ORGANIC POLLUTION OF LAKE DABIE WATERS IN 1997-2000

Arkadiusz Nędzarek, Jacek Kubiak, Agnieszka Tórz
Department of Hydrochemistry and Water Protection, Agricultural University of Szczecin, Poland

ABSTRACT

The hydrochemical survey of Lake Dabie – an integral part of the upper estuary of the Odra River – was carried out in the years 1997-2000. The study was aimed at demonstrating a relationship between anthropogenic pollution and environmental conditions. Water samples were collected at six stations in each season of the temperate climate zone. The analyses covered water temperature, concentrations of phenol, surface-active agents, and petroleum ether-extractable substances, gene-ral measures of organic matter load (BOD₅, COD_Mn, COD_Cr), chlorophyll a, and dissolved oxy-gen. Water temperature ranged between 3.8°C and 23.4°C (annual mean temperature 13.2°C). Dissolved oxygen content ranged between 2.2 and 16.3 mg O₂·dm^-3, with oxygen percent satur-ation ranging between 40.3 and 164.0%. The measures of organic matter concentrations were as follows: COD_Cr 10.0-49.2 mgO₂·dm^-3, BOD₅ 1.1-7.4 mgO₂·dm^-3, chlorophyll a 2.2-327.9 mg·m^-3. The concentrations of petroleum ether-extractable substances ranged from 3.0 mg·dm^-3 to 22.7 mg·dm^-3, phenol from 0.013 to 0.100 mg·dm^-3, and anionic surfactants from 0.018 to 0.147 mg LAS·dm^-3. Statistical analyses has revealed a weak influence of water flow on the values of the analysed parameters. The correlations between surfactants, ether extract and the flow were negative (respectively, r = -0.40 and r = -0.20). The phenol-flow correlation was positive (r = 0.30). Chemical oxygen demand was highly positively correlated with the substances extracted with petroleum ether (r = 0.60) and negatively correlated with anionic surfactants (r = -0.40). Biochemical oxygen demand was positively correlated with the content of chlorophyll a, particularly in lake Dabie (r = 0.70).

Key words: lake Dabie, organic matter, petroleum ether-extractable substances, phenol, surface-active agents.

INTRODUCTION

The flow of organic matter across a riverine ecosystem is a complex process, changing dynamically with the growing season, hydrological conditions, or the level of human impact [16]. Transboundary flow in the rivers plays a key role in both the sources of organic matter and the seasonal changes. According to the studies by Veyssy et al. [33], only 8% of the organic matter transported through the mouth of the Garonne river results from the local production, the rest comes with the surface run off (54%) and wastewater (38%). It is estimated that 25% of carbon that reach the world’s rivers is processed by the ecosystem, 25% is deposited in the sediments, while another 50% is transported to the oceans [17].

Natural organic substances that occur in the environment, unlike those produced by man, are usually easily broken down during a series of biochemical processes referred to as biodegradation [29]. Organic matter concentration in highly-polluted or eutrophic bodies of water represent a measure of the pollution level. Measuring the content of organic matter is important for the assessment of the biodegradation processes of (either allochthonous or autochthonous) organic matter and the resulting oxygen shortages, especially those occurring in the bottom layer of the water column [16, 26]. The purity of the surface layer of water is also affected by an inflow of pollutants that contain specific organic compounds posing a serious threat to the natural environment. Those compounds, which include surface-active substances, ether extract, and phenols, may negatively influence reproduction of the organisms or disturb any stage of their development [21, 32].

The Odra is one of the largest rivers of the Baltic drainage basin and exhibits a considerable bed retention (about 5 days) as well as low water turbidity (a lowland river). Along its entire course, the river is subjected to a strong anthropogenic impact that result in considerable pollution of the water starting as high as in the upper reaches of the river. This effect is particularly intense in the area of the Szczecin agglomeration, which produces municipal sewage and industrial wastewater, discharged untreated into the riverine waters complex that represents the upper estuary of the Odra. As little as some 7% of municipal sewage discharge undergo a highly-efficient treatment process [3, 4, 6, 7, 8, 23].

The estuarine part of the Odra is a complex hydrological system; the river splits into two arms interconnected by a number of channels, wetland islands and the adjacent flow-through lake Dabie. This makes the actual flow volume difficult to estimate, and the hydrometric cross-section at Gozdowice (645.3 km) provides a representative site for flow measurements [5]. The unique hydrographic system of the Odra estuary, its specific water dynamics, and man-modified hydrochemical regime, are all the factors that underlie both the static and the dynamic environmental conditions that provide a habitat rich in terms of biodiversity [6].

Recently published results of a long-term monitoring carried out by the State Inspectorate for Environmental Protection (PIOS) allow concluding that the Odra water quality is improving. The total length of the parts of the river considered as excessively polluted has decreased, which is especially evident if we analyse the compulsory indicators. Although not very rapid, the changes are perceptible and systematic. According to the report by the PIOS [23], the indicators of organic matter concentration (BOD₅ and COD_Cr) measured at Krajnik (690.0 km) do not authorise disqualification of the river’s water. The mean BOD₅ and COD_Cr values at this Odra cross-section were, respectively, 3.7 and 30.4 mg O₂·dm^-3 (for the period 1990-1996).

