Evaluation of the vulnerability of a leaky aquifer considering the retardation effect of an aquitard for specific pollutants: case study in the Tongzhou Plain, China

Abstract

The effect of an aquitard above a leaky aquifer on retarding characteristic pollutants is very important for evaluating groundwater vulnerability. A case study is presented of the leaky aquifer in the Tongzhou Plain of China. On the basis of analyses of the environmental and hydrogeological characteristics of the study area, an indicator system to evaluate the leaky aquifer’s vulnerability was constructed, comprising eight indicators: the depth to groundwater (D), the net recharge (R), the impact of the vadose zone (I), the hydraulic conductivity of the phreatic aquifer (C), the pollution load (P), the phreatic water quality (Q), the ability of the aquitard to retard NH₄⁺ (A), and the ability of the aquitard to retard NO₃⁻ (N). The retardation capacity of the aquitard for the characteristic pollutants in the study area (ammonia nitrogen and nitrate nitrogen) was obtained by numerical modelling with a HYDRUS simulator. The results show that the retarding effect of the aquitard on nitrogen pollutants and the hydraulic conductivity of the phreatic aquifer have significant effects on the vulnerability of leaky aquifers, and that other factors such as groundwater depth and pollutant load, have relatively limited effects. The research can provide a reference methodology for evaluating the vulnerability of porous leaky aquifers in alluvial plain areas.

Résumé

L’incidence d’un aquitard, situé au-dessus d’un aquifère semi-captif, sur le retard de certains polluants caractéristiques est très important pour évaluer la vulnérabilité des eaux souterraines. Une étude de cas est présentée sur l’aquifère semi-captif de la plaine de Tongzhou en Chine. Sur la base de l’analyse des caractéristiques environnementales et hydrogéologiques de la zone d’étude, un système d’indicateurs destinés à évaluer la vulnérabilité de l’aquifère semi-captif a été élaboré; il comprend huit indicateurs: la profondeur des eaux souterraines (D), la recharge nette (R), l’impact de la zone non saturée (I), la conductivité hydraulique de l’aquifère superficiel (C), la charge polluante (P), la qualité de l’eau de l’aquifère superficiel (Q), la capacité de l’aquitard à retarder NH₄⁺ (A), et la capacité de la couche semi-imperméable à retarder NO₃⁻ (N). La capacité de l’aquitard à retarder les polluants caractéristiques de la zone d’étude (azote ammoniacal et azote nitreux) a été obtenue par modélisation numérique à l’aide du code HYDRUS. Les résultats montrent que l’effet retardateur de l’aquitard sur les polluants azotés et la conductivité hydraulique de l’aquifère superficiel ont des impacts significatifs sur la vulnérabilité de l’aquifère semi-captif, et que d’autres facteurs, comme la profondeur des eaux souterraines et la charge polluante, ont une incidence relativement limitée. Cette recherche fournit une méthodologie de référence pour évaluer la vulnérabilité des aquifères poreux semi-captifs dans les zones de plaine alluviale.

Resumen

El efecto de un acuitardo sobre un acuífero semiconfinado en el retraso de contaminantes característicos es muy importante para evaluar la vulnerabilidad de las aguas subterráneas. Se presenta un estudio de caso del acuífero semiconfinado en la llanura de Tongzhou, China. Sobre la base de los análisis de las características ambientales e hidrogeológicas del área de estudio, se construyó un sistema de indicadores para evaluar la vulnerabilidad del acuífero semiconfinado, que comprende ocho indicadores: la profundidad del agua subterránea (D), la recarga neta (R), el impacto de la zona no saturada (I), la conductividad hidráulica del acuífero freático (C), la carga contaminante (P), la calidad del agua freática (Q), la capacidad del acuitardo para retardar NH₄⁺ (A) y la capacidad del acuitardo para retardar NO₃⁻ (N). La capacidad de retardo del acuitardo para los contaminantes característicos de la zona de estudio (nitrógeno amoniacal y nitrógeno nítrico) se obtuvo mediante modelización numérica con un simulador HYDRUS. Los resultados muestran que el efecto retardante del acuitardo sobre los contaminantes de nitrógeno y la conductividad hidráulica del acuífero freático tienen efectos significativos sobre la vulnerabilidad de los acuíferos semiconfinados, y que otros factores, como la profundidad del agua subterránea y la carga contaminante, tienen efectos relativamente limitados. La investigación puede proporcionar una metodología de referencia para evaluar la vulnerabilidad de los acuíferos semiconfinados en áreas de llanuras aluviales.

