IMPACT OF THE WATERSHED DEVELOPMENT ON THE CHEMICAL PROPERTIES OF WATER IN SMALL RESERVOIRS OF THE OLSZTYN CITY

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

ABSTRACT

The studies included forty-five small reservoirs located in the administrative borders of the Olsztyn city. The reservoirs were grouped according to the watershed development. Because the direct surroundings of the reservoirs were quite diverse, four main groups of the reservoirs were distinguished, and further divided into sub-groups. The main groups were: “municipal reservoirs” (built-up areas, waste land, neighbourhood of allotments, parks), “forest reservoirs” (mid-forest, field and forest, forest and meadow), “meadow reservoirs” (field and meadow, mid-meadow), and “other reservoirs” (mid-field, farm development). Water samples for the physico-chemical analyses were taken two times i.e., in May and October 2003. The chemical composition of water in the reservoirs depends a great deal on the watershed type. Water in the reservoirs sited in the urban and built-up watersheds was characterised by high electrolytic conductivity (218–1,182 µS cm^-1) and high concentrations of chloride ions (to 193 mg dm^-3). Water in the “forest reservoirs” was characteristic of high colour (to 320 mg Pt dm^-3) and contained much allochthonous organic matter (to 64 mg O₂ dm^-3). Water in the reservoirs surrounded by coniferous trees was slightly alkaline or close to neutral whereas in those surrounded by broadleaf trees – alkaline. In the mid-meadow reservoirs low primary production was determined (chlorophyll a below 35 mg m^-3, dry weight of seston from 0.4 to 11 mg dm^-3). In the reservoirs adjoining farms, quality of the water was very low, as displayed mainly by the very high chlorophyll a content (to 717 mg m^-3) and the super-saturation of water with oxygen (to 227 %).

Key words: small reservoirs, watershed, nutrients, trophic condition.

INTRODUCTION

The Olsztyn urban agglomeration comprises an example of the intertwining natural and strongly urban landscapes. Districts with single-family houses, districts with blocks of flats, industrial development, shopping malls, are next to lakes, forests and parks. Far from the main streets intensively cultivated allotments can be found.

Unquestionably, the small reservoirs, so-called “water eyes”, add to the city’s charm. They revive the municipal landscape, enhance its valour and make space for rest and recreation. They play also other important roles in the natural environment, such like: hydrologic (water storage), accumulative, and economic (especially those located in the peripheries).

Due to small surface area, usually lower than 1 ha, and small depth [4, 20], the small reservoirs, compared to other elements of the nature, undergo more dynamic changes and are more exposed to external factors and their effects [11]. The distinctive feature of the small reservoirs is the tendency to dry out periodically. The periodical reservoirs, filled with water only in spring, comprise a high percentage [9].

Small reservoirs are very vulnerable to degradation, usually accelerated by new development in a watershed [24]. Improper and thoughtless activity of man results in deliberate clearance of the reservoirs which seems detrimental from the protection viewpoint of both nature and landscape [22].

The recent research of the small reservoirs allows better recognition and appreciation of the role they play in the man’s surroundings.

The goal of this study was to examine in which way the localization and watershed’s development affects the hydro-chemical properties of these valuable but impermanent environmental elements.

MATERIAL AND METHOD

The study included forty-five small reservoirs located in the administrative borders of the Olsztyn city (Fig.1). The location was determined on the 1:10 000 map and in the field. Depths of the reservoirs were measured with an echo depth finder and a manual depth probe. Data regarding the surface areas were obtained from the Olsztyn City Hall.

The reservoirs located in the Kortowo district were marked with the letter “K” (K1 – K7), in Jaroty with “J” (J1 – J4), in Centrum with “C” (C1 – C5), in Gutkowo with “G” (G1 – G5), in Redykajny with “R” (R1 – R7)), in the eastern city outskirts with “W” (W1 – W9), and these surrounding Lake Skanda with “S” (S1 – S8).

Water samples for the physico-chemical analyses were taken twice, in May and October 2003, from the surface layer, with the Ruttner apparatus or directly to bottles. Electrolytic conductivity and pH were measured with the Multi Line F/SET 3 (WTW) meter. Iron, manganese and ammonium were determined with the help of the MERCK SQ 118 spectrophotometer, and chlorophyll a with the Strickland-Parsons method. Other analyses were done in accordance with the Standard Methods [27].

Statistical processing of the data was done with the help of Pearson’s coefficient r.

RESULTS AND DISCUSSION

Small reservoirs of the Olsztyn city were first put on record, systematised and surveyed in 1994 by Gawrońska et al. [6]. That survey included sixty-four land hollows. Again, they were surveyed in 2000 (Gawrońska, Lossow, unpublished).

