Review Geophysics

Status and trend of hydrogeological experiments concerningthe side-wall flow effect in porous media

Fugang Wang • Zhenkai Gao • Yilin Yuan •

Jing Jing • Xinxin Geng • Yuqing Cao

Received: 24 October 2013 / Accepted: 22 December 2013 / Published online: 21 January 2014

� Science China Press and Springer-Verlag Berlin Heidelberg 2014

Abstract Hydrogeological experiments provide an

important means to understand groundwater seepage and

solute transport problems. Basic hydrogeological parame-

ters can be obtained for use in assessing groundwater

resources. In current porous-media research, studies of

side-wall flow are in a preliminary and qualitative phase,

and lack systematic and comprehensive understanding.

Side-wall flow refers to the non-uniform infiltration and

solute transport that occurs near the seepage device

boundary. Flow dynamics depends on the fluid under the

specific hydraulic conditions, physical properties and

chemical composition, medium permeability, the rough-

ness of the side wall, geometric features, and physical

chemistry. Such phenomena not only occur in indoor water

flows and solute transport, but also under natural conditions

in the field. Side-wall flow has features both in common as

well as distinct from preferential flows. In porous-media

experiments, once side-wall flow commences, it affects the

groundwater flow field and chemistry field, resulting in

parameter values deviating from actual values. Based on a

comprehensive analysis of the influence of side-wall flow,

a definition of the side-wall flow effect in porous media is

given. Three directions of research are identified concern-

ing side-wall flow: the mechanism of the side-wall flows

effect, the study of its quantitative impact on seepage flow

and solute transport, and the methods and measures that

need to be taken in hydrogeological experiments to reduce

(or prevent) side-wall flow development.

Keywords Seepage � Solute transport �Hydrogeological experiment � Porous medium �Side-wall flow

1 Introduction

1.1 The significance of hydrogeological experiments

Groundwater resources are an important part of the world’s

water resources. They have important consequences in

economic and social development, especially in arid and

semiarid regions. In the development and utilization of

groundwater resources, correct simulation, and prediction

of the spatial–temporal evolution of groundwater seepage

(the seepage field) and solute transport (the chemical field)

are key preconditions for the sustainable development and

scientific utilization of groundwater resources.

Hydrogeological experiments are one means of under-

standing groundwater seepage and solute transport problems.

Although computer simulations that provide quantitative

studies of groundwater flow and water quality problems are

playing an increasingly important role, and are gradually

dominating various aspects [1], hydrogeological experiments

are still important for acquiring basic hydrological and hydro-

geological parameters needed to establish and run computer

models. Meanwhile, such experiments help in validating

hydrological and hydrogeological parameters derived from

mathematical or statistical methods. In addition, for some

complex hydrological and geological problems, hydrogeolog-

ical experiments have an irreplaceable role.

1.2 Side-wall flow effect and its significance

The most common seepage devices are the seepage column

and seepage tank used in experiments investigating

F. Wang (&) � Z. Gao � Y. Yuan � J. Jing � X. Geng � Y. Cao

Key Laboratory of Groundwater Resources and Environment,

Ministry of Education, Jilin University,

Changchun 130021, China

e-mail: wangfugang@jlu.edu.cn

123

Chin. Sci. Bull. (2014) 59(8):715–721 csb.scichina.com

DOI 10.1007/s11434-014-0123-y www.springer.com/scp

groundwater seepage and solute transport in porous media.

Considering the time scales involved in seepage, and based

on the analog simulation theory, researchers often use sand

as an experimental medium owing to its high permeability

[1]. In an extensive number of current hydrogeological

experiments, except of a few researchers mentioned and

gave a little concerning on the side-wall flow [2–5], the

influence of side-wall flow on seepage flow and solute

transport has not been given sufficient attention.

Moreover, in hydrogeology, this side-wall flow effect

does not yet have an explicit definition. Based on com-

prehensive analysis, the following definition is suggested.

The side-wall flow effect is the non-uniform infiltration and

solute transport that occurs near the seepage device

boundary, and which depends on the fluid under some

given hydraulic condition, physical properties and chemi-

cal composition, medium permeability, roughness of the

side-wall, geometric features, and physical chemistry. Not

only can this kind of flow occur in controlled water flow

and solute transport experiments, but also in natural con-

ditions in the field such as the contact interface of two

strata with different permeabilities.