The aim of this study, carried out during 1997-2000, was (i) to determine the concentrations of organic matter (BOD₅, COD_Mn, COD_Cr) and particular organic compounds (anionic surfactants, ether extract, phenols) in the upper estuary of the Odra (the part of the river located within the administrative boundaries of Szczecin plus lake Dšbie) and (ii), basing on statistical analyses, to evaluate the relationships between the measured hydrochemical indicators and the hydrological and thermal regimes as well as the primary production.

MATERIAL AND METHODS

The hydrochemical survey was carried out from June 1997 until May 2000 according to field and laboratory methodology given in the Standard Methods [28] and included all the seasons characteristic for the temperate zone. Water samples were collected at six stations (Fig. 1) from the surface and the bottom layers of the water column, using the Ruttner water sampler. Site 1: the centre of lake Małe Dabie at 2.5 m depth. Site 2: the Eastern Odra, 400 m upstream from its outlet to lake Wielkie Dabie, in the middle of the river bed, depth 6 m. Site 3: Zatoka Zachodnia bay, 300 m away from Przesmyk Orli passage and 200 m from Plaża Mielecka beach, depth 2 m. Site 4: the centre of lake Wielkie Dabie, depth 3 m. Site 5: the Iński Nurt stream, 200 from its connection with the Western Odra, depth 7 m. Site 6: the Western Odra at 100 m upstream the Skolwin channel, mid river site, depth 12 m. The dates of water samplings are presented in Table 1.

Water physical-chemical indicators were determined using the following methods: Winkler test for dissolved oxygen (mg O₂·dm^-3); biochemical oxygen demand (BOD₅, mg O₂·dm^-3) through 5-day incubation of the samples in the dark at a constant temperature 20°C; chemical oxygen demand (mg O₂·dm^-3) using bichromate (COD_Cr) and permanganate (COD_Mn, oxidizing) methods; chlorophyll a was extracted with acetone, λ= 665 nm. Ether extract (mg·dm^-3) was assayed by weight. Anionic surface active agents (µg LAS·dm^-3) were assayed using methylene blue with chloroform extraction, λ = 652 nm. Phenols (µg·dm^-3) were measured using 4-aminoantipyrine with potassium ferricyanide, chloroform extraction, λ = 460 nm. For the colorimetric analyses, we used the LAMBDA 10 UV/VIS spectrophotometer (PerkinElmer).

The data resulting from the analyses were subjected to statistical analysis. We applied Pearson linear regression with coefficients of correlation (r_xy). The significance was tested at the level α_0.05.

RESULTS AND DISCUSSION

Over the entire period of the studies, i.e. during the years 1997-2000, water temperatures of the Odra parts and lake Dšbie ranged between approx. 4°C and nearly 23°C, with the annual average about 13°C (Table 2). No significant differences were found between the temperatures of the benthic vs. surface layer of the water column (Table 2), which is typical for polymictic lakes and turbid rivers [12, 34].

Seasonal variations in temperature, due to the volume and dynamics of the water, did not occur abruptly and were even delayed in relation to changes in air temperature. Moreover, slightly higher values of this parameter were found in the Eastern Odra compared with the Western Odra (Table 2), as observed by Tórz and Nędzarek [31] in the upper course of this branch, below the “Dolna Odra” Power Station cooling effluent discharge channel.

The waters of the studied estuary exhibited good oxygen conditions. Dissolved oxygen concentration ranged from about 3 to more than 16 mg O₂·dm^-3, with the mean 9 mg O₂·dm^-3 calculated for the entire period of the study (Table 2). The oxygen percent saturation ranged between about 30 and 120%. The survey has revealed a slight vertical oxygen zoning in the studied bodies of water, with the highest differences noted in the waters of the Iński Nurt stream (Site 5); the difference reached 3.5 mg O₂·dm^-3 (May 1998). Highly dynamic bodies of water (such as polymictic lakes or rivers) that show vertical oxygen gradient are considered as highly trophic, where oxygen is consumed excessively at the bottom layer by decomposition of deposited organic matter and the photosynthesis is inefficient to compensate for this oxygen loss [10, 15]. A high trophic level of the studied estuary has been also confirmed by a correlation, usually positive, which we found between dissolved oxygen and water temperature (Table 3). Only pristine waters exhibit a strict negative correlation between dissolved oxygen and temperature [12, 34].

The studies revealed higher oxygen deficiencies in the samples collected from both Odra branches compared with those from lake Dabie. This can be due to varied water organic matter concentrations at particular sampling stations. In general, the Western Odra exhibited higher organic matter content as a result of sewage discharged from the left-bank districts of Szczecin [2, 1, 19, 18].

