Studying External Load in Water Reservoirs on the Example of Slawskie Lake

Adam Malecki, DEng e-mail: a.malecki@iis.uz.zgora pl	Jacek Bojarski, D e-mail: j.bojarski@wmie.uz.zgora.pl
Uniwersytet Zielonogórski, Zakład Teorii Prawdopodobieństwa i Procesów Stochastycznych

Abstract: The structure of the catchment basin of Lake Slawskie, in particular the amount of nutrients, as released therefrom and, consequently, the quality of the lake’s Walters, is adversely influence by inappropriate agricultural practice and municipal economy, especially concerning sewage treatment. The basic source of nitrogen is agricultural area contamination, whereas the source of phosphorus are point pollutant discharges from unsewered areas. Water examination included measuring physical parameters (pH, temperature), oxygen conditions (oxidization, dissolved oxygen), content of organic substances (BZT₅, ChZT_Cr), as well as concentration of biogens (nitrogen and phosphorus compounds), as well as sulphate and chloride ions. The results, as obtained, pertaining to the quality of the waters, were subjected to appropriate, adequate statistical analysis. The article presents the results of total nitrogen and overall phosphorus. Variability of the studied elements within the tributary water, and existence of a linear trend, if any, or lack thereof concerning variation of the factor in question, has been studied by means of regression analysis. For the purpose of trend analysis within five-year measurement series and change forecasting, an advanced statistical tool has been used, as applied for time series analysis. It is SARIMAX, a modification of the non-stationary SARIMA model, taking into account descriptive variable lag within the autoregressive model.

Introduction

Protection of lakes and water reservoirs is, in fact, a matter of limitation of external loads of biogenous substances drained from the catchment basin. Implementation of difficult and costly protective measures must always be preceded by calculation of external loads, especially phosphorus and nitrogen. It allows, on the one hand, to estimate the hazards for the lake and define a necessary degree of biogen reduction, and on the other – it enables designing appropriate protective methods.

Calculation of loads of biogenous compounds, as loading a given lake, may be effected by means of:

- direct examination

- estimation

Direct examination implies execution of a great number of hydrological measurements and physicochemical analyses of waters of all surface affluents and sewage drained into the lake. Being work-consuming and long-lasting, such research is most frequently replaced by application of estimation methods, by means of which to determine external load in water reservoirs. These imply estimation of the amount of nitrogen and phosphorus of sewage origin (point and diffuse sources), farming-related (area sources), draining from roads, as used within the basin (linear sources) and biogens introduced onto the lake surface from the atmosphere (atmospheric sources). This requires adequate recognition of the basing management methods and the number of inhabitants populating the catchment in question which is largely problematic with vast basins. Moreover, it is obvious that biogens which are drained in the vicinity of the basin edges are oftentimes much more likely to reach the lake than of the ones led out many kilometres away. Estimating methods often do not take it into account at all. Bearing the above in mind, external load in Lake Slawskie has hereby been presented, as calculated with the direct measurement method and estimation.

Object of the Study

The object of the study comprises the catchment basin of Lake Slawskie (817.3 ha, max. Depth 12.1 m, mean depth 5.3 m, capacity 43 x 10⁶ m³), located in the south-eastern outskirts of the Lubuski Region. The lake has the basin (Graph 1) of the area of 206 km² (Fig. 1). Out of the lake’s six surface affluents, the decisively dominating one is Czernica, introducing its waters into the eastern part of the lake and draining an area of 60.5 km². The river’s catchment area, like the other ones’, is of agricultural and forest character; there are four populated localities within the area with 275 farms. The Czernica catchment is inhabited and managed by 1,840 people. The population of the basin areas is 57 people per 1 km².

Methodology of the Study

The direct examination was conducted between November 1999 and December 2003. Within the period, measurement of flow density was taken, at monthly intervals, at estuaries of the six surface affluents, alongside with physicochemical examination of the waters. Calculations allowed for the direct catchment of Lake Slawskie.

