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You are here: Home / Data and maps / Indicators / Oxygen consuming substances in rivers / Oxygen consuming substances in rivers (CSI 019/WAT 002) - Assessment published Feb 2015

Oxygen consuming substances in rivers (CSI 019/WAT 002) - Assessment published Feb 2015

Indicator Assessment Created 16 Sep 2014 Published 23 Feb 2015 Last modified 25 Feb 2015, 11:39 AM
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This indicator is updated by 2012 data reported by countries in autumn 2013. The next update will be based on 2013 and 2014 data to be reported by countries in autumn 2015.

 
Contents
 

Indicator definition

This indicator illustrates the current situation and trends regarding biochemical oxygen demand (BOD) and concentrations of total ammonium (NH4) in rivers. The key indicator for the oxygenation status of water bodies is BOD, which is the demand for oxygen resulting from organisms in water that consume oxidisable organic matter.

Units

Annual average BOD after 5 or 7 days incubation (BOD5/BOD7) is expressed in mg O2/l and annual average total ammonium concentrations in micrograms N/l.


Key policy question: Is organic matter and ammonium pollution of rivers decreasing?

Key messages

Concentrations of biochemical oxygen demand (BOD) and total ammonium have decreased in European rivers in the period 1992 to 2012 (Fig. 1), mainly due to general improvement in waste water treatment.

Rivers - European trends

Dashboard
Data sources: Explore chart interactively
BOD5
Data sources: Explore chart interactively
Ammonium
Data sources: Explore chart interactively

Rivers - Biochemical Oxygen Demand

Chart
Data sources: Explore chart interactively
Table
Data sources: Explore chart interactively

Rivers - ammonium

Chart
Data sources: Explore chart interactively
Table
Data sources: Explore chart interactively

Key assessment

Introduction

Biochemical oxygen demand (BOD) and ammonium are key indicators of organic pollution in water. BOD shows how much dissolved oxygen is needed for the decomposition of organic matter present in water. Concentrations of these parameters normally increase as a result of organic pollution caused by discharges from waste water treatment plants, industrial effluents and agricultural run-off. Severe organic pollution may lead to rapid de-oxygenation of river water, high concentration of ammonia and disappearance of fish and aquatic invertebrates. Some of the year-to-year variation can be explained by variation in precipitation and runoff.

The most important sources of organic waste load are: household wastewater; industries, such as paper or food processing; and silage effluents and manure from agriculture. Increased industrial and agricultural production in most European countries after the 1940s, coupled with a greater share of the population connected to sewerage systems, initially resulted in increases in the discharge of organic waste into surface water. Over the past 15 to 30 years, however, the biological treatment (secondary treatment) of waste water has increased, and organic discharges have consequently decreased throughout Europe. See also CSI 024: Urban waste water treatment.

Present concentrations per country

See the WISE interactive maps for information displayed by countries on BOD in rivers and ammonium in rivers

In 2012 (or the latest reported year), countries with an average BOD concentration in the lowest category (less than 1.4 mg/l) are Slovenia (1.0 mg/l), the United Kingdom (1.2 mg/l), France (1.3 mg/l) and Ireland (1.4 mg/l).

In 2012 (or the latest reported year), countries with an average ammonium concentration in the lowest category (less than 40 µg/l) are Norway (11 µg/l), Finland (26 µg/l), the United Kingdom (32 µg/l), Sweden (39 µg/l), Slovenia (40 µg/l) and Ireland (40 µg/l).

Overall trend in BOD and total ammonium

In European rivers, oxygen demanding substances have been decreasing throughout the period 1992 to 2012 (Figure 2). Total  BOD concentration decreased by 1.6 mg/l from 1992 to 2012. By using the filter in figure 2 the river BOD trends for the individual countries are illustrated.

