Hydrogeology of the Yucatán Peninsula

Abstract

The northern Yucatán Peninsula, Mexico is a partially emergent carbonate platform with an extensive continental shelf. Mesozoic-and Cenozoic-era limestone, dolomite, and anhydrite overlie deeply buried Paleozoic-era crystalline and sedimentary rocks. The peninsular aquifer system has developed in nearly horizontal, highly-permeable rocks that are dominantly Tertiary period limestones and dolostones. These are thinly covered by Holocene and Pleistocene epoch carbonate rocks and sediments along the coast, as well as by a thin cover of soil inland. Rocks are both porous and permeable, and permeability exists on two scales: cavernous (fracture) and intergranular (porous medium). Exceptionally permeable zones are developed along faults, perhaps generated by Eocene epoch Caribbean plate movements in the east and, in the northwest, by crustal relaxation and/or basin loading caused by the impact of a large meteorite or comet (Hildebrand et al. 1995). An additional prominent fault, the Ticul Fault Zone (Figure 7.1), whose origin is difficult to assign to a specific tectonic event, is present in the northwest portion of the peninsula. These faults are important as channels for groundwater movement. Eugene Perry acknowledges financial support from NSF Grants EAR-9304840 and EAR-9902971, as well as from the Northern Illinois University (NIU) Graduate School. Velazquez-Oliman acknowledges a graduate fellowship from the Consejo Nacional de Ciencia y Tecnologia and from the Direccion General de Asuntos del Personal Academico (DGAPA). We thank Ing. Ismael Sanchez for providing us with a sample of rainwater from Tropical Storm Mitch and Willard Moore for permitting us to include his 226 Ra measurements.

Full text

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Chapter 7
Hydrogeology of the Yucatán Peninsula
Eugene Perry
Guadalupe Velazquez-Oliman
Richard A. Socki
GENERAL GEOLOGY OF THE NORTHERN
YUCATÁN PENINSULA
The northern Yucatán Peninsula, Mexico is a partially emergent
carbonate platform with an extensive continental shelf. Mesozoic- and
Cenozoic-era limestone, dolomite, and anhydrite overlie deeply buried
Paleozoic-era crystalline and sedimentary rocks. The peninsular aquifer
system has developed in nearly horizontal, highly-permeable rocks that are
dominantly Tertiary period limestones and dolostones. These are thinly
covered by Holocene and Pleistocene epoch carbonate rocks and sediments
along the coast, as well as by a thin cover of soil inland. Rocks are both
porous and permeable, and permeability exists on two scales: cavernous
(fracture) and intergranular (porous medium).
Exceptionally permeable zones are developed along faults, perhaps
generated by Eocene epoch Caribbean plate movements in the east and, in
the northwest, by crustal relaxation and/or basin loading caused by the
impact of a large meteorite or comet (Hildebrand et al. 1995). An additional
prominent fault, the Ticul Fault Zone (Figure 7.1), whose origin is difficult
to assign to a specific tectonic event, is present in the northwest portion of
the peninsula. These faults are important as channels for groundwater
movement.
Eugene Perry acknowledges financial support from NSF Grants EAR-
9304840 and EAR-9902971, as well as from the Northern Illinois University
(NIU) Graduate School. Velazquez-Oliman acknowledges a graduate
fellowship from the Consejo Nacional de Ciencia y Tecnologia and from the
Direccion General de Asuntos del Personal Academico (DGAPA). We thank
Ing. Ismael Sanchez for providing us with a sample of rainwater from Tropical
Storm Mitch and Willard Moore for permitting us to include his 226Ra
measurements.

116 THE LOWLAND MAYA AREA
FIGURE 7.1 Map of the northern Yucatán Peninsula showing the major
geostructural features and an outline of hydrogeochemical terrains discussed
in the text. (Inset map of Mexico shows location of study area). Radium
(226Ra) isotope values in dpm/l are shown for three seaward traverses,
starting from Celestun, Progreso, and Dzilam de Bravo, and extending 20 km
seaward. 226Ra is also plotted for “Bomba” (a coastal spring) and for the
Celestun Water Works (WW). Faults are indicated by lines with serrated
edges, dashed where uncertain. Groundwater flow directions are indicated by
large arrows, dashed where uncertain.
Over almost the entire northern region, a thin freshwater lens is
underlain by a marine saline intrusion. North of the Champoton River, in
Campeche, there are no permanent streams of more than a few hundred
meters length along the west and north coasts. On the east coast of the
peninsula, significant streams are not present in Mexico, but do occur in
Belize. An extensive aquitard has developed within the surface calcrete
layer along the entire north coast from Isla Arena (Campeche) to Holbox

Hydrogeology of the Yucatán Peninsula 117
(Quintana Roo); but, with this exception, there are no extensive aquitards
beneath the northern peninsula.
The distinctive geology and hydrogeology of the region have had a
strong effect on its biota as well as on the culture of its early indigenous
inhabitants. Weathering residue of the exceptionally pure carbonate rocks
has produced remarkably little soil cover, and may have significantly
limited available trace nutrients. Furthermore, even though rainfall is
seasonably abundant over much of the peninsula, water is readily available
only at specific sites because of the absence of surface streams. Access to
water by humans during the pre-colonial period was thus limited to water
that could be obtained from sinkholes in limestone [cenotes, after the Maya
“dzonot”], from primitive hand-dug wells, or from precipitation collected in
various storage devices like those sketched by Frederick Catherwood
(Figure 7.2) and described at Uxmal by Stevens ([1843] 1962).
Except for agriculture, the region offers limited natural resources. Easily
worked building stone was one factor facilitating monumental pre-Colum-
bian architecture (ambitiously recycled as churches and monasteries after
FIGURE 7.2 Aguada, natural water-filled depression at Uxmal as sketched by
Frederick Catherwood. Storage capacity of such natural features was
developed and enhanced by Maya hydraulic engineering in the water-stared
region south of the Sierrita de Ticul where depth to the water table exceeds
100 m, and cenotes are not present. (Source: From an illustration in
Stephens [1843] 1962.)

