Two in one leachate plume in a karstic aquifer

ORIGINAL ARTICLE

Two in one leachate plume in a karstic aquifer

Roger Gonzalez-Herrera • Rodolfo Gomez-Lopez

Received: 9 September 2011 / Accepted: 28 July 2012 / Published online: 14 August 2012

� Springer-Verlag 2012

Abstract Some sites formerly used for waste disposal

purposes, even if they are closed, continue generating

leachate that seeps into the ground and contaminates

groundwater in the area where they are located. It is

believed that the rainfall being in contact with waste

becomes a source of leachate. This fluid seeps into the

aquifer and may identify sources of pollution. A modeling

work which determined the migration times of solutes in

the karstic aquifer of Merida, Yucatan, Mexico, is pre-

sented. Both existing and generated information was ana-

lyzed; a leachate plume was identified. The methodology

used in the study is described, the application of which

allowed concluding that this plume was generated from

two sources: a waste disposal site and the oxidation

lagoons located next to it. The procedure consisted of

performing simulations considering the sources that con-

tribute to the development of the pollution plume and

forecasting their behavior. With the developed methodol-

ogy, similar cases can be analyzed to avoid locating

catchment zones of drinking water in inappropriate places

and/or develop projects to place waste disposal sites that

could affect existing catchment areas and to preserve this

resource, essential for life.

Keywords Groundwater � Leachate � Pollution �Karstic aquifer � Simulation

Introduction

The Yucatan Peninsula is geographically distinctive by its

shape and size; it is bounded by the Gulf of Mexico to the

north and west, and by the Caribbean Sea to the east

(Fig. 1).

Northern Yucatan, where Merida is located, is the best-

known part of the peninsula, because most of its human

population has been concentrated there for the past 1,000

years. During the recent period of accelerated diversifica-

tion, Merida, with a population of about 830,000 inhabit-

ants, has emerged as the major urban center for the

southeast, and as such, Yucatan’s capital has become the

focal point of the region’s industrial, commercial and

professional services (INEGI 2011).

In Merida, one of the permanent problems is the dis-

posal of domestic and industrial wastes that are carried out,

dumping the solids and injecting the liquids on and into the

ground.

A landfill was opened in November 1997; however, an

old municipal solid waste disposal site existed which

started operations in 1979. This dump site was situated

on the northern outskirts of the city of Merida (Fig. 2)

and was operated by the municipality. The site had a

long history of uncontrolled dumping and waste burning

including commercial, industrial and domestic wastes.

From 1993 to 1998, an attempt was made to place the

waste into a compacted raise with a soil cover. This

methodology reduced the risk of the waste catching fire

and also nuisance from air-borne pollution and smells.

Toxic materials were burned in another plant dedicated

exclusively to this practice; however, the resulting

ashes were sent to the old municipal solid waste disposal

site. There is also a composting plant located west of the

city.

R. Gonzalez-Herrera (&) � R. Gomez-Lopez

Facultad de Ingenierıa de la UADY,

Avenida Industrias no Contaminantes por Periferico Norte,

Tablaje Catastral, 12685 Merida, Yucatan, Mexico

e-mail: [email protected]

123

Environ Earth Sci (2013) 68:1945–1953

DOI 10.1007/s12665-012-1882-x

The Merida dump site was emplaced directly onto a

karstic, Tertiary, marine limestone of Miocene–Pliocene

age (DGGTN 1984), with a thin (\1 m; DGGTN 1985) to

non-existent soil cover. The following hydraulic data cor-

respond to the local unconfined aquifer: (1) the hydraulic

conductivity is very high due to the fissured nature of the

rock. Mendez (1993) indicates values of the order of 10-3–

10-4 ms-1, while Sanchez (1992) quotes values ranging

from 3.7 9 10-5 for calcarenites to 3.38 9 10-8 ms-1 for

recrystallized limestone; (2) both matrix and fissure

porosity are high, the former ranging between 40 and 50 %

(Sanchez 1992; Brewerton 1993); (3) the water table is

about 5 m below the ground level; (4) the hydraulic gra-

dient is low at 7.5 9 10-5 ms-1 with flow in a north-

westerly and westerly direction toward the coast; (5) most

of the recharge occurs in August and September. Rapid

bypass flow can be assumed during rainfall events and a

high percentage of rainfall is estimated to go to recharge,

i.e., 150 mma-1.

Thus, the main characteristic of the aquifer in the

Yucatan Peninsula, Mexico, is that it is shallow and

unconfined. The hydraulic conductivity is high because the

rock formation is of the karstic type, which facilitates the

infiltration of contaminants toward the water table. No

waste has been dumped in the old disposal site since April

1998 because of a landfill opening; site remediation and

definite closure is underway. However, it is considered that

the rainfall in contact with the waste becomes a source of

leachate. This fluid seeps into the aquifer allowing the

identification of the source of pollution.