The average yearly decrease in BOD is 0.08 mg/l (-2.9 % per year). A significant decrease is evident at 62% of river stations, with an additional 6% of stations showing a marginally decreasing trend (see Rivers - BOD - statistical analysis). On the other hand, a significantly increasing BOD trend is recorded at only 3% of the stations, with marginally increasing BOD at an additional 1% of the stations. Countries where more than 60% of the stations show a negative trend in BOD concentrations are Ireland (100%), Luxembourg (100%), Slovenia (92%), Slovakia (87%), France (81%), the United Kingdom (75%), Denmark (74%), Austria (66%), Bulgaria (66%) and Lithuania (63%).

Likewise, the relative trend calculation for ammonium shows that the average ammonium concentration decreased by 231 µg N/l in the period 1992–2012 (Figure 3). By using the filter in figure 3, the river ammonium trends for the individual countries are illustrated.

The average yearly decrease is in ammonium is 11.6 µg N/l (-3.5 % per year). Significantly decreasing concentration trends have been observed at 59% of the stations, with an additional 5% of stations showing a marginally decreasing trend (see Rivers - ammonium - statistical analysis). A significantly increasing trend is evident at 3% of stations and a marginally increasing trend at 1% of stations. Countries where more than 60% of the stations show a negative trend in ammonium concentrations are Luxembourg, the former Yugoslav Republic of Macedonia, Slovenia and the United Kingdom (all 100%), Germany (92.6%), Lithuania (88.5%), Ireland (75%), Poland (75%), France (71.7%), Bulgaria (71.6%), Belgium (70.4%), Norway (70%) and Austria (66%).

The decrease is mainly due to improved sewage treatment resulting from the implementation of the Urban Waste Water Treatment Directive and national legislation. The economic downturn of the 1990s in central and eastern European countries also contributed to this fall, as there is an ongoing decline in pollution from manufacturing industries. This suggests that either further improvement in waste water treatment is required or that other sources of organic pollution, for example from agriculture, require greater attention, or both.

BOD and total ammonium time series and trends per geographical region

Link: BOD concentrations in rivers in different geographical regions of Europe

The largest absolute decrease of BOD from 1992 to 2012 has occurred in southeastern European rivers (61%), where concentrations are at their lowest level to-date. They are still, however, the highest in Europe (about 3.1 mg O2/l). The largest yearly decrease is evident in western Europe (3.4% per year). Concentrations in northern European rivers (represented by rivers of Finland only) are the most stable (less than 2 mg O2/l), with an average yearly decrease of 0.8%. The largest proportion of rivers with a negative BOD trend is in western Europe. Since BOD records are traditionally low in the north, the decreasing trends are less pronounced there. However, the share of rivers with an increasing trend is relatively high both in the north and the east.

Link: Ammonium concentrations in rivers in different geographical regions of Europe

The decreasing trend of ammonium from 1992 to 2012 is largest in southeastern (5.0 % per year on average) and western European (4.5 % per year on average) rivers. This is followed by a similarly decreasing trend in the eastern Europe (3.7 % per year on average). Concentrations in northern European rivers are stable, where the smallest decrease, of 1% per year on average, is observed.

The concentrations in eastern European rivers, as assessed for the period 1992 to 2012 (around 80 µg N/l), are significantly lower than those in the previous assessment (made in 2012, for the period 1992-2010: around 200 µg N/l). The reason is that in the 1992 to 2010 assessment, data for 96 monitoring stations in Poland were included, whereas in the 1992 to 2012 assessment, only four stations in Poland were included. Monitoring stations in Poland had an important impact on the assessment of the indicator for the eastern European geographical region as a whole. The same difference can be observed for the southern European region due to the larger number of river monitoring stations in Spain, the only stations representing southern Europe (82% decrease in the 1992 to 2012 period, compared to a 20% decrease in the 1992 to 2010 period) included in the assessment. Southeastern and western European rivers also saw a significant decrease in ammonium concentrations (both around 75%), however, southeastern European rivers still have the highest ammonium concentrations in Europe (around 300 µg N/l).

