Impacts of Municipal Wastewater Treatment Plants on Water Quality in the Berze River Basin

RURAL AND ENVIRONMENTAL ENGINEERING, LANDSCAPE ARCHITECTURE DOI:10.22616/rrd.23.2017.022

IMPACTS OF MUNICIPAL WASTEWATER TREATMENT PLANTS ON WATER QUALITY IN THE BERZE RIVER BASIN

Linda Dambeniece-Migliniece, Ainis Lagzdiņš Latvia University of Agriculture [email protected]

Abstract Eutrophication caused by water pollution with nutrients (nitrogen (N) and phosphorus (P)) is one of the main environmental problems nowadays. Poor water quality might be caused by many natural and anthropogenic factors. The most common anthropogenic factors are water pollution caused by intensive agriculture (e. g. intensive fertilizer use, high density of livestock) and discharges from municipal waste water treatment plants (WWTP). In this study, nutrient load from WWTP to the river Berze basin in time period from the year 2005 to the year 2015 was described. In total, 23 WWTPs of the study area were analysed using descriptive statistics to calculate average values of nutrient amount and concentrations discharged to river Berze basin. From 2005 to 2015 average N and P load from WWTP to the river Berze basin have been reduced, but concentration of N and P in treated wastewater (WW) have increased. The largest WWTP of the study area – Krigeri (WWTP of city Dobele) is subject to the regulations of treated WW quality set by Republic of Latvia Cabinet Regulation No. 34. The WWTP Krigeri meets the regulations, but overall situation in the study area suggests that strict control of smaller WWTP should be made. Key words: waste water treatment, point source pollution, nitrogen, phosphorus.

Introduction shouldn’t exceed 15 mg l-1 for N and 2 mg l-1 for P. Important environmental issue in the European Smaller WWTP (PE<10 000) is not controlled, but is Union is eutrophication of the Baltic Sea. The Baltic subject to HELCOM recommendations. Treated WW Marine Environment Protection Commission, also concentration of total nutrients for WWTP with PE known as Helsinki Commission (HELCOM) as its 300-2000 should not exceed N - 35 mg l-1; P – 2 mg aim has highlighted good environmental status of the l-1; WWTP with PE <300 N - 25 mg l-1; P – 5 mg l-1 Baltic Sea, this includes concentrations of nutrients (HELCOM, 2007). in the water close to the natural levels of a region The aim of this study was to determine municipal (HELCOM, 2007). In order to reach these goals water waste water treatment plant impact on water quality quality has to be improved. affected by nutrient load to the Berze River basin Water quality at a river basin scale can be affected during the time period of 2005-2015. by many natural and anthropogenic factors (Vega et al., This study has been done as a part of author’s 1998). These factors can be furthermore categorized as doctoral thesis. Data used in this study have been point source or diffuse pollution. Example of typical obtained and prepared for hydrological modelling diffuse pollution can be fertiliser use in agricultural with HYPE model (Lindström et al., 2010) as part of lands or forestry. These inputs are usually continuous, the MIRACLE project. with a little variability over time (Carpenter et al., 1998). Point sources are relatively easy to locate Materials and Methods and include the direct inputs from municipal sewage The Berze River basin was used as a study area systems and/or industrial discharges (Thyssen, 1999). (Figure 1). The Berze River is situated in the central Both diffuse and point sources cause significant water part of Latvia and is the tributary of the Svete River. pollution with nutrients (N and P) (Vega et al., 1998; The length of the Berze is 109 km with an elevation Pieterse, Bleuten, & Jørgensen, 2003; Withers et al., difference of 108 m per 109 km. The Berze River has a 2011). contributing drainage area of 872 km2. The river starts Municipal wastewater (WW) is a very composite around 120 m above the sea level in drained meadows mixture. It consists of a high number of substances. in the southern part of the Eastern-Courland highland. Mainly these components are domestic WW (sewage In the middle part of basin there is a hydro-power from households, municipal establishments, and small plant ‘Annenieki’ with a reservoir that can possibly businesses (<50 employees)), sewage from larger (>50 influence nutrient retention. employees) industrial and commercial companies and According to CORINE land cover data, the study hospitals (Pescod, 1992). In Latvia, the treated WW area consists of mostly agricultural areas (56.6%) quality is controlled and set to fit concentrations of and forest and semi-natural areas (384%), remaining nutrients (Ministru kabineta noteikumi Nr. 34, 2002). area: wetlands (1.2%), water bodies (1.1%), artificial For WWTP (waste water treatment plant) larger than surfaces (discontinuous urban fabric, industrial or 10 000 PE (population equivalent) concentration commercial units, dump sites, green urban areas, sport of total Nitrogen and Phosphorus in treated WW and leisure facilities) (28.0%).

