Heavy clay sediments are regarded "safe" from the hydrological point of view due to their low hydraulic conductivities. However, the formation of desiccation cracks in dispersive clays may dramatically change their bulk hydraulic properties. The impact of desiccation cracks on water percolation, dissolved salts and contaminants transport and redox related reactions (microbial ammonium oxidation and denitrification)were investigated in 6 -12 m clay layer near a diary farm waste lagoon. The study implemented unique vadose-zone monitoring systems that enable in-situ measurements of the temporal variation of the sediment's water content along with frequent sampling of the sediment's pore water along the entire vadose zone (> 30 m). Results from four years ofcontinuous measurements showed quick rises in sediment water contentfollowing rain events and temporal wastewater overflows. The percolationpattern indicated dominance of preferential flow through adesiccation-cracks network crossing the entire clay sediment layer. Highwater-propagation velocities (0.4 - 23.6 m h-1) were observed,indicating that the desiccation-crack network remains open and serves asa preferential flow pathway year-round, even at high sediment watercontent (~0.50 m3 m-3). The rapid percolation bypassed the mostbio-geo-active parts of the soil, transporting even highly sorptivecontaminants (testosterone and estrogen) in to the deep sections of thevadose zone, accelerating the underlying groundwater contamination. Theammonium and nitrate concentrations in the vadose zone and the highnumber of nitrifying and denitrifying bacteria (~108 gene copiesgdry-sediemt-1, each) found in the sediment indicated that the entirevadose zone is aerated even at high water content conditions (~0.55 m3m-3). The dissolved salts concentration in the pore-water and theδ2H-H2O and δ18O-H2O values of the pore-water substantiallyincreased with depth (becoming less depleted) in the clay sediment,indicating deep soil evaporation. Daily fluctuation of the airtemperature in the desiccation cracks supported thermally induced airconvection within the cracks void and could explain the deep soilsalinization process. Combination of all the abovementioned observationsdemonstrated that the formation of desiccation cracks network indispersive clay sediments generates a bulk advection dominatedenvironment for both air and water flow, and that the reference to claysediments as "hydrologically safe" should to be reconsidered.
|Title of host publication||EGU General Assembly 2012, held 22-27 April, in Vienna, Austria., p.310|
|State||Published - 1 Apr 2012|