increase in pH if NO3− is used as nitrogen source (Eq. (1); Steinberg et al., 1998). With NH4+ -N and with CO2 (autotrophic assimilation) or acetate (photoheterotrophic assimilation), a decrease in pH can be expected (Eqs. (2) and (3)). 106CO2 +16NO3− +PO34 − +141H2O (CH2O)106(NH3)16H3PO4 +19OH − + 138O2
(1)
106CO2 +16NH4+ +PO34 − +141H2O (CH2O)106(NH3)16H3PO4 +130O2 + 13H +
(2)
53CH3COO − +16NH4+ +PO34 − (CH2O)106(NH3)16H3PO4 + 12H +
(3)
In the phosphate rock mesocosms, no evidence was found that the L. teres population associated with the band of O2 supersaturation above the sediment had any measurable effect on pH. If these algae were utilizing NO3− -N in the mesocosms, the resulting pH change at a pH of 3.2 would be less than 0.1 pH unit based on the measured NO3− -N concentrations. Addition of NO3− to a phosphate rock-treated mesocosm had no clear effects on chemistry and was not expressed in algal growth. Nitrate concentrations remained high until NH4+ -N was depleted. Although reducing conditions are likely present in the sediment, denitrification did not lead to extensive net reduction of NO3− .
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Potatoes were used as a decomposable organic carbon source since such material has been shown to be effective for the generation of anaerobic conditions above the sediment (Fyson et al., 1996, Kalin et al., 1997). The potatoes contained approximately 65% starch which was readily decomposed in anoxic conditions through fermentation and the subsequent anaerobic bacterial respiration of the volatile fatty acids produced. The potatoes also contained sufficient phosphorus (2.25 mg g − 1 dry weight P) to have a substantial effect on the chemistry and biology of the mesocosm system. This phosphorus content was similar to that reported for potatoes in the literature (Scherz and Senser, 1994). Input of phosphorus to the lake from decomposition of potatoes added at a rate of 30 g per mesocosm or 3.6 kg m − 2 would be 8 1 g m − 2 (based on a potato phosphorus content of 2.25 mg g − 1 dry weight). This would give a mean phosphorus concentration of 765 mg l − 1 in the mesocosms or 52 mg l − 1 for the whole lake (based on 25 m depth) if all remained in solution and was mixed throughout the water column. An additional input through sediment release in anoxic conditions may occur but as the sediment surface contains only 0.8 mg g dry weight or approxiately 6 g m − 2, this input would be less than from the potatoes. This study has shown that amendment addition can result in proliferation of algae. The phosphorus concentration resulting from potato addition is unlikely to promote excessive eutrophication. Potatoes added to mesocosms decomposed with a clear sequence of associated changes in the ecosystem: Consumption of O2. Release of nutrients and their diffusion up the water column. Increase in pH and removal of acidity. Precipitation of black particles. Reoxygenation of water. Overall, titratable acidity was removed and primary productivity was stimulated. Potatoes added reduced carbon and nutrients to the mesocosm ecosystem. With 200 g potatoes per mesocosm, potato decomposition generated anoxic, reducing conditions throughout the mesocosm. With 30 g potatoes per mesocosm, anoxic, reducing conditions were confined to the non-circulating water above the sediment. However, nutrients were supplied to the circulating water above and acidity was removed. Potatoes generated substantial changes in water chemistry within 3 weeks and removed most of the acidity within 7 weeks. A substantial improvement in water chemistry was maintained through the period of observations. Addition of phosphate alone may be inadequate to remove all acidity from the water column and generate a near-neutral pH as was achieved in a less acidic, acid rain affected lake (Davison et al., 1995). Organic carbon in the form of potatoes or other decomposable organic materials is likely necessary to initiate sediment activities which result both in elevated nutrient conditions and removal of acidity in the water column. Once such conditions are established, primary productivity may maintain acidity removal through continued support of alkalinity generating processes in the sediment and deep hypolimnion. Remediation of deep, acidic lakes has only been carried out on a large scale in acid rain affected lakes (Blomquist et al., 1993, Davison et al., 1995). However, the
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acidity of these lakes is lower and the pH higher (\ 4) than in the Tagebaurestseen. Use of macrophytes as a source of organic carbon in Tagebaurestseen is limited because of the large size and consequent lack of shallow water in these lakes. In these circumstances, phytoplankton can have an important role as a fuel for sediment alkalinity generating processes and more directly through alkalinity generation from assimilation linked to uptake of NO3+ -N as a nitrogen source. This study demonstrates that organic wastes remove acidity in the short term in asssociation with generation of reducing conditions around the sediment surface and so phosphorus will not be sequestered to the sediment. Whether the resulting increased algal growth in the water column can result in self-sustaining acidity removal is the subject of ongoing studies.
Acknowledgements The authors would like to thank Mike Kuhne, Joerg Koebcke and Dr Dieter Lessmann of BTU, Cottbus, and Hartwig Krumbeck, Dr Michael Hupfer and the Chemistry Laboratory of IGB, Berlin, for chemical analysis.
