The Field-Scale Investigation of the Low Mobility of Drainage Canal Sediments Polluted by Copper in Lowland Area of Croatia

The sedimentation of drainage canals is a common process and its intensity depends on several geographical and hydrological factors. Drainage canal sediments are frequently polluted by heavy metals or other pollutants; they need to be periodically dredged and ultimately, have to be safely disposed of. Furthermore, pollution in smaller drainages may go undetected because under the Water Framework Directive (Directive 2000/60/EC), catchment areas < 10 km2 do not require monitoring. We investigated the hypothesis that water resources of small sub-catchments exposed to agricultural pollutants accumulate sediment for a longer period (several years) and severely enhance environmental risks. We analyzed the data on sediment mobility in drainage canals for a small lowland catchment in Croatia during 2013–2017. We conducted sediment transport modelling for actual precipitation episodes of a 10-year return period and design precipitation of a 50-year return period. The results indicated that sediments and associated copper pollution persist at the canal bottom for several years, which increases the risk of polluting groundwater and the environment in general. Only copper present at the maximum downstream section of the canal has the possibility of moving to the recipient stream and would only be detected in catchment areas bigger than 10 km2. We proved that smaller water bodies evaluated according to monitoring standards prescribed for the closest larger water can enhance environmental risks.

Analysis of drainage canal defects and review of canal cleaner designs

The drainage system elements, namely, the canals, lose their initial design characteristics over time. In general, the canals with unstable profiles are often in the reclamation construction. The problems of unstable profiles of the canals are, primarily, related to the physical and mechanical properties of soils, formation of internal mechanical, as well as chemical and biological processes occurring by the action of natural conditions. Maximum stability is characteristic for canals formed in cohesive organic soils, as well as in various kinds of peat with their thickness considerably exceeding the depth of the canal. With insignificant peat thickness above mineral soils, the stability of the drainage canal slopes will be rather insignificant. Wood peats are the most stable among organic soils. The canals laid in sapropelic soils characterized by the natural flow have rather low stability. That is why, the required stability in wet soils can be achieved by removing the excess moisture. The slopes of the canals made in various clays and loams with their large thickness considerably exceeding the depth of the canal have relatively high stability.