RESEARCH FOR RURAL DEVELOPMENT 2017, VOLUME 1 153 IMPACTS OF MUNICIPAL WASTEWATER TREATMENT PLANTS ON WATER QUALITY Linda Dambeniece-Migliniece, Ainis Lagzdiņš IN THE BERZE RIVER BASIN

Figure 1. Location of the study area: WWTP in the Berze River basin. (Map shows only still operating WWTP and the location of sub-basins).

Table 1 List of wastewater treatment plants in the Berze River basin

No. WWTP title Sub-basin ID Population equivalent (year 2014) Reconstruction/opened 1 ‘BAP Zebrene’ 2 0* Opened in 2011 2 ‘Zebrene’ 3 20 2004 3 ‘Annenieki’ 3 76 2015 4 ‘Stūri’ 3 14 1988 5 ‘Blīdene’ 4 450 1985 6 ‘Leveste’ 7 119 2003 7 ‘Saule’ 7 37 2004 8 ‘Biksti’ 8 23 2010 9 ‘Jaunpils’ 8 7000 2002 10 ‘Jurģi’ 8 55 2003 11 ‘VLIS Jaunpils’ 8 1 1968 12 ‘Lielbērze’ 9 0* 1980 13 ‘Gardene’ 9 427 2014 14 ‘Aizstrautnieki’ 9 26 2010 15 ‘Krigeri’ 12 12187 2001 16 ‘Zelta Druva’ 12 - Closed in 2007 17 ‘Auri’ 13 24 1987 18 ‘Ziedugravas’ 13 27 2010 19 ‘Penkule’ 14 21 1989 20 ‘Krimunas’ 14 44 2013 21 ‘Ķirpēni’ 14 51 1982 22 ‘Strauti’ 14 38 2010 23 ‘Šķibes’ 15 - Closed in 2007 * Non-municipal WWTP.

154 RESEARCH FOR RURAL DEVELOPMENT 2017, VOLUME 1 IMPACTS OF MUNICIPAL WASTEWATER TREATMENT PLANTS ON WATER QUALITY IN THE BERZE RIVER BASIN Linda Dambeniece-Migliniece, Ainis Lagzdiņš

