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Hydrogeology Journal (1998) 6:293–301 Q Springer-Verlag

Hydrochemistry of waters from five cenotes and evaluation of their

suitability for drinking-water supplies, northeastern Yucatan, Mexico

Javier Alcocer 7 Alfonso Lugo 7 Luis E. MarínElva Escobar

Received, October 1996Revised, June 1997; March 1998Accepted, July 1997

Javier Alcocer (Y) 7 Alfonso LugoLimnology Laboratory, Environmental Conservation andImprovement Project, UIICSE, Universidad NacionalAutonoma de Mexico, Campus Iztacala, Av. de los Barrios s/n,Los Reyes Iztacala, 54090 Tlalnepantla, Estado de Mexico,MexicoFax:c52-5-277-1829e-mail: jalcocer6servidor.unam.mx

Luis E. MarínGeophysics Institute, Universidad Nacional Autonoma deMexico, Ciudad Universitaria, 04510 Mexico, D.F. Mexico

Elva EscobarBenthic Ecology Laboratory, Institute of Marine Sciences and

Limnology, Universidad Nacional Autonoma de Mexico, ApdoPostal 70-305, Ciudad Universitaria, 04510 Mexico, D.F. Mexico

Abstract Waters from five cenotes that are currentlybeing used for aquatic recreational activities and thatlie along the Cancun–Tulum touristic corridor, Mexico,were evaluated hydrochemically to determine whetherthe cenotes may be considered as potential drinking-water sources. Several parameters exceed the MexicanDrinking Water Standards (MDWS), but since they donot pose a significant health threat, four of the five ce-

notes may be used as drinking-water sources. The com-mon contaminants in the Yucatan Peninsula, fecal coli-forms and nitrate, are in most cases below the MDWS(0–460 MPN/100 ml and 0.31–1.18 mg/L, respectively).Although these four cenotes meet the MDWS, a carefulgroundwater management policy needs to be develop-ed to avoid contamination (fecal and nitrates) and salt-water intrusion.

Résumé Les eaux de cinq cénotés, qui sont normale-ment utilisées pour des activités de plein air, dans larégion touristique de Cancun–Tulum (Mexique), ont

été soumises à analyses chimiques pour savoir si les cé-notés peuvent être considérés comme des sources d’eaupotable. Plusieurs paramètres dépassent les normesmexicaines en matière d’eau potable; mais commeceux-ci ne posent pas de problème réel de santé, quatredes cinq cénotés peuvent être captés pour l’eau pota-ble. Les contaminants habituels dans les eaux de lapresqu’île du Yucatan, coliformes fécaux et concentra-

tions élevées en nitrate, sont la plupart du temps au-dessous des normes (respectivement 0 à 460 germes/100 ml et 0,31 à 1,18 mg/l). Bien que ces quatre cénotéssatisfassent aux normes, il est nécessaire de mettre enplace des règles précises de l’utilisation de l’eau souter-raine, afin d’éviter la contamination par les germes fé-caux et par les nitrates, ainsi que l’intrusion marine.

Resumen Se analizó hidroquímica y bacteriológica-mente el agua de algunos cenotes localizados a lo largodel corredor turístico Cancun–Tulum, que actualmentese utilizan para diversas actividades recreativas, paradeterminar su potencial de uso como fuente de abaste-cimiento de agua potable. La mayor parte de los pa-rámetros excedieron los criterios establecidos en laNorma Mexicana para Agua Potable (NMAP), sin em-bargo, como éstas no representan una riesgo para la sa-lud, el agua de cuatro de los cinco cenotes puede seremplada como fuente de abastecimiento de agua pota-ble. Los contaminantes comúnes del agua subterránea

de la península de Yucatán, coliformes fecales y nitra-tos, se encuentran en la mayoría de los casos por deba-

jo de la NMAP (0–460 NMP/ 100 ml y 0.31–1.18 mg/l,respectivamente). A pesar de que estos cuatro cenotescumplen con la NMAP, es necesario desarrollar unapolítica de manejo adecuada del agua subterránea paraevitar la contaminación de este recurso (fecal y por ni-tratos), así como la intrusión de agua salina.

