Impacts Of The Preferential Flow Paths On The Nitrate Reactive Transport Processes In Aquifer

Nitrate contamination in groundwater is one of the environmental problems in the world.The excessive nitrate in groundwater poses a serious threat to the stable development of ecosystem,water quality safety and human health.Therefore,it is urgent to carry out the remediation work of nitrate contamination in groundwater,and understanding the influence of hydrodynamic conditions in heterogeneous media on the nitrate reactive transport processes is the key of the remediation of nitrate contaminated sites.Preferential flow paths affect the distribution of groundwater velocity in heterogeneous aquifers and thus control the process of nitrate migration and transformation.At present it is generally believed that the existence of preferential flow paths is not conducive to the removal of nitrate.However,it is not clear whether there is a significant difference in the influence of preferential flow paths on nitrate reactive transport processses under different flow conditions and the control mechanism of preferential flow paths on nitrate migration and transformation need to be understood more deeply,what’s more,the related basic research is very lack.Based on the above background,the tongzhou site in Beijing with strong heterogeneity was used as the research site in this study,focusing on the scientific question that“how do preferential flow paths in the aquifer control the process of nitrate transport and reactive,and then how do they affect the removal of nitrate?”.Firstly,the spatial distribution of the hydraulic conductivity of the aquifer is inverted by combining the inert tracer tests with the numerical model,and based on which the connectivity of different paths was evaluated,so as to identify the preferential flow paths in the aquifer of the Tongzhou site.Subsequently,nitrate reactive transport tracer tests were carried out under natural and forced hydraulic conditions respectively,and the characteristics of nitrate reactive transport processes were identified by means of hydrochemistry and isotope analysis.Finally,the nitrate reactive transport model under natural and forced hydraulic gradients were established,and the indexes related temporal and spatial moment were calculated and analyzed based on the simulated breakthrough curves and solute plume,so as to quantitatively evaluate the impacts of preferential flow paths on the nitrate reactive transport processes and reveal the relevant control mechanisms.The results can not only expand the control theory of the hydrodynamic conditions on the nitrate reactive transport processes in the three-dimensional flow system,but also provide scientific and reasonable suggestions for the scheme design of the nitrate contamination remediation and the risk management related to nitrate pollution.Through the above studies,the following aspects can be learned:（1）Characteristics of the aquifer heterogeneity and the preferential flow pathsThe results of bromine and heat tracer tests show that relative to the aquifers at the depths of 5 m and 15 m,tracer penetrates further in the aquifer at the depth of 10m,and in each monitoring well between the injection and extraction well,bromide concentration and temperature breakthrough curves at the depth of 10 m generally have larger peaks and earlier arrivel time of the peaks,indicate that the permeability is better in the aquifer at the depth of 10 m.In addition,the spatial distribution of the hydraulic conductivity inverted by the bromine solute transport and the water-heat coupling models show that the permeability of the aquifer at the depth of 10 m is relatively high,with the value of hydraulic conductivity rangs from 2.2 m/d to 7.5 m/d;the rangs of the hydraulic conductivity values of the aquifers at depth of 5 m and 10 m are 0.5-3 m/d and 0.45-3.5 m/d respectively.The calculation results of the minimum hydraulic resistance between the injection well and each monitoring well show:（1）at the depth of 5 m,compared to the path between well 3#3 and the injection well,the minimum hydraulic resistances of paths between well 4#3,4#5,5#3 and the injection well are smaller,indicating better connectivity and they are preferential flow paths;（2）at the depth of 10 m,firstly,compared to the path between well 4#5 and the injection well,the minimum hydraulic resistances of path between wells 4#3,3#3 and the injection well are smaller,indicating better connectivity and they are preferential flow paths;secondly,compared to the path between well 5#3 and the injection well,the minimum hydraulic resistance of path between well 2#3 and the injection well is smaller,indicating better connectivity and they are preferential flow paths;（3）at the depth of 15 m,during all paths,the minimum hydraulic resistance of path between well 2#3 and the injection well is smallest,indicating it is a preferential flow path.