Water quality in the basin is affected by Results and Discussion agricultural diffuse pollution and variable point Nutrient load changes in time are shown in sources. The largest point sources of nutrient (nitrogen Table 2. Amount of nutrients loaded into river Berze and phosphorus) loading in the Berze River basin are basin varies, reasons of it should be examine considering wastewater treatment plants (WWTP). Overall 23 error made by missing values (data missing in the waste water treatment plants are located in the Berze reports provided by WWTP operators) in the data set. River basin area. The largest of WWPT serves the city Also, the date of making water sample is not specified, of Dobele (10 000 PE (population equivalent)). Other but seasonality strongly affects waste water quality WWTPs service small communities with a PE <2000. (Redeker, 2011; Vega et al., 1998). Another possible WWPT of Dobele draws up to half of all waste water reason of differences in amount of nutrients loaded discharged form WWTP in the Berze River basin. to river Berze basin might be reconstruction of In this study, the data obtained from 23 WWTP WWTPs (date of WWTP reconstruction can be seen is used (Table 1). Two of the WWTPs (‘Zelta Druva’ on Table 1). and ‘Šķibes’) was in operation until 2007. Two new Most of the Nitrogen from point sources to river WWTPs have been in operation (‘BAP Zebrene’) Berze are loaded to sub-basin ID 12 (76.252 t). The since 2011, due to negligible discharges these two lowest amount of nitrogen has been loaded from sub- WWTPs have been reported as one in this study. basin ID 2 (0.055 t – sub-basin ID2 is not displayed WWTP ‘BAP Zebrene’ serves for a landfill ‘Zebrene’ on the fig. 2) and 15, but WWTPs in these sub-basins and is not municipal WWTP. The largest WWTP have been closed (sub-basin ID 15) or opened recently (‘Krigeri NAI’ – WWTP of city of Dobele) is located (sub-basin ID 2, also not municipal WWTP) causing in the sub-basin ID 12. total amounts being the lowest. The lowest amount of Data on wastewater discharges and nutrient phosphorus from still existing municipal WWTP have concentrations was obtained from the publicly been loaded to river Berze sub-basins ID 4 (4.165 t) available data source of ‘2 – Ūdens’ maintained and 7 (4.981 t) (Fig. 2). by the State Limited Liability Company ‘Latvian Most of the phosphorus from point sources to river Environment, Geology and Meteorology Centre.’ In Berze are loaded to sub-basins ID 8 (10.131 t) and this study data reported from 2005 to 2015 was used. 12 (8.770 t). These sub-basins are mostly agricultural In most of the cases waste water sampling are done areas (Corine land cover data). Livestock research once a year and values of nutrients are calculated to station is located on sub-basin ID 8 most likely tons per year. Microsoft Excel used to describe data causing large amounts of phosphorus leaking to river from 23 WWTPs of river Berze basin. Descriptive Berze basin. It is known that the livestock causes P statistics used to describe the data – average values accumulation in soils (Bouwman et al., 2013). These of nutrient amount and concentrations and standard results suggest that large amounts of livestock affect errors for the nutrient concentrations have been waste water as well most likely sourcing from farm calculated using Microsoft Excel. sewage systems.

Table 2 Total yearly nutrient (Nitrogen and Phosphorus) load from WWTPs to River Berze basin in time from 2005 to 2015

Year Total Nitrogen (tons) Total Phosphorus (tons) 2005 21.578 5.246 2006 20.924 3.841 2007 17.421 3.986 2008 16.786 3.133 2009 9.375 1.104 2010 16.008 0.968 2011 7.274 1.829 2012 10.434 1.864 2013 8.894 1.613 2014 12.584 2.193 2015 9.994 1.533

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2015 9.994 1.533

Most of the Nitrogen from point sources to river Berze are loaded to sub-basin ID 12 (76.252 t). The lowest amount of nitrogen has been loaded from sub-basin ID 2 (0.055IMPACTS t – sub-basinOF MUNICIPAL ID2 is notWASTEWATER displayed on the fig. 2) and 15, but WWTPs in these sub-basins have been closedTREATMENT (sub-basin IDPLANTS 15) or ONopened WATER recently QUALITY (sub-basin Linda[TypeID 2, Dambeniece-Migliniece, also here] not municipal WWTP) Ainis Lagzdiņš causing total amounts being the lowest. TheIN lowest THE amountBERZE ofRIVER phosphorus BASIN from still existing municipal WWTP have been loaded to river Berze sub-basins ID 4 (4.165 t) and 7 (4.981 t) (Fig. 2).