118 THE LOWLAND MAYA AREA
the Spanish Conquest). Also useful was a distinctive and ubiquitous
weathering product of the limestone of the Yucatán Platform—sascab (as
follows), which can easily be processed to make building mortar.
Petroleum, which might be expected in the geologic environment of the
peninsula, has not been reported. Metals are notably absent. Even flint,
which commonly occurs in carbonate rocks, is lacking. It would have been
important in a pre-Columbian economy, as evidenced by tiny and delicate
imported blades of obsidian found at Maya archaeological sites. The major
exportable mineral resource from the area in pre-colonial time was salt, the
production of which was aided by the nature of coastal hydrogeologic
processes then, as it is now.
SOILS
Distinctive characteristics of the soil and surficial carbonate layers of the
Yucatán Peninsula strongly affect evapotranspiration, groundwater
infiltration, and recharge. It is ironic that although rock exposure on
Yucatán is almost complete in some areas, detailed stratigraphy is not well
known, in large part because of the extensive alteration of surface rock. The
result of this alteration is a nearly impermeable surface layer of calcrete up
to about 3 meters (m) thick that covers much of the northern peninsula.
Quiñones and Allende (1974) call this calcrete layer a “carapace” and
attribute its formation to recrystallization of aragonite and high-magnesium
calcite to form a more stable mineral, low-magnesium calcite. Beneath the
carapace is a layer of low magnesium calcite, almost completely lacking in
cement, known as “sascab.” Gerstenhauer (1987) notes that the
development of these two layers is correlated and attributes sascab
development to infiltration of water. Tulaczyk (1993) reports three
occurrences of sascab in northern Quintana Roo in which no overlying
carapace is present, but which contain “clasts” that could be residues of
such a layer destroyed by weathering. These occurrences of what he refers
to as diamictic sascab may tentatively support Gerstenhauer’s model in
which sascab and calcrete developed together during the Mio-Pliocene
epoch under weathering conditions different from those of today.
In many parts of northern Yucatán, the thickness of the soil cover over
bedrock is a few centimeters (cm) at most. Rarely is this thickness more
than a meter, except in karst depressions such as “aguadas.” The
hydrogeologic significance of scarce soil cover is that meteoric precipitation
can move quickly from the surface directly into the aquifer through
fractures or sinks in the almost ubiquitous calcrete layer. This is a major
reason for the absence of surface drainage on the peninsula. Whatever

Hydrogeology of the Yucatán Peninsula 119
rainfall is not evaporated or absorbed in the sascab layer moves almost
immediately into the aquifer.
Variants of three possible sources exist for the soil that is present in
northern Yucatán: (1) residual insoluble material derived from dissolution
of carbonate rock, (2) volcanic dust from Central American volcanoes, and
(3) dust from more remote sources. Gmitro (1986), who examined the
insoluble residue from acid dissolution of several Yucatán carbonate rocks,
reported that values close to 0% were most common, and that values seldom
exceeded 10 percent.
Pope et al. (1996) have correlated soil maps and geologic maps of the
Yucatán Peninsula and report a clear relation between soil type and bedrock
age, consistent with persistence of residual soil that is, in some cases, as old
as the Eocene. Few analyses of Yucatán soils or argillaceous sediments are
available. Schultz et al. (1971) examined three clay beds, used as pottery
clays, that are 1–2 m thick and are interbedded with limestone of probable
Eocene age. One occurrence is from near Ticul, and the other two
occurrences are from the vicinity of Becal, which is about 50 kilometers
(km) to the west. These clays, dominated by particles smaller than 0.25
microns (μ), consist predominantly of mixed layered kaolinite-
montmorillonite, with quartz as the dominant mineral or dominant
additional mineral in the sample fraction greater than 1μ in diameter. The
authors presume the deposits to be residual material derived indirectly from
airborne pyroclastic material. The insoluble residue left after standard acid
leaching of samples of limestone that stratigraphically overlie the clay
horizon at Ticul constitutes 0.5–5 percent of the rock mass and is composed
of montmorillonite (Schultz et al. 1971).
The slow rate at which soil has accumulated over the Yucatán Peninsula
raises the possibility that an appreciable soil component comes from wind-
blown dust originating in Africa. The amount of mineral dust from West
Africa that is precipitated each year in Miami, Florida, has been estimated
to be 1.25 gm·m-2yr-1 (based on measurements during 1982–1983; see
Prospero 1999). Dominant minerals in West African dust are illite and
kaolinite, with lesser amounts of smectite, montmorillonite, and chlorite.
Relative percentage of these minerals varies with latitude, with kaolinite
dominating at low latitudes; because the West African dust plume extends
over Yucatán, the Miami measurements may be relevant to the rate and
source of dust deposition in Yucatán. Oxygen isotope analysis of the quartz
fraction of soils, and of insoluble residues from carbonate rocks of the
Yucatán Peninsula, could, perhaps, distinguish between these sources (Rex
et al. 1969).

