CHEMISM OF BOTTOM SEDIMENTS FROM A LAKE TREATED WITH VARIOUS RESTORATION TECHNIQUES

Renata Tandyrak
Faculty of Environmental Protection and Fishery, University of Warmia and Mazury, Olsztyn, Poland

ABSTRACT

The small (7 ha) and deep (23 m) Lake Starodworskie in Olsztyn, with restricted water dynamics, is an object that was already used a few times for restoration experiments.

In 1967–68, 1972–74 and 1986–87 the lake was artificially aerated with thermal destratification, in 1988–89 it was aerated without the thermal stratification destruction, in 1994 and 1995 phosphorus was inactivated in the lake, using aluminium sulfate. This paper investigates into the effects of restoration, especially of introduction of many tonnes of chemicals, on the bottom sediments and inhibition of P internal loading.

Sampling station 1, located in the shallowest part of the lake, was characteristic of good oxygen conditions, highest content of silica and lowest content of other measured parameters. The highest concentrations of organic matter, aluminium, iron, calcium and phosphorus were not found in the deepest spot but on sampling station 2. Assumingly, it was caused by the artificial aeration and the aerator’s placement over the deepest spot, and by the deposits replacement to the shallower bottom areas.

Significant correlations were determined between organic matter content and calcium (r = 0.813) and phosphorus (r = 0.997). Additionally determined were the correlations between Al-PO₄ and aluminium (r = 0.575), organic matter (r = 0.923) and total P (r = 0.696). There is also a significant correlation between the content of Al-PO₄ in the bottom sediments and phosphorus concentration in the interstitial waters (r = 0.798).

In the sediment cores, sampled from the area subjected to the inactivation experiment, the layer of aluminium hydroxide formed during the treatment was still observable. The sediments of approximately 4 cm, deposited on top, were created during the 7 years after the restoration. Since the layer of hydroxide makes up a physical barrier inhibiting phosphorus release, the process can occur only from fresh sediments.

Key words: lake, restoration, bottom sediments, organic matter, phosphorus.

INTRODUCTION

Bottom sediments comprise an important abiotic element of aquatic ecosystems, the living environment to benthic organisms, participating in the sediments transformations. Sediments are also the source and the trap to nutritional substances thus important in nutrients cycling. Bottom sediments influence chemical properties of an aquatic environment and to a considerable degree they create the living conditions.

In highly eutrophic lakes, exhibiting the major problem of massive phytoplankton blooms, high increase of organic matter causes disturbance in the oxygen conditions. As a result internal loading is initiated, i.e. nutrients are released to the water, mainly phosphates deposited in the bottom sediments [34]. Among the key factors controlling this process are: oxygen content, redox potential, temperature, reaction and concentration gradient of an element at the water-sediment interface [27, 28].

Only the surface layer of sediments participates in substance exchange between sediments and water, however the thickness of this layer has not been univocally defined. Hosomi & Sudo [15] report that the layer is 3 cm thick, Kajak [19] and Hupfer & Uhlman [16] share the opinion that it’s 10–15 cm, whereas according to Søndergaard et al. [30] it is 20 cm.

Taking into consideration that bottom sediments constitute inexhaustible source of nutrients [19], there is a necessity to limit the source. All restoration methods practised in lakes are aimed to do so, i.e.: removal of the sediments from the ecosystem, improvement of oxygen conditions and increase of redox potential at the water-sediment interface or permanent nutrients inhibition in the bottom sediments [24].

MATERIALS AND METHODS

The object of the study was the small (7 ha) and deep (H max = 23 m) Lake Starodworskie in Olsztyn (photo 1). The lake is of little economical use but favourably located, i.e. near the research facilities and thus it was used for practising various restoration techniques. In 1967–68, 1972–74 and 1986-87 the lake was artificially aerated with thermal destratification while in 1988-89 it was aerated keeping the thermal strata undisturbed [25].

In 1994 and 1995 the experiment of phosphorus inactivation was conducted, using aluminium sulfate. The treatment was run twice and in total 15 tonnes of the coagulant was added (average of 18.6 g Al·m^-2 of bottom surface). Only the selected middle section of the lake, demarcated with the 10-m depth curve (fig.1), was subjected to the treatment (3.6 ha), [32]. The added coagulant created on the bottom of the lake a gray, gel-like layer of aluminium hydroxide which was clearly noticeable during the filed investigations (photo 2 a,b). The layer was covered with 4-cm thick sediments created during the next 7 years.