BOD and total ammonium time series and trends per sea region

Link: BOD concentrations in rivers in different sea regions of Europe

The decreasing BOD trend is observed in all sea regions. It is largest in the Mediterranean Sea catchment, where it is decreasing on average by 4.4% per year. The decreasing trend is also strong in the Black Sea (3.6% per year on average), the Greater North Sea (including the Kattegat, and the English Channel; by 2.7% per year on average), and the Celtic Seas, Bay of Biscay and Iberian Coast (by 2.8% per year on average). It is less pronounced in the Baltic Sea (by 0.9% per year on average). The present BOD is highest in the Black Sea (above 2 mg/l) and lowest in the Celtic Seas, Bay of Biscay and Iberian Coast (less than 1.5 mg/l).

Link: Ammonium concentrations in rivers in different sea regions of Europe

Concentrations of ammonium in rivers are highest in the Black sea (142 µg N/l) and the Greater North Sea regions (141 µg N/l). Somewhat lower concentrations can be found in the Mediterranean Sea (134 µg N/l). The Baltic Sea region has a lower record of 52 µg N/l, while the Celtic Seas, Bay of Biscay and Iberian Coast have 58 µg N/l. The concentrations are by far the lowest in the region of the Arctic Ocean (5 µg N/l). A trend comparison shows that concentrations are decreasing in all sea regions, with the largest decrease in the Celtic Seas, Bay of Biscay, Iberian Coast (5.7% average decrease per year), the Greater North Sea, including the Kattegat, and the English Channel (3.8%), the Black Sea (4.9%) and the Mediterranean Sea (3.5%). The decreasing trend is somewhat lower in catchments of the Baltic Sea (2.1% per year on average) and the Arctic Ocean (1.3%).

Data sources

Policy context and targets

Context description

There are a number of EU directives that aim to improve water quality and reduce the loads and impacts of organic matter. First, the Water Framework Directive requires the achievement of good ecological status or good ecological potential of rivers across the EU by 2015 and repeals, step-by-step, several older water related directives. Alongside this, the following directives stay in place: the Nitrates Directive (91/676/EEC), aimed at reducing nitrate and organic matter pollution from agricultural land, the Urban Waste Water Treatment Directive (91/271/EEC), aimed at reducing pollution from sewage treatment works and certain industries (see also CSI24 Urban waste water treatment) and the Integrated Pollution Prevention and Control Directive (96/61/EEC) aimed at controlling and preventing the pollution of water by industry.

Targets

The indicator is not directly related to a specific policy target but shows the efficiency of wastewater treatment (see CSI024). The environmental quality of surface waters with respect to organic pollution and ammonium and the reduction of the loads and impacts of these pollutants are, however, objectives of several directives, including the Surface Water for Drinking Directive (75/440/EEC), which sets standards for the BOD and ammonium content of drinking water, as well as other directives mentioned in the previous chapter.

Related policy documents

Methodology

Methodology for indicator calculation

Data source: Data on rivers is collected annually through the WISE-SoE data collection process. WISE SoE was previously known as EUROWATERNET (EWN) and EIONET-Water. Biological quality elements in rivers have been integrated into the reporting of river water quality, starting from the 2012 reporting period. A formal request is sent to NFPs and NRCs every year with reference to templates to use and guidelines.

The data requested on rivers includes the physical characteristics of the river monitoring stations, proxy pressures on the upstream catchment areas, as well as chemical quality data on nutrients and organic matter, and hazardous substances in rivers. It also includes the biological data (primarily calculated as national Ecological Quality Ratios), as well as information on the national classification systems for each Biological Quality Element and waterbody type. This reporting obligation is an EIONET Priority Data flow.

Station selection: No criteria are used for station selection (except for time series and trend analysis; see below)  

Determinants: The determinants selected for the indicator and extracted from Waterbase are BOD5, BOD7, total ammonium and ammonium.