Key words Contamination 7 hydrochemistry 7 karst 7Mexico 7 water supply

Introduction

The Yucatan Peninsula, southeastern Mexico, is a li-mestone plain with a significant proportion of evapo-rites. Nearly the entire peninsula is underlain by porousand fissured limestone with a veneer of soil and xero-phytes. The southernmost part is covered by a typicaltropical rain forest; temperature variations are small,and seasonality is, therefore, defined by the rainy/dryseason. Quintana Roo, at the eastern part of the Yuca-tan Peninsula, is characterized by two climatic periods

that last six months each; the rainy season is fromMarch/April to October/November, and the dry season



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Fig. 1 Location of study area,northeastern Yucatan Penin-sula, Mexico

is from October/November to March/April. Few sur-face-water bodies exist, and the rivers are short. Waterflow occurs primarily underground. Locations areshown in Figure 1.

Solution lakes, locally known as “cenotes,” are com-mon features of the Yucatan Peninsula. “Cenote” is asomewhat loosely defined term that refers to varioustypes of water bodies contained in limestone cavities.

Pearse et al. (1936) describes four types of cenotes ac-cording to their shape, i.e., jug-shaped, vertical-walled,aguada-like, and cave-like. Cenotes are further classif-ied into two types according to their water characteris-tics. The most common has clear water, a clean sandyor rocky bottom, and a homogeneous well oxygenatedwater mass. In contrast, some cenotes are stagnant, tur-bid, and stratified thermally. In these, the upper layer isalkaline and oversaturated with dissolved oxygen,whereas the bottom layer is acid, lacking dissolved oxy-gen, with H2S in the deeper waters.

Historically, cenotes served as the only sources of

water supply and as important ceremonial places forthe ancient Maya culture. Without them, the Mayanswould have been without sufficient water (Back 1995).For the growing urban and tourist industry of the Mex-ican Caribbean region (e.g., the Cancun–Tulum touris-tic corridor, which is the coastal area between the citiesof Cancun and Tulum, Fig. 1), cenotes play an impor-tant role as potential drinking-water resources and re-creational sites, such as swimming and cave-diving.

Although Quintana Roo is one of the least popu-lated states in Mexico (29th place out of 32 states), itspopulation increased by 43% from 1990 to 1995, and

the number of houses increased by 55%. Populationand housing statistics are given in Table 1. In addition,

Quintana Roo is today one of the most highly urban-ized Mexican states (5th urbanization level out of amaximum of 7, where Mexico City is at level 7; INEGI1995). This accelerated urbanization and increase intourism (Table 1) require large amounts of fresh water.Seemingly, Yucatan characteristics “should provideabundant supplies of water; however, factors of climateand hydrogeology have combined to form a hydrologic

system in which fresh water is scarce and whose chemi-cal environments – seawater intrusion and groundwaterpollution – decrease even that restricted supply” (Wardet al. 1985).

Water is scarce throughout the Yucatan Peninsula,even though rainfall is sometimes as great as 1500 mm/yr (mean is 1050 mm/yr). The rainy season is brief, and85% of the precipitation returns to the atmosphere byevapotranspiration. In addition, the peninsula is under-lain by extremely permeable carbonate rocks that havebeen faulted and folded. The insoluble fraction of thelimestones produces thin residual soils, and bedrock

outcrops are common. Soils and bedrock do little to im-pede the rapid infiltration of meteoric water. Even dur-ing intense storms, surface retention is rarely ob-served.

The high infiltration characteristics of the surface,large permeability of the rocks, and low relief of thearea combine to produce a regional aquifer with a verysmall hydraulic gradient. For example, the gradient innorthwestern Yucatan is about 7–10 mm/km (Marín1990). Thus, Yucatan’s only source of potable water is athin, fragile aquifer that underlies the peninsula(Doehring and Butler 1974) and is underlain by denser,

saline water that occurs more than 100 km inland(Steinich and Marín 1996). High sulfate concentrations



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Table 1 Population and housing statistics from Quintana Roo state and some of the largest cities along the Cancun-Tulum touristiccorridor near the study area, 1990 (INEGI 1991), and 1995 (INEGI 1996).

Parameter Quintana Roo Cancun Playa del Carmen Tulum

1990 1995 1990 1995 1990 1995 1990 1995

Population 493277 703536 167730 297183 3098 17621 2111 3603Houses 106094 163894 39866 75445 711 4646 481 806Potable water1 82588 127228 29358 50895 441 2325 312 632Drainage2 58906 129576 27145 71333 346 3205 145 487

1 Number of houses with potable-water pipe line2 Number of houses with drainage

Table 2 General characteristics of the five cenotes. (mbslpmeters below surface level; Recreational usepswimming, scuba-diving,etc.; Domestic usepdrinking water, bathing, laundry, etc.)