Such intervention in the aquatic ecosystem, especially the introduction of many tonnes of chemicals, should be displayed in the chemical properties of the bottom sediments.

Samples of the sediments were taken in August 2002, with the use of a Limnos apparatus (95 mm diameter), on three sampling stations distributed at the depths of 5 m (st. 1), 15 m (st. 2) and 22 m (st. 3). Sampling stations 2 & 3 were located in the area under the introduced chemical’s activity. On st. 1 the sediment core of 14 cm was taken, on st. 2 & 3 the cores had 18 cm.

The near-bottom water was obtained by decantation of a 10-cm water layer over the bottom sediments.

The sampled cores of sediments were divided into 2-cm sections. After thorough mixing and centrifugation (20 min, 3000 rpm) water over the sediments was decanted and regarded as interstitial. Analyses of the interstitial and the near-bottom waters were done in accordance with the methods commonly recognized in the hydrochemical analyses [31].

Centrifuged sediments were dried in room temperature and grinded in a porcelain mortar. Chemical composition of the sediments was determined after Januszkiewicz [18]. Special attention was paid to phosphorus determination and its fractions with calcium (Ca-PO₄), aluminium (Al-PO₄) and iron (Fe-PO₄), as provided in the methods of Golachowska [11, 12].

RESULTS AND DISCUSSION

The alarmingly poor quality of lake waters imposes the necessity to seek effective methods that would return the lakes to the state before the strong man-induced pollution or at least slow down the eutrophication processes.

In heavily eutrophic lakes where the main source of nutrients are bottom sediments, even radical reduction of external loading usually brings no expected results [3, 26] and restoration of such lake becomes a necessity. Such activities were performed in Lake Starodworskie in Olsztyn.

Redox potential is a very important factor determining ion exchange between water and sediments. Since redox potential is regulated mainly by DO concentration in water, the most common method of restoration is artificial aeration.

At present, the method applied more and more frequently is nutrients inactivation. The method is used to reduce phosphorus content in a lake through its removal from the water column and inhibition of its release from the bottom sediments. To achieve that, metal salts are used, mainly aluminium, iron and calcium [23, 24] however recently the new-generation coagulants, such like PIX, PAX, PAC, have been applied more frequently [9].

Bottom sediments in Lake Starodworskie are classified as siliceous [4]. Silica dominated on all sampling stations and in all analysed cores’ sections (fig. 2); it was found in amounts typical for eutrophic lakes, i.e., 90.55–93.84 % d.w. on the shallowest station, 57.17–74.56 % d.w on st. 2.

Regarding phosphorus inhibition and release from the bottom sediments, their chemical composition is very important, especially the structure of sorptive complex. Bottom sediments in Lake Starodworskie were characteristic of poor sorptive properties. The evidence was the low content of iron, calcium and aluminium (fig. 2) and small, compared to other eutrophic lakes, concentration of total P (fig. 3). Modification of the sediments, through enrichment in aluminium, was first of all aimed to improve the sorptive capacity.

Aluminium content in the lake’s sediments in the individual sections of the bottom was highly variable. The lowest aluminium content was determined on the shallowest station and it was increasing as the sediments depth increased. The maximum amounts were determined in the sediment layers on st. 2 with the maximum concentration in the 4–6 cm section (4.61 % d.w.). In section 6-8 cm the quantity of this element was also higher (2.84 % d.w.) than in the deeper sections (fig.2). Aluminium content in the sediments sampled on st. 3, compared to st. 2, was lower which might result from the way the coagulant was added to the lake and (despite efforts) unevenly distributed over the bottom. Significant correlation (r = 0.575, p ≤ 0.05) was found between aluminium occurrence in the sediments and Al-PO₄.

The per cent iron content in the bottom sediments was similar on all sampling stations and only slightly varied in the individual cores’ sections. The increase on st. 2, in the 6–8 cm section, can be explained by the fact that when oxygen concentration in the near-bottom waters and the bottom waters is low, iron reduction occurs, the Fe(III)-P complex is dissolved and both phosphorus and iron return to the water column [21]. As deoxygenation proceeds, iron can be precipitated in the form of insoluble iron (II) sulfide, deposited in the bottom sediments. Iron content increased also in the section 12–16 cm, sampled on the same station. However, it is difficult to determine explicitly when those deposits were created; it might have resulted from iron precipitation in water during the artificial aeration [2]. No correlation was determined between iron content in the sediments and phosphorus occurring in the fraction with iron (Fe-PO₄).