To date most of the research work conducted in the

aquifer beneath the former waste disposal site has been

mainly focused on determining: (1) the amount of leachate

generated (Ku 1998); (2) the amount of hazardous sub-

stances and health risks (Casares 2002); (3) a methodology

and the feasibility of detection of a pollution plume (Perez

2006); and (4) different types of pollutants, the hydro-

chemical reactions and their concentrations (Vazquez

2007). The spreading process of contaminants has been

ignored.

This paper reports on a research work which determined

the migration times of solutes in the Yucatan karstic

aquifer with the aim of studying the transport process of

pollutant plumes identified with the chloride ion. The field

Fig. 1 Location of the study

area

Fig. 2 The Merida old waste disposal site and monitoring well

locations

1946 Environ Earth Sci (2013) 68:1945–1953

123

work was carried out in the former waste disposal site of

the city of Merida, Mexico. Data generated from 2000 to

2005 were available. The existing information was updated

with recent data to know the status of the pollution plume.

Methodology

The former city dump in Merida, Mexico, dates back from

the early 1980s and ceased operations in 1998. During the

period 1993–1998, the site was used to dispose of solid

waste. The waste was accumulated in the place applying

the area method of disposal. Using this method, cells of

waste were built in mounds on the ground surface. During

the time of operation, the dump site did not have any liner

to protect the aquifer. Garbage was deposited directly on

the rock face devoid of soil. This led to infiltration of

leachate into the aquifer and the resulting groundwater

pollution. This contamination has been identified as a

pollution plume, unprecedented in this type of water sys-

tem (Gonzalez et al. 2004).

Groundwater monitoring

When the study area was visited, it was noted that liquids

from septic tanks and those coming from washed maize

processing were discharged into the oxidation ponds,

located next to the solid waste disposal site (Fig. 2).

Sometimes, the discharges were made even on the hill of

garbage. During the rainy season (from May to November),

the oxidation ponds overflowed spilling the liquid con-

tained in them.

The boreholes used in this work were drilled for a pre-

vious research Gonzalez (2003). They are identified as S1,

S2, S3, S4, S5 and S6 and located at a distance of 10, 25,

50, 100, 300 and 500 m from the ‘‘mountain’’ of garbage,

respectively (see Fig. 2). In 2004, the following were

drilled: S1I, S2I, S3I, S4I, S5I, S6I, S1D, S2D, S3D, S4D,

S5D and S6D, to the left (I) and right (D) of each of the six

boreholes. This makes a total of 18 wells of 8 in. in

diameter and 50 m in nominal depth along the preferential

groundwater flow line determined by Gonzalez (1996).

The groundwater level was first measured in each bore-

hole. The monitoring was performed from the water table to

the total borehole depth. Groundwater sampling was carried

out in each monitoring borehole starting from the water table

where the first sample was collected. Samples were then

collected every 5 m, covering the whole freshwater aquifer. It

is worth noting that the coast is 30 km away from the study

area, thus the saline intrusion was located 50-m deep,

approximately, as depicted in Fig. 3 using electric conduc-

tivity profiles. A minimum of seven samples per borehole

were obtained beginning from the more distant to that closest

to the hill of the waste disposal site in order to avoid cross

contamination. Samples were then labeled and stored in the

laboratory to subsequently analyze for chloride. The titration

method was employed to determine chloride using silver

nitrate (APHA et al. 1998).

Water table contours

A commercial contouring program was used to represent

the geometry of the medium and hydraulic heads. Surfer V.

8 is a computer software developed by Golden Software,

Fig. 3 Electric conductivity

profiles

Environ Earth Sci (2013) 68:1945–1953 1947

123

Inc. It is a geostatistical tool that generates contours, based

on data located in a mesh, applying numerical methods.

The kriging method was used in this work because it

estimates the average cell-centered value of a rectangular

block in a mesh and generates contours that the program

handles to illustrate in its graphical interface. These cal-

culated values can be exported to a file with extension grd,

which can be imported to the groundwater model used.

Modeling

Groundwater flow modeling was carried out using MOD-

FLOW, a computer program developed for the U.S. Geo-

logical Survey in the form of a modular three-dimensional

groundwater flow model. MODFLOW is able to simulate a

wide range of flow through porous media with standard and

varieties of systems including groundwater flow and

transport of contamination (Harbaugh 2005). MODFLOW

solves the distribution of hydraulic head within the model

domain and from the results the velocity components of

flow are calculated. The formula used to predict flow is:

o

oxKxx

oh

ox

� �þ o

oyKyy

oh

oy

� �þ o

ozKzz

oh

oz

� �¼ Ss

oh

otð1Þ

where Ss is the specific storage; h is the hydraulic head; t is

the time and Kxx, Kyy and Kzz are the hydraulic conductivity

values in the principal directions x, y and z.

