Cases and solutions
Effects of urbanization on
groundwater resources of Merida,
Yucatan, Mexico
C. E. Graniel 7 L. B. Morris 7 J. J. Carrillo-Rivera
Abstract Groundwater quality in the city of Merida, Yucatan, Mexico, where dependence on groundwater supply is 100%, is affected by urbanization.
Data from the sampling of shallow wells and boreholes along with water level records are used to
study the aquifer. Chemical changes in time of the
bottom and top half of the freshwater zone are the
basis for hydrogeochemistry. The comparison of
1970 data (which represent information prior to urbanization) to 1991, suggests that the most affected
(contaminated) areas coincide with that of urbanization.
Key words Merida 7 Recharge 7 Carbonates 7
Quality 7 Water balance 7 Freshwater 7 Urbanization 7 Contamination 7 Vulnerability
Introduction
The aquifer in the Peninsula of Yucatan has been studied
by several researchers in the last 30 years. Back and Lesser (1981), Back and Hanshaw (1970), and Gaona Vizcayno and others (1980) defined the hydrogeochemistry of
the region related to the position of the sea/freshwater
interface as well as chemical constituents of the groundwater. Perry and others (1989) have contributed to the
understanding of the chemical evolution of groundwater
at the discharge areas (sea shore) defining the calcite precipitation processes. Marin (1990) developed a computer
Received: 1 October 1997 7 Accepted: 23 February 1998
C. E. Graniel (Y)
Facultad de Ingenería, UADY, Apto Postal 150 Cordemex,
Mérida, Yucatán y Posgrado en Ciencias de la Tierra, UACPyP,
UNAM, Cd Universitaria 04510, México DF
L. B. Morris
British Geological Survey, Maclean Building,
Wallingford, Oxon OX10 8BB, UK
e-mail: [email protected]
J. J. Carrillo-Rivera
Posgrado en Ciencias de la Tierra, UACPyP, UNAM,
Cd Universitaria 04510, México DF
model for the hydrological behaviour of the freshwater
lens between the Merida and the Puerto Progreso area.
Several institutions have contributed to knowledge of the
aquifer through studies of the peninsula where the Merida area is given restricted interest. Recently, Steinich and
Marin (1996) defined hydraulic conductivity anisotropy
in the north-western section of the Yucatan peninsula using Schumberger and Wenner methods.
Most studies have defined with the hydrogeological conditions at the time of the investigations. Although chemistry has commonly been included, the comparison in
time and space of groundwater quality has not defined
any contaminant processes that might have changed the
water quality of the aquifer along vertical and horizontal
planes. The objective of this paper is to describe the effects of rapid urbanization on the groundwater resources
of Merida, where development and management of water
resources in the city is 100% reliant on groundwater for
domestic, commercial, industrial and recreational use.
The methods adopted are based in part on risk assessment procedures for in situ sanitation systems described
by Foster and Hirata (1988) and Lewis and others (1988).
The study area
General characteristics
The Yucatan Peninsula comprises the most extensive carbonate units in the Central American and Caribbean region, with a surface area approaching 7.5E6 Hectare (Ha).
A thick sequence of near-horizontally-bedded late Tertiary and Quaternary carbonates underlie most of the peninsular States of Yucatan, Quintana Roo and Campeche
(Bonet and Butterlin 1962). Merida is in northern Yucatan (Fig. 1) where Oligocene to Pleistocene limestone
units form an extensive featureless low-lying plain. Surface drainage is absent as the result of extensive karstification (Lesser and Weidie 1988). There is negligible soil
cover and seasonal rainfall averages 1000 mm/year at
Merida.