Most countries monitor BOD5. Finland monitors BOD7. Lithuania monitored BOD5 up to 1995 and started monitoring BOD7 in 1996. Latvia monitored BOD7 from 1996 to 2001. Estonia monitored BOD5 in 2010, while it monitored BOD7 up to 2009. BOD is commonly used for BOD5. For countries reporting BOD7, these values have been converted to BOD5 (BOD7 = 1.16 BOD5) for reasons of comparability.

All countries reported total ammonium until 2006. In 2007, Greece and Liechtenstein started reporting ammonium instead of total ammonium. Instead of total ammonium, Cyprus, Lichtenstein and Slovenia began reporting ammonium in 2008, Austria and Netherlands in 2009, Bulgaria and Latvia in 2010, and Estonia, Norway and Poland in 2011.  Besides total ammonium, Slovakia also started to report ammonium for some stations in 2008. Belgium, Germany, Italy, Luxembourg, Slovakia and the United Kingdom report either ammonium or total ammonium for an individual station in a selected year from 2008 on. Data of either of the two determinants was included in the assessment. For those stations in Slovakia where both were reported, total ammonium data was included in the assessment.

All values are labeled as BOD5/total ammonium in the graphs, but it is indicated in the graph notes for which countries BOD7/ammonium data are used.

An automatic QA/QC procedure excludes data (stations*year) from further analysis. This is based on flagging in Waterbase, deriving from QA/QC tests. In addition a semi-manual QA procedure is applied, to identify outliers that are not identified in the QA/QC tests. This comprises e.g. values deviating strongly from the whole time series, values not so different from values in other parts of the time series, but deviating strongly from the values closest in time, consecutive values deviating strongly from the rest of the time series or whole data series deviating strongly in level compared to other data series in the country. If not explicitly confirmed valid by reporting countries, such values are flagged in Waterbase, but only excluded from the following year’s assessment due to timing issues. More details on the QA/QC procedure can be found here:

    • groundwater QA/QC description
    • rivers QA/QC description
    • lakes QA/QC description


Quality checked data: In the table on nutrients ("Waterbase_rivers_v12_Nutrients"), QA-fields are treated as follows:

      • Field "QA_MVissues": all flagged values are excluded from the indicator calculation, except for zero values (flag 103).
      • Field "QA_LRviolation": all flagged values are allowed, except for flagged values that break the rule “Mean >= Minimum” (flag 201) and “Mean <= Maximum” (flag 202). 
      • Field "QA_outlier": all flagged values are excluded from the indicator calculation, except for outliers confirmed by country (flags 491, 493).  
      • Field "QA_station_issues: all flagged values are allowed (including wrong coordinates or missing coordinates), except for "Water Category value is incompatible with this particular dataset” (flag 511) and “station is not defined in the station table" (flag 599).
      • Field "QA_CR violation": all flagged values are allowed.

Mean: Annual mean concentrations are used in the time series and present concentration graphics. Countries are asked to substitute any sample results below the limit of detection/determination by a value equivalent to half of the limit of detection/determination before calculating the station annual mean values. Mean concentration values of zero are included in the indicator calculation as zero (0).


Inter/extrapolation and consistent time series

For time series (Fig. 1-5) and trend analyses, only series that are complete after inter/extrapolation (i.e. no missing values in the station data series) are used. This is to ensure that the aggregated data series are consistent, i.e. including the same stations throughout the time series. In this way assessments are based on actual changes in concentration, and not changes in the number of stations.

Changes in methodology: Station selection and inter/extrapolation. 

Until 2006, only complete time series (values for all years from 1992 to 2004) were included in the assessment. However, a large proportion of the stations was excluded by this criterion. To allow the use of a considerably larger part of the available data, in 2007 (i.e. when analysing data up until 2005), it was decided to include all time series with at least seven years of data. This was a trade-off between the need for statistical rigidity and the need to include as much data as possible in the assessment. However, the shorter series included might represent different parts of the whole time interval, and the overall picture may therefore not be reliable. In 2009, it was decided rather to inter/extrapolate all gaps of missing values of 1-2 year for each station. At the beginning or end of the data series one missing value was replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values were replaced by the values next to them for gaps of two years and by the average of the two neighbouring values for gaps of one year.