The total organic matter load in the Odra estuary as measured with BOD₅, COD_Mn, and COD_Cr averaged, respectively, approx. 4, 11, and 25 mg O₂·dm^-3 over the entire period of survey and was the highest in the samples collected from the Western Odra (Table 4). The results show that the proportion of native organic matter was higher in lake Dabie compared with the riverine waters. The primary production of periphyton, macrophytes, and phytoplankton, whose intensity depends on light intensity, trophic level, temperature, and morphology of the body of water, represented an important source of organic matter. Shallow lakes allow higher levels of primary production compared with deeper lakes, and if their surface is in large part carpeted with vascular vegetation, such lakes are referred to as macrophyte-dominated with the macrophytes-phytoplankton interaction determining the production and eutrophication rate of the lake [9, 11, 16]. Chlorophyll a concentration is an indicator of primary production [10, 24, 25, 27]. The studied estuary has revealed high concentrations of chlorophyll a and a significant correlation has been found between BOD₅ and chlorophyll a (Tables 3 and 4). In the water of lake Małe Dabie, which is under the least influence of the Odra compared with other sites, and which is also the shallowest body of water, this correlation was very strong.

Unlike the lacustrine water, the Western Odra exhibited an increase in the mean BOD₅ during winter and spring, along with water rise observed during this period, as well as a drop in these concentrations with reduced flux during summer and autumn (Table 5). This variation is typical for the rivers in which organic matter concentrations mostly depend on the flow rate rather than on biological processes that undergo in the water [2]. In the Western Odra, with its medium level of correlation between BOD₅ and chlorophyll a, we have found a weak positive correlation between BOD₅ and the flow rate (with negative correlations recorded at the remaining stations) (Table 3). This implies that organic matter is fed to this Odra branch from its upper reaches (probably the area of Międzyodrze) as well as from the organic carbon washed out from the flood areas. Namely, interstitial waters within the shore zones of lakes and rivers are rich of organic matter, such as suspended particles of detritus and bacteria, and contribute to higher values of hydrochemical indicators measured in the shore zones, especially of large rivers [11, 14, 33]. The reversely proportional relation between the flow and BOD₅ observed at the remaining sites indicate that waters of the Eastern Odra have a diluting effect on the concentration of organic matter produced by autotrophic organisms in the lacustrine water, especially during the periods of increased flow. Similar associations with the flow rate in the Odra were found for COD_Mn and COD_Cr, as well as anionic surface active agents (Table 6). Our analysis of correlation with the flow has also demonstrated that COD_Mn and COD_Cr particularly strongly depended on the diluting effect of high water levels in the Western Odra (Table 6), which receives the bulk of municipal sewage from Szczecin. Organic matter concentrations of both Odra branches in the area of administrative boundaries of the city increased downstream as far as the river mouth, as a consequence of addition of pollution discharged from particular sewage outlets [5, 19, 30].

Specific organic compounds, such as anionic surface active agents, phenols, or petroleum ether-extractable substances are particularly noxious for the organisms inhabiting aquatic environments [21, 22, 32, 35]. Such compounds were found during the survey, however at low concentrations (Table 7), not threatening the biocenoses of the studied estuary. The measured concentrations of ether extract were higher than those found in the Vistula mouth, where Niemirycz et al. [20] recorded maximum concentration of ether extract of approx. 3 mg·dm^-3. A very high level of this parameter for the entire Odra estuary was measured only in September 1997 (maximum values in concentration ranges presented in Table 7), as a result of the catastrophic flood (maximum 44.6 mg·dm^-3). Seasonal vari-ations of the mean values exhibited a constant increasing trend for both petroleum ether-extractable substances and anionic surface active agents during the summer (Table 5). A similar seasonality for a range of toxic organic pollutants was found by Niemirycz et al. [21] at different sites located along the entire course of the Odra. Moreover, the water of the Western Odra exhibited higher levels of the discussed parameters compared with the remaining sites.

Statistical analyses have revealed that petroleum ether-extractable substances in the Western Odra were of local origin. Their concentrations were reversely proportional to water flow rate. At the remaining sites, this parameter was proportional to the flow (Table 6). The relation was proportional also between phenols and the flow at all the sampling sites, which allows concluding that these compounds are brought in with the inflowing water, unlike anionic surface active agents which were negatively correlated with the flow rate (Table 6).

CONCLUSIONS

Over the period 1997-2000, Lake Dabie and the adjacent riverine waters of the Western and Eastern Odra were characterised by similar hydrochemical conditions; oxygen concentrations and their changes, organic matter load, and the level of primary production implied underlying intense biological processes, which are characteristic for eutrophicated bodies of water. The waters of flow-through lake Dabie exhibited increased primary production in relation to the waters of both Odra arms. A diluting effect of the waters flowing into the lake from the Western Odra was found in relation to the organic matter produced by autotrophic organisms. We have observed the process of slow self-purification of the water in lake Dabie, which is characteristic for rivers and undergoes through sedimentation of suspended matter and aerobic biochemical reactions (good oxygen conditions across the horizontal and vertical profiles of the lake resulting from its polymictic character due to a lack of thermal stratification, exposure to wind, and the inflow from the Eastern Odra). We have found a negative effect of anthropogenic pollution discharged from the municipal agglomeration of Szczecin on the quality of the Odra estuary waters, especially in the Western Odra. The waters of the Western Odra were characterised by the highest organic matter concentrations, which resulted in considerable oxygen deficiencies throughout the water column, especially during the growing season.

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Accepted for print: 1.08.2007

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