The freshwater part of the aquifer is less than 40 m thick
below the city and underlain by a brackish mixing zone
at 45 m which gives way to saline groundwater at about
60 m depth (Villasuso and others 1988). The solution features typical of well-developed karst and the high primary porosity of much of the matrix of the limestone result
Environmental Geology 37 (4) April 1999 7 Q Springer-Verlag
303
Cases and solutions
Fig. 1
Location of Merida, Yucatan,
Mexico
in a very productive aquifer. Boreholes of 40 m deep frequently have specific capacities in excess of 10 l/s/m
(Morris and Graniel 1992). The shallow depth to water
table (F7 to 8 m) the near absence of soil and the extensive existence of a karstic unconfined aquifer have rendered the Yucatan water resources exceptionally vulnerable to contamination (Butterlin and Bonet 1960). Urbanization is one of the most important challenges facing water-resource planners in the states of the Yucatan Peninsula.
to water table across the city varies between 5 and 9 m
below ground level, effluent residence times in the vadose
zone are minimal and transit to the underlying saturated
zone is almost immediate.
As Merida has negligible land surface relief, stormwater
is locally disposed of, typically by soakaway drains at
street intersections. The preponderance of low-rise building (mostly less than two storeys) has kept the popula-
City profile
Merida is the largest city in the eastern portion of Mexico. It is 32 km south of the coast with a population of
about 535 000 in habitants (INEGI 1992). Although the
population of the city has increased rapidly in the last 30
years, more than 98% of the urban population is within
the 15 800-Ha area encompassed by the ring-road, which
is the boundary of the study area. The population densities vary from semi-rural levels of 2 persons/Ha to over
110 persons/Ha in some low-cost housing complexes, the
average density of the centre and main suburbs is approximately 35 persons/Ha and in the main residential
areas is in the range 50–100 persons/Ha (Fig. 2). More
than 80% of urban households have access to a piped water supply, and over 65% have supplied-mains by toilet
facilities (INEGI 1992).
There is no city-wide system of piped sewerage, and wastewater is disposed directly to septic tanks, soakaways,
cesspools and in a very few districts by local collectors to
deep disposal wells open to the saline underlying water
zone. The 1990 national census (INEGI 1992) lists over
83 000 such tanks/soakaways serving properties within
Greater Merida.
Typical domestic septic tank designs comprise dual solids
settling chambers with retention storage time of a few
Fig. 2
hours and adjacent soakaway wells 4–7 m deep. As depth Population density map, Merida, Yucatan, Mexico
304
Environmental Geology 37 (4) April 1999 7 Q Springer-Verlag
Cases and solutions
tion density relatively low compared to other Mexican
cities, yet the extent of new suburbs, paved streets, parking and public areas, and small domestic lots are dominated by the houses and patio. Flooding in these areas
after heavy rains is frequent.
Although there are numerous private wells and boreholes
supplying industrial and commercial premises, almost all
water in the city is publicly supplied by the state JAPAY
(Junta de Agua Potable y Alcantarillado de Yucatan).
Boreholes and shallow wells in the total freshwater zone
are constructed to collect groundwater. The water supply
sources are three peri-urban borehole fields located
beyond the limits of the city, that provide about twothirds of a mean total daily supply of about 242 000 m 3/
day, while 20 individual intra-urban single or dual well
installations provide the other third. It has been the policy of JAPAY eventually to phase out the urban and suburban boreholes in favour of borehole field sources in
more protected catchments beyond the ring-road. Distribution water looses are considered to be about 40% of
the total supply.
Although Merida is not heavily industrialized, it is a regional centre, and food processing and agro-industrial
businesses are well represented. While many of the larger
companies are in the industrial zone to the south-east of
the city, most smaller businesses are dispersed.
Groundwater regime
The carbonate units in and around Merida comprise an
unconfined, double permeability aquifer. This aquifer has
a variable lithology, ranging from friable coquinas to
chalky calcilutites to well-cemented and partially recrystallized detrital limestone.