Figures 1, 2, 3 and 4 are some research results by

authors of this paper. Figure 1 shows a common device for

experiments with groundwater seepage and solute trans-

port. Figure 2 shows the observation well layout for a

medium sand seepage experiment conducted by the

authors. The difference in water level between the

upstream water supply area and the downstream drainage

area is 4 cm and the water has a steady state flow. A tracer

is put into the water in the tracer mixing area and imme-

diately stirred so that the tracer becomes uniformly dis-

tributed in the water. Figures 3 and 4 show the horizontal

distribution of the electrical conductivity (solute concen-

tration) after 45 and 715 min, respectively. The electrical

conductivity data are measured in the observation wells

(Fig. 2).

In the experiment, the medium sand in the seepage area

is sieved and filled carefully to maintain homogeneity. In

the absence on a side-wall flow effect, the electric con-

ductivity distribution should be parallel to the boundary

line of water supply area, as shown by the dotted line in

Fig. 4. However, the actual concentration field distribu-

tions (Figs. 3, 4) show that the side-wall flow effect has a

significant influence on seepage (solute transport)

experiments.

If there is no side-wall flow in the homogenous medium

experiment mentioned above, the solute transport pattern

will have a piston style. The tracer concentration at every

point in the same cross section should have the same value,

and the data should only be a function of the observation

distance from the upstream tracer input area. Thus, water

samples taken near the side wall can represent the same

Fig. 1 Schematic diagram of experimental device (symbols of I, II,

III, and IV represent the water supply area, seepage area, drainage

area, and tracer mixing area respectively)

Fig. 2 Top view of the distribution of observation wells (symbols of

1–1 to 4–3 represent the codes of the observation wells)

0 100 200 300 400 500 600

Length (mm)

100

200

300

Leng

th (

mm

)

20

120

220

320

420

520

(µS/cm)

Fig. 3 Horizontal distribution of electrical conductivity at 45 min

0 100 200 300 400 500 600

Length (mm)

100

200

300

Leng

th (

mm

)

10

50

90

130

170

210

250

(µS/cm)

Fig. 4 Horizontal distribution of electrical conductivity at 715 min

716 Chin. Sci. Bull. (2014) 59(8):715–721

123

cross sectional circumstance. The real experimental results

show that, owing to the side-wall flow effect, the solute

concentrations in water samples collected near the side wall

are significantly different than those from the central part of

seepage area, even when they are measured in the same

cross section across the flow; additionally, the difference

value changes over time (Figs. 3, 4). These phenomena pose

a scientific question for the hydrogeological experiment.

How can the layout of the observation (sampling) wells be

undertaken to produce data that are closer to actual data and

parameters? Adequate study of the side-wall flow effect is

needed before this question can be answered.

In summary, when the side-wall flow effect occurs in

groundwater experiments, the formation, distribution and

evolutionary processes of the seepage and chemical fields

will be influenced. In such cases, the related hydrogeological

parameters obtained will deviate from realistic settings,

leading to inaccurate results due to the use of these basic

parameters in groundwater resource analyses, evaluations,

and forecasting. Therefore, this should be of concern to

hydrogeologists. To systematically, comprehensively, and

quantitatively study the side-wall flow effect in porous

media and its onset, frequency and development, the clari-

fication of influencing factors and underlying mechanisms,

and their influence on the seepage and chemical fields, has

theoretical and practical importance to research on ground-

water flow, solute transport and water resource assessment.

2 Domestic and international status of side-wall flow

effect research

2.1 Side-wall flow effect in non-hydrogeological fields

Hydrogenation reactors in fields involving petrochemicals

[6], industrial oxygen production [7], and power and

thermal engineering [8] involve the study of side-wall flow

and have captured the attention of both domestic and

international researchers, who have conducted many in-

depth studies. For example, Ni and Li [9] conducted an in-

depth study of the influence of wind tunnel side-walls on

two-phase flow in wind-tunnel experiments. In the field of

hydraulics, in both theoretical and practical engineering

experiments, side-wall flows have been given detailed

study [10]; in such research, experiments involving the

one-way resistance coefficient of artificial rough tubes by

the famous hydraulicist Nikuradse are typical.