The field study of the summer 2003 revealed that only forty-eight of all previously described hollows were filled with water which confirmed their ongoing disappearance from the city’s landscape. In an urban agglomeration such phenomenon results among other things from the new land acquisition for districts and roads construction. In rural areas (such like the outskirts of Olsztyn, especially the Redykajny and the Gutkowo districts), disappearance of the small reservoirs is caused by hydrologic conditions control, partial drainage and urban development. Equally important are the processes of plants and lakes succession [25]. Intensity of the small reservoirs’ transformations is also affected by the climatic changes. Many reservoirs, recognized as permanent, dry out, and restoration of the water resources occurs in the subsequent more humic years [23, 5]. Such thesis was confirmed by the autumnal survey of 2003. The number of small reservoirs filled with water was by seven lower than in the summer.

According to the way of the watershed development, the small reservoirs in Olsztyn were divided into four main groups (tab.1). Obviously, this division should be treated as rough because the land actually surrounding the reservoirs was usually developed in a very diverse way.

The largest group is the so-called “municipal reservoirs” (twenty-seven in total). The group includes land hollows located in the urban areas (photo 1) but also the typically municipal park reservoirs and those placed near the intensively cultivated allotments.

The small reservoirs (0.16–0.19 ha) located in the Centrum district (C2, C3) were used primarily as a municipal water reserve. At present, their watersheds are typical housing districts. In the Centrum district two large reservoirs can be found, located in parks (C4 – 0.61 ha and C5 – 1.9 ha).

The land hollows filled with water and located near Lake Skanda (S1, S4, and S6), and the deepest (5 m) of all examined K1 in the Kortowo district, are the unexploited open casts, once used to get clay for brickyards.

The small reservoirs located on the eastern outskirts of the city were formed naturally and occupy areas strongly transformed by man, mainly waste land [7]. A dense group of the reservoirs (W3 – W6) can be found on the moto-cross area.

One of the most typical features of the small reservoirs is the particularly high diversity of the chemical composition of water [4, 12, 28]. Content of the chemical substances is the resultant of the enrichment processes (horizontal input of water and matter from the watershed, vertical moisture exchange with the atmosphere) and of the internal conversions (plant decay, nutrients’ exchange across the water-sediment interface, sedimentation, biological sorption) in the relatively low water volume.

The reservoirs located in the urban and built-up watersheds, and amid the waste land, were characterised by the electrolytic conductivity in the range 218 – 665 µ cm^-1. Only in S4 the value was extremely high – 1,182 µ cm^-1 in the summer and 984 µ cm^-1 in the autumn (fig. 2). High values of this parameter (665 and 543 µ cm^-1, respectively) were determined also in W1. Both reservoirs are situated near the very busy access roads to Olsztyn. Water in these reservoirs contained also a lot of chloride ions; in S4 as much as 193 mg dm^-3.

Generally, the elevated values of electrolytic conductivity were typical for the reservoirs placed in the vicinity of the allotments. High electrolytic conductivity indicates pollution with inorganic substances and qualifies reservoirs as heavily eutrophied [16, 18]. In the “municipal reservoirs”, the relation between the concentration of chloride ions and the electrolytic conductivity was determined as statistically significant (r = 0.661, p ≤ 0.01).

The content of calcium and magnesium – the elements determining total hardness in the water – varied in a broad range (fig. 3). As a rule, calcium values were higher in the reservoirs neighbouring the allotments. According to Marszelewski [19], the increased content of calcium results from the spring water run-off in watersheds of that kind. High concentrations of this element were found also in the reservoirs located near the busy roads (W1, S4) and in the industrial area (S1, W2).

Colour of water in this group of reservoirs oscillated between 20 and 180 mg Pt dm^-3 in the summer and between 10 and 320 mg Pt dm^-3 in the autumn. Normally, the highest values were typical for the reservoirs localised on the eastern outskirts of the city (fig. 4). The highest values were observed in the autumn in W8 and J4. In W8 it was accompanied by the high content of allochthonous organic matter (73.6 mg O₂ dm^-3) whereas in J4 the elevated colour may have been caused by the considerable content of manganese i.e., 2.02 mg dm^-3, and iron i.e., 2.0 mg dm^-3.

In all reservoirs of this group, the high colour was related to the content of manganese (r = 0.583, p ≤ 0.01).