To simulate solute transport into groundwater, MT3DMS

was used. MT3DMS simulates advection, dispersion and

chemical reactions of contamination in groundwater. It uses

cell-by-cell data which are computed and output by MOD-

FLOW to establish the results (Zheng 2006). The governing

differential equation in three-dimensional form to model

contaminant transport is:

Dxo2C

ox2þ Dy

o2C

oy2þ Dz

o2C

oz2

� �� vx

oC

oxþ �vy

oC

oyþ �vz

oC

oz

� �

¼ Rd

oC

ot

ð2Þ

where C is the concentration; v is the seepage velocity; Rd is

the retardation factor, whose value equals 1 for conservative

tracers; x, y and z are the coordinates, and t is the time.

The next part of the methodology was to simulate the

actual conditions in the study area over time. To do this, it

was necessary to collect information to faithfully reproduce

the natural conditions of the aquifer.

Results and discussion

Simulating the groundwater flow was not an easy task

because there were different values of hydraulic

conductivity (K) reported in the literature and each of these

values yielded its own set of water flow patterns and

speeds.

Sanchez (1999), Gonzalez et al. (2002) and Marın et al.

(2004) applied different techniques to model this complex

aquifer; they have shown that the best calibration is

achieved by modeling the aquifer system as homogeneous

and isotropic. The same assumption was considered to

simulate the hydrodynamic behavior of the aquifer in space

and time.

To define the boundaries of the model, the work of

Sanchez (1993) was very useful. The wells used in his

research were successfully identified. Taking those values

as a starting point to simulate the groundwater flow, the

hydraulic potentials were reproduced in the study area

using the Surfer program V 8 to serve as input to the model

(see Fig. 4). Equipotential lines were constant head

boundaries. The arrows shown in Fig. 4 are second type of

boundaries (lines representing no flow or contaminant

transport) in the model; they define the surficial domain to

the study. The freshwater part of the aquifer, which is

unconfined, is less than 40-m thick below Merida and

underlain by a brackish mixing zone at 45 m, which gives

way to saline groundwater at about 50-m depth (Villasuso

et al. 1989).

Once the boundary conditions were established, the

properties of the medium were defined, highlighting the

values that were used in the model. Thus, the value of

hydraulic conductivity K in the area was taken from the

work of Gonzalez et al. (2002) with a value of 96,336 m/

day, which corresponds to the maximum value reported in

the numerical model of the aquifer of Yucatan. The

porosity considered is that reported by Gonzalez (1996)

with a value of 30 % (n = 0.3). A uniform recharge of

150 mm/year was assigned (Lesser and Weidie 1988) for

the Yucatan karstic aquifer. The dispersivity of 29.75 cm

was taken from the work of Graniel et al. (2003), deter-

mined in the small town of Santa Gertrudis Copo, a place

near the study area. The specific storage Ss = 1.35 9

10-6 m-1 and the specific yield Sy = 0.25 were obtained

by Casares (2006). With these properties, groundwater flow

was simulated to obtain its general direction.

The aquifer was modeled as a homogeneous and iso-

tropic system. To determine the degree of correspondence

between simulated and field values, a sensitivity analysis

was performed after calibration as follows: the ground-

water flow obtained with the model was compared with

that originally reported by Sanchez (1993). To achieve the

above, monitoring wells used by Sanchez (1993) located in

zone 16 with UTM coordinates were positioned in the same

coordinates of the model. Equipotential contours, gener-

ated applying Surfer V. 8 software, were matched with

those obtained after simulation with Visual Modflow 3.0.


123

The best fit was obtained when the minimum difference

values were computed between both contour maps.

When the groundwater flow model was calibrated

(Fig. 5), to adequately carry out the transport modeling

exercise, pollution plumes, identified with the chloride ion,

were simulated for the above conditions. Three scenarios

were considered as the source of pollution of the aquifer:

(1) the former waste disposal zone, because of the leachate

percolating into the aquifer; (2) only pollution from liquid

spills from the oxidation ponds; and (3) both sources, the

waste disposal zone and the spills from the oxidation

ponds, were polluting the aquifer.

The natural concentration of chloride ion in groundwater

underlying the area is 19 mg/l. It was considered as the

background concentration because it was determined in a

water sample taken in a borehole located upgradient of the

site (Casares 2002). Gonzalez (1996), Ku (1998) and

Casares (2002) report leachate chloride concentrations in

Fig. 4 Hydraulic heads in the

study area

Fig. 5 Boundaries and groundwater flow calibrated model


123

the former waste disposal site, whose data was taken as the

initial concentration in the model. Chloride concentrations

in representative leachate samples were analyzed for three

periods: (1) during the operating time of the dump site

(Gonzalez 1996); (2) in the period of closure (Ku 1998);

and (3) a couple of years after closure (Casares 2002).

