July 14, 2023
The project “Protecting streams for a clean Black Sea by reducing sediment and litter pollution with joint innovative monitoring and control tools and nature-based practices” (Acronym: Protect-Streams-4-Sea). The project is realized under the “Joint Operational Programme Black Sea Basin 2014-2020”, which is one of the programmes, established in the framework of the European Neighborhood Instrument (ENI) 2014-2020. As such, the Project was focused on the environmental monitoring of nonpoint source pollutants and litter, which end up in the Black Sea, and the efforts on its cleaning are focused not only on the sea itself, but also along coastal areas where many pollutants and litter are provided by the river watersheds.
Therefore, the idea of the project was to stop the nonpoint source pollutants and litter from their reaching the rivers and consequently - the Black Sea. This goal was being achieved mainly by using different innovative methods, most of which were applied in Moldova practically for the first time. Additionally to Moldova, its project’s partners were universities and NGOs from Greek, Romania, Armenia and Turkey.
The project overall objective was the environment protection and reduction of pollutants and litter in Black Sea. Its specific objectives included development of new tools for joint monitoring of pollutants and litter, identification their sources and volumes, as well as development and suggestion of the best management practices. The project duration was 36 months: from 20.07.2020 to 19.07.2023.
The main purpose of the current press-conference is to inform the general public about the project activities in Moldova, and the results obtained, with a special attention to the new research methods applied. When presenting the results of carried out researches, the emphasis is on the application of innovative technologies, taking into account their novelty and availability.
As a study area for addressing the Project’s tasks in Moldova the Bălțata River — a fairly typical small river of Moldova — was selected. This river is a right tributary of the Dniester River, which flows into the Black Sea where all surface pollution and litter that enter its main stream are directly transported to. Moreover, Bălțata presents the ecological situation in other analogous small river basins of the country. Its basin area has 153.9 km2, length - 27.47 km, width - 7.74 km and includes 13 rural settlements. From the project’s viewpoint two factors were important here: the high level of soils degradation and their intensive erosion processes (above 29%).
The study of these processes included:
1. Hydrological modeling of the Bălțata’s streamflow and sedimentation.
Because the movement and storage of water at watershed scales is a complex system affected by climatic, geologic, soil, land use, anthropogenic and other factors, a nature of these drivers is best investigated by the hydrologic models simulating them over different spatial and time intervals, and in different physiographical conditions. In the project there were used two hydrological models: SWAT (Soil and Water Assessment Tool) and WEPP (Water Erosion Prediction Project). Their implementation pursued a specific task for each: SWAT was aimed at estimating the current and expected Bălțata flow as the main transporter of pollution, while WEPP modeling was aimed at stimulating soil loss and sediment yield in the basin.
In particular, the SWAT modeling of Bălțata potential annual runoff in the current climate (1981-2020) can reach 0.048 cubic kilometers. Although, given an undoubted bias in these estimates due to an anthropogenic load on the watershed, these results should be considered with some caution, they provided a basis for assessing the expected climate change impacts. SWAT modeling showed that depending on a time horizon and radiative load, the Bălțata flow can change from increase by 7-8% in the first half of 21th century to a decrease from about 8% to more 25% in 2071-2100. On average, an increase in runoff can reach about 2% in 2021-2050, and a decrease - up to 17% in 2071-2100. Moreover, SWAT has simulated the water accumulation in three reservoirs in its riverbed. This annual water accumulation can amount to 65% of the watershed total runoff.
WEPP modeling stimulated soil loss and sediment yield in the Bălțata basin, caused by sheet and rill erosions occurring in channels due to hydraulic shear. For their assessment two approaches were used: offsite and onsite. The offsite, or watershed method has provided simulation results focused on the sediment yield delivered through channels to certain outlet point. On the whole, according this approach the sediment discharge from Bălțata outlet amounted about 5430 ton/yr (0.4 t/ha/yr), with fairly even distribution by territory. In most of the watershed, the annual sediments per hectare are less than 1/4 t, followed by areas with sediments from 1/4 to 1/2 t, and only in the riverbed’ lower they reach 1-2 tons. The total value of soil loss, estimated through onsite approach amounted ~21542 ton/yr). Moreover, this method has provided a high degree of soil loss detail across the basin.
The carried out study has clearly demonstrated the enormous potential of hydrological modeling for assessing the scale and spatial distribution of soil and sediments accumulation resulted from erosion processes. The further development and wide use of this method can not only supplement, but sometimes replace the existing expensive and labor-intensive field experiments. However, its wide use in the Moldova is possible only with creating the relevant and freely accessible databases on soils, landuse and climate for the entire territory.