For the purpose of estimation, average land management of the Commune of Slawa was assumed (Table 1, 2), as well as unit drain coefficient values, according to Giercuszkiewicz-Bajtlik (1990). Simultaneously, a limnological study of the waters of Lake Slawskie was being done. In order to estimate the critical phosphorus and nitrogen load of the lake, the Vollenweider model was applied (1968, 1976), accounting for hydraulic load and water retention time in the reservoir.

Results and Discussion

The results of the five-year study of the waters of Lake Slawskie demonstrate a progressing process of its eutrofisation (Table 3). The list of biogen sources reaching Lake Slawskie shows that accumulation of phosphorus in the lake was, within the period included in the study, 50,278 kg/year (average value), whereas the average nitrogen load amounted to ca. 182,367 kg. This is, among others, evidenced by the rising concentration of total phosphorus and nitrogen (Fig. 2 and 3).

The lake’s degradation vulnerability, as calculated by Kudelska et al. (1994) (Table 4) demonstrate average dependence on external influences. The data included in Table 4 shows that Lake Slawskie is to be classified as Category IV with respect to susceptibility to degradation, i.e. being highly exposed to catchment influence, thus creating a landscape system which is characterised by a progressing process of water eutrophication. Vollenweider’s equation (1976) allows to estimate to what extent the present state of the lake concords with its load and to what degree this should be limited in order to achieve a satisfactory condition thereof.

Table 5 compares the permissible and hazardous values of phosphorus and nitrogen in the waters of Lake Slawskie according to Vollenweider’s criterion (1976). As one may see in the table, the actual phosphorus load in Lake Slawskie, as measured year-by-year within the study, exceeds the hazardous level a few times, whereas the nitrogen load exceeds, except for the year 2001, the permissible values, comprising – on average – 58% of the hazardous load. This is an extremely important problem with respect to the strategy of lake water quality management. The quantity of phosphorus load to be eliminated from the lake inflow comprises a difference between the external load of the lake and the permissible level. This estimation constitutes also a foundation for all reclamation measures based on elimination or inactivation of phosphorus. Lakes of more favourable natural characteristics (Category I) stand a better chance for restoration than ones which are vulnerable to external influences (Category III). Janczak (2002) studied degradation susceptibility of Lake Slawskie in the years 1991-1997. The results are similar: 1991 – 1.5 points/Category II; 1992 – 2.2 points/Category II; 1993 – 2.2 points/Category II;1994 – 2.8 points/Category III; 1995 – 2.9 points/Category III; 1996 – 3.1 points/Category III; 1997 – 3.1 points/Category III.

One should state that the lake is at very high risk of biogen delivery from external sources, as compared to its natural resistance. The estimation for the years 1999-2003 enables to forecast that improvement of the lake’s water quality is possible. It is practicable to apply a number of protective measures, however the first step to be taken is to arrange for the water and sewage management in the catchment area.

The value of balanced concentration in Lake Slawskie, corresponding to its load in the years 1999-2003, was 0.052 mg P/m²/year, whereas the measured value amounted to 0.,50 mg P/m²/year. This shows that the condition of the lake is far from equilibrium, taken the existing external load, which, as per Vollenweider’s equation, calls for reduction of its mean concentration. This would require, apart from providing for full drainage of the direct catchment basin, that the amount of drained post-sewage water be largely reduced, with buildings located within sparsely-populated areas being equipped with individual treatment facilities. In addition, it is necessary to lower agriculture-related load contributions, as well as abide by the principles of good practice in farming. Implementation of the abovementioned activities could reduce the lake’s biogen load by 31,731 kg P and 80,917 kg N, in aggregate, provided that inflowing waters were of the first class of purity, which – for the time being – would not call for any technical interference within the lake’s sheet of water. Contribution of individual sources before and after application of accessible measures is illustrated in Table 6.

Conclusions

The basic problem of the catchment basin of Lake Slawskie is insufficient equipment of the infrastructure with facilities for collection, drainage and treatment of sewage. The waters of Lake Slawskie are characterised by a very high level of trophy. This is a result of the progressing eutrophication, i.e. constant high load with polluted affluent waters.

