engleski

NUMERICAL ANALYSIS OF ESCHERICHIA COLI TRANSPORT THROUGH THE KARST ENVIRONMENT (EXAMPLE OF BOKANJAC- POLIČNIK AQUIFER, REPUBLIC of CROATIA)

This paper analyzes a pollutant transport (the principal component is E. coli) through the karst aquifer Bokanjac-Poličnik, located in pumping site of the Zadar water supply system. The main pollution sources are dysfunctional septic tanks (no public sewerage system is available) in the immediate vicinity of the pumping site. E. coli concentrations in water were monitored in the B5 well. Numerical model Hydrus-1D was used in the analysis of the pollution transport through the vadose zone („dual-permeability“ module for flow and „physical nonequlibrium“ module for transport). The model was forced by daily precipitation data from two ombrographic stations. The model assumes flow and transport through two mutually interactive pore zones - matrix and fracture zones. The formulation is based on the Richards-Richardson equation. Transfer of dissolved or colloidal substances comprises convection processes (through water flow) and diffusion (through concentration gradient). The spatial domain of the transport model through the vadose zone was 8 m long and was discretized in vertical direction by 800 elements, each 1 cm in length. Effect of plant uptake and evaporation process was not taken into consideration. The transport through the horizontal (saturated) section was analyzed assuming that the uniform and stationary flow velocity was 1 cm/s, which corresponded to the results of an in-situ measurement of apparent horizontal flow velocity from the rural settlements to the B5 well. In the horizontal section, sorption was not considered due to the relatively large diameter of the fractures in the approximately horizontally fractured channels. The results confirm that the surrounding settlements with no public drainage system and dysfunctional septic tanks are the main source of fecal contamination, which is transported to the water supply wells. The analysis shows that the increased E. coli concentrations in the well occurs after 7 days of intensive precipitation.

karst aquifer, Escherichia coli transport, numerical model

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