The aquifer facies diversity is reflected in highly variable
hydraulic properties. Laboratory measurements on core
samples from the upper 35 m of the saturate zone (boreholes at the landfill site on the north-western edge of the
city) show primary porosities of 8 to 55% and matrix hydraulic conductivities ranging four orders of magnitude
from 0.003 m/day to more than 30 m/day (Brewerton
1993) (Table 1). It is noteworthy that the matrix is almost
isotropic; this implies vertical groundwater movement
could take place as readily as horizontal flow.
Extensive carbonate dissolution has superimposed numerous solution features on the lithological system. Although several caverns are known to exist within the city
boundaries at the water table, these features have not
been mapped, so lateral and vertical distribution of karstic development is not known. The secondary permeability thus imposed accounts for the bulk of the hydraulic
conductivity and values well in excess of 700 m/day have
been inferred from pumping tests analysis (Bucley and
Macdonald 1994). This extremely permeable substrate
comprises a most productive aquifer which has been extensively tapped. The hydraulic conductivities appear to
be sufficiently high to cause under prevailing extraction
rates a negligible regional watertable depression, while allowing high yielding boreholes with minimal risk drawdown of interference effects. Coupled with an extremely
low horizontal regional gradient (0.00003 around Merida,
SARH 1988), these features have diluted the need for rigorous pumping tests, so reliable zonal hydraulic conductivity, storage coefficient and effective storage values are
not available.
Surface drainage landforms are absent in the extremely
flat northern Yucatan plain, and it has traditionally been
inferred that groundwater recharge from rainfall is relatively high as a result of infiltration of runoff/sheetflow
through karst-derived features. However, a contrasting
dissolution/precipitation feature which may be of equal
importance to the groundwater recharge in the Merida
area is present at the surface, where a dense recemented
limestone carapace, of variable thickness but typically
about 1.5 m deep, is frequently found. It forms a lime-
Table 1
Laboratory measurements of matrix hydraulic characteristics of
core samples from Merida limestone aquifer. (Site: municipal
landfill (tiradero) located on ringroad, NW edge of Merida
urban area. Samples from core retrieved from four site
investigation boreholes)
sample 8
borehole
8
depth (m)
porosity (%)
hyd cond (k)
@ 20 7C (m/day)
lithological description
1472-IH
1472-IV
1474-IH
1474-IV
1475-IH
1475-IV
1475-2H
1475-2V
1473-IH
1473-IV
1473-2H
1473-2V
2
2
4
4
5
5
5
5
3
3
3
3
6.5
6.5
9.5
9.5
9.5
9.5
33.0
33.0
35.0
35.0
40.0
40.0
50.0
55.0
55.0
50.0
50.0
50.0
40.0
40.0
45.0
45.0
9.8
8.6
8.0
5.4
7.9
3.1
5.1
3.3
0.003
0.003
36.7
32.6
0.013
0.006
shelly white 1st.
shelly white 1st.
shelly white 1st.
shelly white 1st.
shelly white 1st.
shelly white 1st.
white chalky calcilutite
white chalky calcilutite
cream coquina 1st
cream coquina 1st
Beige, well cemented partially recryst. Lst
Beige, well cemented partially recryst. Lst
range
6.5–40
8.6–50
0.003–36.7
* all porosity data above 10% quoted to nearest 50% due to rapidity of drainage during testing, H horizontal plug, V vertical plug
Environmental Geology 37 (4) April 1999 7 Q Springer-Verlag
305
Cases and solutions
stone pavement, sometimes crossed with infield fissures
carapace, being excavated into soft and porous strata
in those areas where soil is absent or present only as a
(Table 1). Poor maintenance of the systems – drains
scanty partial cover. Rainfall has been observed to pond
blocked by debris/suspended solids – would cause
temporarily on such areas after the heavier rainfall
stormwater to remain at the surface subject to for
events. Such surface detention would reduce recharge by
evaporation. There is some evidence that this occurrs
increasing actual evaporation/evapotranspiration. This efin the central business district.
fect could be especially important in the northern part of 3. Imported water supply: this is the most important inthe Yucatan plain where rainfall totals are only 5001–1000
fluence on recharge patterns in Merida; an equivalent
mm/year and potential evaporation rates increase from
of 370 mm/year recharge is imported into the city
about 1700 mm/year at the coast to over 2400 mm/year
from the three borehole fields.
