Holocene and Recent Alluvial Sediments in Ogosta River Basin (Northwestern Bulgaria) and Anthropogenic Impact Assessment Dora Angelova Geological Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria email: [email protected] Abstract The catchment basin of Ogosta river takes an area of 3,110 sq km. It involves well developed river system of 40 feeders inclusive. In the region of Stara Planina mountain and the Fore Balkan the rivers are with narrow beds but in some places form large valleys. In the Danube lowland these rivers form wide floodplain terraces and strongly meandering river beds. The thickness of Holocene and recent alluvial sediments varies into wide ranges (from 1 m up to 40 m), as well as the sedimentation environment is changeable, too. The floodplain terraces width varies from 1 m to 2 km. The alluvial sediments in the Ogosta river basin are subjected to strongest anthropogenic alterations as a result of the urbanization, agricultural activities, mining industry, etc. Ninety four recent terrains strongly polluted with substandard substances in soil and water as a result of the anthropogenization. Some of the terrains are so strongly polluted with heavy metals and arsenic that they are defined as non- reclaimable. Generally, the pollution came in the period 1964-1979 from mining and flotation plants, which are located in Ogosta river upstream. Heavy metals and arsenic transportation is realized by river water during the high water, and their accumulation in alluvium comes in low water periods. The strong intensification of living style and agriculture, especially in the second half of 20th century also increased the alluvium pollution with chemical substances, living and construction waists. The poly- functional usage of alluvial sediments from the floodplain terraces of rivers from Ogosta river basin extremely reduced their quality. In the last years this necessitates fulfillment of extensive and integral studies and monitoring, which to the present remain still incomplete. The work presented makes assessment of Holocene alluvial sediments, natural and anthropogenic impacts and their stability. Keywords: Holocene, sediments, anthropogenic pollution, Ogosta river, Northwestern Bulgaria. Introduction The Ogosta river basin covers an area of 3,110 sq km. The Ogosta river itself springs from the West Stara Planina mountain. It is characterized with well-developed river system, which involves about 40 bigger feeders. The Ogosta river length exceeds 144 km. It runs into the Danube river. The bigger feeders of Ogosta river are as follows: Dagodelska Ogosta river (average annual water flow of 2.10 m3/s), Barziya river (1.95 m3/s) and Skat river (1.81 m3/s). In the region of Stara Planina mountain and the Fore Balkan the rivers are with narrow beds but in some places form large valleys. In the Danube lowland these rivers form wide floodplain terraces and strongly meandering river beds. The floodplain terraces width varies from 1 m to 2 km. The thickness of Holocene and recent alluvial sediments varies into wide ranges (from 1 m up to 40 m), as well as the sedimentation environment is changeable, too. These sediments had been studied by Mishev (1959), Stoilov (1970), Angelova (2001), etc. The alluvial sediments in the Ogosta river basin are subjected to strongest anthropogenic alterations as a result of the urbanization, agricultural activities, mining industry, etc. Some of the terrains are so strongly polluted with heavy metals and arsenic that they are defined as non- reclaimable (Kotsev, Yokova, 2002; Kotsev et al., 2005). Ninety-four recent terrains strongly polluted with substandard substances in soil and water as a result of the anthropogenization (Tsatchev et al., 1973). The poly-functional usage of alluvial sediments from the floodplain terraces of rivers from Ogosta river basin extremely reduced their quality. The work presented makes assessment of Holocene alluvial sediments, natural and anthropogenic impacts and their stability. Brief notes about the geology and tectonic of Ogosta river basin The territory of Ogosta river basin represents a variegated puzzle of strongly deformed and several times eroded Caledonian, Hercynian and Alpine structures being in autochthonous and allochthonous position. The tectonic structures are built by sequences various by age (from Paleozoic to Quaternary) and lithological types’ rocks deposited in different geodynamic environments. The main lithological BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 1/7 types subjected to erosion and denudation and building the floodplain terraces of most of the rivers from Ogosta river basin are as follows: phyllites, marbles, schists, basic volcanites, granitoids, dolomites, siltstones, shales, breccias-conglomerates, marls, sandstones, clays, quartz, etc. The geological-tectonic evolution within the studied territory of Northwestern Bulgaria resulted in the forming and discovery of various mineral resources – metal, non-metal and fuels. The ore deposits are formed under conditions of collision. They