2. Remote sensing techniques in the identification and mapping erosion-prone areas
The use of remote sensing techniques aimed to identify the erosion-prone areas, using the Earth's surface historical satellite monitoring. Two tasks were addressed: implementation of normalized difference indexes to map areas the most vulnerable to soil erosion and to use satellite images in the assessment of time trends in stream bank erosion.
The normalized difference indexes quantify the presence of a certain material entity in a remote sensing image based on the differences in this entity's reflectance in different spectral intervals. In this study two indexes were used: Normalized Difference Vegetation Index (NDVI), which assesses the fraction of a vegetation cover within a pixel and the Normalized Difference Water Index (NDWI), which reflects water content in soil and vegetation. Identification of erosion hot spots in the Bălțata River basin was based on the freely available satellite images and analysis tools, with the contribution of some ancillary information, such as land use and soil types as well as gullies and landslides obtained as a result of manual interpretation of satellite images and field research. The satellite images were acquired by Thematic Mapper (TM) and MultiSpectral Instrument (MSI) sensors on the board of Landsat 5 and Sentinel-2 missions, accordingly. On the whole, six scenes were used, by three for spring and autumn, covering the period between 1986 and 2020.
Analysis of the research outputs has shown that just 8.37% of the Bălțata basin’s area is affected by erosion or exposed to it. The dominant erosion forms are landslides, which cover two times larger areas than gullies, occupying 40% of the erosion-prone area where moderate exposure is observed on 32%, low exposure - on 23%, and high exposure - on slightly above 4% of this area. The analysis included also the distribution of erosion depending on slopes, soils and land use.
The assessment of time trends in stream bank erosion aimed to determine and map its potential areas, using a long-term series of satellite images. The carried out study has shown the medium-resolution satellite images represent an optimal balance between the scale of analysis, its duration and time consumed for data collecting and processing. In particular, there were used satellite images, acquired by Thematic Mapper (TM) and Operational Land Imager (OLI) sensors on the board of Landsat 5 and Landsat 8 missions, accordingly. The Landsat images allowed increasing the study period from 1985 to 2022 and relative compatibility of two instruments: a quite coarse spatial resolution of Landsat scenes (30 m) is compensated by available 37 years sample. On the whole there were used 27 TM yearly scenes for 1985-2011 and 10 OLI scenes for 2013-2022 in the period March, 15 — October, 15 for each year. For each scene, the Normalized Difference Railway Erosivity Index (NDReLI) was computed, which falls within the range [-1; +1]; the higher this index, the higher erosivity. Then, a linear regression model NDReLI vs. time was ran. A positive trend, as a result of increasing NDReLI over time, suggests a growing trend of stream bank erosion, a negative trend points to its decreasing.
The modeling output highlighted a significant increasing trend in stream bank erosion in the Bălțata basin: almost 80% of the valid pixels and more than half of all subjected to this process. Very few sectors, accounting for just 0.3% of the main streams length, represented a decrease in bank erosion in the analyzed period. The results also show that croplands might have the strongest impact on stream bank erosion over time, while relatively large water-covered areas are much more important for an erosion increasing than any other land use type.
3. Drone mapping of erosion hotspots
According to the project’s program, the drones should have been used to map surface erosion at watershed levels. While the large high erosion areas were identified by the indices, the drones provided their further study at small scales through mapping the surface hot spots and evaluating the stream bank erosion. For this study a new version of the DJI's smartest drone Phantom 4 PROSpess was used. This drone is equipped with two dual rear imaging sensors and infrared sensing systems that detect obstacles in 5 directions and fly around them in 4 directions. The technique of drone erosion mapping involved capturing drone images of a target area and their processing and analysis, using specialized software. In the study, the collected high-resolution images were analyzed through a variety of software tools, including GISs, which was need to create detailed maps necessary to identify areas at risk of erosion and pollution and measure their current and potential future adverse effects.
This study included the assessment of drone stream bank and surface erosions. The first assessment was demonstrated on the example of the Bălțata watershed’s two plots, conventionally named the Middle Bălțata and the Lower Bălțata – Recea mouth confluence. The orthomosaic of the Middle Bălțata combined 25 orthophoto images, with the keypoints’s average number of about 73.3 thousand at a resolution of 1.1 cm/pixel. Its Digital Surface Model (DSM) has shown that here plots of low intensity erosion occupy 0.013 ha (1%), medium - 0.0026 ha (0.21%), and strong intensity - less than 0.005% of the entire area. The length of the bank sections subject to erosion is 55% for the Bălțata right bank, and 31% - for the left one. On the second plot, practically both banks of the Bălțata and Recea rivers are subject to erosion processes, including several microerosive forms of linear erosion. In general, the bank erosion on these two rivers is developing along their lengths of about 2800 m and about 2300 m from the mouths, respectively, in a narrow riparian strip.