It is worth mentioning that the amount of leachate per-

colating into the aquifer varied in the source of contami-

nation and with time because of the amount of waste

present in the site. Therefore, three stages were clearly

identified in the waste disposal site: operation, closure and

abandonment. During the operation time, chloride con-

centrations were between 3,065 and 6,880 mg/l. At closure,

concentrations varied between 3,695 and 5,000 mg/l.

Therefore, a wide range of concentrations could be con-

sidered as initial input to the transport model. Given the

above, it was decided to vary the initial chloride concen-

tration in the transport model to represent field values.

The assumptions are reflected in the inputs of the model,

which illustrates the boundaries (hydraulic potentials) and

point sources. The domain is divided into 80-m intervals in

the X axis and 56-m intervals in the Y axis. A finer mesh

was built, in the section where the boreholes are located, of

16 m in the X axis and 11.3 m in the Y axis (see Fig. 6).

The vertical dimension (Z axis) was defined in 5-m inter-

vals from the water table to the bottom of the aquifer.

To illustrate the behavior of chloride, when monitoring

was carried out, plots, like the one shown in Fig. 7, were

generated with the information collected. With the help of

graphs similar to these, results were compared with those

reported by Gonzalez (2003) who located the center of

mass of the plume at a depth of 21 m below the water table.

The graphics obtained in this research were also compared

with profiles of chloride concentrations for six monitoring

wells considered in a research carried out by Vazquez

(2007), covering the period October 2000 to September

Fig. 6 Model grid. Mesh is refined in the area of interest

Chloride Concentration at a depth of 20 m

0.0050.00

100.00150.00200.00250.00300.00350.00400.00450.00500.00

1998 2000 2002 2004 2006 2008

YEARS

S1_20M

S2_20M

S3_20MS4_20M

S5_20M

S6_20M

Fig. 7 Chloride profiles in central boreholes at a depth of 20 m


123

2005, who identified that the contaminant plume was 25 m

below the water table. Therefore, looking at the results of

these researchers, greater attention was paid to the behavior

of the plume at 15- to 25-m depth below the water table.

No significant differences were observed between

chloride concentrations, in boreholes S5 and S6, with time

from 2003 to 2007 at a depth between 15 and 25 m;

therefore, it was thought that the plume was not in the

process of attenuation or that there was an additional

source of chloride entering the aquifer. Neither of the

boreholes S3 and S4 had a significant reduction in chloride

concentration during the same time period and depth

interval. However, boreholes S1 and S2 showed a down-

ward trend in the concentration of chloride ion from 2002;

this is consistent with the ideas expressed by Gonzalez

(2003), who mentioned that the contaminant plume gen-

erated by leachate from waste accumulated, identified in

his research, was attenuating.

On analyzing all these graphs, the pattern could not be

explained when considering the original idea that there was

only one plume generated by leachates of the old waste

disposal site (Fig. 8). In other words, the high values of

chloride in boreholes S5 and S6 were not only a contri-

bution of such a plume. The resulting behavior can be

explained if an additional source of contamination is

considered.

When the chloride in excess is attributed to the oxida-

tion ponds, given that they are located near the study area

and in the model domain, and no other source of chloride is

present, the behavior of the local groundwater contamina-

tion can be reproduced (Fig. 9). The simulation (Fig. 10)

shows that the contaminant plume emanating from the old

disposal site is currently stable and their concentrations

decrease slowly. The plume generated from the oxidation

ponds will continue growing should conditions be the same

over time.

Conclusions

Carrying out model simulations of both groundwater flow

and contaminant transport, it was possible to identify the

presence of a plume generated by the oxidation ponds

located next to the old waste disposal site of Merida,

Mexico, which had not been identified in previous inves-

tigations in the karstic aquifer below the site. The model

simulations show that the contaminant plume of the old

waste disposal area is currently stable and contaminant

Fig. 8 Plume generated, assuming the old disposal site as the only source of contamination


123

concentrations are slowly decreasing. On the other hand,

the plume generated in the zone of the oxidation ponds will

increase over time if conditions remain similar to those of

the model. The boreholes, of the monitoring network,

located far from the old waste disposal site are more

influenced by the plume emanating from the oxidation

ponds.

Acknowledgments The authors express gratitude to: the Engi-

neering School of the Autonomous University of Yucatan, Mexico,

for the provided facilities, equipment and transportation to carry out

field and laboratory work; the National Council of Science and

Technology (CONACYT) for funding the research projects from

which this work derives and the fellowship for Rodolfo Gomez to

complete his graduate studies. This work was part of his master’s

degree thesis in environmental engineering. The authors also thank

the reviewers for their invaluable comments for improving this paper.

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Documents

Two in one leachate plume in a karstic aquifer