120 THE LOWLAND MAYA AREA
REGIONAL HYDROGEOLOGY
Porous, permeable karst carbonate of the Yucatán Peninsula contains a
fresh groundwater lens underlain by a saline intrusion whose depth is
defined approximately by the Ghyben-Herzberg relation
di = 40·dx,
where dx = elevation above mean sea level (msl) and di = depth of interface
between fresh and saline water [density 1.025 grams per cubic centimeter
(gm·cm-3)]. The nearly flat water table (gradient 2 centimeters per kilometer
[cm/km]) is controlled by sea level and, to a lesser extent, by recharge from
the annual precipitation of 500 to 1500 millimeters (mm) (Chavez-Guillen
1986, 1988). Ion content of the groundwater comes primarily from mixing
with the seawater intrusion and from dissolution of minerals.
Water for human use is accessible where cenotes developed along
geologic structures (Cenote Ring, Holbox Fracture Zone), near the coast
where the water table is near the surface, and in north-central Yucatán
where weathering has resulted in extensive karstification. The depth of the
water table increases inland, from greater than 20 m in Ticul to deeper than
100 m in the Puuc region south of the Sierrita de Ticul.
The terminal Cretaceous Chicxulub Impact Crater, centered
approximately on Chicxulub Puerto (lat. 21o20' N, long. 89o35' W), has
influenced hydrogeology by producing a basin of subsidence that has
partially escaped erosion and karstification (Hildebrand et al. 1995;
McClain 1997; Perry et al. 1995; Pope et al. 1996). Geologic structures that
influence groundwater movement (see Figure 7.1) include the Cenote Ring
(or “Ring of Cenotes”), a permeable zone surrounding the Chicxulub
Sedimentary Basin; the Ticul Fault Zone (delineated by the Sierrita de
Ticul); and the Holbox Fracture Zone in northeastern Quintana Roo
(Tulaczyk et al. 1993; Southworth 1985).
Other notable geomorphic/hydrogeologic features are (1) the north coast,
characterized by a shallow ramp, nearly-continuous dune ridge, cienaga
(saline swamp), and exposed rock (tsekel); (2) the fault-bounded east coast;
(3) the north-central plain with strongly developed karst features
(“Pockmarked Terrain”); (4) a region of poljes—large, flat enclosed basins
whose geologic origin is uncertain—south of the Sierrita de Ticul; and (5)
the zone of high-sulfate groundwater, located south and east of Lake
Chichancanab in Quintana Roo. Aspects of regional hydrogeology are
presented in papers by Back and Hanshaw (1970); Marin et al. (1990);
Moore, Stoessell, and Easley (1992); Perry et al. (1989, 1995); Pope,
Rejmankova, and Paris (2001); Reeve and Perry (1994); Stoessell et al.
(1989); and Stoessell (1995).

Hydrogeology of the Yucatán Peninsula 121
EVAPORITES/BRECCIA AS A SOURCE
OF GEOCHEMICAL TRACERS
Chemical tracers are proving useful in understanding groundwater
movement and rock-water interaction in the Yucatán aquifer. These include,
in particular, the major ions calcium (Ca2+), magnesium (Mg2+), sulfate
(SO42-), bicarbonate (HCO3-), and chloride (Cl-), as well as the minor ion
strontium (Sr2+). Analyses have been presented by Perry et al. (1995);
Stoessell et al. (1989); Moore, Stoessell, and Easley (1992); and Velazquez-
Oliman (1995). The brief summary presented here is based on about 60
major element analyses and 65 additional SO42-/Cl- ratios of ours that have
not yet been published. Several isotopes also provide useful hydrogeologic
information. These include oxygen (δ18O)–hydrogen (δ2H) composition of
groundwater, surface water, and precipitation; δ34S composition of sulfides
and sulfate (SO42-); 87Sr/86Sr ratios in rocks and water; and various isotopes
of radium (e.g., 226Ra).
Almost all fresh groundwater of the Yucatán platform is at (or near)
saturation equilibrium with calcite and (depending on the value assumed for
the solubility product constant, Ksp) with dolomite. This is expected in an
aquifer system dominated by carbonate rocks.
As reported by Perry et al. (1995), ratios of SO42-/Cl- (or Cl-/SO42-) have
helped determine mixing and flow patterns of Yucatán groundwater. The
ratio 100·SO42-/Cl-, expressed in chemical equivalents, is 10.3 for seawater;
ratios close to this are observed in much of the northern part of the
peninsula. This implies two things:
1. Over much of the peninsula, a major source of ions in the fresh
groundwater lens comes from mixing with the underlying saline
intrusion, which has been determined to be present as far inland as
St. Elena (about 100 km; see Figure 7.1).
2. In most cases, SO42- behaves as a conservative ion. Exceptions to
the latter observation, to be discussed subsequently, are
a. unusual cases in which oxidation–reduction (REDOX)
reactions have converted SO42- to H2S and HS-, and
b. the case of Lake Chichancanab, a few kilometers from the
southeast border of Yucatán (see Figure 7.1), which is in
saturation equilibrium with gypsum; thus, the SO42-
concentration in this lake is governed by gypsum solubility.
The evaporite beds that produce high-sulfate water in Lake Chichan-
canab also affect the groundwater geochemistry of a considerable area in
central Yucatán, with consequences for tracing groundwater movement; for
rock diagenesis; and, in practical terms, for agricultural productivity. Two