The article defines main sources of hazards for purity of Lake Slawskie’ waters, alongside with necessary data comprising a basis for development of guidelines concerning protection of the waters of the basin and the lake against further degradation within the municipal, industrial and agricultural sectors which, if implemented, will contribute to reduction of biogenous substance discharges.

Total nitrogen in the tributaries shows slight variation, with essential seasonality of up to three years, which may indicate both increased intensity of inflowing contaminants and spontaneous production of this impurity. As far as outflows are concerned, total nitrogen is characterised by lack of autoregression and significant linear correlation with the content of total nitrogen in the lake 11 and 12 months earlier, as well as a lower level than the one detected in the lake. This would suggest that the lake acts like a battery, which is also evidenced by negative coefficient values at Ac11 and Ac12.

In case of phosphates, the situation is analogous. Levels of this contaminant is similar in both out- and inflowing waters, whereas the lake water level is significantly lower.

In case of total phosphorus, the lake acts like a battery: 1999,2000,
and in the years 2001,2002 – it is also a biogen generator. In general, one may observe frequent exeedance of the mean values of the examined pollutants in the lake, as compared to the content of these components in outflowing waters, wherein the tendency is rising.

In the light of the conducted research, the results, as acquired, allow for the following conclusions:

1. Average retention loss is 221 mm per year. Lake Slawskie has its whole area situated within an aquifer. The water-bearing layer is a balance-loss compensator. Having assumed that the waters of Lake Slawskie are replenished with underground waters in the amount of ca. 10% - the result is restoration of water balance.

2. The results indicate clearly that origin of nitrogen and phosphorus loads, as drained into the surface water from individual partial catchments (with the Czernica basin being the major source of biogens flowing into Lake Slawskie), is spatially diverse. Within the period under study, this made average loads of 0.89 mg P/dm³ and 3,.29 mg N/dm³, respectively. The content of biogens flowing in with the Czernica waters comprises 76.8% of nitrogen and 84.7 % of phosphorus, as drained into the lake. The river delivers a mean yearly amount of ca. 21.5 tonnes of phosphorus and 36.4 tonnes of nitrogen. The Jeziorna river catchment loads Lake Slawskie the least, bringing in 0.19 mg P/dm³ and 1.1 mg N/dm³. This supports the assumption that afforested areas protect surface water against biogen drain to a larger degree. Farmed land dispose of a surplus of biogens (ca. 106.5 kg N/ha/year and ca. 18.2 kg P/ha/year), which, with intense rain- and snowfall, infiltrates into underground and surface waters.

3. The obtained results comprise evidence of gradual degradation of Lake Slawskie caused by excessive inflow of biogens from partial catchments, which also indicates a trend for the coming five and ten years in case hitherto method of management of the catchment should prevail.

4. The water of Lake Slawskie is, with respect to its physicochemical properties, classified as “impaired” (undergrade). Improvement of the quality of the lake’s waters may only be achieved by increasing purity of all watercourses which feed it, as these deliver a majority of pollutants. Hence, one may argue that elimination of sources of contaminants may improve the existing quality of the waters, as well as maintain a higher standard thereof in the future.

5. Adequate water and sewage management by the Town of Slawa, as well as the numerous holiday resorts and homes, is a determinant for the future of the lake’s waters. Moreover, it is necessary to promptly develop a Lake Slawskie protection program, including a detailed analysis of the reasons of degradation, alongside with a set of propositions as to what counteracting measures should be undertaken. The first stage, however, should focus on protective activities.

6. The incoming phosphorus load is, according to Vollenweider’s criterion, higher (0.50g P/m²) than the permissible level (0.052 g P/m²), exceeding the hazardous one as well (0.104 g P/m²). The low N to P ratio testifies to the presence of municipal sewage in the lake’s waters, which explains rapid growth of algae and cyanobacteria n summer. Internal supply, as defined by a ratio of total phosphorus in the surface layer within summer, at the peak of stagnation, to its concentration in spring, at the beginning of the circulation period, is 1.15, showing that biogenous substances are released from bottom residues during summer, which may be distributed within the whole of the lake’s capacity and used in the photic layer by plant complexes in the same way as compounds which are drained into the lake from its catchments. This estimation comprises basis for all reclamation measures based on elimination or inactivation of phosphorus.

