SWICA 2 FIELD TRIP to the

Northwest Yucatan Peninsula2-3 April, 2003

Eugene Perry* & Julia PachecoTrip Leaders

*e-mail: t60ecp1@wpo.cso.niu.edu

Photos & field assistance:Richard Socki and Charles Galindo

Consequence of 2: Groundwater flows preferentiallythrough faults and fracture systems. In particular, there is extensive flow through the Cenote Ring.

Characteristics of theNorthern YucatanPeninsula

1) Shallow Shelf2) Flat-lying, homogeneous, porous, permeable carbonaterocks3) Almost no surface drain-age4) Extensive saline intrusion5) Little soil

NASA PIA03379

Field Tripbegins here

Relief image showing Cenote Ring

Ends here

Field Trip begins on the evening of 2 April

Leave Merida 5:15 PM. Arrive Celestun 7:15 PM.We will stay at the Hotel Maria del Carmen, a Modest but Clean and Comfortable Hotel righton the Beach.Dinner 7:30 PM8:30-9:45 PM Lecture.

Celestun

Dead mangrove forestbehind town

A Fishing Village on a Sandspit

7:30 AM, 3 April. Embark for Celestun Estuary. Bring a hat! Continental Breakfast served enroute. The Estuary receives groundwater flowing through the western arm of the Cenote Ring.Mixing of this groundwater with seawater arguably produces anaggressive water capable of dissolving aragonite and high-Mgcalcite in the shell-derived sand brought by westward-flowinglongshore current (Perry et al, 2003). Thus, chemical erosion isprobably an important mechanism maintaining the estuary. Groundwater here is confined. We will visit one or more springs in the Estuary. Groundwater under positive head also enters theEstuary through the sand and silt in the bottom.

Flamingo sightings are likely but not guaranteed.

[Photo courtesy of Ken Knezick-Island Dreams:http://www.islandream.com/phot0302b.htm]

Kopoma water is typical ofwater flowing westward inthe Cenote Ring. Whenmixed with seawater theresulting water is capable of dissolving calcite and aragonite.

Water seeping through bottomsilt of the estuary, when mixedwith estuarine surface water,forms a mixture at equilibriumwith aragonite.

[Photo courtesy of Ken Knezick-Island Dreamshttp://www.islandream.com/phot0302a.htm]

Spoonbill

North Coast Confining Layer

(Numbers are water table elevations abovemean sea level (m).)

This diagram (fromPerry et al, 1989)shows the zone confined by a caliche layer northof Merida. AtCelestun the coastalconfined zone isabout twice as wide.

Explanation:1) Water table (WT)is closely controlled by sea level.2) WT reaches groundsurface near coast.3) Evaporating waterprecipitates CaCO3.4) Cracks formed in caliche layer are immediately healed.Layer becomes impermeable.

Saline intrusion

From Perry et al, 1989

This is a general schematic for the north coast. The CelestunEstuary is different in that, at least in places, bottom silt restsdirectly on permeable corroded and punky limestone. Thecaliche confining layer is present beneath the town, and groundwater beneath the Estuary is confined.

DoubleCenoteRing

Quarry

Dzaptun Ha

Old Water Works

Celestun

LongshoreCurrent

9:30. Disembark at Estuary bridge. (Note that the only formal“rest(?) room” facilities for a long time are at the Embarcadero!Drive 11.2 km east to the old Celestun municipal waterworks. Here we can see that the entire water supply for Celestun was pumped (until about the year 2000) from a single well open for only a meter or so in the highly permeable limestone & beneath about a meter of confining caliche. Groundwater here responds directly to tides and, except for slightly higher NaCl content is similar to groundwater from Kopoma (also on the Cenote Ring).

Frequently there is a layer of surface water above the caliche layer.When present, it has a completely different chemistry from the well water.

9:55(+/-) Head east for 4 km (on the only possible road) to a dirt road on the south side of the highway. Turn right and proceed to the end of this road. (The clue to the turnoff is that a high tension power linecrosses the main road here.) This is Cenote Dzaptun Ha located atN20°51'01". W90°14'08." It now supplies water for Celestun.

Dzaptun Ha is more than 30 m deep. Like many cenotes, it has a dome-like ceiling below the caliche cap, and it has a rubble pile from collapse material in the center. It extends below the saline interface. (See chemical profile.)

Although Dzaptun Ha is about 20 km from the coast, it responds to tides.In fact, it shows a complex behavor, which, although not analyzed, isconsistent with a shallow response to tides at the nearest coast and a deeper response to tides further north where the Cenote Ring encounters the coast.

10:20 Head for small quarry in caliche near Celestun-Chunchucmilcrossroad (20.8 km east of the Celestun Estuary bridge). In this quarryit is possible to get a close look at the caliche confining layer (which is what was quarried). Although subterranean erosion is less vigoroushere than elsewhere along the coast, it is possible to see how cementation by seasonal evaporation of groundwater maintains the coherence of the impermeable caliche layer.

Groundwater in this quarry has a tidal fluctuation, but, in contrast to Dzaptun Ha, it is directly correlated with the tide at Celestun.

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CalicheThickness

Brecciated caliche(detail)

Brecciated caliche

Caliche

11:00 Drive south on road to Chunchucmil. At 5.1 km we pass a small, shallow cenote that opens into a cave at one end. This cenote also responds to tides.

At 24.9 km from Celestun road, we pass through the small village of Chunchucmil with its hacienda and satellite cottages. This hacienda is more or less typical of estates in the heyday of the henequen boom.