2.2 The side-wall flow effect in hydrogeological

porous-media experiments

In hydrogeology, a large number of indoor seepage

experiments and solute (e.g., pollutant) migration

experiments, undertaken both domestically and abroad,

have been conducted to investigate the influence of a

porous medium on groundwater seepage and pollution

issues. Abroad, experiments on groundwater seepage

through porous media were established much earlier.

Among these were the pioneering works of French engi-

neer Darcy in 1856 that led to the establishment of the

basic law of groundwater movement theory, Darcy’s Law

[1]. Abroad, research began on solute transport and the

migration of pollutants in groundwater from the beginning

of the twentieth century [11]. Related domestic research

began in the early 1980s [12]. To date, researchers at home

and abroad have performed many related experiments

under different media conditions (homogeneous and het-

erogeneous [13–15], coarse- and fine-grained [14–16],

consolidated and unconsolidated [16–21], layered and non-

layered [22, 23], flow area and immobile watershed

[24, 25], saturated and unsaturated [14, 25]), and dif-

ferent environmental conditions (such as rainfall [26],

sloped terrain [27], surface runoff [26, 27], model scale

[14, 28, 29], boundary conditions [28, 30], and static and

dynamic settings [31]).

The main focus and core issues related to porous med-

ium seepage flow and solute transport experiments [24, 29,

32] have focused on the following aspects: (1) obtaining

relevant hydrogeological parameters (such as the perme-

ability and dispersion coefficients); (2) hydraulic conduc-

tivity and solute (or pollutant) migration process, as well as

some of the major problems, such as water content trans-

formations, the solute migration rule, adsorption and

desorption of solute transport, the retarded rule [32–34],

tracing the process of seepage flow and solute transport

[35] and process visualization [36]. In recent years, nuclear

magnetic resonance imaging and X-ray technology have

attracted attention for use in experiments that visualize the

spatial distribution of solute concentrations [18, 36, 37].

Considering the time needed for these experiments, the

media used most often are sandy soils with good perme-

ability. The experimental apparatus used for simulations

are the one-dimensional flow column and the two- or three-

dimensional flow seepage tank. Although research aimed at

improving or designing new experiment devices has been

undertaken [20, 26, 38], side-wall flow effect has not yet

been incorporated in such devices.

In the published literature, Mao et al. [2] and Liu et al.

[3] both mentioned the issue of side-wall flow in their sand-

box experiments investigating embankment seepage and

piping, and have also performed a qualitative analysis of

onset conditions, of side-wall flow, embankment destruc-

tion form and destruction hydraulic gradient. In his book

‘‘New Stone Slag Dam—theory and practice of a coarse

grain soil dam’’, Qu and He [4] mentioned that, to prevent

water penetration along the side wall in controlled indoor

Chin. Sci. Bull. (2014) 59(8):715–721 717

123

penetration tests, he adopted the method and the measure

of laying clay or vaseline between the side-wall and test

samples. In indoor controlled experiments on Second

Songhua River seepage, Wang found a main priority side-

wall seepage in medium sand [5]. From solute transport

experiments, Ye et al. [12] mentioned that the diameter of

the seepage column should not be too small as side-wall

flow affects contaminant transport. Zhu [21], in his mas-

ter’s thesis ‘‘Non-Darcy seepage experiments and mathe-

matical description’’, discussed capillary boundary-layer

effects on the seepage velocity and medium permeability of

crude-oil at low speed in a porous medium. He found that

the boundary layer thickness decreased as pressure

increased, and the thickness tended to be stable at certain

pressure values. Additionally, a calculation formula for

boundary layer thickness is proposed. Qiao [39], in his

master’s thesis, mentioned that effects related to soil col-

umn size on side-wall flow and on salt transport should be

given appropriate consideration. He found that the solute

transport rate is lower in a smaller-sized device.

With the support of the National Natural Science

Foundation of China, the authors of this paper are carrying

out systematic research on the side-wall flow effect.

Research to date has shown that a side-wall flow effect can

occur even in homogeneous and isotopic media (Figs. 3, 4).

