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C10 – Biological Oxygen Demand and the concentration of ammonium in rivers of the Republic of Moldova

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Key messages

The Moldovan rivers are moderately to highly polluted by organic matter and ammonium. The pollution is slightly increasing. 

Levels of pollution of freshwater with ammonium and organic matter in the Nistru basin is high. There is no indication of improving trends. The pollution of freshwater in the Prut basin with ammonium has decreased from a high level towards a low level, but pollution with organic matter is still at a moderate level, although there has been a slow decrease recently.

Is organic matter and ammonium pollution in the rivers of Moldova decreasing?

Figure 1 – Changes in average ammonium and BOD5 concentrations in rivers of Moldova (1992-2017)

Data sources 

Note: the data series are calculated as the averages of the annual means of ammonium concentrations (mg NH4-N/l) (left) and BOD5 (mg O2/l) (right) for river sites in the two river basins of the Republic of Moldova (Prut basin and Nistru (Dniester) basin) and in the Black Sea basin (direct tributaries of the Black Sea), as well for the whole country for the period 1992-2017. The number of river sites is given in parenthesis. The total number of river sites is 19. For 13 sites the specification is given in regard to their location upstream (amonte) or downstream (aval) of a city or town; six sites are located upstream and seven downstream.

The average concentration of ammonium in rivers in the period 1992-2017 was 1.77 mg N/l, ranging from 1.09 (1997) to 2.49 mg N/l (2001). The concentrations are increasing on average by 0.012 mg N/l each year. This is because of sites on the Nistru basin rivers. Here, the average increase for the period 1992-2017 was 0.048 mg N/l. Six sites on the Prut rivers and two sites on the Danube and its tributary (Cogîlnic) have shown a decreasing trend (on average a decrease of 0.042 mg N/l each year). The overall trend in biological oxygen demand after 5 days (BOD5) in rivers in Moldova is positive as well. Nevertheless, the current concentration of dissolved oxygen needed to break down organic matter in rivers has not changed much since the start of the period. In 1992 it was 4.5 mg O2/l and in 2017 it was 4.35 mg O2/l. The average annual BOD5 for the period 1992-2017 was 4.12 mg O2/l. 

 

There were three distinctive trends between 1992 and 2017. After 1992 the concentration started to fall until 1998, which had the lowest average concentration (3.15 mg O2/l). Then there was a period of increasing concentrations, which finished in 2008. There were 3 years that had average peaks of high BOD concentrations: 2003 (4.92 mg O2/l), 2004 (4.79 mg O2/l) and 2008 (5.11 mg O2/l). The average concentration in the last 9 years was 4.25 mg O2/l and it remains stable. 

 

River water downstream of a city or town is significantly more polluted then water upstream as is demonstrated by 13 sites in Moldova (Figure 1). Ammonium concentrations in river waters upstream of cities have shown a decreasing trend, but non-treated effluent from cities is clearly causing a permanent increase downstream. The BOD5 concentration shows a slight increasing trend in upstream sites and is higher after effluents from cities have entered the rivers. Nevertheless, the difference between upstream and downstream is not as high as it is for ammonium.  

The overall water quality situation in Moldova is dominated by the river Nistru, which is more polluted than the river Prut. The average ammonium and BOD5 concentrations in the period 1992-2017 on the Nistru river were 2.75 mg N/l and 5.09 mg O2/l, respectively. Both parameters indicate high levels of organic matter pollution. On the Prut river, the average ammonium concentration in the period 1992-2017 was 0.40 mg N/l, but from 2010 it was only 0.11 mg N/l. The ammonium pollution has decreased from a high level towards a low level. The average BOD5 concentration in the period 1992-2017 in the river  Prut was 2.72 mg O2/l and the in last 5 years was 2.54 mg O2/l. This parameter shows that there is constant moderate pollution of freshwater in the Prut river basin. 

What is the current state of organic matter and ammonium pollution of rivers in Moldova?

Figure 2 - Current concentrations of ammonium and BOD5 in rivers in Moldova (2017)

Data sources 

Note: The graphs show the ammonium (left) and BOD5 (right) concentration classes at river monitoring sites in the two river basins of the Republic of Moldova (Prut basin and Nistru (Dniester) basin), in the Black Sea basin in Moldova (Danube and Cogîlnic) and for the whole country, based on the average of the annual mean concentrations for 2017. The number of monitoring sites per river basin is given in parenthesis.

At present, 15 % of the sites in rivers in Moldova have low ammonium concentrations (< 0.04 mg N/l), and one third have moderate concentrations (between 0.04 and 0.1 mg N/l). Almost half of sites have high or very high concentrations of ammonium; all except one site are located in the Nistru (Dniester) basin. Two sites out of six on the Prut basin have low ammonium concentrations. These six sites are moderately polluted by ammonium. The nitrate measured in the Danube river in Moldova falls within the moderately polluted class (between 0.1 and 0.2 mg N/l). The  ammonium pollution in the River Cogîlnic is between moderate and high (0.2 mg N/l).  

Almost 70 % of river sites in the Republic of Moldova are highly polluted with organic matter, the remainder are moderately polluted (2.0-3.0 mg O2/l). In the Nistru (Dniester) basin, two sites are moderately polluted with organic matter and three are highly polluted. Six sites show very high pollution with organic matter (BOD5 > 4 mg O2/l). In the Prut basin, three sites are moderately polluted with organic matter and three are highly polluted. The BOD5 measured in the Moldavian part of Danube river is 2.51 mg O2/l (moderate pollution). The River Cogîlnic is highly polluted with organic matter  BOD5 is 3.35 mg O2/l).  