70 km inland (Rodriguez 1984). These features are shown 4. Unsewered sanitation: effluent from industrial and
in Fig. 3.
commercial premises which use mains, mains leakage
and unsewered sanitation are three sources of imImpact of new recharge sources on urban water
ported water to the upper part of the aquifer beneath
balance
Merida.
The six main factors which modify recharge quantities in 5. Storage/disposal of effluents and residues: like private
any urban area (Foster and others 1992) are summarized
abstraction, the volume of industrial and non-domesin Table 2. In the particular case of Merida, the relative
tic effluent reaching the aquifer is not known. Howevimportance of these factors and their interaction can be
er, although many of the major industrial sites have
indicated as follows:
their own supply boreholes, they also generally dispose
1. Surface impermeabilization: despite extensive paving/
of process effluent on-site. The effects on local water
roofing areas, the naturally occurring recemented caraquality may be major, as is the water disposal due to
pace may have retarded diffuse infiltration under natelectrical generation. Changes in net recharge in most
ural conditions, but could increase local (point) recases would be slight, abstraction being almost
charge. Further investigations are needed to undermatched by effluent percolation except where indusstand recharge in Merida, from where there might
trial processes, such as brewing, result in significant
more water for ready infiltration.
consumptive use.
2. Stormwater soakaways: these are likely to be impor6. Amenity irrigation: this inflow is compensated as the
tant for recharge as both municipal (street intersecsupply is usually obtained locally from shallow wells.
tion) and domestic (roof runoff) systems bypass the
The impact from different quantities of groundwater recharge are substantial. The new water balance includes
volumes that are deteriorating the original chemical and
bacteriological characteristics of the local groundwater.
Fig. 3
Thus new its to the general exploration must be considHydrogeological regime, Merida
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Cases and solutions
Table 2
Summary of impact of urbanization processes on groundwater [adapted from Foster (1992)]
urban process
rates
recharge
modification
area
time basis
implications
for quality
principal
contaminants
surface
impermeabilisation
stormwater
soakaways*
imported mains water
supply
unsewered sanitation*
reduction
extensive
permanent
minimal
none
increase
extensive
intermittent
increase
extensive
continous
margainall
negative
positive
Cl, HC, FP, N-NO3
(ClHC spills)
none
increase
extensive
continous
negative
marginal
increase
increase
restricted
continous
negative
restricted
seasonal
variable
N-NO3, FP, DOC,
ClHC, Cl, HC
DOC, HM, N-NH4,
HC, ClHC
N-NO3, Cl
land storage/disposal
effluents and residues
irrigation of amenity
areas
* Important industrial component
major ions
HC hydrocarbon fuels
hydrocarbons
HM heavy metals
DOC dissolved organic carcon
Cl Choride and other
N nitrogen compounds (nitrate or ammonium)
ClHC chlorinated
FP faecal pathogens
A digital modelling of the flow regime applied numerical
two-dimensional steady-state code FLOWPATH. The
modelling was intended as a conceptual tool to assess
(future) strategies for potable water supply and wastewater disposal. Detailed modelling of the groundwater flow
Methodology
regime leads to uncertainties concerning key elements of
the groundwater balance. Although the karstic limestone
A comprehensive dry season sampling was carried out in aquifer of the Yucatan Peninsula is the focus of much inApril 1991 of 39 shallow wells, two underground caverns terest by karst hydrologists, much of the research emphasis is also concentrated on aspects of geomorphological,
(which enter the uppermost 2–4 m of aquifer) and 17
sedimentological and geochemical evolution, along the
boreholes (which generally tap the bottom half of the
coastal margins and a highly developed karst zone formfreshwater zone) (Fig. 4) at the end of the 1991 dry season. Analytical results provide a baseline to compare with ing a semi-circular ring of dolines inland. However, in
the earliest available chemical data for 1970. Specific con- terms of regional hydrogeological resource evaluation,
several important parameters (such as hydraulic conduccentrations measured from wells in rural parts of the
tivity, effective porosity, head potential and recharge) instudy area down-gradient of the city were considered as
key indicators as no data on 1970 sampling and analytical cluded in the hydrological balance remain poorly quantified; consequently the effects of urbanization on the
techniques are available. The indicator values of TDS,
groundwater resources of Merida requires the integration
chloride and nitrate are considered reliable. The 1970
of the environmental geological studies, chemical (such
analytical results are used to characterize development
as Cl P, NOP
and related contamination for the city population of
3 ) and social investigations to define the area.