The drone surface erosion was estimated on two plots, tentatively named North Bălțata and Sagaidac. The drone survey has shown that in the first plot the most affected by erosion processes is its middle, most steep part, while manifestations of the medium and low intensity erosion occur almost along the entire area. The total area of identified low erosion intensity is approximately 1.2 ha, the medium intensity erosion — 0.97 ha and of high intensity — 0.91 ha. In the Sagaidac study plot the surface erosion of different intensity predominates in its lower and middle parts of this slope. The total area of identified areas of low intensity erosion is 0.013 ha, with medium intensity erosion - 0.006 ha and of strong intensity - 0.003 ha.
4. RUSLE method
For assessing soil erosion from a hectare per year we had used RUSLE method which obtains the mean annual soil loss taking into account by precipitations, soil cover, topographical factor, i.e. slope length and slope steepness factor, and land cover.
The resulting RUSLE model for the project area allows us to estimate stream banks susceptibility to erosion. The right bank of the Baltata River has the largest share of steep slopes in the entire river basin, however, due to the fact that the same area accounts for most of the extended areas with forests, as well as half of the orchards and vineyards of the basin, the annual loss of soil under these plantations is quite small (0 - 8 t/ha/yr). At the same time, where the protective effect of the vegetation cover is absent, we observed the most prone to erosion zone (to the south of the village of Baltata).
We were able to demonstrate, that 82.6% of the area of Baltata River basin are comprised by areas with low and very low erosion susceptibility (<1– 8 t/ha/year), which can be explained by the fact that areas with the largest slopes or the largest runoff in many cases fall on forested areas in the Baltata River basin, which neutralizes the effect of erosion.
Our findings are also confirmed by the observational data, since the most dramatic changes occurred in the villages of Balabanesti, Cruzesti and Tohatin, where the share of moderately eroded soils had increased by at least 10% over the period of 16 years.
5. Fingerprinting
To identify the sources of suspended sediments in the Bălțata river, the fingerprint method was used. This method relates the physical or geochemical properties of sediments to corresponding sources within the watershed, based on two major assumptions: (a) potential sediment sources are distinguished by some fingerprinting properties, and (b) relative source contributions to suspended sediment can be determined by comparing sediment imprinting properties with source material samples. The common approach that was used in this sediment footprinting study was to select many footprinting properties and apply statistical procedures to optimize them to best distribute suspended sediment to different potential sources. The footprint assessment included assessment of total sediment contributions from three primary sources: hillslopes, gullies and riverbanks.
The samples from the sediment sample, necessary for their footprinting in the Bălțata river basin, were collected by grabbing its outlets and sub-basins at a depth of 5.0 cm in September 2022, after intense rainfall events here, on the different use area, using the ISO 10381-1 standard methodology :2002. In total, 26 soil and 3 sediment samples were selected. Different statistical procedures were adopted to quantify the relative contribution of each potential source of sediment delivered to the main channel of Bălțata Stream. The total value of heavy metals in the soil samples, except for lead and zinc, is below the maximum permissible concentrations, primarily in settlement areas. Concentrations of heavy metals in sediments are close to soil samples.
6. Litter trap
The collector for capturing floating waste, as a tool to prevent river pollution, was installed on the Răut River, the middle-size Dniester River tributory. The installation allows the collection of plastic waste and other floating physical pollutants from the surface of a portion of the river. The catchment collector was designed, built and installed by the NGO “Renașterea Rurală”, the subcontractor of the project (Chișinău). The collection manifold is composed of two 12-meter-long tubes, each anchored to heavy concrete blocks placed at the bottom of the river. The tubes direct the waste to the trash can constructed of metal mesh. The catchment allows boating or kayaking along the river without special actions and does not present an impediment to flora and fauna. The catchment collector is installed on the Răut River, on the territory of the "Orheiul Vechi" National Park, which has assumed responsibility for its maintenance and the periodic cleaning of the garbage can. The catchment collector collects approximately one ton of waste per month, preventing it from reaching the Dniester River and then the Black Sea. All collected waste is sorted and the plastic is transported to the recycling plant.
For more information, please, visit the link: https://eco-tiras.org/docs/Brochure_Final-2023.pdf
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