122 THE LOWLAND MAYA AREA
possible SO42- sources are present in the area. Eocene gypsum-bearing
evaporite has been reported in Quintana Roo (Lopez Ramos 1973), and gyp-
sum and anhydrite are also abundant in impact breccia from the Chicxulub
Crater (Rebolledo-Viera et al. 2000; Ward et al. 1995). Either of these could
be the source of the 2,600 parts per million (ppm) SO42- that has found in
water of Lake Chichancanab and of high SO42- values in groundwater of
what is labeled the Evaporite Terrain in Figure 7.1. Gypsum is regularly
precipitated around the lake bank, and celestite (SrSO4) is in saturation
equilibrium in the lake water. It is reasonable to postulate that the steep
eastern bank of Lake Chichancanab is a fault—upthrust to the east—and
that this fault exposes K/T breccia to dissolution by shallow groundwater. A
reconnaissance study of groundwater south and east of Lake Chichancanab
indicates an apparent lineament of water of high and variable SO42-/Cl- ratio
extending from Lake Chichancanab to Cenote Azul on the southeast coast
of Quintana Roo, near Bacalar Pueblo.
SO42-/Cl- ratios show that groundwater from the vicinity of Lake
Chichancanab moves northwestward through the Ticul Fault Zone (see
Figure 7.1), then northward through the western arm of the Cenote Ring
into Estuario Celestun (Velazquez-Oliman 1995; Perry et al. 1995). It is
also possible that one or more additional sources of shallow evaporite-
bearing rock is present as a SO42- source for groundwater south of Lake
Chichancanab in the Evaporite Terrain.
The Cenote Ring is a major channel for groundwater movement in
northern Yucatán. This is confirmed by SO42-/Cl- determinations of
groundwater and by groundwater table elevations that were reported by
Marin (1990) and Marin et al. (1990). Subsequent unpublished
measurements by Perry and Zhang (based on first-order INEGI
benchmarks) indicate that multiple groundwater divides exist in the system
(cf Steinich et al. 1996), and that the highest groundwater elevation on the
northern peninsula occurs approximately at Lake Chichancanab (4 m above
msl), indicating the flow directions shown in Figure. 7.1.
COASTAL REGION
The hydrogeology and many of the physical and geochemical
characteristics of the coastal area can best be understood with reference to
two features. First, water arrives at this coast exclusively as groundwater,
and much of this groundwater is channeled into specific zones in response
to structural features such as faults or lineaments. This explains the presence
of three of the important north coast freshwater discharge zones: Estuario
Celestun, Bocas de Dzilam, and Laguna Conil. Second, a coastal aquitard
has produced a sort of groundwater “sandwich” along the north coast.

Hydrogeology of the Yucatán Peninsula 123
Where fully developed, as at Celestun, the persistent sand dune that
overlies this coastal aquitard has the following hydrologic components
(from the uppermost downward; see Figure 7. 3):
A. a thin layer of freshwater produced by local precipitation;
B. a thin layer of saline water in direct contact with seawater;
C. the calcrete aquitard upon which the dune rests; the aquitard
prevents vertical water movement.
Beneath it, the multilayer hydrogeologic “sandwich” is completed by
D. the coastal edge of an extensive freshwater lens that constitutes
the major peninsular aquifer (with a dynamic hydrostatic head
that remains higher than sea level);
E. the dispersion zone, a zone of variable thickness of active mixing
between the freshwater lens (D) above and the saline intrusion (F)
below; and
F. the saline intrusion that penetrates many kilometers inland. Here,
as elsewhere, the freshwater lens floats on the saline intrusion, but
mixing of these two layers may be particularly active beneath the
confining aquitard because the whole system responds to tides
(Reeve 1990).
Seawater flows inland within the unconfined upper part of the system, and
through the coastal dune, in response to solar-induced evaporation in the
cienaga (saline swamp) that occurs on the landward side of the dune ridge
(Figure 7.3). The development of this coastal system is discussed below.
The distinctive and widespread north-coastal aquitard formed from the
coastal portion of the ubiquitous surface calcrete layer of the peninsula in
response to several factors (Perry et al. 1989). These include the low
gradient of the land surface (measured in cm/km), the steady rise in sea
level for the past 17,000 years (Fairbanks 1989; Coke, Perry, and Long
1991), direct control of the groundwater table by sea level, saturation of
groundwater with respect to calcite, and tropical climate with high rate of
evaporation of surface water. Perry et al. (1989) have incorporated these
factors into the following model to explain observed characteristics of the
aquitard.
As shown in Figure 7.4, the groundwater table near the north coast
intersects the land surface along a broad band where water comes to the
surface and evaporates. Land surface at Mérida (35 km inland) is only 8 m
above msl—yielding an average land gradient of about 20 cm/km. (East of
Progreso [see Figure 7.1], the slope of the land surface is somewhat steeper
than 20 cm/km, whereas in the vicinity of Celestun to the west, the gradient
is much less.) The slope of the land is so small that seasonal variation of the

