Transport model of potentially toxic elements through unsaturated zone at regional wellfield Kosnica : doctoral thesis

Abstract

Predmet istraživanja ove doktorske disertacije je nesaturirana zona na području regionalnog vodocrpilišta Kosnica. Ciljevi ovog doktorskog rada bili su: (i) identificirati dominantne procese transporta u nesaturiranoj zoni; (ii) odrediti parametre toka vode i transporta onečišćivala nesaturirane zone; (iii) odrediti geokemijsko ponašanje potencijalno toksičnih elemenata u nesaturiranoj zoni; (iiii) izraditi prognozni model toka vode i transporta potencijalno toksičnih elemenata (olovo, kadmij, cink) kroz nesaturiranu zonu. Temeljem brojnih laboratorijskih analiza svojstava tla i eksperimenata definirani su parametri toka vode (kapacitet tla za vodu, hidraulička vodljivost) i transporta onečišćivala (sorpcija, disperzija, volumna gustoća, koncentracija potencijalno toksičnih elemenata). Brojnim kemijskim i eksperimentalnim analizama uzoraka iz nesaturirane zone određeno je geokemijsko ponašanje potencijalno toksičnih elemenata. Za primjenu numeričkih simulacija korišten je program HYDRUS-1D. Kapacitet zadržavanja vode u tlu najveći je u A horizontu i iznosi ~45%. Na temelju istraživanja hidrauličke vodljivosti utvrđene su najniže vrijednosti za A i IVGso/Gr horizonte. Veće vrijednosti hidrauličke vodljivosti određene su u donjem dijelu profila u kojima je zastupljena pjeskovito-šljunkovita i šljunkovita tekstura. U AC-IC horizontu je određena manja vrijednost disperzivnosti koja iznosi 1 cm, dok je za horizont IIC/Gso određena veća vrijednost i iznosi 1,7 cm. Utvrđene su male razlike u sorpciji između pojedinih elemenata. Analizom otopina porušenih uzoraka dobivenih digestijom zlatotopkom utvrđene su najveće koncentracije elemenata (Cd, Pb i Zn) u najplićem dijelu profila. Na temelju dobivenih parametara toka vode i transporta potencijalno toksičnih elemenata kreiran je prognozni model na istražnoj lokaciji. Ukoliko se usporede koncentracije potencijalno toksičnih elemenata dobivene s prognoznim modelom transporta na dnu profila tla odnosno na granici s otvorenim vodonosnikom s MDK granicama pravilnika za iste elemente onda se može zaključiti da postoji velika opasnost od onečišćenja na istražnoj lokaciji. Mehanizmi sorpcije imaju glavni utjecaj na transport elemenata u saturiranu zonu. Uslijed akcidentnih situacija izlijevanja koncentracije potencijalno toksičnih elemenata od 1000 mg/l, rizik od onečišćenja vodonosnika s time i regionalnoga vodocrpilišta Kosnica je visok. Ovim istraživanjima nesaturirane zone na lokaciji Kosnica definirani su parametri toka i transporta potencijalno toksičnih elemenata te je napravljen numerički model kao osnova za buduća istraživanja akcidentnih situacija.The subject of this thesis is the unsaturated zone in the area of the regional water well Kosnica. The objectives of this thesis were: (i) to identify dominant processes in the unsaturated zone; (ii) to determine water flow and solute transport parameters in the unsaturated zone (iii) to determine geochemical behaviour of potentially toxic elements in the unsaturated zone; (iiii) to build prognostic water flow and the transport model of potentially toxic elements (lead, cadmium, zinc) through the unsaturated zone. The research was conducted in the area of the Kosnica water well, dominated by Fluvisol soil type. In order to better understand the water flow and the transport of the potentially toxic elements through an unsaturated zone detail field, laboratory analyses and experiments were made. For numerical simulation HYDRUS-1D program was used. The program models the water flow using Richard's equation. The transport of potentially toxic elements is described using the advection and dispersion equation. Based on the laboratory analyses of soil properties, the parameters of water flow and solute transport are defined. The bulk density rises with depth while the water holding capacity is the largest in the A horizon due to particle size distribution. Minimum values of hydraulic conductivity were determined in the A and IVGso/Gr horizons. The experimental results of the sorption processes were used for the determination of correlation coefficients for individual elements. The order of decreasing correlation coefficients was: Pb> Zn> Cd. Small differences in sorption were found between the individual elements. The soil column dispersivity was 1 cm for AC-IC horizon, and 1.7 cm for horizon IIC/Gso. The higher cation exchange capacity, determined in the top of the profile, indicates the possibility of more pronounced sorption of elements in shallow horizons. The highest element concentrations, extracted by aqua regia, were found in the A and AC-IC horizons (for Zn 70 mg/kg and for Pb 51 mg/kg). The lead concentration obtained using sequential extraction analysis is mostly bound to the organic fraction, while zinc is mostly bound to the residual fraction. Illitic material is the most common mineral phase in the fraction <2 μm and correlates with the sorption of lead in top horizons (A and AC-IC). Measured values of water content in the soil using TDR probes for the period from September 1, 2011 to October 31, 2012 were used for the water flow model calibration. The comparison of water simulations’ results and the measured data was performed using the HYDRUS 1D software. The simulated water content was less variable than the measured values. Low correlation (~ 0.40) was revealed between the measured and the simulated values of the water content. Such a low correlation between the measured and the simulated water content in the soil could be a consequence of the difference between the meteorological data obtained from a weather station located 3.5 km from the investigated location. Based on the chloride tracer experiments on soil columns in the laboratory, the calibration of the solute transport model was made. The best correlation of the measured and the simulated values of tracer concentration is 0.43 for AC-IC and 0.74 for IIC/Gso horizon. Based on the parameters of the water flow and the transport of potentially toxic elements, the prognostic model was created at the investigation site. If the concentrations of potentially toxic elements, obtained by the transport prognostic model at the bottom of the soil profile are compared with the MAC limits ordinance for the same elements, it can be concluded that there is a high risk of contamination at the investigative site. Sorption mechanisms have a major impact on the transport of potentially toxic elements in the saturated zone. In case of an accidental spill of potentially toxic elements with concentrations of 1000 mg/l, the risk of contamination of the aquifer is high. This research of unsaturated zone at the Kosnica location defined the water flow and the transport of potentially toxic elements’ parameters and designed a model for the future research and accident mitigation