7
Hydrology on the Edge of the Chicxulub Crater

Background and Study Region

Chunchucmil, Ucí, and Cansahcab are ancient Maya sites situated just outside and to the southwest, and just inside and to the north, of the Chicxulub impact crater, a 65-mya bolide-impact site crossing the northwest corner of the modern Yucatán Peninsula (Luzzadder-Beach 2015a; Pope, Ocampo, et al. 1996; Perry et al. 1995). Our research in this region began in 1994 in the Chunchucmil vicinity and in 2011 in the Ucí-Cansahcab area. Chunchucmil was densely populated at the end of the Early Classic and beginning of the Late Classic periods (see chapter 4), and depended heavily on groundwater (Luzzadder-Beach 2000). Ucí and Cansahcab were Late Preclassic and Early Classic sites linked by an 18k-m-long network of raised stone causeways (sacbeob). The overall goal of the studies was to investigate the sites’ economic status and integration, and to evaluate the natural resource base.

The Chicxulub crater is relevant to human history because its geologic structure controls groundwater flow through the region, including flow through a ring of cenotes, or sinkholes, in the karstic limestone surface, mirroring the slumping and faulting around the impact crater structure below (Pope, Ocampo, et al. 1996). The series of cenotes, also known as the zona de cenotes, offers access to groundwater, and also due to greater porosity and permeability in their vicinity, to groundwater flows through the arc of cenotes and out to sea via springs or ojos de agua at the ends of the crescent. This permeable and porous zone serves as a hydraulic diversion, drawing water quickly through the limestone aquifers circling northwest to the zona de cenotes, thereby diverting water that otherwise would pass northwest into the inside of the crater. Hence the inside of the crater zone has slower groundwater flow, and a deeper water table closer to the permeable cenote zone (Luzzadder-Beach 2000, 2015a, 2015b). In addition to ready access at natural cenotes, the ancient Maya also dug wells and constructed casings of cut stone (figure 7.1).

Figure 7.1. Typical ancient Maya wells at (a) Chunchucmil, edged with ancient pillar stones from a former Puuc-style building, and (b) Ucí, stone-edged and -lined. (Photos Courtesy of Sheryl Luzzadder-Beach.)

We mapped the depth to the groundwater table and assessed water quality to develop a model of what kind of water supply was available to the ancient Maya for domestic and agricultural use, in addition to seasonal rainfall. Seasonal rainfall is meager, averaging 900 mm per year, with a distinct dry season between January and April, and peak rainfall between June and September (Luzzadder-Beach 2000; chapter 6, this volume). With variable amounts of rainfall throughout the year, and despite rainwater-runoff capture and storage in ancient structures such as chultunes, groundwater offers a ready and more consistent supply.

Luzzadder-Beach (2000) explained the field sampling strategies and the laboratory methods used for the Chunchucmil study area (figure 7.2), and provided the first comprehensive water-quality and water-table data for the Chunchucmil archaeological research site. We used these same methods from Luzzadder-Beach (2000) in the Ucí-Cansahcab study (figure 7.3). In the course of our summer 2011 field season, we sampled 41 water wells and cenotes in a transect from Ucí in the west, to Tizimin in the east, crossing the crater interior and the eastern boundary of the Ring of Cenotes. At each well we took GPS points, measured the depth to the groundwater surface (in meters, m, as an indicator of ease of access), and collected samples for field and laboratory study (figure 7.3). The parameters we measured included electrical conductivity (EC µS), total dissolved solids (TDS, mg/liter), salinity (Sal, ppt), and the mineral elements and compounds chloride (Cl), sulfate (SO₄), calcium (Ca), magnesium (Mg), and nitrate as NO₃ (all measured in mg/liter). These are diagnostics for water use, aquifer sources, for mapping flow direction, and testing for seawater influence (Luzzadder-Beach 2000).

Figure 7.2. Chunchucmil groundwater sample sites, 1994 and 1997. Stars represent wells. (After Luzzadder-Beach [2000], S. Hutson, Cartography.)

Figure 7.3. 2011 Hydrologic study area: (a) Ucí-Cansahcab water sample well sites (1:2,400,000 DEM) and (b) digital elevation model (close up). Dots represent wells. (After Luzzadder-Beach et al. [2012], K. Doctor, Cartography.)

Earlier studies from Perry et al. (1995) and Back and Hanshaw (1970, 11 wells) provided regional contexts and comparison for Yucatán regional water quality and hydrology. The 1994–1997 Chunchucmil results (Luzzadder-Beach 2000) provide a comparison with the 2011 results from the Ucí-Cansahcab study for a broader regional understanding of water resources.

