# Evaluation of groundwater pollution in a mining area using analytical solution: a case study of the Yimin open-pit mine in China

- Tianxin Li
^{1}, - Li Li
^{1}, - Hongqing Song
^{1}Email author, - Linglong Meng
^{1}, - Shuli Zhang
^{2}and - Gang Huang
^{1}

**Received: **8 October 2015

**Accepted: **17 March 2016

**Published: **31 March 2016

## Abstract

### Introduction

This study focused on using analytical and numerical models to develop and manage groundwater resources, and predict the effects of management measurements in the groundwater system. Movement of contaminants can be studied based on groundwater flow characteristics. This study can be used for prediction of ion concentration and evaluation of groundwater pollution as the theoretical basis.

### Case description

The Yimin open-pit mine is located in the northern part of the Inner Mongolia Autonomous Region of China. High concentrations of iron and manganese are observed in Yimin open-pit mine because of exploitation and pumping that have increased the concentration of the ions in groundwater. In this study, iron was considered as an index of contamination, and the solute model was calibrated using concentration observations from 14 wells in 2014.

### Discussion and evaluation

The groundwater flow model and analytical solutions were used in this study to forecast pollution concentration and variation trend after calibration. With continuous pumping, contaminants will migrate, and become enriched, towards the wellhead in the flow direction. The concentration of the contaminants and the range of pollution increase with the flow rate increased.

### Conclusions

The suitable flow rate of single well should be <380 m/day at Yimin open-pit for the standard value of pollution concentration.

## Keywords

## Background

With the rapid development of mining, problems related to groundwater in mining areas are becoming increasingly prominent. For example, due to drainage of groundwater, a series of environmental problems, such as pollution of groundwater and decline of aquifer levels, in mining areas have occurred (Dhakate et al. 2008). In order to achieve sustainable development and reach a secure status regarding both quantity and quality of groundwater bodies, some management methods for environmental protection should be introduced, particularly in the prediction of groundwater changes and the control of contamination prior to mining activities (Jiménez-Madrid et al. 2012). It is crucial to accurately understand the factors governing pollutant migration, in order to produce reasonable guidance for environmentally sustainable mining in the future.

In recent decades, groundwater flow and contaminant transport play an important role in the complicate groundwater systems (Regli et al. 2003). According to the Williams (1985) details of site’s geology investigation and hydraulic properties measurement are the main characteristics of groundwater flow. Most previous studies have focused on the trend of groundwater flow and the regularity of contaminant transport. A sharp drop in hydraulic head occurs at the center of the model area, and this generates a cone of groundwater depression and a continuous decline of head, with respect to time, as a result of the high rate of groundwater abstraction (Seyf-Laye et al. 2012). Dawson et al. (2007) introduced a two-dimension numerical model to simulate the movement of polluted water in the sub-grade of a highway and investigate the main factors which influence the movement of contaminants in the highway environment. According to his research, factors that have big influence on the groundwater contamination are the permeability of sub-grade soil, suction-water content characteristics, partitioning coefficient, and sorption capacity of aggregates. Some researchers have focused on the assessment of contamination of groundwater via heavy or toxic metals, nitrate, chromium, and the reactive transport of contaminations from mines to groundwater (Bicalho et al. 2011; Li et al. 2012; Hajhamad and Almasri 2009). A mathematical model was proposed for groundwater pollution in a leaky aquifer system, and this model was applied to describe groundwater pollution in the Taiyuan Basin (Xue et al. 1997). Hoeks (1981) proposed an analytical approach that considered the main properties of the soil system. It is useful of applying this approach to predict contaminants transport in groundwater; however, little quantitative information is available for the soil in terms of homogeneity, dispersion coefficients, and complex interaction processes. Moutsopoulos and Tsihrintzis (2005) researched the turbulent and transient flows in confined porous media and two equations for both short- and long-time conditions were introduced. In addition, the inertia effects and non-Darcy flow conditions also were considered in some analytical solutions (Sen 1989; Moutsopoulos 2007; Wen et al. 2008a, b; Sedghi-Asl et al. 2014; Miracapillo et al. 2014). A mathematical model about contaminant transport due to well injection was developed by Hsieh and Yeh (2014) in a confined aquifer. Potsane et al. (2014) considered macroscopic deterministic models that describe contaminant transport in saturated soils, under uniform radial water flow conditions, through comparisons with numerical solutions.