Indicator specification

Indicator definition

The level of oxygen concentration in water bodies, expressed as biochemical oxygen demand (BOD) – which is the amount of dissolved oxygen required for the aerobic decomposition of organic matter present in water – and the level of concentrations of ammonium (NH4/N-NH4) in rivers.

Units

The annual average BOD after 5 days of incubation (BOD5) at 20 °C is expressed in mg of O2/l; the ammonium concentration is expressed in mg of N/l.

 

Rationale

Justification for indicator selection

Large quantities of organic matter (microbes and decaying organic waste) can result in reduced chemical and biological quality of river water, impaired biodiversity of aquatic communities and microbiological contamination that can affect the quality of drinking and bathing water. Sources of organic matter are discharges from waste water treatment plants, industrial effluents and agricultural run-off. Organic pollution leads to higher rates of metabolic processes that demand oxygen.

 

Scientific references

Policy context and targets

Context description

National policy context

  • Water Law No 727 of 23 December 2011.

  • Regulation on the monitoring of systematic evidence on the status of surface water and groundwater (Guideline No 932 of 20 November 2013).

  • Regulation on environmental quality requirements for surface water (Government Decision No 890 of 12 November 2013).

 

International policy context

Targets

National targets

The National Environmental Strategy provides for the implementation of a river basin management system to improve the quality of surface water by 50 % (2023).

According to the Regulation on environmental quality requirements for surface water, Moldova should improve the quality of surface water by one class.

International targets

The UN Sustainable Development Goal 6 target 6.3 aims to achieve, by 2030, improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally.

Related policy documents

Environmental Strategy for the years 2014-2023 and of the Action Plan for its implementation. Republic of Moldova Government Decision No 301 of 24 April 2014.

 

Methodology

Methodology for indicator calculation

In Moldova, the monitoring programme for surface water is based on the Water Law and its regulations transposing the Water Framework Directive. The monitoring of freshwater quality is done by the Environment Quality Monitoring Division of the State Hydrometeorological Service of the Ministry of Agriculture, Regional Development and Environment of the Republic of Moldova. Monitoring data from 19 locations on eight rivers and three lakes was used for the analysis. The frequency of surveillance monitoring was four times a year.

An annual time series for each site was calculated by averaging the values for individual samples per year. Aggregated time series are calculated as the average of the individual annual time series and are calculated for the two major river basins, Nistru (Dniester) and Prut, and the basins of two rivers draining into the Black Sea (Dunerea (Danube) and Cogîlnic), as well as for the whole country. Only the time series that were complete after gap-filling are included in the aggregated time series. This is to ensure that the aggregated data series are consistent, i.e. including the same sites throughout the time series. In this way assessments are based on actual changes in concentration and not on changes in the number of sites. 

For the present-state analysis, the monitoring sites were assigned to different concentration classes, based on the average of the annual mean concentrations for 2017. All sites with data from 2017 were included in the analysis.

Water quality samples at the monitoring sites for surface water are collected in accordance with International Organization for Standardization (ISO) 5667. The determination of BOD5 (biochemical oxygen demand in 5 days) is performed using the iodometric method, based on an assessment of the amount of oxygen dissolved in water by keeping the sample for 5 days in an incubator, where it is maintained at a temperature of 20 °C.

The colorimetric method for determining the concentration of ammonium ions is based on the reaction with the Nessler reagent, which is treated with Seignette’s salt solution. The intensity of the yellow colour is measured in a spectrophotometer at a wavelength of 425 nm.

Additional information can be found on the environmental indicators public portal of the environmental ministry (http://data.gov.md/ckan/dataset/14794-biochemical-oxygen-demand-and-concentration-of-ammonium-in-rivers).

 

Methodology for gap-filling

For time series and trend analyses, only series that were complete after interpolation (i.e. no missing values in the site data series) are used. This is to ensure that the aggregated data series are consistent, i.e. including the same sites throughout the time series. In this way assessments are based on actual changes in concentration and not on changes in the number of sites.

Methodology references

  • EEA, 2005, EEA core set of indicators guide, EEA Technical Report No 1/2005, Office for Official Publications of the European Communities, Luxembourg.

  • UNSD and UNEP, 2013, ‘Questionnaire 2013 on environment statistics, section: water’, United Nations Statistics Division and United Nations Environment Programme.

  • UNECE, 2018, ‘Guidelines for the application of environmental indicators’, description of C10: biochemical oxygen demand (BOD) and concentration of ammonium in rivers, United Nations Economic Commission for Europe.

  • UNECE, 2018, ‘Guidelines for the application of environmental indicators’, glossary of terms — C10: biochemical oxygen demand (BOD) and concentration of ammonium in rivers, United Nations Economic Commission for Europe.

 

Uncertainties

Methodology uncertainty

No uncertainty has been specified.

Data sets uncertainty

No uncertainty has been specified.

Rationale uncertainty

Biochemical oxygen consumption and total ammonium are appropriate to illustrate water pollution by organic matter. However, the use of annual mean values may not fully illustrate the severity of low oxygen conditions.

Data sources