Parameter Carwashcenote

Cristalcenote

Mayan Bluecenote

Nohochcenote

Casacenote

Size (m!m) 50!15 15!9 50!10 75!5 25!20Maximum depth (m) 6 5 5 7 7Water level (mbsl) 0 0 0 0 0Present water use Recreation Recreation Recreation Recreation, Domestic RecreationalBottom characteristics Rocky Sand to Muddy Rocky Rocky Rocky with detritus

Vegetation Sparse Cabomba, Benthic algae Nymphaea, Sagittaria Sparse Mangroves

occur in waters of some cenotes in the Yucatan Penin-sula, resulting from solution of gypsum deposits thatunderlie the region (Perry et al. 1996; Velázquez 1995;Steinich et al. 1996).

The aquifer is highly vulnerable to contaminationdue to its hydrogeologic characteristics and to anthro-pogenic activities, including the discharge of both

domestic and industrial wastes into the aquifer and theindiscriminate use of pesticides and fertilizers (Marínand Perry 1995).

The objective of this paper is to evaluate the hydro-chemistry of water in five cenotes along the Can-cun–Tulum touristic corridor to determine whether thewaters (1) meet the Mexican Drinking Water Standards(Diario Oficial de la Federación 1989) for drinking-wa-ter supplies, and (2) meet the Mexican Standards foruse as aquatic recreational centers.

Methodology

The five cenotes that were studied are (1) Carwash(20716.48bN, 87729.74bW); (2) Cristal (20712.50bN,87728.98bW); (3) Mayan Blue (20711.61bN,87729.74bW); (4) the main entrance of the Nohoch(20717.93bN, 87724.20bW); and (5) Casa (20715.97bN,87723.41bW). Locations are shown in Figure 1, and dataare given in Table 2. Locations were determined using aMagellan Field Pro V GPS (Global Positioning System)instrument calibrated at Puerto Aventuras (Fig. 1). TheNohoch cenote is cave-like, and the other four are well-like (Table 2).

To detect maximum possible chemical variationsduring the dry/wet periods, sampling was conducted atthe end of the dry season (March 1995), when maxi-mum concentrations were expected, and at the end of the rainy season (October/November 1995), when max-imum dilution was anticipated. At each cenote pool, amidday profile of temperature, pH, conductivity (K25),

dissolved oxygen, and percentage of dissolved oxygenwas made using a calibrated Hydrolab Datasonde3/Sur-veyor3 multiparameter water-quality datalogger andlogging system.

Temperature, conductivity, and dissolved-oxygenvertical profiles were checked in situ for possible stra-tification (thermo-, halo-, or oxyclines). When the wa-ter column was homogeneous, a mid-depth sample wascollected with a 5 l Van Dorn water bottle for furtherchemical and microbiological analyses; otherwise, twoor three samples were taken, one from each stratum. Ateach site, three splits were taken, for alkalinity, anions,

and cations (acidified). Water samples were refriger-ated until analyzed in the laboratory.Anions analyzed include carbonates and bicarbon-

ates (through alkalinity), sulfate, and chloride; cationsinclude calcium, magnesium, sodium, and potassium.The anions were measured with a HACH DREL/2000water-quality portable laboratory. Nutrients (NO2,NO3, PO4) were analyzed following the techniques de-scribed by Strickland and Parsons (1972), Parsons et al.(1984), and Stirling (1985). Total dissolved solids(TDS) were estimated by adding the individual species.Microbiological analyses (fecal coliform) were done byusing the technique described by the APHA et al.(1985).



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Table 4 Mean ionic salinity of the five cenotes during the dry and rainy seasons. Values are in % meq/L

Parameter Casa cenote(top)

Casa cenote(bottom)