2015 9.994 1.533 90 80Most of the Nitrogen from point sources to river Berze are loaded to sub-basin ID 12 (76.252 t). The lowest amount of nitrogen has been loaded from sub-basin ID 2 (0.055 t – sub-basin ID2 is not displayed on the 70 fig. 2) and 15, but WWTPs in these sub-basins have been closed (sub-basin ID 15) or opened recently (sub-basin ID 2, also60 not municipal WWTP) causing total amounts being the lowest. The lowest amount of phosphorus from still existing municipal WWTP have been loaded to river Berze sub-basins ID 4 (4.165 t) and 7 (4.981 t) (Fig. 2). 50

Tons 9040 8030 7020 6010 500 3 4 7 8 9 12 13 14 15 Tons 40 Sub-basin ID 30 Figure 2. Total amount of Nitrogen loaded from WWTPs to river Berze basin from 2005 to 2015. 20 Figure 2. Total amount of Nitrogen loaded from WWTPs to river Berze basin from 2005 to 2015. 10 The lowest amount of phosphorus has been might be reason of Nitrogen load peak, if the WWTP loaded from0Most sub-basin of the phosphorus ID 2 (0,000 from tons point – sub-basin sources to riverdid not Berze function are loaded properly to sub-basinsat the time. ID 8 (10.131 t) and 12 ID2(8. 770is not t) . displayedThese3 sub-basins on the4 fig. are mostly3) and7 agricultural15 (0.3268 t), areas 9 (CorineThe maximum 12land cover permitted13 data). Livestockconcentration14 research15 of N station and is located on sub-basin ID 8 most likely causing large amounts of phosphorus leaking to river Berze basin. It is but WWTPs in these sub-basins have been closed P in treated WW are set by local legislation (Ministru known that the livestock causes P accumulation inSub soils-basin (Bouwman ID et al., 2013). These results suggest that large (sub-basin ID 15) or opened recently (sub-basin ID kabineta noteikumi Nr.34, 2002) only applies to amounts of livestock affect waste water as well most likely sourcing from farm sewage systems. 2, also not municipal WWTP). The lowest amount of WWTP larger than 10 000 PE. For these WWTP The lowest amount of phosphorus has been loaded from sub-basin ID 2 (0,000 tons – sub-basin ID2 is phosphorus from still existing municipal WWTP have maximum N concentration is 15 mg l-1 and maximum not displayedFig onure the 2 .fig. Total 3) amountand 15 (0.326of Nitrogen t), but loaded WWTPs from in WWTPsthese sub-basins to river haveBerze-1 been basin closed from (sub-basin2005 to 2015. ID 15) beenor openedloaded recentlyto river Berze(sub-basin sub-basins ID 2, IDalso 4 not(0.850 municipal t) P concentrationWWTP). The is lowest 2 mg lamount. Fig. 5of shows phosphorus reduction from of still andexisting 7 (0.998 municipalMost t) (Fig. of the 3).WWTP phosphorus have beenfrom loadedpoint sources to river to Berze rivernutrient sub-basinsBerze concentration are loaded ID 4 (0.850 to in sub-basins treated t) and WW7 ID(0.998 8from (10. t)131 (Fig.WWTP t) 3and). 12 (8.As770 seen t). Theseon Fig. sub-basins 4 average nutrientare mostly (N agriculturaland P) loads areas ´Kriger’ (Corine over land time. cover Thedata ). WWTP Livestock Krigeri research meets station N is fromlocated single on WWTP sub-basin to River ID 8 Berzemost basinlikely havecausing reduced large amountsconcentration of phosphorus regulations, leaking but peaksto river in Berze2010 basin.(15,90 It is 12 -1 inkn time,own butthat notthe livestock significant. causes Nitrogen P accumulation spikes inin soils year mg (Bouwman l ) when et theal .,WWTP2013). Thesewas under results reconstruction. suggest that large 2010amounts to average of livestock 1.21 tons affect of Nitrogenwaste water (15.458 as well tons most of likelyStarting sourcing with from 2006 farm WWTP sewage Krigeri systems. meets regulation Nitrogen10 loadedThe lowest to sub-basin amount ID of 12).phosphorus In year 2010has been the loadedfor Phosphorus from sub-basin concentration ID 2 (0,000 in treated tons –WW. sub-basin ID2 is largestnot displayed WWTP on(Krigeri the fig. – 3)WWTP and 15 of (0.326 city of t), Dobele) but WWTPs inTotal these average sub-basins nutrient have beenconcentration closed (sub-basin in treated ID 15) wentor opened under8 reconstructionrecently (sub-basin (Table ID 1). 2, Reconstructionalso not municipal WW WWTP). loaded The to riverlowest Berze amount basin of havephosphorus not reduced from still existing municipal WWTP have been loaded to river Berze sub-basins ID 4 (0.850 t) and 7 (0.998 t) (Fig. 3).