124 THE LOWLAND MAYA AREA
FIGURE 7.3. Schematic cross section of an aquifer system at the coast. A = a
thin layer of freshwater produced by local precipitation; C = calcrete aquitard.
The flow of salt water through the dune ridge results from the pumping effect
of evaporation in the swamp/lagoon (cienega). Other units are lobeled as they
appear in the text.
water table causes its intersection with the land surface to migrate on the
order of 1 km or more during each yearly rainfall cycle (the “transition
zone” of Figure 7.4). Because virtually all Yucatán groundwater is saturated
with respect to calcite, evaporation of water along this transition zone
results in the precipitation of calcite, which fills pore spaces and fractures in
the nearly ubiquitous layer of surface calcrete. This has produced a wide
swath of impermeable surface calcrete that is almost devoid of soil (the
tsekel zone in Figure 7.3 and 7.4) along much of the north coast. Over time,
slowly rising sea level has propagated the tzekel zone inland and upward to
produce the confining layer or aquitard shown in Figure 7.3 and 7.4.
As postulated by Gerstenhauer (1987), and supported by observations of
Tulaczyk et al. (1993), the calcrete layer that extends over Yucatán is
perhaps millions of years old. Surface water can penetrate this calcrete layer
only through fractures produced by subsurface weathering and collapse. At
inland sites where the calcrete layer is well exposed, it is seen to be
composed of individual blocks (with an average size of several meters) that
are separated from each other by narrow, continuous cracks. Within the
tsekel these cracks are filled by calcite, more or less as they form; this

Hydrogeology of the Yucatán Peninsula 125
FIGURE 7.4. Detail of the coast showing water table elevations and coastal
zones. Groundwater table comes to surface seasonally in the “transition
zone”. Numbers are depth to the groundwater table in meters (m). (Source:
Perry et al. 1995.)
sealing process, which must be continuous, may be the most important
factor in keeping the confining layer intact. Nevertheless, only spaces with
widths measured in millimeters can be filled by cementation. Thus, cenotes
and large fractures that have become incorporated in the tsekel are not
sealed by cementation.
The aquitard formed by filling of voids in the calcrete layer is typically
about 0.5-m thick (Sanborn 1991), and it extends into the Gulf of Mexico
for an undetermined distance, creating, in effect, a dam that groundwater
must flow under or over to reach the gulf. As a consequence, groundwater
in the coastal region is under a positive head, which at Chuburna is about 40
cm (Perry et al. 1989).
As noted above, a beach of carbonate sand (consisting in part of high
magnesium calcite and aragonite) has developed seaward of the tsekel and
above the coastal aquitard. Saltwater from the Gulf of Mexico moves inland

126 THE LOWLAND MAYA AREA
through this dune ridge and forms an evaporative lagoon (or coastal swamp)
between the dune ridge and the tsekel (Figure 7.3). Under suitable
circumstances, evaporation proceeds to the point at which halite and other
evaporite minerals precipitate. This natural process has been enhanced by
the construction of evaporating ponds (charcas) in Celestun, San Crisanto,
El Cuyo, and other places, and is the basis for indigenous salt harvesting
that has now been developed on a modern commercial scale at Las
Coloradas. (The fact that seawater must move through the coastal dune to
produce observed concentrations of halite is an aspect of the coastal system
that may have escaped the notice of some people studying the coastal
swamp and lagoons, and of those people harvesting salt.)
In cenotes that have, in effect, been moved into the tsekel or coastal
swamp by rising sea level, groundwater flow is reversed, and these cenotes
become springs of fresh or brackish water. Around some cenotes within the
coastal swamp, salinity gradients persist over time and have produced
circular bands of vegetation with distinct salinity tolerances. Roots of this
vegetation trap organic matter and sediments to form small circular islands
(petenes) (Febles and Batllori 1995). The aquitard persists for an
undetermined distance into the Gulf of Mexico; drowned cenotes, in
shallow coastal waters, form submarine springs that are well known to local
fishermen (Figure 7.5).
ORIGIN OF THE COASTAL LAGOONS
Before modern coastal development, the north coast of the
Yucatán Peninsula consisted of an almost continuous dune ridge, broken
only in a few places by largely subterranean outflows of freshwater that
created shallow, swampy, brackish-to-saline estuaries. Based on
observations of the effects of Hurricane Gilbert, the dune ridge may have
been frequently (but temporarily) breached by tropical storms.
Diego de Landa ([1566] 1998) mentioned Ría Lagartos, and
Stevens ([1843] 1962) described Bocas de Dzilam. Estuario Celestun is
another such system. Estuario Celestun and Bocas de Dzilam are located at
the seaward extensions of the Cenote Ring and are fed by groundwater
flowing through the fracture system associated with the Chicxulub Impact
Crater (Perry et al. 1995; Pope, Rejmankova, and Paris 2001). The most
open of these systems is Laguna Conil on the easternmost north coast; there,
groundwater (and occasional overland flow) is channeled through the
Holbox Fracture Zone (Tulaczyk et al. 1993) extending from Lake Coba to
the coast. Groundwater movement to the coast is more diffuse at Ría
Lagartos, but aligned wetlands appear south of El Cuyo on the map
accompanying Lillo et al. (1999).

Hydrogeology of the Yucatán Peninsula 127
FIGURE 7.5. Xbulla, a drowned cenote about 0.5 km seaward from the coast
and about 10 km east of Dzilam de Bravo, The vigorous outflow of freshwater
from this cenote/spring into the Gulf of Mexico that is apparent in this
photograph results from a hydrostatic head of about 0.5 m.
Development of the unusual coastal lagoons of the Yucatán Peninsula is
based, in part, on the geochemistry of carbonate rocks and specifically on
their behavior in coastal aquifers. Back et al. (1979) demonstrated that
“caletas” of the Caribbean coast of the peninsula, such as Xel-Ha, formed
(and continued to develop) by solution of solid carbonate rock along
fractures intersecting the coast. The dissolution results from the decidedly
nonlinear behavior of freshwater and saltwater when they are mixed. As
pointed out by Badiozamani (1973), the mixing of two waters, both
saturated with respect to calcium carbonate, can form an aggressively
unsaturated water.
Back et al. (1979) to demonstrate that the unsaturated mixed water that
forms where large freshwater discharges occur on the east coast is
responsible for coastal erosion and caletas development along that coast.
There, discharges occur through fractures in solid rock, and corrosion of
cavern walls and collapse of roof blocks are evident.
In contrast, almost all sites of groundwater discharge along the gently
sloping north coast are hidden by sand and silt. Nevertheless, conditions for
chemical corrosion are present there as well. Much of the north-coast sand
and silt is derived from the shells of marine organisms and is composed of
high magnesium calcite and/or aragonite—minerals that are relatively

