填埋场黏土衬垫中起始水力梯度对污染物运移影响

Influence of initial hydraulic gradient on contaminant migration in seepage-proofing clay liner of landfills

垃圾填埋场黏土衬垫对防止渗沥液的渗漏和运移起到至关重要的作用,其中有些黏土渗流呈现非达西渗流现象,存在起始水力梯度i0,然而起始水力梯度对黏土衬垫防污性能的影响尚未明确。为此,开展了起始水力梯度对污染物运移的影响研究。首先,在不同围压作用下,采用去离子水对黏土进行渗透试验;然后,在围压σ=200 kPa的条件下,分别采用水力梯度为0,12.5,25.0和375.0对4个平行黏土试样进行Na+溶液渗透,定期测定渗出液Na+浓度,并最终分层测定试样中Na+的含量。结果表明:随着固结压力的增大,起始水力梯度越大;当水力梯度分别0,12.5和25.0时,渗出口无渗透液产生,并且Na+在黏土中的迁移规律基本相同,即分子扩散系数基本相等;当水力梯度为375.0时,水动力弥散系数远远大于分子扩散系数。因此,可以认为当水力梯度小于i0时,渗沥液不渗透或渗透很小,污染物运移以分子扩散为主;当水力梯度大于i0时,污染物运移逐渐转为以对流为主。

Clay liners play a crucial role in preventing leakage and migration of landfill leachate.Non-darcy seepage often appears in clay,with an initial hydraulic gradient.However,the effect of initial hydraulic gradient on the anti-fouling performance of clay liners is not yet clear.Therefore,the influence of initial hydraulic gradient on contaminant migration was studied in this paper.Firstly,the deionized water was used for the clay penetration test under different confining pressure.Secondly,under the confining pressureσof 200 kpa,Na+solution was used for permeability test under 4 parallel compacted clay samples with hydraulic gradients of 0,12.5,25 and 375,the concentration of Na+in the leachate was determined regularly.Finally,the concentration of Na+in the samples was determined in layers.The results show that the initial hydraulic gradient increases with the increase of consolidation pressure.When the hydraulic gradient is 0,12.5,and 25,there was no osmotic liquid at the seepage outlet and the Na+migration rule is basically the same,i.e.,the molecular diffusion coefficient is almost equal.When the hydraulic gradient was 375,the hydrodynamic dispersion coefficient is much larger than the molecular diffusion coefficient.Therefore,it can be considered that when the hydraulic gradient is less than i0,the leachate is impermeable or small permeable,and the pollutant migration is dominated by molecular diffusion.When the hydraulic gradient is greater than i0,the pollutant migration gradually turns to convection.