摘要

位于越流含水层之上的隔水层对特征性污染物阻滞作用对于评估地下水的脆弱性非常重要。本研究以中国通州平原越流含水层为例。在对研究区环境和水文地质特征分析的基础上,构建了评估越流含水层脆弱性的指标体系,其中包括八个指标:地下水深度(D),净补给量(R),包气带影响(I),潜水含水层的渗透系数(C),污染负荷(P),潜水水质(Q),隔水层对NH₄⁺的阻滞能力(A)以及对NO₃⁻的阻滞能力(N)。利用HYDRUS模拟器进行数值建模,获得了研究区隔水层对特性污染物(氨氮和硝酸盐氮)的阻滞能力。结果表明,隔水层对氮污染物的阻滞作用和潜水含水层的渗透系数对越流含水层的脆弱性有着显著影响,而诸如地下水深度和污染物负荷等其他因素的影响相对有限。该研究可为评价冲积平原地区孔隙越流含水层的脆弱性提供参考方法。

Resumo

O efeito de um aquitardo acima de um aquífero semiconfinado no retardamento de poluentes característicos é muito importante para avaliar a vulnerabilidade das águas subterrâneas. É apresentado um estudo de caso do aquífero semiconfinado na planície de Tongzhou, na China. Com base nas análises das características ambientais e hidrogeológicas da área de estudo, foi construído um sistema de indicadores para avaliar a vulnerabilidade do aquífero semiconfinado, composto por oito indicadores: a profundidade das águas subterrâneas (D), a recarga líquida (R), o impacto da zona vadosa (I), a condutividade hidráulica do aquífero freático (C), a carga de poluição (P), a qualidade da água freática (Q), a capacidade do aquitardo de retardar NH₄⁺ (A) e a capacidade do aquitardo retardar NO₃⁻ (N). A capacidade de retardo do aquitardo para os poluentes característicos da área de estudo (nitrogênio amoniacal e nitrogênio nitrato) foi obtida por modelagem numérica com simulador HYDRUS. Os resultados mostram que o efeito retardador do aquitardo nos poluentes nitrogenados e a condutividade hidráulica do aquífero freático têm efeitos significativos na vulnerabilidade dos aquíferos semiconfinados, e que outros fatores, como profundidade da água subterrânea e carga de poluentes, têm efeitos relativamente limitados. A pesquisa pode fornecer uma metodologia de referência para avaliar a vulnerabilidade de aquíferos semiconfinados poroso em áreas planas aluviais.

Appendix

Quantitative evaluation of the capacity of the aquitard above the leaky aquifer to retard NH₄⁺ and NO₃⁻

Simulation scheme

To investigate the capacity of the aquitard above the leaky aquifer to retard NH₄⁺ and NO₃⁻ transport in the study area, a simulation scheme was constructed. The construction was on the basis of division of the hydrogeological units in the study area, combined with the strata structural characteristics and lithological characteristics obtained from borehole data, the dynamic characteristics of the shallow groundwater, the hydraulic gradient between the phreatic aquifer and the leaky aquifer, and the degree of pollution in different parts of the phreatic aquifer. The simulation schemes of migration of the characteristic pollution components NH₄⁺ and NO₃⁻ through the phreatic aquifer-aquitard-leaky aquifer system were then constructed. A total of 15 simulation schemes were constructed, as shown in Table 4. The corresponding division of the hydrogeological units is shown in Fig. 4.

Table 4 The simulation schemes

Quantitative simulation of the retardation capacity of the aquitard on NH₄⁺ and NO₃⁻