7. Analysing the impact of area pollution upon the quality of surface waters, one should examine every catchment, starting at its source area, in order to determine, based on differences concerning the chemical composition of water between the profiles of the watercourse under study, the degree of change triggered by the influence of contaminants. Only having significantly reduced biogen inflow from the catchment, one can consider practicability of application, in selected reservoirs, of a coagulant precipitating excessive contents of phosphorus, and resulting in inactivation of the element within residues.

In the author’s opinion, in order to achieve the abovementioned goals it is necessary to implement the following activities:

- the lake is a functional part of the catchment landscape system, hence preservation of quality of its ecosystems and purity of waters is only possibly through protection of the whole hydrological system, including the land area,

- arranging for the water and sewage management in such a way that the inflowing rivers should be of the first class of purity,

- collection and treatment of all sewage from the neighbouring localities and B&Bs within the direct catchment will protect the lake against pollution as well as result in averting the degradation processes,

- implementation of principles of integrated farming may significantly contribute to reduction of the process of biogen release from the catchment area into the surface waters,

- water usage should be confined within small areas, so that large amounts of contaminants are not carried out of the place where these are produced,

- boggy complexes and the lake litoral vegetation is a productive contributor to reducing the lake’s eutrophication load; such habitats should be preserved or even restored,

- large load of the lake’s waters with biogens undermines technical reclamation, unless the amount of contaminants drained from the catchment is significantly reduced – then supporting activities within the lake’s sheet of water will be possible,

- it is suggested that research work be continued concerning the broadest possible scope of protection of the waters of Lake Slawskie, this work comprising a practicable foundation for such undertakings.

The research, as conducted, shows straightforwardly that there is a need to improve the natural qualities of the Lake Slawskie catchment basin and adopt forms of development and management of the area to the economic, cultural and aesthetic virtues of this place. One should pay particular attention to remedying the effects of contaminants and degradation, as well as the origin thereof. While protecting the environment, just like in case of health protection, we call for preventive measures, especially counteracting harmful practices, to be the most effective way of dealing with the problem.

Bibliography

Giercuszkiewicz-Bajtlik M., 1990: Prognozowanie zmian jakosci wod stojacych. Instytut Ochrony Srodowiska, Warszawa [Warsaw], pp.1-69.

Kudelska D., Cydzik D., Soszka H., 1994: Wytyczne monitoringu podstawowego jezior. Biblioteka Monitoringu Srodowiska. Warszawa.

Malecki A., 2008: Oddzialywanie zlewni czastkowych Jeziora Slawskiego na bilans wody i biogenow. Uniwersytet Zielonogorski [University of Zielona Gora].

Vollenweider R. A., 1968: “Scientific fundamentals of the eutrophication of lakes and flowing waters, with particular reference to phosphorus and nitrogen as factors in eutrophication”. OECD Technical Report DAS/CSI/68.27, pp.159, Paris.

Vollenweider R. A., 1976: Advances in defining loading levels for phosphorus in lake eutrophi-cation. Mem. Ist. Ital. Idrobiol., No 33, pp.53-63.