Carwashcenote

Mayan Bluecenote

Cristalcenote

Nohochcenote

DS1 RS2 DS RS DS RS DS RS DS RS DS RS

Sodium 78.6 75.8 78.4 83 38.7 55.1 59.5 67.3 62.4 63.9 53.1 55.2Potassium 2.4 2.4 2.4 2.6 0.6 0.7 1.4 1.2 1.2 1.4 0.5 0.7Calcium 4.5 5.9 3.2 3.2 39.8 25.9 20.3 18.2 19.7 18.8 12.1 26.1Magnesium 14.5 15.9 16 11.1 20.9 18.3 18.7 18.3 16.7 16 34.3 17.9Bicarbonate 0.9 1.4 0.3 0.6 40 13.5 14.5 7.8 19.2 9.1 9.1 13.4Carbonate 0 0 0 0 0 0 0 0 0 0 0 0Chloride 88.3 89.2 90.2 93.4 54 77.1 75.9 83.5 74.3 80.1 82.6 78Sulfate 10.8 9.1 9.5 5.8 5.6 6.4 9.4 7.9 6.2 8.6 8.2 6.6

1 DS, Dry seasonb RS, Rainy season

Table 3 Minimum-maximum concentrations of physical, chemi-cal, and biological parameters of five cenotes, compared with the

Mexican Legislation standards. Concentrations in mg/L, ex-

cept pH (pH units), total dissolved solids (g/L), temperature ( 7C),and fecal coliforms (MPN/100 mL)

Parameter Casacenote

Carwashcenote

Mayan Bluecenote

Cristalcenote

Nohochcenote

DWS1 RPC1

Alkalinity 134–292 348–440 344–696 350–696 332–342 400 NE3

Chloride 6237–16200 207–689 880–1273 936–1070 689–1050 250 NEPhosphate ND4–0.05 0.007–0.01 ND–0.004 0.02–0.03 ND–0.01 0.1 NENitrate nitrogen 0.314–1.023 0.627–1.09 0.492–1.000 0.982–1.222 0.703–1.03 5.0 NENitrite nitrogen ND–0.006 0.0025–0.003 ND–0.0122 ND–0.003 0.003–0.006 0.05 NEDissolved oxygen 2.17–7.02 2.98–4.68 0.64–4.07 0.93–2.62 1.88–2.345 4.0 NESulphate 900–2400 30–80 145–170 110–162.5 82.5–145 500.0 NEAesthetic NOT NOT NOT OK OK * *Oil and grease ABS5 ABS ABS ABS ABS ABS ABSFloating matter ABS ABS ABS ABS ABS ABS ABS

Odor ABS ABS ABS ABS ABS ABS ABSpH 6.76–7.92 6.73–7.47 6.77–7.31 6.76–6.83 6.88–6.95 5–9 NETotal dissolved solids 12.5–34.6 0.5–1.7 1.7–2.9 2.0–2.5 1.7–2.0 0.5 NETemperature 25.6–28.3 25.1–28.1 25.2–26.6 25.2–26.2 24.7–26.4 NC6

c2.5 NEFecal coliform 8–110 0–174 8–46 0–460 15–30 1000** 200

1 DWS, Drinking-water supply standards2 RPC, Recreational use with primary or direct contact3 NE, Not established4 ND, Not detected5 ABS, Absent6 NC, Natural conditions

* Water body must be free of substances that: (1) adversely mod-ified the water physical characteristics or make deposits, (2) holdfloating matter which produces nasty appearance, (3) impartodor, taste, or turbidity, (4) favors nasty or undesirable aquaticlife** With conventional treatment

Results

The results of each of the physical, geochemical, andbiological parameters, including temporal and spatialvariations, are shown in Tables 3 and 4, and in Figure 2,and they are discussed below.

Temperature The water temperature of the cenotes is fairly constant(Table 3). Temperature differences between dry andrainy seasons are negligible. Water in the Carwash ce-note exhibited the maximum temperature variation(26.40B1.03 7C) in the rainy season. However, thismaximum variation was associated with an ephemeralthin lens of rainwater that had formed over the “stand-ard” pool water prior to sampling. This stratum was al-

kaline, more dilute and warmer than the pool waterthat it overlay.

The average temperature for most springs (exceptthermal springs) is nearly equal to the average mean airtemperature in the area (van der Kamp 1995). Temper-ature of the water in the cenotes (24.65–28.29 7C) issimilar to the mean air temperature calculated by Wardet al. (1985) and ranges from 23 7C in January to 28 7Cin May. The data are similar to the mean water temper-ature reported by Pearse et al. (1936) of 25 7C(21.9–28.5 7C) and Herrera-Silveira et al. (1997) of 26.4 7C (22.0–33.5 7C).

pH Pearse et al. (1936) report pH values of 6.8–8.6 for wa-ter in cenotes in northwestern Yucatan state, and Her-