In the “municipal reservoirs” pH was the least variable in the park reservoirs and in these located near the allotments (6.8 – 7.9 pH). The land hollows in the built-up areas and amid the waste land were characterised by quite considerable pH variations i.e., from 5.53 pH (K3) to 9.77 pH (S1). K3 is placed near the Municipal Power and Heating Plant (photo. 2). After the start of the heating season, water in K3 did not display any drastic increase of the mineral nitrogen therefore the lower pH may have been caused by the water’s enrichment in sulphur dioxide. Hessen & Tranvik [8] report that the majority of SO₂ and NOx emitted to the atmosphere and having the ability to lower drastically pH in the nearby water reservoirs, come from the industry.

The high pH in W4, W5, W6, and S1 (fig. 5) was caused by the intensified primary production, indicated additionally by the deficit of carbon dioxide.

Dojlido [3] reports that concentration of oxygen in the surface waters is usually lower in summer due to the lower solubility of oxygen at higher temperatures. Such tendency was observed in most of the surveyed reservoirs. The reversed phenomenon was noted in the W3 – W6 group, located on the moto-cross area. In the summer, they were visibly super saturated with oxygen (over 130 % O₂). As already mentioned, this phenomenon may have been caused by the intensive algal growth and intensified photosynthesis (high pH, depletion of carbon dioxide, high content of chlorophyll a, and high dry weight of seston (fig.6, 7)). Similar trends were observed in reservoirs K1, K4, K6, S1, and S6 (fig. 6,7).

The nutritive elements, nitrogen and phosphorus, occurred mainly in the organic form. Typically, the concentrations of organic phosphorus in the reservoirs located in the built-up areas and on the waste land were high in the autumn.

In W2, sited near a very large intensively cultivated vegetable and flower garden and near the housing development, the concentration of mineral phosphorus in the autumn was very high (1.420 mg P dm^-3) and possibly resulted from the dump of domestic wastewater. The high concentration of phosphates in the autumn was determined also in the park reservoir C4 (0.404 mg P dm^-3) and in the summer in J4, placed near the housing estate (0.888 mg P dm^-3).

The highest concentrations of nitrogen, both mineral and organic, occurred in the reservoirs sited near the allotments (fig. 9). The dominant mineral form was ammonium. The autumnal increase of the nitrates concentration (0.230 – 0.540 mg N-NO₃ dm^-3) indicates their relationship with the intensive agricultural production [14].

In the group of the reservoirs sited on the built-up land the lowest nitrogen concentrations were measured in the reservoirs of the Centrum district (fig. 9).

In the discussed group, K4, K5, and C1 (photo. 3) are the flow-through reservoirs, connecting with Lake Kortowskie. They play the accumulative role thus contributing to the lake’s protection. Lossow [17] and Koc & Nowicki [13] report that in a small reservoir even low water exchange can contribute to a significant reduction of the load of many components imported to the lake.

The “forest” group includes fourteen reservoirs (tab.1), located mainly on the city outskirts. Beside the typically mid-forest (G5, J1, J2, R1, K5, K6, and K7) (photo. 4), some reservoirs had mixed watersheds, including meadows (G4, S2, and S5) or cultivated land (R2 and R4). K5, classified above as “municipal”, is discussed with this group because of the proximity of the allotments.

Typical for this group is the high water colour. The extreme value (160-280 mg Pt dm^-3) was determined in R4 (cultivated fields and forests with the dominance of coniferous trees in the watershed). Attention should be paid to the mid-forest K5 (140 – 160 mg Pt dm^-3), surrounded by a mixed forest, and G4 situated in the forest and meadow watershed (160 mg Pt dm^-3). According to Allan [1], water in the forest area contains much organic substances, mainly humic, colouring the water light yellow to dark brown. The statistical significance between the water colour and the allochthonous matter was high (r = 0.655, p ≤ 0.01).

Organic matter, and humic and fulvic acids, and humins it contains [2], can contribute to pH reduction. pH is influenced also by the type of a tree stand in the watershed. Dominance of coniferous trees in the watershed (S7, S8, R2, and R4) results in the light acidic or close to neutral pH of the run-off waters whereas dominance of broadleaf trees in the watershed shifts the pH of the run-off towards alkaline (S5).

The humic compounds responsible for water colour considerably increase the permanganate value which was especially high in S7 (64.4 mg O₂ dm^-3) and in R4 (64 – 54 mg O₂ dm^-3). It may have indicated an occurrence of much allochthonous organic matter.

The range of the electrolytic conductivity values in this group of reservoirs was relatively wide (tab. 2). The extreme values were measured in the reservoirs near Lake Skanda (S2 and S5) and in the Kortowo district (K5). The reason may have been the import of inorganic pollutants [18]. In these reservoirs the statistical significance between the electrolytic conductivity and the content of chloride ions (r = 0.792, p ≤ 0.01), and the calcium ions (r= 0.813, p ≤ 0.01), was very high.