6 Tons 12 4 10 2 8 0 6 2 3 4 7 8 9 12 13 14 15 Tons Sub-basin ID 4

0 2 3 4 7 8 9 12 13 14 15 Sub-basin ID

Figure 3 Total amount of Phosphorus loaded from WWTPs to river Berze basin from 2005 to 2015.

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Figure 3 Total amount of Phosphorus loaded from WWTPs to river Berze basin from 2005 to 2015.

As seen on Fig. 4 average nutrient (N and P) loads from single WWTP to River Berze basin have reduced in time, but not significant. Nitrogen spikes in year 2010 to average 1.21 tons of Nitrogen (15.458 tons of Nitrogen loaded to sub-basin ID 12). In year 2010 the largest WWTP (Krigeri – WWTP of city of Dobele) went under [Typereconstruction here] (Table 1). Reconstruction might be reason of Nitrogen load peak, if the WWTP did not function properly at the time.

Figure 3 Total amount of Phosphorus loaded from WWTPs to river Berze basin from 2005 to 2015. 1.4 As seen on Fig. 4 average nutrient (N and P) loads from single WWTP to River Berze basin have reduced 1.2 inIMPACTS time, but notOF significant.MUNICIPAL Nitrogen WASTEWATER spikes in year 2010 to average 1.21 tons of Nitrogen (15.458 tons of Nitrogen TREATMENT PLANTS ON WATER QUALITY loaded to 1sub-basin.0 ID 12). In year 2010 the largest WWTP (Krigeri – WWTP of city of Dobele) went under reconstructionIN THE BERZE (Table RIVER 1). BASINReconstruction might be reason of NitrogenLinda load Dambeniece-Migliniece,peak, if the WWTP did Ainis not Lagzdiņš function properly at0.8 the time.

tons 0.6 1.4 0.4 1.2 0.2 1.0 0.0 0.8 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 tons 0.6 year

0.4 Average of Nitrogen Average of Phosphorus

0.2 Figure 4. Average N and P load from WWTPs to the Berze River basin changes in time (2005 – 2015). 0.0 The2005 maximum2006 permitted2007 concentration2008 2009 of N 2010and P in 2011treated WW2012 are set2013 by local2014 legislation2015 (Ministru kabineta noteikumi Nr.34, 2002) only applies to WWTPyear larger than 10 000 PE. For these WWTP maximum N concentration is 15 mg l-1 and maximum P concentration is 2 mg l-1. Fig. 5 shows reduction of nutrient concentration in Average treated of WW Nitrogen from WWTPAverage ´Kriger’ of Phosphorus over time. The WWTP Krigeri meets N concentration regulations, but peaks in 2010 (15,90 mg l-1) when the WWTP was under reconstruction. Starting with 2006 WWTPFigure Krigeri 4. Average meets N regulationand P load forfrom Phosphorus WWTPs toconcentration the Berze River in treated basin changesWW. in time (2005 – 2015). Fig ure 4. Average N and P load from WWTPs to the Berze River basin changes in time (2005 – 2015).

