Figure 1
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Data sources
- Data were provided by the Environment Quality Monitoring Division of the State Hydrometeorological Service of the Ministry of Agriculture, Regional Development and Environment, under the ENI SEIS II East project activities.
- Water quality database of Dniester river basin, State Hydrometeorological Service.
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.