Xu et al. (2014) established both a groundwater seepage model, and solute transport model, for a factory district and surrounding areas, in Qianan City. The Cr^{6+} have been used to be an index of contamination by researchers which predicted the range of pollution halo and its maximum value. There are some factors influenced the contaminant migration in groundwater such as flow direction, hydraulic conductivity, and the concentration of contaminant at the source (Kjeldsen et al. 1998; Klinck and Stuart 1999). Therefore, it is meaningful to use flow simulation models to research the direction of groundwater flow, distribution of hydraulic heads and flow magnitudes (Banejad et al. 2014). The range of pollutants that exceeds standards can be controlled by a constantly updated groundwater, which was obtained through the flow model and solute transport model that was used in Yishui iron mine in Shandong province (Zhou et al. 2013).

This paper based on the previous research proposed a solute model of pollutant concentration and obtained the analytical solution. The present solute model is able to delineate, analyze, and assess contaminant migration and enrichment during the development of an open-pit mine. The analytical method, in comparison with a numerical solution, is accurate to predict the contaminants migration in the underground water system, and does not require a geological model. It is also more economical than a numerical solution. Therefore, in this study, we used the analytical method to analysis the question, and the model has been validated in its application to the study and prediction of the trend of contaminant flow commences before development in an open-pit mine.

## Study area

^{2}which is a synclinal basin surrounded by low mountains and hills.

According to geological survey reports, the iron minerals in the study area exist in the strata in the form of compounds that are difficult to dissolve. Because of exploitation by open-pit mining in the research area, which caused the groundwater funnel, and the farther the distance from the ground, the lower the oxygen content. This causes a large amount of the dissolved state of the two valence ions of iron to be present in groundwater. This can be presented as follows: FeO + H_{2}O + 2CO_{2} = Fe (HCO_{3})_{2}.

## Methods

### Confined–unconfined flow model

With the continuous pumping, the study area of the confined aquifer will be changed into unconfined aquifer which named the confined-unconfined flow. Based on the generalized large well method, a confined–unconfined flow model has been established to simulate the groundwater flow trend in the Yimin open-pit mine and the problem was described as follows with the same assumptions of the previous research (Li et al. 2014):

*F*: area of mine, m

^{2};

*H*

_{0}: initial hydraulic head, m;

*K*: permeability coefficient, m/s;

*M*: confined aquifer thickness, m;

*Q*: flow rate of the whole pumping wells, m

^{3}/s;

*r*: radius of groundwater, m;

*r*

_{0}: reference radius, m;

*T*

_{ m }: different period corresponding to average hydraulic conductivity, m

^{2}/s;

*t*: time, s;

*μ*

_{ m }: different period corresponding to average specific yield; \(\varphi\): Jilin Gaussian potential function.

### Solute transport model

In this paper, convection, dispersion and chemical reaction will be considered, and the problem will be derived as a stable well flow problem of a planar radial flow field. The following conditions are assumed: (1) the pollutant is mainly distributed in a homogeneous, isotropic and equal-thickness confined aquifer; (2) the aquifer has a complete pumping well, constant flow pumping and an established stable well flow is centered on the well; (3) the diffusion coefficient of pollutants in the medium is assumed to be constant; and (4) pollutants transfer into groundwater from the porous medium by solid phase chemical reactions.

*C*: solute concentration in groundwater, kg/m

^{3};

*Cs*: solute concentration in solid, kg/m

^{3};

*Dr*: radial dispersion coefficient of porous media, m

^{2}/s;

*r*: radius of groundwater, m;

*u*

_{ r }: actual fluid velocity, m/s;

*r*

_{ w }: radius of pumping well, m;

*n*: porosity of porous media;

*f*: hydrodynamic dispersion, kg/m

^{2}s;

*λ*: chemical reaction rate constant;

*ρ*

_{ b }: density of porous media, kg/m

^{3}.

## Analytical solution

### Analytical solution of flow model

Based on the characteristics of groundwater flow in open-pit mines, an analytical model of special pumping flow transformation from confined groundwater to unconfined groundwater (Li et al. 2014) and velocity solutions can be derived as follows: Boltzmann function transformation (Zhang et al. 2010), equation \(u = \frac{{r^{2} }}{4at}\), where \(a = \frac{{T_{m} }}{{\mu_{m} }}\). Through substitution, Li et al. (2014) gave the expression of potential function can be presented as,

*T*: Confined aquifer conductivity, T = KM.

### Analytical solution of Solute transport model

*C*

_{1}and

*C*

_{2}can be expressed as

## Results and discussion

### Solute transport model calibration

Exploitation quantity of coal from 2007 to 2014

Year | Exploitation quantity (tons) | Year | Exploitation quantity (tons) |
---|---|---|---|

2007 | 9.41 × 10 | 2011 | 1.50 × 10 |

2008 | 1.383 × 10 | 2012 | 1.53 × 10 |

2009 | 1.42 × 10 | 2013 | 1.55 × 10 |

2010 | 1.453 × 10 | 2014 | 1.23 × 10 |

*C*

_{ s }is the initial average value of Iron ion concentration in rock stratum.