Total hardness in this group of reservoirs varied considerably. The lowest values (0.7 – 1.0 mval dm^-3) were measured in reservoirs S7 and S8, G5 and J2 which are fed mainly by water from the atmospheric deposition [4]. The higher total hardness was typical for the reservoirs adjoining the allotments.

Alike in the “municipal” group, nutritive elements in this group occurred mainly in the organic form (tab. 2).

Nitrates comprised the dominant form of mineral nitrogen, with the highest concentrations determined in R4 and K5. The content of nitrates was maximal in the reservoirs surrounded by the cultivated land.

In the Gutkowo district – the western fringes of Olsztyn – three mid-meadow reservoirs (G1, G2, and G3) can be found. Meadows and cultivated land surround also S3 (near Lake Skanda) and R5 (the Redykajny district). All these are characterised by the low primary production (chlorophyll a below 35 mg m^-3, dry weight of seston 0.4 – 11.07 mg dm^-3).

In G3, the values of electrolytic conductivity were the highest of all reservoirs surrounded only by meadows; simultaneously, the concentration of chloride ions in G3 was the lowest.

The chemical composition of water in R5 was on one hand affected by its boggy character and on the other by the pasture-dominated watershed (controlled grazing of cows). In R5, the colour of water was very high (240 mg Pt dm^-3) and was caused by the content of iron ions (1.74 – 2.32 mg dm^-3) and high permanganate value (57.6 – 68 mg O₂ dm^-3). According to Kowal [15], iron contained in water easily forms chelates with humic substances.

The concentration of nutritive elements was also exceptionally high in the water of R5 (tab. 3). The concentrations of total phosphorus ranged from 1.47 to 3.05 mg P dm^-3. Total nitrogen was measured in the range from 3.86 to 9.21 mg dm^-3.

W7 is located on the eastern outskirts of Olsztyn. It is quite deep (2.2 m) and totally surrounded by arable fields (photo. 5). Quality of the water in W7 was rather good; only the manganese content was somewhat high (1.27 mg dm^-3).

The next group is the “farm” reservoirs (R3 and R6) (photo. 6), accumulating nutritive substances from the farms, nearby fields and pastures. The main distinctive feature for this group is the trophic condition, displayed by the content of chlorophyll a (91.2 – 717.25 mg m^-3) and the high saturation with oxygen (74.1–227.7 %). The low trophic state confirms also the high permanganate value (51.2 – 73.6 mg O₂ dm^-3), an indicator of much accumulated organic matter resistant to degradation.

The hitherto studies allow concluding that small reservoirs disappear from the municipal landscape. They are the most exposed to the man-induced acceleration of the eutrophication process, both in the aquatic environment and in comparison to the wetland ecosystems [10, 21]. In cities, many reservoirs have been demolished, filled with soil, or converted into construction land. In the rural areas large number was dewatered and allocated for cultivation. Reduction of the number of these valuable reservoirs deprives the land of natural valour and decreases the biological diversity.

Small reservoirs should be treated as an important element necessary to upkeep the balance in the cultural landscape [26]. Municipal areas should intertwine with highly natural areas. “Water-eyes” can successfully play such role, making specific islands surrounded by the homogenous development.

CONCLUSIONS

The study confirms the thesis that watershed development has an effect on the physico-chemical parameters of water in the small reservoirs. The best quality water was found in the reservoirs surrounded directly by waste land.

Water colour is related to the content of iron, manganese, and resistant to degradation organic matter of the allochthonous origin. The type of trees growing in the direct vicinity also has its effect.

In the so-called “municipal” and “forest” reservoirs the content of chloride ions was correlated with the electrolytic conductivity. In the other groups such relationship was not observed.

The lowest productivity was typical of the “meadow reservoirs”, displayed by the low concentrations of chlorophyll a and low dry weight of seston.

Nutritive elements occurred mainly in the organic forms. The highest concentrations were determined in the reservoirs under the strongest pressure of the agriculture: cultivation of grains, vegetables and flowers. Confirmed was the influence of agriculture on the surface waters pollution with nitrates.

ACKNOWLEDGEMENTS

Authors make acknowledgements to Piotr Grochowski and the following students, participating in the field studies and laboratory examinations: Beata Dobrowolska, Przemysław Duszkiewicz, Kinga Guzowska, Tomasz Paluch, Anna Pawlikowska, Marta Wilczyska.

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Accepted for print: 02.01.2007

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