128 THE LOWLAND MAYA AREA
soluble when compared to low magnesium calcite. Mixing of fresh and
saline water can occur anywhere within the plumbing system, including
within the sediment column. Although direct evidence is lacking, Perry and
Velazquez-Oliman (1996) calculated that a wide range of mixtures of
seawater with groundwater from Kopoma (which is typical of the water
delivered by the Cenote Ring to Estuario Celestun) is capable of dissolving
both calcite and aragonite. Surface water collected from the estuary in the
dry month of May was found to be supersaturated with respect to both
calcite and aragonite, as was a sample of groundwater taken from a
piezometer driven through the mud and silt of the estuary. However, a
mixture of roughly equal parts of these two waters produces a water still
supersaturated with respect to calcite, but approximately at saturation with
respect to the more soluble aragonite.
Perry and Velazquez-Oliman (1996) interpreted this result to mean that
groundwater arriving at Estuario Celestun mixes with seawater and
dissolves as much of the aragonitic and high magnesium calcite fraction of
the carbonate silt and sand as it is capable of before discharging into the
Gulf of Mexico. This implies that lagoons of the north coast are maintained
by a balance between the supply of carbonate sand by the steady westward
current, physical transport of particulate matter by freshwater discharge, and
chemical dissolution of the least stable carbonate minerals in the sediment.
A still untested corollary of this hypothesis is that sediments within the
estuary are predicted to have a higher percentage of stable low magnesium
calcite than the sediment load carried by the longshore current.
It is noteworthy that the piezometer used to collect the groundwater
sample within the estuary was driven for about one meter into soft
sediment. At that depth it encountered a layer of coarse, friable material.
Water within the friable layer is partially confined by fine sediment and, in
this case, rose in the piezometer tube to a level several centimeters above
the water in the estuary. Such piezometers can be set in many parts of the
estuary with similar results. It seems probable that the coastal aquitard has
been eroded (or corroded) here (although the aquitard is present beneath
Celestun, a town built on the coastal dune), and that corrosion of sediment
grains may be widespread within the sediment column.
Although Ría Lagartos is not associated with the conspicuous structural
features observed at Celestun and Bocas de Dzilam, it does have a large
number of brackish water springs within the estuarine channel (Pope and
Duller personal communication; Perry personal observation), including one
that has several centimeters of head that has been encased in a large
concrete drain pipe to supply water to fishermen. A moderate-sized salt
extraction facility exists at Las Coloradas. Evaporation ponds have been
placed along the sand dune, a placement that may have the undesired

Hydrogeology of the Yucatán Peninsula 129
consequence of limiting entry of seawater [and, hence, sodium (Na+) and
chloride (Cl-) ions that form halite, NaCl] to Ría Lagartos.
Geochemistry of groundwater of the northern part of the east coast,
especially in the Xcaret-Tulum area, has been studied in detail by Stoessell
et al. (1989) and Stoessell (1995), and in the Xel Ha Zone by Back et al.
(1979) and Back et al. (1986). It is similar in many respects to the
geochemistry of water on the north coast of the peninsula, in that calcite-
saturated groundwater combines with seawater to produce a mixed water
that causes coastal erosion. The principal difference between the
development of the two coasts appears to result from structural and
stratigraphic differences. The east coast contains Pleistocene dunes that are
raised, lithified, and fault-bounded (Weidie 1985). That contrasts with the
gently sloping north coast described above. Farther south along the east
coast (near Bacalar), there are freshwater lakes and cenotes very close to the
coast whose waters have low chloride (Cl-) content, indicating little contact
with seawater [e.g., 104 parts per million (ppm) Cl- for Lake Bacalar and 44
ppm for nearby Cenote Azul, compared to > 19,000 ppm for seawater].
Lake Bacalar is long, narrow, and low-lying, and its lack of contact with the
sea is in apparent contrast to Estuario Celestun (on the west) or Xel-Ha (on
the east). The difference may result from the chemistry of groundwater
reaching this lake. Lake Bacalar water contains 1070 ppm SO42-, and
adjacent Cenote Azul contains 1240 ppm SO42-.
Cenote Azul is a deep, well-mixed lake with a bottom measured at 64 m,
and it apparently reflects closely the composition of groundwater arriving at
this coast. Our analysis of Cenote Azul water shows it to be nearly saturated
with respect to gypsum and strongly supersaturated with respect to both
calcite and aragonite. (The saturation index for aragonite in this water is
0.58.) In contrast with Celestun or Xel-Ha, mixing of this water with
seawater does not produce an aggressive water—a 50/50 mixture is still
supersaturated with respect to both minerals. This absence of an aggressive
water may be the major factor distinguishing Lake Bacalar from other
coastal water bodies on the northern peninsula.
OTHER STUDIES
Radium
Carbonate minerals incorporate an appreciable concentration of uranium
(U) into their structure. Consequently, intermediate radioactive decay
products such as radium (Ra) are released when the host carbonate
dissolves. Moore (1996a, b) has shown that several radium isotopes, which