The18 maximum permitted concentration of N and P in treated WW are set by local legislation (Ministru kabineta noteikumi16 Nr.34, 2002) only applies to WWTP larger than 10 000 PE. For these WWTP maximum N -1 -1 concentration14 is 15 mg l and maximum P concentration is 2 mg l . Fig. 5 shows reduction of nutrient concentration12 in treated WW from WWTP ´Kriger’ over time. The WWTP Krigeri meets N concentration -1 regulations,1 but peaks in 2010 (15,90 mg l ) when the WWTP was under reconstruction. Starting with 2006 - 10 WWTP Krigeri meets regulation for Phosphorus concentration in treated WW.

mg l 8 6 18 4 16 2 14 0 12 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 1 - 10 Year

mg l 8 Nitrogen (mg l-1) Phosphorus (mg l-1) 6 Figure4 5. Concentration of nutrients (N and P) in treated WW from WWTP ‘Krigeri’ changes over time. 2 Figure 5. Concentration of nutrients (N and P) in treated WW from WWTP ‘Krigeri’ changes over time. over time0 (Fig. 6). Average nitrogen concentration recognized as one of the main eutrophication causes has met 2005Totalregulations average2006 (concentration nutrient2007 concentration2008 of 2009nitrogen in treated2010 in WW the2011 Balticloaded2012 sea to river region2013 Berze (Kiedrzyńska 2014basin have2015 et not al .,reduced 2014). over -1 -1 15time mg (Fig.l ) only 6). Averagein years nitrogen2011 (14.56 concentration ± 2.54 mg has l Year )met Results regulations of this (concentration research stress of the nitrogen importance 15 mg to ltarget-1) only in and 2012 (14.04 ± 13.19 mg l-1) -1and since then have small WWTPs-1 as the main pollutant with nutrients of years 2011 (14.56 ± 2.54 mg l ) andNitrogen 2012 (mg (14.04 l-1) ± 13.19 mgPhosphorus l ) and (mgsince l-1) then have increased to 32.57 ± 4.34 increased to 32.57 ± 4.34 mg l-1 in year 2015. Average the river basins in Latvia. phosphorus concentrations have reduced from 7 ± 1.35 mg l-1 (in year 2005) to 3.69 ± 0.44 mg l-1 (in year Conclusions 2015) andFig urethese 5. concentrationsConcentration ofare nutrients higher than (N and set P)in in treatedIt was WW recognised from WWTP in this ‘Krigeri’ study changesthat the over average time. the state regulations. amount of nutrients (N and P) loaded to river Berze All Totalthe results average indicate nutrient that concentration to reduce Nin andtreated P WWbasin loaded from toWWTP river Berze (waste basin water have treatment not reduced plants) over time (Fig. 6). Average nitrogen concentration has met regulations (concentration of nitrogen 15 mg l-1) only in loads to river Berze basin from-1 point sources small have reduced-1 over time. yearsWWTP 2011 (PE (14.56 < 10000) ± 2.54 have mg to l be) and targeted. 2012 (14.04The largest ± 13.19 mgThe l ) largestand since amount then haveof nutrients increased is loadedto 32.57 to ±sub- 4.34 WWTP ‘Krigeri’ meets all the legislation set by basin ID 12 where city of Dobele and its WWTP are state government and have reduced nutrient load to located. In time from 2005 to 2015 in total 76.252 tons river Berze basin, but overall situation in the basin on nitrogen and 8.770 tons of phosphorus have been shows nutrient concentration increase suggesting loaded to the river Berze basin sub-basin ID 12. other (PE > 10000) WWTP causing the problem. The WWTP Krigeri (the larges WWTP in the study In Poland, poorly treated waste water has been area) meets N and P concentration in treated WW