200 000 habitants.
A set of samples was collected according to field procedures recommended by Foster and Gomes (1989).
Groundwater samples were analysed by The British GeoResults and discussion
logical Survey (BGS) and the Autonomous University of
Yucatán. Laboratory techniques are those recommended
Changes 1970–1991
by Standard Methods (APHA 1992).
A comparison of contour maps of 1991 to data for 1970
A flame photometer was used for the analysis of major
cations, except for Ca c and Mg c which were determined revealed an increase in the concentration of groundwater
by a standard two-stage hardness titration with EDTA.
constituents, confirming extensive contamination of the
P1
c
,
NH
and
Cl
;
The analysis of anions including HCOP
upper part of the aquifer. Cl P and TDS concentrations
3
4
P
SO4 were determined using the turbidimetric method.
have significantly increased areas underlying dense urOrganic nitrogen was determined by the macro-Kjeldahl
banized districts of the city (Fig. 5). Field measurements
method and UV spectrometry was used for NOP
(of pH, temp, dissolved oxygen, electrical conductivity
3.
ered other than the inducement of saline water from beneath.
Environmental Geology 37 (4) April 1999 7 Q Springer-Verlag
307
Cases and solutions
Fig. 5
Chloride (mg/l) upper part of the Merida aquifer
Fig. 4a, b
Location of a shallow wells and b boreholes in Merida
and faecal coliform) in shallow well discharges confirm
that dissolved oxygen concentrations are significantly reduced in these two areas (Fig. 6). This response to organic and inorganic loading is echoed in the distribution
P
of N species, with NHc
4 as well as NO3 as detected in a
central zone ringed by a high NOx- area (Fig. 7).
A third zone of high salinity was identified between the
airport-main industrial zone and the western boundary
ring as indicated by higher TDS in water from privately
owned industrial boreholes. Private abstraction in this
area is undocumented but is likely to be high from the
nature of the industries involved (power generation, beverage production, food processing).
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Environmental Geology 37 (4) April 1999 7 Q Springer-Verlag
Fig. 6
Dissolved oxygen (mg/l) upper part of the Merida aquifer
Salinization processes
A network of observation boreholes has since been established to monitor seasonal changes in groundwater quality. Bacteriological analyses of samples from these boreholes show faecal coliform counts varying widely both
seasonally and between boreholes (Fig. 8). These together
with total organic carbon concentrations of more than
Cases and solutions
Rises in contaminant concentrations after recharge events
are a commonly observed phenomenon as the aquifer is
P
shallow and unconfined, increments of Cl P, SOP
4 , NO3 ,
respectively, have been recorded in rural areas to the
northeast of Merida (Pacheco 1985) during recharge
events. However, it is interesting to note that this phenomenon can be observed in the urban water balance of
Merida as rainfall has significantly decreased in recent
years. The effect of recharge other than from direct infiltration of excess rainfall is considered in the next two
sections.
The overall hydrogeological framework suggests that salinity changes could be caused by two main factors: (1)
input of drainage waters to the shallow aquifer and (2)
the upward movement of saline water from below. These
processes are relevant to groundwater management.