130 THE LOWLAND MAYA AREA
can be detected at low concentration, can be used to estimate hitherto
hidden fluxes of groundwater to the ocean.
Because groundwater is virtually the only form of discharge to the ocean
from the northern Yucatán Peninsula, radium (226Ra) isotopes were tested to
quantify Yucatán groundwater discharge. Enhanced carbonate dissolution in
the mixing zone was expected to produce a strong, easily measured signal.
We made three traverses into the Gulf of Mexico, in directions
approximately perpendicular to the coast, out to approximately 20 km (see
Figure 7.1): one seaward from near Celestun, another from Progreso, and a
third (collected by a former graduate student, J. Zhang) from Dzilam de
Bravo (near Bocas de Dzilam). Preliminary results of 226Ra measurements
of these samples show that, at 20 km, 226Ra values of 0.1 and 0.08
disintegrations per minute per liter (dpm/l) were comparable to values of
0.07 to 0.09 dpm/l for open water in the Gulf of Mexico (Moore personal
communication). The most shoreward sample from the Dzilam de Bravo
traverse has an exceptionally high value for a marine sample; the two
reference samples taken on land are also exceptionally high. This is good
news for future modeling of groundwater discharge, but perhaps not so
good news for residents of the peninsula—the terrestrial background sample
taken from the Celestun water works exceeds U.S. Environmental
Protection Agency (EPA) guidelines for drinking water.
Hurricanes and groundwater
Measurement of oxygen (18O) and hydrogen (2H) isotopes in natural
waters has become part of the standard set of geochemical tools used in
hydrologic studies. Both isotopes are being used as natural tracers in order
to refine our understanding of the water cycle on the Yucatán Peninsula. In
particular, Lawrence and Gedzelman (1996) have observed that hurricanes
and severe tropical storms deliver rain that is 18O-depleted in comparison to
normal tropical precipitation.
Because of the frequency with which hurricanes (and tropical storms that
evolve into or degenerate from hurricanes) pass over the Yucatán Peninsula
(Figure 7.6), we began in 1997 to collect background data from municipal
wells, cenotes, lakes, the ocean, and rain to test whether severe tropical
precipitation can serve as a tracer for the recharge of groundwater. Factors
that make the peninsula a particularly good place to attempt groundwater
tracer tests include storm frequency, lack of soil (and consequent rapid
infiltration), lack of surface runoff, low hydraulic gradient, extensive saline
intrusion, and high aquifer permeability. Evidence that the aquifer responds
quickly to severe tropical storms comes from the report by Marin et al.
(1990) that Hurricane Gilbert, which passed directly over northern Yucatán

Hydrogeology of the Yucatán Peninsula 131
in 1988, was followed immediately by a general rise of approximately one
meter in the water table.
Opal and Roxanne passed over the Yucatán Peninsula in 1995, as did
Dolly in 1996. In 1998, Hurricane Mitch devastated much of Nicaragua,
Guatemala, and parts of the Mexican state of Chiapas. Then, with
diminished intensity, it passed over Yucatán as a tropical storm, depositing
4 cm of precipitation at Mérida. Even though most of the storm’s energy
had dissipated before it arrived in Yucatán, groundwater and surface water
was collection immediately afterward. Because the isotope sampling
program began in the middle of an unusually frequent series of storm
events, sufficient background data for rigorous analysis are lacking.
Nevertheless, accumulated qualitative evidence shown that the stable
isotope composition of Yucatán groundwater does respond to tropical storm
events; demonstrating that the isotope composition of water in the
freshwater lens changes rapidly. From the latter observation, residence time
of water in the freshwater lens is relatively short.
Stable isotope tracer studies of groundwater are possible because, as a
result of isotope fractionation related to evaporation and precipitation, there
FIGURE 7.6. Tracks of recent hurricanes and tropical storms across the
Yucatan Peninsula (from the National Atmospheric and Space Administration
[NOAA]). Tropical storms/hurricanes are Dolly (∆), Mitch (□), Opal (◊),
Roxanne (○). Filled symbols: 0000 Universal Time (UTC); open symbols:
1200 UTC.