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e c e n o t e s c o m p a r e d w i t h t h e M e x i c a n D r i n k i n g W a t e r S t a n d a r d s

( M D W S

) . D o t t e d l i n e s o r a n a r r o w

i n d i c a t e v a l u e s o f M D W S



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rera-Silveira et al. (1997) report a mean pH value of 7.5(7.2–8.6) from a variety of aquatic ecosystems, mostlyin the Yucatan state. The pH of water in the five ce-notes sampled is constant and slightly acid (i.e., ~7;Table 3; Fig. 2). The pH differences between dry andthe rainy season are insignificant (6.73–7.66 and6.74–7.92 in the dry and the rainy seasons, respective-ly). The highest pH values (17) correspond to the wa-ter in the lower water stratum of the Casa cenote,which is seawater. The remaining samples, includingthe upper brackish stratum of the Casa cenote, wereacid (pHp6.73–6.95).

During the rainy season, the pH of water from theMayan Blue and Carwash cenotes is slightly alkaline.This condition is probably related to particulate CaCO3

material that probably washes into the cenotes. Thepresence of these layers in both cenotes was confirmedby their lower conductivity and salinity values andhigher temperature. For both cenotes, the deeper wateris acid (pHp6.74–6.82).

Total Dissolved Solids (TDS)

Concentrations of TDS are uniform and characteristicof fresh water. An exception is water from the Casa ce-note, which varies temporally from brackish to saline,and where surface brackish water overlies seawater(Tables 3 and 4; Fig. 2). In the rainy season, the brack-ish-water layer of the Casa cenote is thicker (almost6 m) than in the dry season (about 4 m). The brackishsurface stratum is about 12.5–13.5 g/L in both seasons.In the rainy season, a thin, diluted stratum overlies themain water bodies of the Mayan Blue and Carwash ce-

notes (1700 and 500 mg/L, respectively).

Dissolved Oxygen (DO) Characteristically, cenote waters are undersaturated indissolved oxygen (i.e., DO ~50% saturation; Pearse etal. 1936). This undersaturation is associated with bio-logical (respiration and microbial oxidation of organicmatter) and chemical oxidation of the groundwater.The uppermost layer has the highest DO concentra-tions, and DO diminishes gradually with depth (Ta-ble 3; Fig. 2). Dissolved oxygen crosses the atmo-

sphere–water interface, which explains the higher con-centrations in the top layer. In addition, photosyntheti-cally-produced DO (i.e., aquatic macrophytes and pe-riphyton) increases DO concentrations in the poolswhen compared with nearby groundwater. Microbio-logical (respiration) and chemical DO consumption re-duce the DO content of groundwater, which explainsthe occurrence of a lower DO concentration in the bot-tom layer.

Alkalinity

Alkalinity of cenote waters ranges from 134 mg/LCaCO3 (Casa cenote bottom stratum in the rainy sea-

son) to 696 mg/L CaCO3 (Cristal and Mayan Blue ce-notes in the rainy season; Tables 3 and 4; Fig. 2). Wa-ters in the Carwash, Nohoch, and the upper stratum of the Casa cenote have small variations in alkalinity be-tween the two periods (3–27%), whereas waters in theCristal and Mayan Blue cenotes and water in the lowerstratum of the Casa cenote have large fluctuations(98–102%). The alkalinity in the water of the MayanBlue, Cristal, and Carwash cenotes in the rainy seasonis higher than in the dry season, in contrast to waters inthe Nohoch and Casa cenotes, where the dry-season al-kalinities are higher than in the rainy season. The low-est alkalinities (i.e., ~300 mg/L CaCO3) occur in waterin the Casa cenote, especially in the deep stratum.

Sulfate (SO 4 )

The lowest concentration of sulfate (30 mg/L) occurs inwater in the Carwash cenote in the rainy season; thehighest value (2400 mg/L) occurs in the lower waterlayer of the Casa cenote in the rainy season (Tables 3

and 4; Fig. 2). The largest seasonal fluctuation in sulfateconcentration was measured in the Casa cenote(900–2400 mg/L SO4) and the lowest in the Mayan Bluecenote (145–170 mg/L SO4). Waters in the other ce-notes have fresh-water concentrations and lower sulfateconcentrations (170 mg/L).