At Chunchucmil, we turn east toward Maxcanu. Beyond the village chosas (like this one) we come to a large, unrestored Maya city. A good question (possibly related to climate and sea level change) is: Why was it built here?

As we drive east, notice the conspicuous ridge that will appear on the right. This is the Sierrita de Ticul. It marks a fault, upthrown to the south, that is a major conduit for groundwater.

12:00. Arrive at Merida-Campeche highway. Seek out a PEMEX station near Maxcanu for attention to bodily functions.Proceed to Grutas Chalcehtok, struggling not to get lost on new highway. The hope is to arrive here by 12:30 for lunch besidea cave system in the Sierrita de Ticul. This cave would be a cenote if sea level were higher.

Like many Yucatan caves, Grutas Chalcehtok have had a long period of human occupation.

Leave the Grutas about 13:15. There are two optional stops south of Merida:1) in downtown Kopoma, essentially on the Cenote Ring, we can compare the chemistry of the groundwater to that at the old Celestun well. Both are similar, with high sulfate/Cl ratios (an excellent groundwater tracer). This stop depends on whether we can set up portable equipment for rapid chemical analysis (otherwise see Table 1). 2) Cenote Chen Ha, on the Merida highway 5.2 km north of Kopoma is typical of the cenotes in this part of the Cenote Ring. A stop here depends on whether we are on schedule

Early afternoon is the hottest part of the tropical Yucatecan day. There will be plenty of cold drinks and bottled water, but this is when the farmers come home for a nap. That's another reason that some afternoon stops are optional.

A few kilometers to the east is Hacienda Yaxcopoil, now a tourist attraction. This is where an international consortium cored a 1.5 km hole into (and perhaps through) Chicxulub impact breccia.

Afternoon stops will concentrate on showing the effectsof development. We will travel to Chicxulub Pueblo, centerof the eponymous impact crater, which is about 60 kmnorth-northeast of Kopoma, arriving about 15:00. If possible we will stop briefly at a chicken or hog "factory." Whether we stop or not, we will pass a number of these large waste-producing Facilities, a few of which are visible from main highways.

From Chicxulub we will drive to the coast to see coastal development, which involves causeway construction, filling in of the saline swamp, and destruction of the caliche confining layer. A few salt pans are still in operation, continuing an industry first developed by the Maya.

Final stop will be at Yucalpeten, the first, and largest, of a number of dredging operations undertaken to produce safe harbors for fishing boats.

Chicxulub builds a bullring in preparation for itsannual festival.

Sample Collect Date

Ca Mg Sr K Na pH

(meq) (meq) (meq) (meq) (meq)

Celestún Mar 95 7.53 5.99 0.057 0.32 16.33 6.96

Celestún Mar 96 8.85 6.39 0.063 0.32 16.18 6.62

Celestún Apr-94 9.00 7.03 0.060 0.36 15.92 6.75

Celestún Dec-93 9.03 6.34 0.060 0.31 15.55 6.65

Kopoma Sep-93 8.42 6.46 0.059 0.34 15.45 6.88

Kopoma Jun-93 7.75 6.09 0.052 0.36 14.37 6.77

Sample SO4 Cl HCO3 100x Ion Bal T

(meq) (meq) (meq) SO4/Cl (%)

Celestún 5.45 18.53 6.58 29.42 -0.55 26.60

Celestún 6.48 19.38 6.3 33.41 -0.71 26.60

Celestún 6.24 17.44 6.56 35.77 3.43 26.37

Celestún 6.41 18.65 6.41 34.38 -0.30 26.45

Kopoma 7.31 17.25 6.29 42.34 -0.21 27.66

Kopoma 6.72 14.60 6.42 46.01 1.57 27.73

Perry, E., Swift, K., Gamboa, J., Reeve, A., Sanborn, R., Marin, L., and Villasuso, M., 1989, Geologic and environmental aspects of surface cementation, north coast, Yucatan, Mexico: Geology, v. 17. p. 818–821. Perry, E., Marin, L., McClain, J., and Velazquez, G., 1995, Ring of cenotes (sinkholes), northwest Yucatan, Mexico: Its hydrogeologic characteristics and possible association with the Chicxulub impact crater: Geology, v. 23, p. 17–20.

Back, William, Hanshaw, Bruce B., Comparison of chemical hydrogeology of the carbonate peninsulas of Florida and Yucatan, Journal of Hydrology, 10 (4), p. 330-368, illus. (incl. geol. sketch map), 1970.

Back, William (U. S. Geol. Surv., Reston, VA, United States), Hanshaw, Bruce B., Herman, Janet S., Van Driel, J. Nicholas, Differential dissolution of a Pleistocene reef in the ground-water mixing zone of coastal Yucatan, Mexico,Geology, 14, p. 137-140, illus. incl. 1 table, sketch map, 25 refs, 1986.

Perry, E.C., Velazquez-Oliman, G., and Marin, L. E. (2002) The Hydrogeochemistry of the Karst Aquifer System of the Northern Yucatan Peninsula, Mexico. International Geology Review, 44, p. 191-221.

Pope, K. O.; Rejmankova, E.; and Paris, J. F. (2001) Spaceborne imaging radar-C (SIR-C) observations of groundwater discharge and wetlands associated with the Chicxulub impact crater, northwestern Yucatan Peninsula, Mexico. GSA Bulletin, 113, pp.

Ward, W. C.; Weidie, A. E.; and Back, W., 1985, Geology and hydrogeology of the Yucatan and Quaternary geology of northeastern Yucatan Peninsula: New Orleans Geological Society, 160 p