References Blomquist, P., Bell, R.T., Olofsson, H., Stensdotter, U., Verde, K., 1993. Pelagic ecosystem responses to nutrient additions in acidified and limed lakes in Sweden. Ambio 22, 283 – 289. Blouin, A.C., 1989. Patterns of plankton species, pH and associated water chemistry in Nova Scotia lakes. Water Air Soil Pollut. 46, 343– 358. Cook, J.R., 1965. Influence of light on acetate utilization in green Euglena. Plant Cell Physiol. 6, 301. Davison, W., 1985. Conceptual models for transport at a redox boundary. In: Stumm, W. (Ed.), Chemical Processes in Lakes, Wiley, Chichester. Davison, W., George, D.G., Edwards, N.J.A., 1995. Controlled reversal of lake acidification by treatment with phosphate fertilizer. Nature 377, 504 – 507. DEV (Deutsche Einheitsverfahren zur Wasser, Abwasser und Schlammuntersuchung), 1992. Schlammuntersuchung. Verlag Chemie, Weinheim, D11, E5, D19. Dvorak, D.H., Hedin, R.S., M Edenborn, H., McIntire, P.E., 1992. Treatment of metal contaminated water using bacterial sulphate reduction: results from pilot scale reactors. Biotechnol. Bioeng. 40, 609–616. Fyson, A., Kalin, M., Smith, M.P., 1996. Microbially-mediated metal removal from acid mine drainage. In: Moo-Young, M., Anderson, W.A., Chakrabarty, A.M. (Eds.), Environmental Biotechnology: Principles and Practice. Kluwer, Dordrecht. Gruenewald, U., 1996. Bericht zur Erarbeitung von Grobaussagen zur Gewaessergu¨ueteentwicklung von Tagebauseen der Lausitz. LMBV Senftenberg and BTU, Cottbus, 64 pp. Kalin, M., 1993. Treatment of acidic seepage using microbiology. Final report, MEND Proj.3.11.1. CANMET, File: 015SQ.23440-2-9217. Kalin, M., Fyson, A., Smith, M.P., 1997. Microbial acid reduction in sediments — concepts and application. International Conference on the Abatement of Acidic Rock Drainage, Vancouver, 31 May–6 June 1997. Klapper, H., Schultze, M., 1995. Geogenically acidified mining lakes — living conditions and possibilities of restoration. Int. Rev. Ges. Hydrobiol. 80, 639 – 653. Lee, R.E., 1989. Phycology, 2nd ed. Cambridge University Press, Cambridge.
A. Fyson et al. / Ecological Engineering 10 (1998) 229–245
245
Leedale, G.F., 1967. Euglenoid Flagellates. Prentice-Hall, Eaglewood Cliffs, NJ. Luckner, L., Eichhorn, D., 1996. Flutungskonzept fuer die Tagebaurestloecher. Wasserwirtsch. Wassertech. 4, 18–28. Mills, A.L., Bell, P.E., Herlihy, A.T., 1989. Microbes, sediments and acidified water: The importance of biological buffering. In: Rao, S.S. (Ed.), Acid Stress and Aquatic Microbial Interactions. CRC Press, Boca Raton, pp. 1–19. Nixdorf, B., Wollmann, K., 1997. Ecological potential for planktonic developments and food web interactions in extremely acidic mining lakes. In: Geller, W., Klapper, H., Schulze, M. (Eds.), Proceedings of the Workshop Abatement of Geogenic Acidification in Mining Lakes, Magdeburg, 4–6 September 1995. Springer-Verlag, Berlin, in press. Nixdorf, B., Lessmann, D., Gruenewald, U., Uhlmann, W., 1997a. Limnology of extremely acidic mining lakes in Lusatia (eastern Germany) and their fate between acidity and eutrophication. Mine. Proceedings of 4th International Conference on the Abatement of Acidic Rock Drainage, Vancouver, 31 May–6 June 1997, pp. 1745– 1760. Nixdorf, B., Mischke, U., Lessmann, D., 1997b. Chrysophytes and Chlamydomonads: pioneers of colonization in extremely acidic mining lakes (pH B3) in Lusatia (Germany). Hydrobiologia (in press). Olaveson, M.M., Stokes, P.M., 1989. Responses of the acidophilic alga Euglena mutabilis (Euglenophyceae) to carbon enrichment at pH 3. J. Phycol. 25, 529 – 539. Psenner, R., Pucsko, R., Sagar, M., 1984. Die Fraktioierung organischer und anorganischer Phosphorverbindung von Sedimenten-Versuch einer Definition oekologisch wichtiger Fraktionen. Arch. Hydrobiol. 70 (Suppl.), 111–155. Rowley, M.V., Warkentin, D.D., Sicotte, V., 1997. Site demonstration of the biosulphide process at the former Britannia Mine. Proceedings of 4th International Conference on the Abatement of Acidic Rock Drainage, Vancouver, 31 May– 6 June 1997, pp. 1531 – 1548. Scherz, H., F. Senser, 1994. Die Zusammensetzung der Lebensmittel: Na¨hrwert-Tabellen, 5th ed., CRC Press, Boca Raton. Steinberg, C.E.W., Adrian, R., Behrendt, H., Behrendt, H., Hupfer, M., Kasprzak, P., Meinelt, T., Obermann, A., Brueggemann, R., 1998. Fundamentals of Applied Limnology (in German). In: Lecher and Luhr (Eds.), Handbook of Water Management. Parey-Verlag, in press. Stumm, W. and J.J. Morgan, 1996. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, 3rd edn. Wiley, New York, 1022 pp. Vile, M.A., Weider, R.K., 1993. Alkalinity generation by Fe3 + reduction in wetlands constructed for acid mine drainage treatment. Water Air Soil Pollut. 69, 425 – 441.
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