In 2010 this approach was modified, allowing for gaps of up to three years, both at the ends and in the middle of the data series. At the beginning or end of the data series up to three years of missing values are replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values are replaced by the values next to them, except for gaps of one year and for the middle year in gaps of three years, where missing values are replaced by the average of the two neighbouring values. Only time series with no missing years for the whole period 1992-2011 after such inter/extrapolation are included in the assessment. The number of gaps is unlimited, only gap length (size) of three years is defined. This procedure increases the number of stations that can be included in the time series/trend analysis. Still, the number of stations is markedly reduced compared to the analysis of the present situation, where all available data can be used. In Figure 1, the two time series are used: 1992–2012 and 2000–2012.

Aggregation of time series

The selected time series (see above) must be aggregated in to a smaller number of groups and averaged, before the aggregated series can be displayed in a time series plot. Determinants are grouped into five geographical regions of Europe, which contain the following countries: 

Eastern: CZ, EE, HU, LT, LV, PL, SI, SK. 

Northern: FI, IS, NO, SE. 

Southern: CY, ES, GR, IT, MT, PT.

South-Eastern: AL, BA, BG, HR, ME, MK, RO, RS, TR, XK.

Western: AT, BE, CH, DE, DK, FR, IE, LI, LU, NL, UK.

(List of country codes can be found here )

Not all countries listed per region are included in the figures due to no data being reported or no stations with complete time series after inter/extrapolation. Due to changes in the monitoring network (adapting to monitoring networks under Water Directives) the time series are broken and limited number of time series is available for some countries. 

Determinants are in addition grouped into six sea region catchments, which are defined not by countries but by river basin districts or river basin district subunits if consistent with catchment areas of seas. The data thus represents rivers or river basins draining into that particular sea. The sea regions are defined as Arctic Ocean, Greater North Sea, Celtic Seas, Bay of Biscay and the Iberian Coast, Baltic Sea, Black Sea and Mediterranean Sea. The sea region delineation is according to the Marine Strategy Framework Directive (MSFD) Article 4, with the Arctic Ocean added as a separate region. As the catchment area draining into what is defined as the North-east Atlantic Ocean region of the MSFD is very big, it was decided rather to use the sub-region level here, but merging the Celtic Seas and the Bay of Biscay and the Iberian Coast. 

Determinants are also aggregated for the whole of Europe.

Trend analyses

Trends are analysed by the Mann-Kendall method (McLeod 2005) in the free software R (R Development Core Team 2006). The test was suggested by Mann (1945) and has been extensively used with environmental time series (Hipel and McLeod, 2005). Mann-Kendall is a test for monotonic trend in a time series y(x), which in this analysis is nutrient concentration (y) as a function of year (x). The test is based on Kendall's rank correlation, which measures the strength of monotonic association between the vectors x and y. In the case of no ties in the x and y variables, Kendall's rank correlation coefficient, tau, may be expressed as tau=S/D where S = sum_{i<j} (sign(x[j]-x[i])*sign(y[j]-y[i])) and D = n(n-1)/2. S is called the score and D, the denominator, is the maximum possible value of S. The p-value of tau under the null hypothesis of no association is computed by in the case of no ties using an exact algorithm given by Best and Gipps (1974). The tests reported here are two-sided (testing for both increasing and decreasing trends). Data series with p-value < 0.05 are reported as significantly increasing or decreasing ("strong trends"), while data series with p-value >= 0.05 and <0.10 are reported as marginally significant ("weak trends"). Data series with p-value >0.10 have no significant trend. The test is non-parametric which