132 THE LOWLAND MAYA AREA
is a remarkably regular relation between 2H and 18O in worldwide
precipitation as shown:
δ2H = 8δ18O + 10
In this equation, δ2H and δ18O are parts per thousand (‰) ratios of the
heavy isotopes to the more abundant isotopes, 1H and 16O respectively,
compared to the same ratio in Vienna Standard Mean Ocean Water
(VSMOW).
The above relationship, commonly referred to as the “Meteoric Water
Line” (MWL), is remarkably general; as a result, rainwater is accurately
“tagged” as to its origin. The most common process that can move water
away from the MWL is evaporation, which, on a hydrogen-oxygen isotope
plot (with δ2H as ordinate and δ18O as abscissa), shifts water down and to
the right of the line.
Figure 7.7 shows the MWL together with the δ2H-δ18O isotope
composition of Yucatán groundwater and rainwater. The figure includes an
analysis of a sample of rain from Tropical Storm Mitch, which was
collected for us in Mérida on 3 November 1998 by Ing. Ismael Sanchez. Its
δ18O value of –9.11‰ is well within the range of tropical storm values
reported by Lawrence and Gedzelman (1996), and its δ2H value is -60.5‰.
The composition of this water falls close to the MWL. It is more depleted in
2H and 18O than any other sample we have analyzed from Yucatán. Most of
the groundwater samples are from municipal wells that typically pump
water from depths on the order of 20 m within the freshwater lens.
Water samples taken in 1997 from Betania and Dziuche, in the south-
central part of the study area, are more depleted than other samples in this
sample suite (Figure 7.8a and 7.8b). These samples are from an area directly
in the path of Hurricane Roxanne in 1995. This is also a transitional region
where some seasonal streams are found. It may be that hydraulic
conductivity of the aquifer is less here and that these stations were still
recovering from the hurricane.
Of 11 localities for which data are available for both 1997 and 1998,
nine increased in δ2H for an average increase of 5‰ from one year to the
next. The average change in δ18O was less, amounting to only +0.2‰. In no
case did the 1998 waters (sampled after the passage of Mitch) move on a
δ2H-δ18O diagram in a direction indicating mixing with water from this
tropical storm. In three of the localities sampled in both years, it was
possible to sample shallow, completely covered cenotes within a few
hundred meters of deeper municipal wells. In each of these localities, the
isotopic composition of the shallow (cenote) water was significantly
different from the corresponding well water. The general trend of the data
suggests a system recovering from earlier (and locally more severe) tropical
storms or hurricanes such as Opal, Roxanne, and Dolly.

Hydrogeology of the Yucatán Peninsula 133
FIGURE 7.7. Hydrogen (ᵹ2H) and oxygen (ᵹ18O) isotope data from northern
Yucatan plotted with reference to the “meteoric water line” (MWL).
Sulfur isotopes
Socki (1984) examined the sulfur isotope relationship in several Yucatán
wells and cenotes. The most interesting of these is Cenote Xcolak (see
Figure 7.1), which is 120 m deep. The interface between fresh and saline
water is sharp at 50 m. Above that depth, water is thoroughly mixed,
whereas below it is anoxic and contains increasing concentrations of
hydrogen sulfide (H2S) with depth. The cenote collects organic matter from
a catchment area several times its size, and the organic matter reacts with
sulfate-rich saline water below the interface according to the general
reaction:
SO42- + 2CH2O = H2S + 2HCO3-
At the bottom of Cenote Xcolak, the isotopic composition of SO42- shifts
from a seawater δ34S value of 21‰, found in deep Yucatán wells, to δ34S
42.6‰. Sulfide values (total reduced sulfur precipitated by silver nitrate,
AgNO3) were lowest at intermediate depths, reaching a minimum of δ34S –
33‰ at 80 m.
If oxidation-reduction reactions of this type were common in Yucatán
groundwater, it would not be possible to use SO42- or SO42-/Cl- as
groundwater tracers. In fact, ratios of SO42-/Cl- significantly lower than the
seawater ratio are relatively rare. Most variations from this ratio are toward

134 THE LOWLAND MAYA AREA
FIGURE 7.8a and 7.8b. Histograms of oxygen (δ18O) and hydrogen ((δ2H)
isotopic composition of Yucatán waters.
higher values, indicating that such reactions are relatively uncommon
except where the aquifer is exposed to organic matter.
CONCLUSION
The unique combination of tropical climate, a partially emergent carbon-
ate platform, and geologic history (giving rise to such features as specific
faulting patterns and the selective exposure of evaporites) combine to make
Yucatán a valuable environment in which to study the hydrogeology of car-
bonate rocks. There is almost no surface runoff on the northern Yucatán
Peninsula. Because of the relative homogeneity of the near-surface rocks of
Yucatán, their near-horizontal bedding, and the absence of extensive aqui-
tards (other than the coastal aquitard), faults have a major influence in col-
lecting and channeling groundwater.
Most groundwater of the peninsular aquifer closely approaches chemical
equilibrium with calcite. Surface water of Lake Chichancanab is saturated

Hydrogeology of the Yucatán Peninsula 135
with respect to gypsum, but all analyzed groundwater from the freshwater
lens is undersaturated with respect to this mineral. Nevertheless, some
groundwater from the area around Lake Chichancanab does have a high
SO42- content, making sulfate a useful groundwater tracer. Other
groundwater receives its dominant component of anions from the saline
intrusion, as shown by chemical equivalent ratios of 100·SO42-/Cl-, which
are similar to the marine value of 10.3. The fact that values of this ratio
approaching 10.3 are found over a large part of the peninsula confirms that
the saline intrusion is truly extensive. Groundwater has had a major role in
the sculpting of coastal features both by creating the coastal aquitard and
also by providing freshwater that, when combined with seawater, forms an
aggressive mixed water partly responsible for developing and maintaining
coastal openings. This is true not only on the east coast, where it has long
been recognized, but also on such north-coast openings as Celestun and Ría
Lagartos.
Stable isotope determinations of oxygen (18O) and hydrogen (2H)
show not only that severe precipitation from Tropical Storm Mitch was
distinctly different from normal precipitation on the peninsula, but also that
the Yucatán aquifer is a dynamic system. Each station that was sampled in
both 1997 and 1998 showed a significant change in groundwater isotopic
composition, with isotopically “heavier” values in 1998 than in 1997
suggesting that the aquifer was still recovering from being hit by storms
such as Opal, Roxanne, and Dolly in 1995 and 1996. Other isotope tracers
including radium and sulfur, discussed here, and strontium (not discussed)
promise to enhance our understanding of aquifer behavior significantly.
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