Figure 5 shows the result of solute transport experiments in

homogeneous and isotropic sand media. We found that the

electric conductivity in different observation wells (see

Fig. 2) changed over time. The closer an observation well is

to a side wall, the earlier a peak will be found in the electric

conductivity (Figs. 5, 6). This effect is gradually weakened

as the distance from the side wall to the observation well

increases (Fig. 6).

2.3 Hydrogeological experiments on the side-wall flow

effect in fissured media

In recent years, research worldwide involving fissured

medium seepage flow and solute transport has made use of

numerous studies on the influence of side-wall factors, like

fracture wall roughness, gap width, connectivity, contact

area, and the hydraulic characteristics of fractured media

[40–43]. Domestic scholars have systematically summa-

rized research on seepage flow in fissured media and solute

transport, and on theoretical, experimental, and numerical

simulation research, both nationally and internationally

[44–46]. They have posed new methods, such as surface

adhering sand [47] and molding [48], to design various

physical models with different geometric features and

fracture roughness, conducted numerous experiments, and

produced a rich array of results on aspects of experimental

methodology and the theoretical modeling of rough frac-

ture media seepage [44–46].

From seepage flow in fissured media and solute trans-

port experiments, the experience and progress obtained on

the influence of boundary characteristics on seepage flow

and solute transport provides an important reference and a

basis for research on side-wall flow effects in porous

media.

2.4 The difference between side-wall flow

and preferential flow

There are both differences and similarities between side-

wall flow and preferential flow in the fields of hydrogeol-

ogy and agricultural soil water. Preferential flow refers to

the groundwater flow and solute moving quickly along

wormholes, root holes and fissures in seepage zones. In

research related to preferential flow, Lawes et al. published

a landmark paper in 1882, bringing forward for the first

time, the high concentrations of pesticides found in field

experiments related to preferential flow at depths in the

ground. They pointed out that there are two types of flow,

one being preferential flow with higher speed and uneven

flow, and the other being the slower and more uniform

matrix flow. Studies indicated that two types of seepage

Fig. 5 Electric Conductivity over time in a medium-grained sand

medium with a water level difference of 4 cm (symbols of 4–1 to 4–3

represent the codes of the observation wells in Fig. 2)

Fig. 6 Electric Conductivity over time in a fine-grained sand medium

with a water level difference of 12 cm (symbols of 3–1 to 3–5

represent the codes of the observation wells in Fig. 2)

718 Chin. Sci. Bull. (2014) 59(8):715–721

123

influence solute transport, depending on soil type and

rainfall intensity.

Although preferential flow was noticed by others very

early, it was not until the middle of the twentieth century

that preferential flow was gradually recognized and given

attention by scientists as the explanation for the increased

frequency of pesticides polluting deep groundwater sources

(pesticides can rapidly degrade and be adsorbed by soils).

So far, research on preferential flow covering basic theory

[49–52], simulation and tracer technology [52–58], soil

water sampling [59, 60], and other aspects has made sig-

nificant achievements. Specific topics covered include

pesticides [61], heavy metals [62], toxic metallic elements

and nutrients [63], pathogens, and nutrients [64].

In one quantitative study of preferential flow, Chikhaoui

et al. [54] chose conservative ions such as calcium, mag-

nesium, sodium and potassium as tracers, and analyzed the

correlation between preferential flow and conductance by

testing for the four specific ion concentrations and solution

conductivity values.

Thompson found that freon could be used as a good

artificial tracer in the measurement of groundwater flow

velocity (1974). Freon does not react with the stratum

material, and can be detected at minimal concentrations

without endangering the safety of public water sources

[58]. Kung et al. [57], in 2000, quantified different spatial

point velocities by solute concentration and breakthrough

curves determined by adding chloride and bromide

ions, continuous PFBA (5-fluorobenzoic acid), 2,6-DFBA

(2,6, fluorobenzoic acid), o-TFMBA (adjacent three fluo-

rinated methylbenzoic acid), and five types of non-

adsorption, non-reaction inorganic and organic conserva-

tion tracers. Ronkanen and Klove [55], in investigating the

potential for peatland to treat urban wastewater, adopted

stable isotopes of deuterium and oxygen to study prefer-

ential flow. The technology involved in this study, in

addition to X-ray and nuclear magnetic resonance imaging

[36], consisted of time domain reflection (TDR) and

ground penetrating radar [56].