In comparison to other lakes, calcium quantity in the bottom sediments of Lake Starodworskie should be regarded as small. Like in the case of the previously discussed elements, this component was found in the lowest amounts on st. 1 (0.81–0.61 % d.w.) and the highest quantity was determined on st. 2 (4.2–1.4 % d.w.). All sampling stations exhibited calcium depression as the depth of the sediments’ cores increased. In section 8–12 cm on st. 2 and 10–12 cm on st. 3 an exception to this rule was noted – the concentration increased. The major source of calcium to lake deposits is the drainage basin [19]; another is accumulation caused by physico-chemical and biological processes. No significant correlation was determined between calcium and Ca–PO₄.

Settling flocs of aluminium hydroxide, created during the inactivation, stimulated mechanical precipitation of organic matter and increase of its content in the bottom sediments. It has been evidenced in the previous studies [33] and observed by Gawrońska & Brzozowska [8] on Lake Długie in Olsztyn.

Organic matter deposited in the bottom sediments requires very good oxygen conditions for mineralization. The lowest content of organic matter was determined in the sediment layers on st. 1 (fig. 2). Percentage of this component in the sediment core varied between 4.6 and 2.4 % d.w. and decreased in every subsequent section. This station was characteristic of very good oxygen conditions [22] which obviously stimulated intensive degradation.

Two other sampling stations, located in the area subjected to the inactivation experiment, exhibited higher contents of the discussed component. However, the maximum amounts were not measured in the deepest spot, like in most lakes [17], but on st. 2. Assumingly, it was caused by transfer of the deposits to the shallower sections of the bottom. Gawrońska [7] formulated a hypothesis that this might be the result of the previously applied restoration, i.e. artificial aeration, and placement of a pipe aerator over the deepest spot in the lake. A very significant correlation was determined between the content of organic matter and calcium on all sampling stations (r = 0.813, p ≤ 0.01) and between organic matter and iron on st. 1 (r = 0.857, p ≤ 0.01).

In sorption process organic matter can play more or less important role, depending on what organic anions build the complexes with iron (or aluminium) and phosphorus [14]. According to Kenzer [21] the key factor determining the amount of settling phosphorus is the trophic status of a lake whereas Gawrońska [7] argues that phosphorus quantity is determined by the efficiency of its bonding to sediments.

In polluted lakes phosphorus occurs in the form of numerous mineral and organic bonds, in dissolved, colloidal and suspended form and in bottom sediments. In the examined lake, the correlation between organic matter and phosphorus was very significant on st. 1 & 2 (r = 0.977, r = 0.797, p ≤ 0.01, respectively).

Usually, thickness of sediments and content of phosphorus in the sediments increase as the lake’s depth increases, and phosphorus concentration is higher in the surface layers of the sediments [20]. Such phenomenon was observed also in the bottom deposits of the examined lake, especially on st. 1.

The highest content of phosphorus, both mineral and organic, was measured on st. 2 (fig. 3). Since in the section 4-6 cm, as already mentioned, the content of aluminium was the highest and the field observations have confirmed occurrence of the aluminium hydroxide layer in this section (photo. 2 b), the deposits directly underneath contained phosphorus precipitated due to the applied treatment. Despite the conditions favouring phosphorus release, the layer of hydroxide constituted a barrier, effectively inhibiting that process.

Durability of phosphorus deposition in bottom sediments depends on the type of chemical bonds which it makes in the sediments. These bonds are of various permanence and hence of various biological availability. Kenzer reports [21] that analysis of quantitative relationships between fractions differing by bio-availability can provide valuable information regarding durability of phosphorus accumulation and potential for its release to the water column.

The form of the highest bio-availability is the labile phosphorus, loosely bound to sediments [1, 13]. Participation of this form was low and equalled on average 5 % of the sum of all determined phosphorus fractions. The highest concentration was detected on the deepest sampling station, i.e. 0.041 mg P/g d.w. (6.8 % participation in mineral P). Comparison with the results obtained on the same lake in 1985–87 by Gawrońska [7], who determined 17-% participation of labile fraction in mineral P, shows that the content of this form of phosphorus decreased considerably. Similar results were obtained during the restoration of Lake Głęboczek [10]. Very significant correlation was found between this form of phosphorus and organic matter (r = 0.902, p ≤ 0.01).

