井间分溶示踪估计重非水相污染物残留量的影响因素数值分析

Numerical analysis of the influencing factors for estimating DNAPL residual by the partitioning interwell tracer tests

对于重非水相污染场地,传统的钻孔取样方式因经费和取样个数的限制,往往无法准确估计含水层中重非水相污染物的残留量。井间分溶示踪法费用较低,且不会对污染源区造成破坏,可适用于推估重非水相污染物残留量,但目前该方法的准确性尚未得到验证。本文基于数值模拟方法探讨了示踪剂注入速率、示踪剂类型以及污染源区结构等多因素对井间分溶示踪估计重非水相污染物残留量准确性的影响。数值算例结果表明:井间分溶示踪实验估计重非水相污染物残留量的误差小于15%;降低示踪剂的注入速率使得分溶性示踪剂在非水相和水相流体间作用更充分,能够提高井间分溶示踪估计重非水相污染物残留量的精度,实验中示踪剂注入速率由500m^3/d降为250m^3/d后,多种情景的平均精度由91.68%提高至93%;选取低分溶系数的示踪剂有利于提高示踪剂的回收率,从而提高推估精度,实验中示踪剂由2,2-二甲基-3-戊醇改为己醇后,平均精度由87.83%提高至96.85%;复杂的重非水相液体污染源区结构易于出现示踪剂绕流等现象,导致井间分溶示踪估计重非水相污染物残留量的误差增大,实验中含水层由均质变为非均质后,平均精度由93.03%变为91.65%。重非水相污染场地调查时,建议结合数值模拟方法选择适宜的示踪剂和示踪剂注入速率,以提高污染场地的刻画精度。

For dense non-aqueous phase liquids(DNAPL)sites,the traditional geological drilling methods are limited by the cost and the number of samples.It is often unable to accurately estimate the residual amount of DNAPL in aquifers.The partitioning inter-well tracer tests(PITT)has low cost and will not cause damage to the source areas,so it can be used to estimate the residual amount of DNAPL.However,the accuracy of this method has not been verified.Based on the numerical simulation method,the effects of various factors,such as tracer injection rate,tracer type and source zone architectures,on the accuracy of estimating the residual amount of DNAPL in PITT are discussed.The results of numerical examples show that the error of PITT in estimating the residual amount of DNAPL is within 15%.Reducing the injection rate of the tracer makes the partitioning tracer more fully interact with NAPL phase and water phase,which can improve the accuracy of PITT estimation of DNAPL residue.After the tracer injection rate decreases from 500 m 3/d to 250 m 3/d in the experiment,the average accuracy of various scenarios increases from 91.68%to 93%.The selection of tracer with low partitioning coefficient is beneficial to improve the recovery rate of tracer,so as to improve the accuracy of estimation.In the experiment,after the tracer changes from 2,2-dimethyl-3-pentanol to hexanol,the average accuracy increased from 87.83%to 96.85%.Complex source zone architectures are more likely to cause tracer bypass,which makes the error of estimating DNAPL residual larger.In the experiment,after the aquifer changes from homogeneous to heterogeneous,the average accuracy changes from 93.03%to 91.65%.Before using PITT to carry out site investigation,it is advisable to select appropriate tracer and tracer injection rate by combining with numerical simulation method to improve the precision of characterization of contaminated sites.