are tied to the horst structures. The main ore minerals are: pyrite, arsenopyrite, sphalerite, galena, scheelite, native gold, electrum, silver and silver sulfosalts. The erosion, denudation and accumulation processes cause concentration of significant quantities of heavy metals in the alluvial sediments. From the non-metal deposits of local importance are clays at Chiren and Byala Slatina, sands at Bukovets, Rogozen, Banitsa, Montana, etc., and pebble at Montana, Byala Slatina and Butan village. Holocene and recent alluvial sediments The Holocene and recent alluvial sediments build both higher and lower floodplain terraces as well as the river bed (Figures 1 and 2) and have the marks of morphodynamic development of morphostructural units (Angelova, 2001). Figure 1. Geological map of the Quaternary sediments in the watershed of Ogosta river in the Danubian plain (after the Geological map of Bulgaria, 1:100,000 scale, map sheets Kozloduy and Byala Slatina, Filipov et al., 1989, 1993, with amendments): 1-4 Holocene: 1 – alluvium; 2 – proluvium; 3 – gravitation formations and sediments; 4 – diluvium; 5-9 Pleistocene: 5 – alluvium covered with loess; 6 – sandy loess; 7 – typical loess; 8 – clayey loess; 9 – loess-like clays; 10-11 Plio-Pleistocene: 10 – Red clay formation; 11 – Covering gravel formation; 12 – Pre-Quaternary rock complexes. The pie diagrams show the percentage of clay, silt and sand into the loess varieties. BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 2/7 Figure 2. Geological map of the Quaternary sediments in the Ogosta river basin in the Fore Balkan and the Stara Planina mountain (after the Geological map of Bulgaria, 1:100,000 scale, map sheet Montana, Filipov et al., 1993, with amendments): 1 – Pre-Quaternary rock complexes; 2 – Plio- Pleistocene covering gravel formation; 3-5 Pleistocene-Holocene: 3 – diluvium; 4 – proluvium; 5 – gravitation formations and sediments; 6 – Pleistocene-Holocene alluvium. Within the mountain and foreland morphotectonic units the Ogosta river and its feeders flow into their erosional beds. In the places where terrace deposits are preserved they represent roughly processed boulders up to 40 cm in diameter, chaotically deposited in the riverbed facies. In the valley extensions the boulders become finer to medium size (up to 10-12 cm) cobble, as thin intercalations and lenses of sands are observed, too. The thickness of alluvial sediments exceeds 10 m. The river terraces width changes depending on the pliability of lithological varieties. The particles’ content is quite different. In the Danube lowland (Moesian hilly-plateau-like lowland) the alluvial deposits come predominantly into flood facies. The gravel of the riverbed facies in the Kozloduy lowland lies directly on the Pliocene clays. Its thickness is about 6 m, as at the base the particles are bigger and well rounded, while at the top they become smaller, cemented with sandy-clayey cement. Above them 12 m of brown-yellow compact clayey sands are deposited. The total thickness of alluvial sediments amounts to 19 m. The following fauna has been determined there: Theodoxus transversalis Pfeif., Lithoglyphus sp. Pfeif., Amphimelania holandri Ferr. and Fagita acicularis Ferr. These findings define the Holocene age of the sediments (after Koyumdzhieva, National Geofund). The Ogosta river flood terrace is about 1-2 km wide. The differentiation of lower and higher floodplain terraces is provisional one due to the gradual and undistinguished transition between them. The BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 3/7 section penetrated in well N152 at Gromshin village is 11.5 m thick. The lower boundary to the marls is erosional one. At the base of the section fine-grained sands 1.5 thick are deposited. They are covered with 1 m thick gravel, which is followed upward by yellow fine to medium grained cross- bedded sands up to 2 m thick. The top of the section is built by 6 m thick gravel with intergranular space filled up with coarse-grained sand. The gravel particles are well rounded as the sizes vary from 1-2 cm to 7-8 cm. The particles are presented by various rock types but the quartz predominates. The top 1 m cover consists of dark grey compact soil-like clays. Alluvial loess and clay, 2.0-2.5 m thick, superpose the profile from Lehchevo village to the north (wells NN 58 and 75), as their thickness increases northward. The thickness of gravel deposited at the base of the section of well N29 drilled between Nivyanin and Komarevo villages is 3 m, as it lies on the eroded surface of grey marly clay. The gravel is poorly cemented with clayey-sandy cement. The gravel particles are small (1-2 cm size), semi-rounded to well rounded, consisting predominantly of limestone and quartz. The gravel is covered with gray- brown compact fat clay up to 3 m thick. The sand and clay portions in alluvium increase to the north, while the gravel portions decrease (well N72 at Krushovitsa village). The alluvium from the flood terraces of the Danube, Ogosta and Skat rivers becomes mixed in the river mouths.