（2）Nitrate reactive transport processes in the heterogenous aquiferThe results of the nitrate reactive transport tracer test under the natural hydraulic gradient condition show:（1）under the natural condition,the mean hydraulic gradient is～0.014,the groundwater flows from the southeast to the northwest;（2）no significant peak of Br^-concentration and no significant enrichment ofδ¹⁵N andδ¹⁸O in nitrate were observed at most of the monitoring wells,indicating the injected NO₃^--N remained within 2 m around the injection well during the whole test period of 450 h with the natural hydraulic gradient;（3）during the observation phase,the obviours enrichment ofδ¹⁵N andδ¹⁸O in nitrate in the well 4#4 was observed.In addition,in this well,when the peaks of Br-,NO₃^--N and CH₃COO^-concentration were observed,the accumulation of NO₂^--N and NH₄⁺-N also happened.These results suggested that denitrification and reduction of nitrate to ammonium were both occurred in the aquifer.However,NH₄⁺-N only appeared in the early stage when the concentration of CH₃COO^-was high and the concentration of NO₃^--N was still low.When the concentration of NO₃^--N begins to rise,NH₄⁺-N disappeared and NO₂^--N begins to accumulate,indicating that only a small amount of NO₃^--N participated in the process of nitrate reduction to ammonium,and denitrification is the main reduction mechanism of NO₃^--N in the aquifer.The results of the nitrate reactive transport tracer test under the forced hydraulic gradient condition show:（1）under this condition,the mean hydraulic gradient is～0.8,the groundwater also mainly flows from the southeast to the northwest;（2）at depths of 5 m,10 m and 15 m,in wells where peaks of Br^-,NO₃^--N and CH₃COO^-concentration were observed,the accumulation of NO₂^--N was also observed,but there was no NH⁺₄-N produced.At the same time,theδ¹⁵N andδ¹⁸O in nitrate in these wells enriched during the test.These results indicat that the mechnism of nitrate reduction is denitrification and there was no evidence that the reaction of nitrate reduction to ammonium occurred;（3）at depth of 15 m,theδ¹⁵N andδ¹⁸O in nitrate only enriched in well 2#3,but they are more negative than that in aquifers at 5 m and10 m depths,suggesting the a lower degree of denitrification in aquifer at depth of 15m,and a higher degree of denitrification in aquifers at depths of 5 m and 10 m.（3）Impacts of the preferential flow paths on nitrate transport processesFirstly,the calculation based on the simulation data of nitrate reactive transport models with the natural and forced hydraulic gradient conditions show that preferential flow occurred in the recognised preferential flow paths under the forced hydraulic gradient condition,the average groundwater velocity and the mass flux of nitrate in the recognised preferential flow paths increase by 5.1%-64%and 3.6-143%.Based on these results,on the one hand,the feasibility of using the minimum hydraulic resistance as an effective measure to quantify the connectivity of aquifers and than to accurately identify the preferential flow paths in aquifers was verified;on the other hand,it wad enphasized that preferential flow paths can quickly transport a large number of NO₃^-,increasing the potential risk of NO₃^-pollution in the downstream aquifer or waters.Secondly,compared with the difference of velocity between fractures and matrix,difference of the mean velocity between the preferential flow paths and the non-preferential flow paths reported in this research are smaller.The difference of average flow velocity between the preferential flow paths and the non-preferential flow paths reported in this research are significantly smaller than that reported in fractured aquifers,but larger than that reported in soil macropore,indicating the degree of preferential flow is highly related to the degree of aquifer heterogeneity and the scale of the preferential flow paths.The greater the degree of quifer heterogeneity,the more significant the degree of preferential flow in the preferential flow paths.In addition,compared with the small-scale macropore,the large-scale preferential flow paths have more important significance for the preferential transport of nitrate.Finally,the analysis based on the simulaion data also show that the pferential flow paths in aquifer do not always produce preferential flow（faster groundwater flow velocity and larger NO₃^-mass flux）.It depends on the magnitude of the hydraulic gradient and the spatial relationship between the distribution direction of the preferrential flow paths and the direction of the hydraulic gradient.Under the natural condition with the average hydraulic gradient of 0.014,there was no obvious preferential flow.However,under the forced flow condition with the average hydraulic gradient of 0.8,the distribution direction of the preferrential flow path is perpendicular to the direction of the forced hydraulic gradient,consequently,the maximum NO₃^-mass flux does not convergence to the preferential flow path rather to the low-permeability path which develops along the direction of the hydraulic gradient.Thus,the threat of preferential flow paths to environmental pollution can be artificially controlled by changing flow conditions（hydraulic gradient or its direction）.