Highly mobile is also phosphorus bound to iron (fig. 4). On the shallowest station, with good oxygen conditions most of the year, the vertical distribution of this component did not display much variability. The values oscillated in a small range from 0.021 % d.w. in the layers to 4 cm depth to 0.038 % in the deepest section of the core. Other stations were characteristic of higher concentrations, decreasing in the subsequent, deeper sections of the cores. On st. 2, in the section 12–14 cm, where iron increase was noted, the tendency was disturbed.

In regard to the previously mentioned fraction, phosphorus bound to aluminium (fig. 4) was present in higher amounts (22.4–29.2 % in the sum of all marked fractions). However, the content was lower than the results obtained by Tandyrak [33] in 1996. Decrease in the content of this phosphorus bond can result from the fact that with time aluminium salts, heavier than primarily formed in the gel-like layer on the surface, migrate inside the sediments, diminishing their efficiency as a physical barrier preventing phosphorus release [5]. The evidence is the distribution of aluminium and its bonds with phosphorus in the sediments’ cores sampled on the stations located in the experiment area.

In the bottom sediments correlations were determined between Al-PO₄ and organic matter (r = 0.923, p ≤ 0.001), Al-PO₄ and total P (r = 0.696, p ≤ 0.01) and organic matter and total P (r = 0.977, p ≤ 0.01). They indicate occurrence of aluminium-organic bonds in the sediments.

The lowest mobility is typical for calcium-bond phosphorus. Availability of this fraction is to a considerable extent related to water and sediment reaction. Increase of pH is the stimulus for creation of insoluble bonds in water, non-vulnerable to changes in the redox potential [6, 21]. This fraction constituted 40.6–58 % of total P contained in the bottom sediments.

Bottom sediments and water constantly interact, and the exchange of mineral and organic components and gases is very complex [7]. Concentration of phosphorus in the interstitial waters (tab. 1) of Lake Starodworskie was high (1.15–4.52 mg·dm^-3), typical for eutrophic lakes [35]. Nonetheless, significant correlations between Fe–PO₄ and mineral P and total P in the interstitial waters were not found. On the other hand, there is a significant correlation between Al-PO₄ and total P in the interstitial waters (r = 0.798, p ≤ 0.01).

The basic mechanism responsible for migration of substances from interstitial waters to overlaying waters is the molecular diffusion, depending mostly on the concentration gradient at the water-sediment interface. Bottom sediments released to the interstitial waters mainly mineral phosphorus (50–64 % of total P).

In the near-bottom waters (tab. 1) concentration of mineral phosphorus equalled from 0.054 mg P∙dm^-3 on the shallowest station to 0.767 mg P∙dm³ at the deepest spot in the lake (st. 3). In the interstitial waters, on all stations, in section 0-2 cm, phosphorus concentration was higher (0.89-1.79 mg P∙dm^-3). Taking into consideration the anaerobic conditions in the near-bottom waters and the concentration gradient at the interface, there were conditions favourable for phosphorus release from the bottom sediments to the water. Similar phenomenon was observed in the deepest sections of the examined cores, especially on st. 2. In the area covered by the inactivation experiment, in the sampled sediments a gray layer of aluminium hydroxide was found, created during the treatment. The sediments, approx. 4 cm thick, laying above this layer, were created during the 7 years after the restoration. Since the layer of hydroxide comprises a physical barrier to phosphorus release, this process can only occur in the fresh layers of the bottom sediments.

CONCLUSION

The main source of phosphorus in bottom sediments is organic matter of allochthonous and autochthonous origin and chemical precipitation of phosphorus with calcium, iron and aluminium compounds. Content of this element in deeper deposits is lower, as in the transformation processes some portion is returned from the surface layer to water column.

There is a significant correlation between Al-PO₄ and total P in the interstitial waters. No significant correlation between Fe-PO₄ and phosphorus indicates that assumingly in this lake iron is unimportant in the process of phosphates release.

Artificial aeration nearly instantaneously improves the oxygen conditions in the deeper waters and thus inhibits internal loading with nitrogen and phosphorus.

The layer of aluminium hydroxide created in phosphorus inactivation, successfully inhibits phosphorus release from sediments and interstitial waters to lake waters.

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