Actual parameters measured in the study area

Symbols | Value | Symbols | Value |
---|---|---|---|

| 660 |
| 0.1 |

| 3.72 × 10 | \(T_{m}\) | 0.03 |

| 80 | \(\mu_{m}\) | 0.09 |

| 1009.3 |
| 0.75 |

| 0.2 |

Comparison of analytical value and real data of dissolved iron concentration in 2014

Samples | Observed data (mg/l) | Calculated values (mg/l) |
---|---|---|

1 | 0.81 | 0.86 |

2 | 0.79 | 0.81 |

3 | 0.68 | 0.69 |

4 | 0.79 | 0.84 |

5 | 0.93 | 0.96 |

6 | 3.86 | 3.90 |

7 | 0.38 | 0.38 |

8 | 2.00 | 2.40 |

9 | 2.99 | 3.10 |

10 | 0.63 | 0.64 |

11 | 0.43 | 0.43 |

12 | 0.28 | 0.30 |

13 | 1.17 | 1.18 |

14 | 0.33 | 0.36 |

Single well pumping flow rate

Samples | Flow rate (m | Calculated values (mg/l) |
---|---|---|

1 | 432 | 0.86 |

6 | 510 | 3.90 |

8 | 484 | 2.40 |

14 | 389 | 0.36 |

From Table 3 and Fig. 4, it can be concluded that the calculated value and real value were very close for the 14 samples of study, and any error within the allowable range. Therefore, the calculated value has been validated and may be used to predict the trend of contaminant ion migration and enrichment in future years with continuous pumping.

### Analysis of the trend of solute concentration with different flow rates

*Q*= 432 m

^{3}/day, the value is 0.8607 mg/l, and at

*Q*= 691 m

^{3}/day, the value is 101.8 mg/l. Therefore, by changing the values of the flow rate it was discovered that the trend of concentration increased while the value of flow rate increased.

The values of iron concentration when the radius is 1 m with different flow rates in 2014

Flow rate (m | Value (mg/l) |
---|---|

432 | 0.86 |

518 | 4.60 |

605 | 22.20 |

691 | 101.80 |

Sample “1” calculated values in 2014 in the study area

Radius (m) | Values (mg/l) |
---|---|

1 | 0.86 |

10 | 4.56e−6 |

20 | 2.39e−7 |

30 | 4.45e−8 |

40 | 1.37e−8 |

50 | 5.47e−9 |

### Analysis of the trend of solute concentration near the pumping well

*Q*= 432 m

^{3}/day, the value has reached 0.86 mg/l, which exceed the standard value of 0.3 mg/l.

### Prediction of the range of solute concentration with different flow rates

*Q*= 432 m

^{3}/day. Also the distance from wellhead is over 1.9 m, the values of iron concentration are higher than standard value with

*Q*= 518 m

^{3}/day. The flow rate not only impacts the value near the wellhead, but also impacts the range of that the value. Most importantly is that when

*Q*= 380 m

^{3}/day, the value at the radius of 1 m is just 0.3 mg/l, that is to say, the flow rate should not exceed 380 m

^{3}/day when pumping groundwater.

## Conclusion

This paper presents a groundwater flow model and a solute transport model for the confined aquifer of the Yimin open-pit mine. For this purpose, an analytical solution was employed to simulate groundwater flow and solute transport with continuous pumping. The models have been validated correctly and can be used to predict the trend of groundwater in Yimin open-pit mine.

With continuous pumping, contaminants migrate and become enriched towards the wellhead in relation to the flow direction. An increase in flow rate values was found to correspond to an increase in the concentration of contaminants. Finally, we can see that the values of iron concertation don’t exceed the standard value of 0.3 mg/l within the range of 2 m from the wellhead, the flow rate should be 380 m^{3}/day. For Yimin open-pit the suitable flow rate should be less than 380 m^{3}/day, which may make the iron concentration less than the standard value near the wellhead.

From the discussion above, the model is useful in predicting and studying the trend of groundwater change and contaminant migration before exploitation, and enables further efforts to be made to protect groundwater resources in other open-pit mines with similar hydrogeological conditions.

## Declarations

### Authors’ contributions

LL carried out the study; TL and HS supervised the work; LM designed the experiments, LL wrote the manuscript, and established the solute transport model, SZ provided the Hydrogeological data of Yimin open-pit mine. All authors read and approved the final manuscript.

### Acknowledgements

The authors gratefully acknowledge the National Nature Science Foundation of China under Grant 41303059 and 51404024 for financial support. The authors also express gratitude to researchers at the Yimin open-pit mine for their cooperation.

### Competing interests

The authors declare that they have no competing interests.

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

## Authors’ Affiliations

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