Fig. 7
Nitrogen of nitrates (mg/l) upper part of the Merida aquifer
Fig. 8
Faecal coliform (NC!100 ml) upper part of aquifer the Merida
45 mg/l noted during the April 1991 sampling confirm
not only heavy organic loading of the upper aquifer but
also suggest that the annual recharge pattern still retains
some control over temporal variations in groundwater
quality in this upper zone. Boreholes show stable coliform counts regarding precipitation (recharge availability) in time.
Importance and effects of new urban to rainfall
recharge
Several groundwater resource study estimates of mean
annual recharge to the Yucatan aquifer have been made
based on calculations of the excess of mean annual rainfall over evaporation. For the Merida area these recharge
estimates vary from 140 to 200 mm/year (SARH 1988;
Back 1988; Rodriguez 1984). Using public piped water
supply figures, the mean daily volume of water circulating in the city supply mains represents a consumption of
about 460 l/person/day. Assuming 10% consumptive loss,
this is equivalent to an average annual infiltration rate
across the city of 505 mm/year, or about three times the
average current estimates of rainfall recharge. In the particular case of Merida, after allowing for consumptive
use, all leakage recharges the aquifer, either as mains
leakage before entering the domestic cycle, or afterwards
as septic tank/soakaway effluent.
The presence of large quantities of water in the aquifer
provides dilution, which would have the effect of reducing effluent pollutant concentrations. This dilution can be
inferred from the water-quality contour maps; although
typical waste indicators like Cl P and NOP
3 are higher in
the upper part of the aquifer, their concentrations appear
to be les expected from estimates of pollutant leaching
load.
The occurence of NOP
3 illustrates the point. The average
human contribution to wastewater is about 5 kg N-NOP
3/
person/year (Lewis and others 1982) in organic forms
which are subsequently mineralized to inorganic species
during decomposition (in septic tanks or during percolation). In Merida, ambient dissolved oxygen in the upper
aquifer is sufficient to make nitrification to nitrate the
predominant process except in the city centre, where mineralization of organic nitrogen to ammonium is occurring. If all the nitrogen produced from human excreta
were nitrified and dissolved in 170 mm/year rainfall recharge, the nitrate concentration of the resultant recharge
would be 98 mg/l N-NOP
3 . This is about four times more
observed
in the April 1991 sampling
than the high NOP
3
of the upper part of the aquifer, even allowing for the
ammonium content of the central zone.
Environmental Geology 37 (4) April 1999 7 Q Springer-Verlag
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Cases and solutions
selves, proved impossible to reconcile with observed regional groundwater gradient without invoking extremely
high hydraulic conductivity values for the freshwater
zone (Fig. 9). A higher hydraulic conductivity value at
the coast is required to permit the annual flux to discharge seawards.
Secondly, hydraulic conductivity values are scarce. In the
absence of local transmissivities values obtained independently from pumping test data, where both thickness and
hydraulic conductivities scationare reasonably estimated
an approximate mean value of about 75 000 m 2/day was
calculated using specific capacity data from one of the
peri-urban public supply boreholefields. This is equivalent to a hydraulic conductivity of about 2100 m/day in
the south of the modelled area. This figure would be considered very high for regional flow assessment purposes,
although hydraulic conductivity has a value of up to 864
m/day, recorded from the karstic Aymamón limestone of
Modelling urban flow regime
The first calibration runs have encountered the following Puerto Rico (Giusti 1978).