Results and Discussion

Water Table and Field Hydrology Findings

Table 7.1 presents the mean and median depths to the water table of wells measured in Chunchucmil in 1994 (n = 14), 1997 (n = 22), and 1998 (n = 17), and in the 2011 Ucí-Cansahcab transect (n = 41). The mean and median depths were 1.37 m and 1.71 m, respectively, in the Chunchucmil region in 1997, and 6.99 m and 7.0 m, respectively, in the Ucí-Cansahcab transect in 2011. Considering this greater depth and the wide variability of access through bedrock and sascab, the Ucí-Cansahcab region has a greater challenge to reach groundwater via natural access points and well construction. Groundwater depths across the region conform to findings of Perry et al. (1995) for increasing depth closer to the Ring of Cenotes (Luzzadder-Beach 2000). This is because of higher permeability in the limestone aquifer material nearest the Zone of Cenotes (Perry et al. 1995).

In terms of field measurements of EC, the mean and median at Chunchucmil were 1,219 µS and 996 µS, respectively, in 1997 and Ucí’s mean and median were 1,120 µS and 1,072 µS, respectively, in 2011 (table 7.1). Chunchucmil has a wider range of measures, but there is little difference between the two regions. These EC levels (a proxy measure for the total of all dissolved ions, or dissolved solids) indicate high TDS, which is a limit on agriculture (Luzzadder-Beach and Beach 2008). Salinity was measured in the field at Ucí in 2011 and most wells and cenotes had a trace, with an average of 0.55 ppt and a range from 0.2 to 1.6 ppt. There were 19 wells with salinity above 0.5 ppt, three of these ranged from 1.2 to 1.6 ppt. Salinity of 0–0.5 ppt is considered the range for freshwater; from 0.5 pp to 5.0 is considered “oligohaline”; waters in this category compare to estuarine. Ocean water salinity is about 30.0 ppt for comparison (USEPA 2006). In an unpublished 1998 field study we measured groundwater salinity in Chunchucmil wells and cenotes. The 1998 mean value was 0.87 ppt, the median was 0.7 ppt. The 1998 salinity range was 0.1 to 3 (table 7.1), with 11 of 17 wells exhibiting oligohaline salinity (above 0.5 ppt). Six wells of 17 had salinity in the range of 1–3 ppt; these were located from the modern town center of Chunchucmil to northwestward.

Water Quality Findings

We also summarize and compare water chemistry (table 7.2), between Ucí-Cansahcab and Chunchucmil. Nitrate (NO₃) contamination is typically an indicator for animal waste, fertilizers, or sewage, and poses a danger of methemoglobinemia in infants (see Luzzadder-Beach 2000; Rajagopal and Tobin 1989). Since NO₃ contamination comes from fecal sources, it also may have increased E. coli and other potentially harmful microbes. We should note a history of and the potential for cholera, giardia, hepatitis, and typhoid in Yucatán groundwater (Pacheco A., Cabrera S., and Marín 2000; Delgado et al. 2011). NO₃ means and medians are an order of magnitude less in the Ucí-Cansahcab transect inside the Ring of Cenotes, (mean of 0.34 mg/liter and range of 0–1.4 mg/liter) when compared with the NO₃ in Chunchucmil groundwater outside of the Ring (mean of 2.5mg/liter and range of 0–12 mg/liter). No individual well in either region, however, exceeded the health standard of 45 mg/liter. The maximum for Ucí was 1.4 mg/liter and for Chunchucmil was 12 mg/liter; the vast majority of wells in the Ucí region had concentrations of < 1.0 mg/liter, and for Chunchucmil 16 of 22 wells had concentrations of NO₃ below 3 mg/liter. Concentrations in both regions are therefore not as high as might be expected for this karstic region. Nevertheless, waterborne microbial diseases have a steady impact today, and likely were a major problem in the past though ancient Maya populations did not have to contend with the diseases introduced from Europe in the sixteenth century (Delgado et al. 2011).

The nitrate contamination potential of groundwater is high in this karstic region, yet the actual concentrations were moderate in Chunchucmil (modern population 1,000; ancient population from 30,000+ centrally to 60,000+ regionally ca. ad 600, see chapter 8, this volume; Luzzadder-Beach 2000) and even lower in the Ucí region. The modern population of Motul is about 23,000 (Brinkhoff 2015) but cannot serve as a modern analogue for the magnitude of settlement for ancient Chunchucmil because groundwater is lower and farther from contamination by about 5 m of limestone in most places. Also, Luzzadder-Beach (2000) concluded that throughflow in the Ring of Cenotes vicinity effectively dilutes, diverts, and disperses contamination away from the sites.

