Hydrogeological characteristics and protection zones of Osekovo wellfield

The future wellfield Osekovo will present the framework of the Moslavacka Posavina Regional Water Supply System. It consists of five wells, total capacity 200 l/s. It is situated in the Sava River depression. The aquifer is highly heterogeneous regarding its spreading and its depth, which is due to climate changes and neotectonic movements during its depositing (between Middle Pleistocene and Holocene). The aquifer laterally changes into clay deposits to the north, extends to the Sava River to the west and south, and sinks to the east. The Sava impact on the groundwater level in the wellfield site is reduced because of a fault shift that had caused a considerable thinning of the aquifer and weakening of its direct hydraulic connection with the Sava. An average aquifer depth is between 25 and 130 m. The lithological composition is dominantly sand, with some subordinate gravel. The aquifer hydrological parameters were determined using pumping test results for five wells. Transmissivity ranges between 648 and 1235 m/day, and storage from 8.7×10-4 to 9.5×10-4. Hydraulic conductivity (20 to 40 m/day) was determined by grain size analysis of 47 samples from 10 structural boreholes. The overburden lithological composition is dominantly clay with different silt and gravel content. The bottom layer consists of clay. The wellfield protection areas are determined by the relevant Ordinance (OG 66/11) which stipulates three protection zones. The first sanitary protection zone encompasses an area of minimum 10 m from each well in all directions protected by minimum 2 m high fence. Total first protection zone area for the five wells is 1825 m2. The second sanitary protection zone encompasses an area outside the first zone boundary to the line from which groundwater travel to the well lasts minimum 50 days. In case vertical water flow from the surface to the aquifer lasts for more than 50 days, the second zone is not determined. The water particle travelling time through the aquifer overburden by vertical flow in Osekovo is estimated at 1923 days, thus it is concluded that the second protection zone is unnecessary. The third protection zone envisages minimum water retention time in the underground of 25 years of horizontal flow. The third zone was determined by a numerical groundwater flow model and particle tracing. MODFLOW software was used for groundwater flow and MODPATH software for particle tracing simulation. Total area of the third protection zone is 24 km2.