Preferential flow and side-wall flow have common

characteristics: both are groundwater non-uniform infiltra-

tion processes in a certain seepage field or flow path. Both

exhibit differences within the overall groundwater flow: (1)

preferential flow velocities are higher than the average flow

velocity of a groundwater flow field, but velocities asso-

ciated with side-wall flow can be higher or lower than the

average flow velocity of groundwater; (2) preferential flow

mainly occurs in the inner regions of the seepage field,

whereas side-wall flow mainly develops in the flow area

near the boundary. Thus, in studies of side-wall flow

effects, the method used in preferential flow studies pro-

vides a good reference base, but the differences between

these must be considered.

Nevertheless, in groundwater seepage and solute (or

pollutant) transport experiments in porous media, the side-

wall flow effect has not been given sufficient attention by

researchers and remains at a qualitative stage, lacking

systematic and comprehensive analysis.

3 Side-wall flow effect research trends

Considering the importance of hydrogeological experiment

in groundwater studies, as well as the significance of side-

wall flow effects in hydrology experiments, a systematic

study of side-wall flow effects should be carried out in a

timely manner. Research should consider several of the

following aspects.

3.1 Formation mechanism of the side-wall flow effect

in hydrogeological experiments

The side-wall flow effect formation mechanism involves a

range of different environmental and other influencing fac-

tors that contribute to the occurrence, development, and

evolution of side-wall flow. The study of side-wall flow

formation mechanisms is the key premise needed to deter-

mine the effect of side-wall flow on experimental processes

and results. A thorough investigation of side-wall flow effect

mechanisms will help with the scientific design and setup of

experiments, and improve the reliability of hydrogeological

parameters produced by experimentation. For example,

improvements are possible in determining reasonable loca-

tions for water quality sampling, and ideal data collection

points for effectively capturing information on the overall

effect of the seepage flow and solute transport.

Based on a comprehensive analysis of all possible factors

influencing side-wall flow, four main aspects arise: (1)

characteristics of the experimental media (including medium

properties such as grain size, density, and structural char-

acteristics such as grain arrangement and grain sorting), (2)

hydraulic conditions (hydraulic gradient), (3) device char-

acteristics (e.g., characteristics of scale, boundary condi-

tions), and (4) physical properties of the fluid (e.g.,

temperature, viscosity). From these four aspects, experi-

ments should be conducted systematically to analyze the

onset conditions of side-wall flow, evolution of the flow, and

its development with respect to the influencing factors. This

will then lead to a clarification of the formation mechanism

of the side-wall flow effect.

3.2 Quantitative study of the influence of side-wall

flow on seepage flow and solute transport

Quantification of the side-wall flow effect can support the

study of the mechanisms that cause it, and can also provide

Chin. Sci. Bull. (2014) 59(8):715–721 719

123

a reference for laying out sampling well locations. A

quantitative study of side-wall flow influences is required so

that results can be applied in practical engineering settings.

Research should consider different conditions (medium,

hydraulic, device, and fluid characteristics), in parallel with

quantitative study of the evolution of the seepage and

concentration (chemical) fields in the longitudinal, trans-

verse, and vertical three-dimensional space. The evolution

of both seepage and concentration fields is dependent on

water mass exchange and the velocity differences between

the boundary region and the inner seepage region.

3.3 Methods and measures to reduce (or prevent) side-

wall flow influence in hydrogeological experiments

Based on previous research on the formation mechanism and

quantitative study of the side-wall flow effect, probable

methods and measures need to be proposed to reduce (or

prevent) such effects in accordance with the diverse condi-

tions encountered in hydrogeological experiments. Research

results and theory can be applied to the design of actual

hydrogeological experiments involving side-wall flow, and

the parameters and data obtained through experimentation

will have both scientific and practical value.

Acknowledgments This work was supported by the National Nat-

ural Science Foundation of China (41172205) and the Opening Fund

of Key Lab of Groundwater Resources and Environment, Ministry of

Education, Jilin University and the Scientific Research Foundation for

Returned Overseas Chinese Scholars, State Education Ministry.

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