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Transfert des nutriments dans le bassin du Danube et apports a la Mer Noire: Modelisation et bilans

Domenii publicaţii > Ştiinţele pământului şi planetare + Tipuri publicaţii > Tezã de doctorat (nepublicatã)

Autori: Trifu (Raducu) Maria Cristina

Editorial: 2002.

Rezumat:

Un obiectiv major al acestei teze, realizate in cotutela intre Universitatea Paris 6 si Universitatea de Constructii din Bucuresti, a fost intelegerea si cuantificarea fluxurilor de nutrienti transportati de fluviul Dunarea la Marea Neagra, tinand cont de modificarea activitatilor umane din ansamblul bazinului hidrografic, inclusiv Delta Dunarii, de-a lungul unei perioade de timp care include caderea blocului din Est in 1990 (1988 – 1997).

Modelul RIVERSTRAHLER (Billen et al., 1994 ; Garnier et al., 1995), care permite simularea principalelor variabile de calitate a apei, inclusiv a nutrientilor (azot, fosfor, siliciu), a fost aplicat la nivelul bazinului hidrografic al Dunarii. Modelul contine 11 subazine, un ax principal al Dunarii, de la confluenta cu raul Inn pana la intrarea in Delta Dunarii, precum si principalele rezervoare din bazin. Structura modulara a modelului a permis elaborarea in cadrul acestei teze a unui model pentru regiunea inundabila a deltei Dunarii. Variabilele impuse modelului privind geomorfologia, hidrologia, meteorologia si aporturile difuze si punctuale, au trebuit sa fie colectate si analizate la aceasta scara spatio – temporala. Validarea modelului a necesitat de asemenea si colectarea datelor de calitate a apei, tot la aceeasi scara. Din pacate aceasta baza de date a fost cateodata limitata.

Incepand cu anii 90, diminuarea activitatilor economice in tarile din Est a determinat scaderea concentratilor de azot si fosfor in Marea Neagra. Pentru a analiza calitatea apei in urma schimbarilor survenite in sectorul industrial, domestic (reducerea cantitatii de fosfor in detergenti) si agricol (descresterea cantitatilor de ingrasaminte), in cadrul tezei s-a urmarit o analiza comparativa a perioadelor de dinainte si dupa 1991.

De asemenea s-a realizat o analiza detaliata a rezervoarelor din bazinul Olt, un tip extrem de subazin in cadrul bazinului Dunarii din punct de vedere al amenajarilor hidraulice (38 rezervoare). Aceasta analiza dovedeste rolul retentiei de siliciu la nivelul bazinului hidrografic (aproximativ 45% din fluxul raului Olt in prezenta rezervoarelor, pentru anul 1997). In acelasi timp s-a investigat posibila retentie a nutrientilor in Delta Dunarii atat prin bilant, plecand de la masuratorile de calitate a apei si a debitului, cat si prin modelare, cu modelul RIVERSTRAHLER, pentru perioada 1994 – 1997. Rezultatele arata ca aceasta zona umida, in pofida suprafetei sale mari, retine o cantitate redusa de azot (-9 ¸ 20%), dar reprezinta o „rezerva” pentru fosfati (15 ¸ 30% retentie din fluxurile de intrare).

Bilanturile aporturilor la Marea Neagra, calculate prin modelare, pentru toate sistemele hidrice ale Dunarii (subazine, ax principal si delta) dovedesc o retinere a siliciului in toate componentele sistemului fluviului Dunarea si confirma inca o data ca rolul deltei ca „zona tampon” este relativ redus. Fluxurile au fost calculate pentru un an uscat (1993) si unul umid (1996).

In final au fost construite diverse scenarii pentru a se evalua consecintele acestora asupra nivelului poluarii si a calitatii apei. Rolul marilor rezervoare de pe axul principal a fost diferentiat, fata de rolul rezervoarelor din bazinul hidrografic al Dunarii. Calculele arata ca barajul Portile de Fier, adesea desemnat ca principal loc al retentiei de siliciu, joaca numai un rol modest (3% retentie din fluxul de intrare in delta). Scenarii de perspectiva, bazate pe o ameliorare a metodelor de tratare a apelor uzate si o reducere a ingrasamintelor azotoase, conduc la posibile reduceri ale aporturilor de N si P in Marea Neagra (6-12 % et respectiv 60-70 %).
The central goal of these thesis, realised in „co-tutelle ” between University of Paris 6 and University of Bucharest, was to understand and to quantify the nutrients flux deliver by the Danube river system into the Black Sea, by taking into account the human activities modifications in the whole basin and its delta, for a long period of time which include the break-up of the East block in 1990 (1988 – 1997).

The RIVERSTRAHLER model (Billen et al., 1994 ; Garnier et al., 1995), that allows to simulate the main variables of quality water, the nutrients (nitrogen, phosphorus, silica) being included, was implemented to the scale of the Danube basin. The model comprises 11 sub-basins, a main branch of the Danube River, from the confluence with the Inn River to the beginning of the Danube Delta, and the mains reservoirs. The structure of the model did allow, in the framework of these thesis, to elaborate a model for the flood region of the Danube Delta.

There were the geomorphological, hydrological and meteorological constraints, as well as the diffuse and point sources of nutrients, which we must to gather at this spatial and temporal scale. For the validation of the model’s simulations it was also necessary to gather the water quality measurements at the same spatial scale. Unfortunately this database was sometimes very scarce.

Since the beginning of the nineties, the reduction of the economical activities from the East countries has induced an abrupt drop of the nitrogen and phosphorus concentrations in the Black Sea. In order to analyse the changes of the industrial and human activities (e.g. the reduction in the use of P-containing washing powders) as well as of the agricultural practices (the agricultural fertilizers diminution) on the water quality, one attempt, in the framework of these thesis, to realise a comparative study for the periods before and after 1991.

A focus is made on the Olt River, which is an extreme type of sub-basin in the Danube watershed, from the impoundment’s point of view (38 reservoirs). This study shows the role of the reservoirs in the silica retention at the scale of its drainage network (about 45% of the Olt River flux in the presence of reservoirs, for year 1997). Likewise a possible retention of nutrients in the Danube Delta was investigated by budgeting, using the measures of water quality and discharge, and by modelling RIVERSTRAHLER approaches, for the period 1994 – 1997. The results show that this wet area, in despite of their great surface, retain a small quantity of nitrogen (-9 ¸ 20%), but represent a skin for the phosphates (15 ¸ 30% of the influx is retain).

The resulting contribution budget, delivery in the Black Sea, for the whole hydrological systems of the Danube catchment (sub-basins, main branch and Danube delta), shows silica retention in every component of the Danube river system and represent the confirmation of the small role of the Danube delta like a „tampon area”. The contribution flux was calculated for a dry year (1993) and a humid one (1996).

Finally, different scenarios were elaborated in order to estimate there consequences on the pollution level and water quality. The role of the great impoundments of the main branch was differentiated with those of sub-basins. The results suggest that the Iron Gate reservoir, often designed like the major site for silica retention, has only a small role (3% of the influx in the delta is retain). The prospective scenarios, based on the improvement of the wastewater treatment and the reduction of the agricultural fertilizers, show possible reductions of N and P contributions delivery by the Danube River (6-12 % et 60-70 % respectif).

Cuvinte cheie: bazinul Dunarii, modelare ecologica, retentie nutrienti, rezervoare, Delta Dunarii // Danube basin, ecological modelling, nutrient retention, reservoirs, Danube Delta