Hydrogeological conceptual model

The target strata of the model is the shallow aquifer group, including the phreatic aquifer, the aquitard and the leaky aquifer. Since the main purpose of the simulation is to analyse the retardation capacity of the aquitard on the pollutant components NH₄⁺ and NO₃⁻, only the vertical one-dimensional (1D) solute transport model was conducted. The retarding effect of the aquitard was obtained by analysing the time difference of the two monitoring points M1 and M2. A schematic diagram of the hydrological conceptual model is shown in Fig. 15. The aquifer group is saturated, and the upper and lower flow boundary conditions are Dirichlet boundary conditions. The upper concentration boundary is set as the known concentration boundary. The concentration values are assigned as the average values of the concentrations of NH₄⁺ and NO₃⁻ in the groundwater of representative zones of each simulation scheme. The lower concentration boundary is the zero-concentration-gradient boundary. The “three nitrogen” mutual transformation concept was considered in the process of nitrogen pollutant migration from top to bottom. Nitrite (NO₂⁻) is an intermediate product of the nitrogen cycle, which is very unstable in the water system and can be oxidized to nitrate in the presence of oxygen and reduced to ammonia or nitrogen under oxygen deficit or no oxygen conditions. Therefore, the retardation-capacity analysis of the aquitard is only conducted for NH₄⁺ and NO₃⁻. The monitoring points M1 and M2 were set at the bottom of the phreatic aquifer (also the top of the aquitard; M1), and the top of the leaky aquifer (also the bottom of the aquitard; M2; Fig. 15). By analysing the time difference between the pollutant arriving at monitoring points M1 and M2, quantitative information on the retardation ability of the aquitard to the characteristic pollutant components can be obtained. Additionally, by analysing the concentration curves of the pollutants at M1 and M2, one can obtain the process of change in the concentration of the pollutants at the bottom of the phreatic aquifer and the top of the leaky aquifer.

Simulator and model parameters

The simulator HYDRUS 1D was used. The software has been widely used in studies of saturated aquifers, unsaturated soil water, and salt and pollutant migration (Šimůnek et al. 2008; Kirkham et al. 2019). The corresponding parameters in the model were determined as follows: The characteristic moisture parameters of each stratum in the study area were assigned according to the test data of particle analysis and calculated by using the neural network prediction method in HYDRUS software. It includes the residual water content (θ_r), saturated water content (θ_s), saturated hydraulic conductivity (K_s), reciprocal of the air entry value (α), pore diameter distribution coefficient (n) and pore diameter connectivity coefficient (l). The medium bulk density (ρ_b) was set based on the test, and the hydrodynamic dispersion parameters (D_w) and adsorption isotherm coefficient (K_d) were set according to the laboratory test results. The nitration rate coefficient (k₁), nitrosation rate coefficient (k₂) and denitrification rate coefficient (k₃) were assigned according to the experimental results. The molecular diffusion coefficient of nitrogen in pure water was calculated using the empirical values given in the model. The specific model parameter values are shown in Table 5.

Table 5 Model parameters of different lithologies

Analysis of the simulation results

The aim of the simulation here is only to obtain two of the eight indicators in the evaluation indicator system, here, taking Scheme 1 as an example to simply analyse the aquitard retardation capacity on NH₄⁺ and NO₃⁻. The simulation results of Scheme 1 are shown in Figs. 16 and 17, which represent the migration process of NH₄⁺ and NO₃⁻ in the fine sand (phreatic aquifer), silty clay (aquitard), medium sand (leaky aquifer) modes (Table 4). Figure 16 shows that after 14.6 years, the NH₄⁺ reaches the bottom of the phreatic aquifer from the upper part of the phreatic aquifer, and it enters the top of the leaky aquifer through the aquitard layer after 56.2 years. As shown in Fig. 17, NO₃⁻ reaches the bottom of the phreatic aquifer from the upper part of the phreatic aquifer after 0.11 years, and it enters the top of the leaky aquifer through the weakly permeable layer after 0.22 years. According to the thickness of the aquitard layer in Scheme 1 (9.40 m), it can be calculated that under the representative average regional hydraulic gradient of Scheme 1, the retardation capacity of the unit thickness of the aquitard on NH₄⁺ and NO₃⁻ was 0.23 and 86.36 m/year, respectively. The statistical results of other schemes are shown in Table 6. The spatial distribution maps of the retardation capability of the aquitard in the shallow part of the study area to NH₄⁺ and NO₃⁻ are shown in Figs. 12 and 13.

Table 6 Statistical table of the aquitard retardation capacity of the different schemes on NH₄⁺ and NO₃⁻

The spatial distribution of the retardation capacity of the aquitard for NH₄⁺ and NO₃⁻ provides the source of key indicator factors for the establishment of the vulnerability evaluation indicator system of leaky aquifers. In the construction of the vulnerability evaluation system of the leaky aquifer, the retardation ability of the aquitard for NH₄⁺ and for NO₃⁻ were taken as evaluation factors and incorporated into the evaluation system.

Fig. 15

Sketch map of the hydrogeological conceptual model

Fig. 16

NH₄⁺ change curves

Fig. 17

NO₃⁻ change curves