Observation posts Slawa - localities

rainfall- catchment boundaries

watermark- partial catchment boundaries

overflow 1- control-measurement points on the lake

ground waters (piezometers)

ground waters (wells)

Fig. 1. Control-measurement and observation posts within the Lake Slawskie catchment

Table 1. Land application structure within the Lake Slawskie catchment, as set against the Commune of Slawa [acc. to the commune’s own data]

Item	Application	Area
		Commune of Slawa		Catchment
		ha	%	ha	% communal	% catchment
1.	Total area	32,678	100	20,600	63.0	100
2.	Farmland	13,487	41.3	8,181	68.7	39.7
	- arable land	10,987	33.6	5,806	52.8	28.2
	- grassland	2,500	7.7	1,374	54.9	6.7
3.	Forests	15,927	48.7	10,170	63.8	49.3
4,	Urbanised area	432	1.3	255	61.3	1.3
5.	Waters	1,031	3.2	1,003	97.3	4.9
6.	Others	1,801	5.5	991	55.0	4.8
7.	Protected landscape area	11,100	34	11,100	100	53.9

Table 2. Management of the Lake Slawskie partial catchments

Catchment name	Area Tereny								Lakes		Other
	Acc. to IRS*	Author’s own measurement	Forests		Buildings		Farmland
	[km^2]		[km^2]	%	[km^2]	%	[km^2]	%	[km^2]	%	[km^2]	%
Direct catchment	41.10	36.17	15.65	43.3	0.77	2.1	9.00	24.9	8.17	22.6	2.58	7.1
Czernica	60.50	60.53	22.72	21.0	0.33	0.5	36.20	76.3	-	-	1.28	2.1
Radzynska Struga	16.50	16.47	4.37	26.5	0.30	1.8	10.35	62.8	0.10	0.60	1.35	8.2
Cienica	65.00	65.51	47.02	71.8	0.37	0.6	14.31	21.8	1.66	2.50	2.15	3.3
Debogora	20.50	19.51	7.89	40.4	0.53	2.7	9.92	50.8	-	-	1.17	6.0
Jeziorna	4.20	4.31	1.98	25.1	0.15	3.5	0.88	20.4	0.10	2.3	1.20	27.8
Myszkowski Row	-	3.50	2.07	59.1	0.10	2.9	1.15	32.9	-	-	0.18	5.1
Overall	207.80	206.0	101.7	49.3	2.55	1.90	81.81	39.9	10.03	4.9	9.91	4.8

Table 3. Mean yearly biogen loads incoming and outgoing from Lake Slawskie (kg/year) within the period under study 1999-2003

Components	Symbol	kg /year
		P	N
Phosphorus and nitrogen load drained into the lake with sewage originating from point sources	(Iś)	none such
Phosphorus and nitrogen load brought in by inhabitants and tourists within unsewered area of the direct catchment basin	(Il)	8,568.4	53,552,7
Phosphorus and nitrogen load from spatial sources in the direct catchment basin	(Ip)	756.5	16,594,1
Phosphorus and nitrogen load brought in by bathing people	(Ik)	8.5	170,0
Phosphorus and nitrogen load resulting from utilisation of roads within the catchment	(It)	1.8	3,6
Phosphorus and nitrogen load brought in by rain- and snowfall	(Ia)	1,684.0	9,971,0
Phosphorus and nitrogen load drained into the reservoir with affluent waters	(Id)	25,432.4	47,486,5
I = I ś + Il + Ip + Ik+ It + Ia + Id		36,451,6	127,778.0
Within the lake’s depth		21,500,0	76,110.0
Within bottom residue and biocenosis		50,277,6	182,367.4
Outflow from the Lake		7,674,0	21,520.5

Table 4. Estimation of degradation vulnerability of Lake Slawskie in the period 1999-2003

Coefficient	Coefficient value	Category of vulnerability			Category	Point Score
		I	II	III
Mean depth [m]	5.2	≥10	≥5	≥3	II	2
V, Lake [m^3] L, Lake [m]	1.73	≥4.0	≥2.0	≥0.8	III	3
% of water stratification	0	≥35	≥20	≥10	IV	4
P, active bottom [m^2] V, epilimnion	0.19	≥0.10	≥0.15	≥0.30	III	3
% of water exchange per year	50	≤30	≤200	≤1000	II	2
Schindler’s coefficient P, Catchment [m^2]+ P, Lake V, Lake	4.9	≤2	≤10	≤50	II	2
Method of management of direct catchment [%]	Arable land - 24.0 Forests - 36.0 Lake - 20.6 Others -19.4	≥60 of forests	<60 of forests <60 of arable land	≥60 of arable land	I	1
Total degradation vulnerability category [Cat. I: 0.8 points, Cat. II: 0.9-1.6 points, Cat. III: 1.7-2.4 points, Cat. IV > 2.4 points]					IV	2,43