Finally groundwater elevation; although groundwater
problems, which have yet to be solved. Firstly, recharge
estimates have been derived from calculations of the dif- heads have been produced on a regional basis (SARH
ference between measured mean annual rainfall and com- 1988), precise piezometric contours are not yet available,
and they would be invaluable for calibration purposes.
puted annual evapotranspiration which, however, lack a
The production of reliable maps is hindered by the exdescription of recharge processes. Three recharge estitremely low regional piezometric gradient, estimated at
mates are encountered in the literature: a mean value of
only about 0.00003 across the city. Further, piezometric
150 mm/year (Lesser and Weidie 1988; Back 1988) and
205 mm/year SARH (1988) while Rodriguez (1984) cites
about 100–275 mm/year for the Theissen polygons covering the 4535-km 2 rectangle of northwest Yucatan encomFig. 9
passing Merida. Values which represent about 15–25% of Model of aquifer using areal recharge values of 100–275
annual rainfall, while not necessarily excessive in themmm/year
If rainfall and leakage sources were aggregated, the urban
recharge would increase to about 675 mm/year, and the
resultant nitrogen concentration would decrease to 30
mg/l (this calculation takes into account background nitrate concentrations in groundwater supplies from the
periurban boreholefields which are generally below 5 mg/
l N-NOP
3 . Although this is still higher than the levels of
15–25 mg/l N-NOP
3 observed in the most densely populated suburbs, it is reasonable, as dilution effects from
mixing with throughflow have not been included.
The indications are therefore that the present groundwater system beneath Merida has evolved in such a way
that it has become reliant on recirculation of mains-derived water to mitigate the effects of a high diffuse contaminant load on the upper part of the aquifer to be of
extreme vulnerability.
310
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Cases and solutions
Fig. 10
Calibration sensitivity runs of model showing critical effects of
recharge values on regional gradient
values require a correction for the displaced volumes beneath the sea-water level (Ghyben-Herzberg theory)
which could account for higher water volumes involved
in the water balances than those anticipated. Additional
salinity logging could prove to be a reliable source of information.
Even so, the modelling carried out has already highlighted important groundwater management issues. For
instance, a key public health concern is to assess the susceptibility of the major peri-urban borehole fields located
south and east of the city to reversal of the exceedingly
low regional gradient. The lower the hydraulic conductivity and thus the throughflow, the more likely a reversal
of groundwater gradient due to both pumping outside
the city and infiltration of urban wastewater beneath
Merida. The groundwater velocity diagrams in Fig. 10
show two sensitivity runs using a transmissivity of 75 000
m 2/day based on the specific capacity calculations referred to earlier. If the regional transmissivity of the northern Yucatan aquifer is lower than the figure, or 75 000
m 2/day cited earlier, much lower regional recharge is implied; computations indicate that flow reversal would
start to occur if rainfall recharge were below about 50
mm/year around Merida.
Conclusions
Sampling of numerous wells in the unconfined karstic limestone aquifer underlying the city of Merida has confirmed that contamination of the upper part of the freshwater aquifer is occurring. This phenomenon is a result
of the urbanization process.
Recharge beneath the city from anthropogenic sources,
principally from in situ sanitation and effluent disposal,
significantly exceeds that which would have occurred
from excess rainfall prior to urbanization. The extra recharge is a result of the interaction of a generous per
capita supply with entirely on-site wastewater and urban
drainage disposal, and it appears to have mitigated the
effects of a large diffuse contaminant load on a highly
vulnerable aquifer.
As much of this extra recharge results from imports of
water from peri-urban borehole fields, there are implications for the local flow regime. The limestone being both
porous and extremely permeable, the effect of the mass
transfers of water appears to be subdued, but preliminary
sensitivity analysis of the resultant hydrogeological model
of the aquifer indicates that gradient reversal could occur
even if the transmissivity was as high as 75 000 m 2/day.
There is reason to suspect that current estimates of recharge to groundwater of the northern plain of Yucatan
may be higher than anticipated using water divergence
modelling only. Further research on this most important
aspect of the groundwater resource is indicated, given the
100% dependence of the states of the Yucatan Peninsula
on groundwater for all their water needs.
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Cases and solutions
Acknowledgements The authors acknowledge the members of
a joint FIUADY/BGS/CNA team working collaboratively on the
project and would like to acknowledge the contributions made
by the colleagues involved. This paper is published with the approval of the Director, British Geological Survey, NERC.
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