Sulfate (SO₄) levels in Ucí-Cansahcab ranged from 0 to 205 mg/liter, and only 3 of 41 wells exceeded 100 mg/liter. None of these wells was over the domestic consumption limit of 250 mg/liter. For Chunchucmil, sulfate ranged from 0 to 295 mg/liter, with two wells exceeding the USEPA limit of 250 mg/liter, and these were on the far western (seaward) edge of the transect (Luzzadder-Beach 2000). Overall SO₄ concentrations were higher in Chunchucmil than in Ucí. High SO₄ levels could also indicate fertilizer inputs and natural bedrock sources from volcanic and from estuarine sources. These sites at the edge and within the Chicxulub crater are low in sulfur because they consist of limestone deposited after the creation of the impact crater, but ejecta blankets from the impacts to the south in Yucatán, Belize, and Guatemala have extremely high levels of sulfur that imposed limits on some crops (Luzzadder-Beach and Beach 2009, Pope et al. 1999).

The other mineral constituents we measured to compare the two regions’ groundwater quality were Ca, Mg, and Cl. For the Ucí Transect, calcium (Ca, mean = 271 mg/liter) and magnesium (Mg, mean = 81.1 mg/liter) levels were typically double those of the Chunchucmil region (Ca mean = 113.5 mg/liter; Mg mean = 39.3 mg/liter; see table 7.2). On the other hand chloride (Cl) and SO₄ (discussed above) mean concentrations were higher in Chunchucmil (Cl = 188.5 mg/liter) than in Ucí (Cl = 143.2 mg/liter) on average, posing a potential limit on agriculture (see Luzzadder-Beach 2000, and Luzzadder-Beach and Beach 2008 for more discussion on agriculture limitations posed by mineral constituents in groundwater).

Limitations on agriculture can occur from Cl, TDS, and salinity in selected wells across the region (Luzzadder-Beach et al. 2012). These water quality conditions would limit intensive, irrigated, pot agriculture and orchard crops. These levels would have fewer potential impacts on the region’s staple crop, maize, but our scant ancient bone evidence indicates that the prehispanic population relied less on maize here than elsewhere (Mansell et al. 2006).

Potential for Seawater Mixing

The chloride-to-sulfate ratios (Cl epm/SO₄ epm) revealed that groundwater quality is not affected by seawater mixing despite tidal fluctuation in groundwater elevations both at Chunchucmil and elsewhere in the Yucatán Peninsula (Luzzadder-Beach 2000, Perry et al. 1995). Selected wells exhibit high Cl/SO₄ ratios in both regions (see table 7.3), but the majority of wells have a ratio lower than seawater (9.6; see table 7.3), and are low-ion enough to indicate very little seawater mixing. All wells in both regions, except for one in Ucí-Cansahcab at 0.93, had a Ca/Mg ratio > 1, which also indicates no seawater mixing (table 7.3; see Luzzadder-Beach 2000, and Perry et al. 1995).

Conclusions

Although these karstic plains of the Yucatán Peninsula exhibit thin soils (Beach 1998a), low rainfall, and limited surface water (Luzzadder-Beach 2000), the groundwater resources are an asset to settlement and to agriculture and domestic consumption. There are few limitations imposed except for salinity, total dissolved solids as expressed by electrical conductivity, and chloride (Luzzadder-Beach 2000; Luzzadder Beach et al. 2012). Nitrate is remarkably low in this fractured limestone region, whose geology has a high dispersal potential. The Ring of Cenotes plays a significant role in the hydrogeology of the region, and allows dilution and dispersal of these contaminants via throughflow. The next steps of research should focus on biological contaminants and their diffusion and persistence in the system, to have a better idea of modern water quality and safety for human consumption. We do know from other studies that waterborne diseases create a disease burden on the Yucatán region, especially the areas with a water table that is close to the surface. Thus Chunchucmil with its near-surface water table was likely in greater threat from pathogens than was Ucí-Cansahcab, which was about 5 m farther removed from the water table and surface contamination. Ucí-Cansahcab’s water characteristics inside the Ring of Cenotes differ in some specific respects from those of Chunchucmil outside of the Ring and the modern population is an order of magnitude larger, but the quality of water overall is not significantly different. Salinity is a moderate limitation of the groundwater in about half of the wells at Ucí. The findings for the Ucí-Cansahcab hydrology transect support the conclusions for our Chunchucmil study (Luzzadder-Beach 2000) that groundwater quality was likely not severely limited for the ancient populations. Therefore, both ancient and modern settlements on both sides of the Ring of Cenotes have reasonable access to useable water resources, worthy of continuing study and protection from harm.