Chloride (Cl)

The highest measured chloride concentration(6237–16,200 mg/L) occurs in the lower water stratumof the Casa cenote (Tables 3 and 4; Fig. 2), because of

the proximity of this cenote to the sea (about 200 m).Although dilution was expected during the rainy sea-son, the highest surface chloride concentrations in wa-ter from the Casa and Nohoch cenotes were observedin this season. The salinity of the upper water layer of the Casa cenote remains homogeneous throughoutboth seasons, probably because of the continuous mix-ing between the upper brackish and the lower salinestrata. Waters in the other cenotes have lower concen-trations in the rainy season, as expected. The chlorideconcentration fluctuates between the two seasons. Thelargest seasonal variation in concentration is in water

from the Casa cenote (6237–16,200 mg/L), and thesmallest is in water from the Cristal cenote(936–1070 mg/L). Pearse et al. (1936) report lower chlo-ride concentrations (70–560 mg/L) from waters in ce-notes located farther inland in the Yucatan state.

Nitrite Nitrogen (N-NO 2 ) The nitrite concentrations in water of all five cenotesare insignificant compared to the other nitrogen spe-cies, including NH4, which is the second-most abundantform of nitrogen after nitrates (Table 3; Fig. 2). Waters

from the Cristal, Nohoch, and Casa cenotes have highernitrite values in the dry season, and waters in the



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Mayan Blue and Carwash cenotes have higher nitritevalues in the rainy season.

Nitrate Nitrogen (N-NO 3 )

The most abundant nutrient in waters from all five ce-notes is nitrogen in the form of nitrate (Table 3; Fig. 2).Waters from the Mayan Blue and Nohoch cenotes have

higher nitrate concentrations in the rainy season; theremaining cenotes have higher concentrations in thedry period. Nitrate concentrations of these five cenotesare substantially less than groundwater nitrate valuesreported from Yucatan state (145 mg/L N-NO3) by Pa-checo and Cabrera (1997). Nonetheless, as these au-thors mention, large differences in nitrate concentra-tion in waters from adjacent wells in Yucatan suggestlocal rather than regional contamination.

Phosphate (PO 4 )

Phosphate concentrations of waters in the cenotes arelow (n.d.–0.013 mg/L PO4), due to the presence of highconcentrations of calcium. This condition has been ob-served in similar karstic systems elsewhere (Margalef 1983, Sánchez et al. 1998). No significant differences ex-ist between seasons (Table 3; Fig. 2). The lower waterstratum of the Casa cenote has dissolved phosphateconcentrations that are below detection limit in boththe rainy and dry seasons. The highest concentration of phosphate occurs in water from the Mayan Blue cenote(0.0130 mg/L PO4), and the lowest concentrations arein waters from the Mayan Blue, Casa, and Nohoch ce-

notes (not detected).

Fecal Coliform Bacteria Waters from the five cenotes tested contain smallamounts (fewer than 500 bacteria per 100 mL) of fecalcoliform bacteria, and most samples had counts thatwere fewer than 50 bacteria per 100 mL (Table 3;Fig. 2). Other studies (e.g., Doehring and Butler 1974)report that counts of fecal coliforms fluctuate from0–4200 bacteria per 100 mL (1255B1086 bacteria per100 mL). Results of the present study indicate that

counts are 3–460 bacteria per 100 mL. In general, thenumber of fecal coliform bacteria is higher in the rainyseason. Only water from the Cristal cenote has morebacteria in the dry season than in the rainy season (460and 186 bacteria per 100 mL, respectively). Watersfrom the Nohoch cenote (15 and 30 bacteria per100 mL, respectively) and the Maya Blue cenote (19and 46 bacteria per 100 mL, respectively) have aboutdouble the number of fecal bacteria in the rainy seasoncompared to the dry season. The most striking differ-ence in bacteria count is in water from the Carwash ce-note, where fecal coliform bacteria increased from

3–174 bacteria per 100 mL from the dry to the rainyseason.

Aesthetic Characteristics

The cenotes have clear and oxygenated waters most of the time, and rocky or sandy bottoms that are coveredto a variable degree with submersed aquatic macro-phytes (e.g., Cabomba sp., Nymphaea sp., Sagittariasp.). Diverse biota (e.g., fish, crayfish, and turtles) areeasily observed through the transparent crystal-bluewaters in the cenotes. During the rainy season, watersof the Casa, Carwash, and Mayan Blue cenotes have adark-red coloration that is produced by tannic acidleached from the surrounding area. In addition, a largequantity of suspended matter (i.e., particulate organicmatter) adds turbidity to the reddish water. However,this reddish layer disappears within a few days afterrainfall stops. The presence of tannic acid diminishesthe aesthetic attraction and increases the cost of mak-ing the water potable.