Table 5. Actual, permissible and hazardous yearly phosphorus and nitrogen loads in Lake Slawskie

Year	Phosphorus values [mg P/m^2/year]			Nitrogen values [mg N/m^2/year]			Hazard category
	1	2	3	1	2	3	P	N
1999	0.34	0.052	0.104	1.95	1.5	3.0	III	I
2000	0.32	0.051	0.102	1.78	1.5	3.0	III	I
2001	0.74	0.052	0.104	1.44	1.5	3.0	III	I
2002	0.82	0.049	0.098	2.13	1.5	3.0	III	I
2003	0.29	0.059	0.118	1.63	1.5	3.0	III	I
Mean	0.50	0.052	0.104	1.79	1.5	3.0	III	I

Explanation: 1 - actual, 2 - permissible, 3 - hazardous

Table 6. Contribution of individual sources before and after application of accessible measures

Source of contaminants	Present				Following application of measures, no interference within the lake’s sheet of water
	P kg/year	%	N kg/year	%	P kg/year	%	N kg/year	%
Inhabitants and tourists	8,568.4	23.51	53,552.7	41.91	0	-	0	-
Spatial sources	756.5	2.08	16,594.1	13.00	0	-	0	-
Bathing people	8.5	0.02	170.0	0.13	0	-	0	-
Utilisation of roads	1.8	0.00	3.6	0.00	0	-	0	-
Rain- and snowfall	1,684.0	4.62	9,971.0	7.80	1,684.0	36.34	9,971.0	21.28
Affluent waters	25,432.4	69.77	47,486.5	37.16	2,950.0	63.66	36,890.0	78.72
Total	36,451.6	100	127,777.9	100	4,634	100	46,861	100

Lake outlet Affluents Load accumulation= bottom residues +biocenosis (50,277.6)

- water depth (21,500.0)

Fig. 2. Mean yearly phosphorus balance in Lake Slawskie in the years 1999-2003 (kg/year)

Explanation:

- Total phosphorus load brought in by inhabitants and tourists within unsewered area of the direct catchment basin (kg/year);

- Total phosphorus load from spatial sources in the direct catchment basin (kg/year);

- Total phosphorus load brought in by bathing people (kg/year);

- Total phosphorus load resulting from utilisation of roads within the catchment (kg/year);

- Total phosphorus load brought in by rain- and snowfall on the lake surface (kg/year);

- Total phosphorus load drained into the reservoir with affluent waters (kg/year);

Affluents and outlet: L₁- Radzynska Struga, L₂ – Cienica, L₃ – Row Myszkowski, L₄ – Jeziorna, 0 – Czernica, P₁ – Debogora, 0₁- outlet (Obrzyca)

Lake outlet Affluents Load accumulation= bottom residues +biocenosis (188,367.4)

- water depth (76,110.0)

Fig. 3. Mean yearly nitrogen balance in Lake Slawskie in the years 1999-2003 (kg/year)

Explanation:

- Total nitrogen load brought in by inhabitants and tourists within unsewered area of the direct catchment basin (kg/year);

- Total nitrogen load from spatial sources in the direct catchment basin (kg/year);

- Total nitrogen load brought in by bathing people (kg/year);

- Total nitrogen load resulting from utilisation of roads within the catchment (kg/year);

- Total nitrogen load brought in by rain- and snowfall on the lake surface (kg/year);

- Total nitrogen load drained into the reservoir with affluent waters (kg/year);

Affluents and outlet: L₁- Radzynska Struga, L₂ - Cienica, L₃ - Row Myszkowski, L₄ - Jeziorna, 0 - Czernica, P₁ - Debogora, 0₁- outlet (Obrzyca)