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IMPACTS OF MUNICIPAL WASTEWATER -1 -1 mg l in year 2015. Average phosphorus concentrations haveTREATMENT reduced from PLANTS7 ± 1.35 mgON lWATER (in year QUALITY 2005) to 3.69 -1 Linda± 0.44 Dambeniece-Migliniece, mg l (in year 2015) Ainis and Lagzdiņš these concentrations are higher than set in theIN state THE regulations. BERZE RIVER BASIN

40 35 30 25 1 - 20 mg l 15 10 5 0 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Year Average of Nitrogen (mg l-1) Average of Phosphorus (mg l-1)

Figure 6. Average concentration of nutrients (N and P) in treated WW loaded from WWTPs to river Berze basin changes over time. Figure 6. Average concentration of nutrients (N and P) in treated WW loaded from WWTPs to river Berze basin changes over time. (waste water) regulations set by Republic of Latvia Acknowledgements Cabinet Regulation No. 34. But the concentration of This research was supported by BONUS, the joint All the results indicate that to reduce N and P loads to river Berze basin from point sources small WWTP nutrients in treated WW loaded to river Berze basin Baltic Sea research and development programme for (PE < 10000) have to be targeted. The largest WWTP ‘Krigeri’ meets all the legislation set by state government haven’t reduced in time pointing out that the problem years 2010 – 2017 project MIRACLE (Mediating and have reduced nutrient load to river Berze basin, but overall situation in the basin shows nutrient concentration mightincrease be the suggesting smaller WWTP other (PE (PE > <10000) 10000). WWTP Thus, causingto integrated the problem. actions In for Poland, sustainable poorly ecosystem treated waste services water has reducebeen nutrientrecognized load as to oneriver of Berze the mainbasin eutrophicationsmall WWTP causesin a changing in the Baltic climate). sea region (Kiedrzyńska et al., 2014). shouldResults be targeted.of this research stress the importance to target small WWTPs as the main pollutant with nutrients of the river basins in Latvia. References 1. ConclusionsBouwman, L., Goldewijk, K.K., Van Der Hoek, K.W., Beusen, A.H., Van Vuuren, D.P., Willems, J., ... Stehfest,It was E. (2013).recognised Exploring in this studyglobal that changes the average in nitrogen amount and of phosphorusnutrients (N cycles and P) in loaded agriculture to river induced Berze basin fromby WWTPlivestock (waste production water treatmentover the 1900 plants) – 2050 have period.reduced Proceedings over time. of the National Academy of Sciences, 110(52),The 20882 largest – 20887.amount DOI: of nutrients 10.1073/pnas.1012878108. is loaded to sub-basin ID 12 where city of Dobele and its WWTP are 2. located.Carpenter, In time S.R., from Caraco, 2005 to N.F.,2015 inCorrell, total 76. D.L.,252 tonsHowarth, on nitrogen R.W., andSharpley, 8.770 tons A.N., of phosphorus& Smith, V.H. have (1998).been loaded to theNonpoint river Berze pollution basin of sub-basin surface watersID 12. with Phosphorus and Nitrogen. Ecological Applications Ecology, The WWTP Krigeri (the larges WWTP in the study area) meets N and P concentration in treated WW 8(3), 559 – 568. 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10. Thyssen, N. (1999). Nutrients in European ecosystems. N. Thyssen, Ed. Copenhagen: European Environment Agency. 11. Vega, M., Pardo, R., Barrado, E., & Debán, L. (1998). Assessment of seasonal and polluting effects on the quality of river water by exploratory data analysis. Water Research, 32(12), 3581 – 3592. DOI: 10.1016/ S0043-1354(98)00138-9. 12. Withers, P.J., Jarvie, H.P., Hodgkinson, R., Palmer-Felgate, E.J., Bates, A., Neal, M., … Wickham, H.D. (2011). Characterization of phosphorus sources in rural watersheds. Journal of Environmental Quality, 38(5), 1998 – 2011. DOI: 10.2134/jeq2008.0096.

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