（4）Control mechanisms of the preferential flow paths on denitrificationThe preferential flow paths mainly control the denitrification,and further affect the removal of nitrate from two aspects.On the one hand,the calculation results based on the simulated data show that for the different paths located at the same depth under the forced hydraulic gradient condition,the removel rate of nitrate reduced by 6.53%-91.7%,and correspondingly the mean travel time along the preferential flow paths reduced 11.41%-38.94%,indicating that the preferential flowpaths greatly shorten the average residence time of nitrate in the aquifer,so as to shorten the duration of denitrification,makes less of NO₃^-has been removed.On the other hand,preferential flow paths regulate the groundwater velocity,thus affecting the denitrification rate and the removal efficiency of nitrate.However,the cimpacts are quite opposite in natural and forced hydraulic gradients condition reported in this study.Under the natural hydraulic gradient condition with the velocity range 0f 0.0017m/h-0.008 m/h,compared with the average velocity of 0.003 m/h at the depth of 5 m,the average velocity of 0.008 m/h at the depth of 10 m is larger,however,the removel rate of nitrate per hour at depth of 10 m is 0.6%,which is higher than that at depth of5 m（0.21%）.This result indicates that the larger velocity along the preferential flow path can increase the denitrification rate,promote the ocurrence of denitrification and facilitate the removal of NO₃^-.This may because under the natural hydraulic gradient condition,the slow groundwater velocity limits the solute transport rate to the microbial community,greater groundwater flow velocity in the preferential flow path can increase the the amount of available NO₃^-and CH₃COO^-per unit time.Under the forced hydraulic gradient condition with the velocity range 0f 0.01m/h-0.05 m/h,compared with the average velocity of 0.033 m/h at the depth of 5 m,the average velocity of 0.035 m/h at the depth of 10 m is larger,however,the removel rate of nitrate per hour at depth of 10 m is 0.36%,which is lower than that at depth of5 m（0.39%）.This result indicates that the larger velocity along the preferential flow path can reduce the denitrification rate,limite the ocurrence of denitrification and facilitate the removal of NO₃^-.This may because under the forced hydraulic gradient condition,the greater groundwater flow velocity in the preferential flow path extreamly short the contact time between the microorganism and the reaction substrate.（5）Application of the control mechanismFrom the results of the literature research,the groundwater velocity ranges coresponding to the natural and forced hydraulic gradient condition reported in this study can respectively well represent the groundwater velocity(usually less than 10^-2m/h)in low permeability media which is mainly composed of clay or silt,and the groundwater velocity（usually greater than 10-2 m/h）in high permeability medium which is mainly composed of sandy soil.Therefore,the control mechanism and reletive conclusion of this study can provide reference for other studies.In addition,the results of this study also show that when the groundwater flow change from a static state to a high-speed flow state,there will be a critical velocity value under which the influence of groundwater velocity on denitrification will change from a positive effect to a negtive effect.In addition,the results of this study indicate that the change of nitrate concentration may also cause the reversal of the impacts.Therefore,for evaluating the hydrological function of priority flow channels in the ecological environment,it is very important to consider the spatial and temporal variation of nitrate concentration and groundwater velocity caused by the change of flow boundary conditions.Based on the conclusion mentioned above,we suggested that before the design of remediation scheme for a nitrate-contaminated site,identificating and delining the preferential flow paths in the aquifer and the biogeochemical sites with high microbial activity for denitrification firstly,and then controlling the nitrate migration path by applying a forced hydraulic gradient or controlling the groundwater velocity in the preferential flow paths by taking some effective measures,it will help to improve the remediation efficiency of the nitrate-contaminated site.The innovation of this study lies in:（1）The smallest hydraulic resisdence calculated based on the spatial distribution of hydraulic conductivity was used as a quantitative evaluation index to quantify the connectivity of the aquifer,so that the preferential flow paths were accurately identified.This method can provide reference for other studies;（2）the bidirectional influence of preferential flow paths on the denitrification under two different hydraulic gradient conditions were elucidated,and the relative control mechanisms were revealed.These results can enrich the control theory of nitrate migration and transformation process under varying dynamic conditions.