No oil, grease films, or matter was observed floatingon the water surfaces. Stagnant cenotes develop bright-green floating algal blooms, and sometimes leaves orsmall tree branches fall on the surface. However, dur-

ing this study, no anthropogenic floating matter was ob-served. Water in the cenotes is odorless.

Discussion and Conclusions

The five cenotes in this study correspond to the “com-mon” cenote, with clear water, a clean sandy or rockybottom, and a homogeneous, oxygenated water mass.No horizontal differences (physical or chemical) wereobserved in the cenote pools. This characteristic is

probably related to their small surface areas, constantwater flows, and, thus, short retention times. Moore etal. (1992) calculated maximum groundwater flow veloc-ities of 1 cm/s in the Carwash cenote and 3 cm/s in theMayan Blue cenote. In the Cristal cenote and in thefast-flowing current in the Casa cenote, horizontal wa-ter movement from the springs, where water emergesto form the pools, and from the pools into the caves,where water goes back underground, was observed byfollowing the trajectory of disturbed fine silt.

Currently, all five cenotes are being used for watersports, such as swimming and diving, which are desig-

nated as “Recreational Use with Primary or DirectContact.” In the foreseeable future, these cenotes maybe considered as sources for drinking water. The Casacenote is the only one that does not meet the MDWS,due to its high TDS content; thus, it will not be consid-ered as a potential source for drinking water. TheMDWS and the “Standards for Recreational Use withPrimary or Direct Contact” are given in Table 3.

Groundwater salinity in Yucatan Peninsula is400–2900 mg/L, as measured by Doehring and Butler(1974) from various wells throughout the peninsula.Data from this study, excluding the Casa cenote, are

within this range. The bottom water stratum of theCasa cenote is almost pure seawater (Tables 3 and 4); it



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has the highest dissolved-oxygen concentration and DOsaturation values (170%). Water in the Cristal cenotehas a slightly higher bottom DO concentration, becauseit has a larger population of aquatic macrophytes (i.e.,Cabomba sp.), compared to the other cenotes. A fresh-water input to the Mayan Blue and Carwash cenotes inthe rainy season accounts for the higher DO concentra-tion of the shallow water.

Dissolved-oxygen concentrations of the cenotes arebelow the desirable level (4 mg/L), as defined by theMDWS. Oxidation processes, including chemical andmicrobiological, tend to remove most or all of the DOfrom the groundwater (van der Kamp 1995), which ex-plains the low DO concentrations in the pools. Sink-holes along the northeastern coast of Yucatan act as lo-cal basins for the collection of organic debris (Stoessellet al. 1993). DO levels in the pools are higher than inthe springs, because of the series of chemical changesthat occur as oxygen-poor groundwater emerges at theground surface and is suddenly exposed to the oxygen-rich atmosphere, to temperature changes, and to bio-

logical activity (e.g., photosynthesis). Escobar-Brioneset al. (1997) and J. Alcocer (unpublished data) ob-served that DO levels in groundwater of the cave sys-tems of the cenotes average 1.5–2.0 mg/L, which is0.5–1.0 mg/L lower than in the pools. Herrera-Silveira(1994) reports DO concentrations less than 1 mg/L ingroundwater that discharges to coastal lagoons.

Chloride concentrations in waters of the cenotes ex-ceed the MDWS recommendation; the high values sug-gest salt-water intrusion. Two sources of salt water oc-cur in the Yucatan Peninsula (Lesser and Weidie 1988,Perry et al. 1996, Steinich and Marín 1996, Steinich et

al. 1996). The first is the dissolution of evaporite depos-its interbedded in the carbonate rocks, and the secondis seawater. Over-exploitation of the aquifers, com-bined with the shallowness of the fresh-water/salt-waterinterface is modifying the ionic dominance in the shal-low fresh-water lens. In the northwestern and centralparts of the peninsula, dissolution of evaporites is themain source of salt water.

