Surface water quality in the hydroelectric power plant reservoirs on the lower course of the Sava

16 Sep, 2019
Surface water quality in the hydroelectric power plant reservoirs on the lower course of the Sava

Every now and then we come across a comment or post in individual social media on the topic of organic pollution, reducing oxygen levels, and overall deterioration in the surface water quality inside the reservoirs of the hydroelectric power plants (HPPs). All this is supposed to have been caused by the construction of the hydroelectric power plants. But is that really the case?

Everywhere, water quality is constantly changing, depending on a variety of factors. For water quality monitoring purposes, the company HESS has commissioned accredited providers to conduct various measurements and analyses in the area of the reservoirs on the lower course of the Sava River, in compliance with the relevant administrative licences and applicable legislation, all as part of the monitoring of the conditions of surface waters. Let's now take a look at all the factors that affect the surface water quality inside reservoirs, how the monitoring of the conditions of surface waters is conducted, and the findings of the monitoring carried out in the areas of the existing hydroelectric power plants on the lower course of the Sava between Boštanj and Brežice.

Surface water quality influenced by several factors

Surface water quality inside the reservoirs of the hydroelectric power plants is influenced by a number of factors. The most important being: the condition or status of the surface water on entering the reservoirs, the presence of sources of pollution at the reservoirs, and residence times of the water inside the reservoirs. 

All the hydroelectric power plants on the lower course of the Sava are of the combined run-of-the-river and storage type, meaning the residence times of the water inside the reservoirs is significantly shorter than compared to storage-type power plants, since due to the power plants' mode of operation, the flow of the river is practically unchanged and similar to its natural flow. Also, the sizes and depths of this type of reservoirs are substantially smaller than with storage-type HPPs.

Which monitoring programs are carried out by HESS?

The company HESS conducts regular monitoring of the conditions of the surface waters inside the reservoirs of HPP Boštanj, HPP Arto-Blanca, HPP Krško and HPP Brežice, in accordance with individual HPP's environmental protection approvals and by adhering to the national legislation with regard to the monitoring of surface water conditions. This monitoring comprises chemical, physico-chemical, and biological analyses of the surface waters.

To ensure the credibility of the results, we do not carry out the monitoring by ourselves. Instead, it is conducted by authorized professional institutions using trained experts and accredited procedures for sampling, measuring, and laboratory analyses. The monitoring entails field work, laboratory work and office work. In predetermined sampling spots, water measurements and samples are taken for further laboratory analyses. After laboratory analyses are completed, final reports on the work performed are produced.

Organic pollution of the reservoirs

The organic pollution of the reservoirs is assessed based on the following parameters: 5-day biochemical oxygen demand (BOD5) and saprobity. BOD5 indicates the amount of oxygen needed for aquatic organisms – in the presence of oxygen, at a certain temperature and over a certain period of time – to decompose the organic matter in the sample. Saprobity, on the other hand, which is assessed by sampling algae (phytobenthos) and tiny aquatic animals (benthic invertebrates) attached to the bottom of a body of water, indicates the body of water's degree of pollution with organic matter.

The results of measurements carried out from 2012 to 2018 show that the water is of good biological status and that over the years, with the construction of new HPPs and reservoirs, there has been no deterioration in the status of surface water in the lower course of the Sava between Boštanj and Brežice.

The values of the BOD5 parameter fall within the expected range. The only exceptions here are the BOD5 results for the HPP Arto-Blanca reservoir from 2013 and 2017 and for the HPP Brežice reservoir from 2017. In these cases, we have reason to believe that the less-than-favourable results can be attributed to the Sava River's degree of pollution with nutrients at the entrance to the chain of HPPs on the lower course of the Sava, additional input of nutrients through wastewater from agricultural, urban and other inhabited areas, industrial and municipal wastewater treatment facilities, other polluters located by individual reservoirs, and longer residence times of the water inside the reservoirs due to lower flow rates of the Sava during late summer as a direct result of an extended period without any precipitation. Also, the results of the monitoring carried out do not show any deterioration of results from one reservoir to another along the entire chain of HPPs on the lower course of the Sava, since the worst single result (maximum) and the highest average were determined in the second consecutive reservoir downstream – the HPP Arto-Blanca reservoir.

The results of the analysis of the saprobity parameter in the period of observation show high or good ecological status of the water in the reservoirs. The only exception here is the saprobic index for the HPP Boštanj reservoir from 2017, which indicated a moderate status. What's interesting is that the result was worse in the reservoir farthest upstream, while the results for the reservoirs immediately downstream from it (HPP Arto-Blanca and HPP Krško) showed the water to be in good condition. In this case, too, we have reason to believe that the less-than-favourable result can be attributed to the presence of nutrients in the Sava River at the entrance to the chain of HPPs, additional input of nutrients through wastewater from agricultural, urban and other inhabited areas, industrial and municipal wastewater treatment facilities, other polluters located by individual reservoirs, and unfavourable hydrological conditions of the Sava during late summer. The results of analyses conducted in 2018 again showed good ecological status in all the reservoirs.

Water's oxygen levels

The level of oxygen in water is one of the most general physico-chemical parameters determining the ecological status of water. This particular parameter is measured multiple times a year in each of the HPP reservoirs on the lower course of the Sava. The measurement results show that the oxygen levels inside individual reservoirs have been high enough throughout the years of sampling. Also, the oxygen levels are similar among all the reservoirs, which goes to show that the series of reservoirs does not have a negative impact on the oxygen conditions in the water – the oxygen level measured in the HPP Brežice reservoir is not any lower than that of the HPP Boštanj reservoir

Based on the calculated averages for individual reservoirs, we have found the parameter values to be comparable, ranging between 10.00 and 10.30 mg/l. It is also interesting to note that the highest result among the lowest (minimal) oxygen levels in the water was measured in the fourth consecutive reservoir downstream – the HPP Brežice reservoir. According to the data on this parameter, there is again no sign of any deterioration of results from one reservoir to another along the entire chain of HPPs on the lower course of the Sava.

 

Based on the conducted measurements and analyses and surface water condition monitoring, we find the water inside the HPP reservoirs on the lower course of the Sava to be free from organic pollution, to have good oxygen conditions and to be of good quality. Furthermore, there is also no sign of a gradual increase in organic pollution, reduction of oxygen levels or overall deterioration of the surface water quality along the entire series of the existing reservoirs. The construction of new HPP reservoirs on the lower course of the Sava does not cause the organic pollution inside the reservoirs to increase, nor does it reduce oxygen levels or deteriorate the surface water quality in general. The principal reasons for this condition include proportionately smaller sizes and depths of the reservoirs, as well as using run-of-the-river- and storage-type hydroelectric power plants and modes of operation which allow a practically uninterrupted flow of the Sava and shorten the residence times of the water inside individual reservoirs.