Nitrates are often reported as common groundwatercontaminants concurrently with bacteria (Scanlon1990). Fertilizers, pesticides, and organic wastes in-crease nitrate concentrations in karstic systems. Nitrate

contamination plumes emanating from septic tankshave also been reported (van der Kamp 1995). Naturalnitrate sources, such as leguminous and non-legumi-nous terrestrial nitrogen-fixing plants and plant-litterdecomposition, also contribute to nitrate species ingroundwater. An additional source of nitrate ingroundwater is ammonia, as suggested by Stoessell etal. (1993). Ammonia may be oxidized to nitric acid andmay further dissociate into nitrate species in the pres-ence of oxygenated waters. The northeastern part of the Yucatan Peninsula is poorly cultivated, and thepopulation density is still low. Both factors explain the

low nitrate concentrations in waters of the five cenotes,all of which are much less than the MDWS (5 mg/L).

The presence of fecal coliform bacteria suggests thepotential presence of pathogenic viruses that are re-sponsible for various diseases, including infectious he-patitis; the low concentrations reported in this study in-dicate a low risk of infection. Sewage and domestic an-imals are potential sources of biological contamination.The five cenotes are regularly visited by tourists and,thus, are subject to biological contamination. The Casacenote has a small restaurant nearby, and the peoplewho run the restaurant live in small huts near the ce-note. Although wastes from the restaurant dischargeinto a septic tank, the inhabitants and their domesticanimals (turkeys, dogs, etc.) rely on a less formal meansof personal waste disposal. The yards near or adjacentto the living quarters in the Nohoch cenote and to thetouristic facilities (dressing rooms and kiosk) in theother cenotes (i.e., the Cristal, Carwash, and MayanBlue cenotes) are used as latrines. Few domestic ani-mals (dogs) are known near the Cristal, Mayan Blue,and Carwash cenotes, but in the Nohoch cenote, nu-merous dogs, cats, horses, and cows have been ob-

served. Cristal cenote has the highest count of bacteria(460 MPN/100 mL); the high value is probably asso-ciated with the high volume of monthly visitors.

In general, aesthetic characteristics are satisfactoryfor potable water and recreational uses, except duringthe rainy season, when the possibility increases of theclear, blue water turning into a red, turbid one.

Four of the five cenotes (Carwash, Nohoch, MayanBlue, and Cristal) currently meet the MDWS. The Casacenote does not meet the MDWS, due to its high TDScontent. However, two potential problems exist with re-spect to the other four cenotes being used as sources of

drinking-water supplies: (1) unless a proper groundwat-er extraction scheme is developed that minimizes salt-water intrusion, the water quality is likely to be de-graded quickly; such a scheme should be supplementedby monitoring the chloride content carefully; and (2)the alkalinity, chloride, and TDS concentrations in sev-eral of the cenotes exceed the MDWS. The dissolved-oxygen content required by the MDWS is not met;however, this does not pose a major health hazard.Thus, the conclusion is that from a water-quality stand-point the cenotes, except for the Casa cenote, are suita-ble as drinking-water sources.

In summary, low levels of the reduced forms of ni-trogen (i.e., nitrites and ammonium), as well as low ni-trate values, indicate that waters in the Casa, Carwash,Cristal, Nohoch, and Mayan Blue cenotes are not con-taminated. Currently, the main problem is the highTDS content at the Casa cenote, and the potential forsubstantial increases in TDS at the other cenotes, un-less a groundwater withdrawal scheme is conductedthat considers the potential for salt-water intrusion.Furthermore, in order for these cenotes to provide pot-able water for many years to come, proper sanitary fa-cilities should be constructed in the areas surrounding

the cenotes.



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Acknowledgments This project was financially supported by Di-rección General de Asuntos del Personal Académico de laUNAM project IN203894. L.E. Marín acknowledges supportfrom the Consejo Nacional de Ciencia y Tecnología (project0258PT). The authors thank Dr. D. Valdés and Chem. E. Real deLeón (CINVESTAV Unidad Mérida) for carrying out nutrientand microbiological analyses. Special thanks are given to the In-stituto de Ciencias del Mar y Limnología field station PuertoMorelos for providing lodging and laboratory support, and toMike Madden and his team (especially Chuck Stevens) of CE-DAM Dive Center at Puerto Aventuras for providing cave-divingequipment and logistical support. Biologists V. Urbieta, M.E.García, M. Sánchez, L. Peralta, and L. Oseguera are acknowl-edged for helping in the collection of biological and water sam-ples, and for conducting chemical analyses. The authors sincerelythank Dr. H. Mooers for suggestions and comments that greatlyimproved this paper.

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