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Ground-penetrating radar, resistivity and spontaneous potential investigations of a contaminated aquifer near Cancún, Mexico

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Title:
Ground-penetrating radar, resistivity and spontaneous potential investigations of a contaminated aquifer near Cancún, Mexico
Alternate Title:
NCKRI Symposium 2: Proceedings of the Thirteenth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst
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Carpenter, Philip J.
Adams, Ryan F.
Lenczewski, Melissa
Leal-Bautista, Rosa M.
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University of South Florida
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English

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pg(s) 231-237 Geophysical surveys were made over portions of the Cancún municipal well field in the Yucatán Peninsula of Mexico, approximately 20 km southwest of the city of Cancún, in order to identify karst conduits that channel contaminated surface waters into the main aquifer. Specifically, ground-penetrating radar (GPR), vertical electrical soundings (VES) and spontaneous potential (SP) surveys were employed to identify these conduits and detect water movement through them. Cancún s municipal water supply has frequently been affected by fecal coliform bacteria and other contaminants. Water supplies are largely derived from highly permeable fractured karst limestone aquifers characterized by rapid transport of microbial and chemical contaminants from the surface to subsurface unconfined and confined aquifers. Quaternary and Tertiary limestone bedrock outcrops across this entire area, which exhibits less than 3 m of local relief. Schlumberger array VES were made at two locations.. One sounding revealed a 3-layered structure consisting of a 177 ohm-m layer 2.1 m thick, (probably weathered limestone), overlying a high resistivity layer 8.2 m thick (massive limestone with some small caves), overlying saturated limestone (45 ohm-m). The other sounding could not be successfully inverted due to lateral resistivity variations. Twenty-one GPR profiles were also made with 50- and 100-MHz antennas along roads passing through the well field. In the upper 5 m these profiles reveal cut-and-fill structures and a myriad of diffractions that may represent collapsed and filled sinkholes or solution-enlarged fractures. A major interface delineated by GPR at about 6-8 m depth probably represents the water table. An unusual transparent zone (absence of GPR reflections) was also visible in one GPR profile made near a surface conduit. This transparent zone was at least 1.5 m wide and extended over several meters depth. SP measurements near this conduit during a rainstorm revealed a peak-to-peak variation of 16 mV, suggesting SP may also be a viable method for mapping subsurface water movement in this well field. The overall implication of this work is that geophysical methods are valuable in delineating recharge points and shallow contaminant pathways, and should be used more extensively in this part of the Yucatán Peninsula to support groundwater investigations.
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Description
pg(s) 231-237
Geophysical surveys were made over portions of the Cancn
municipal well field in the Yucatn Peninsula of Mexico,
approximately 20 km southwest of the city of Cancn, in order
to identify karst conduits that channel contaminated surface
waters into the main aquifer. Specifically, ground-penetrating
radar (GPR), vertical electrical soundings (VES) and
spontaneous potential (SP) surveys were employed to identify
these conduits and detect water movement through them. Cancn s
municipal water supply has frequently been affected by fecal
coliform bacteria and other contaminants. Water supplies are
largely derived from highly permeable fractured karst limestone
aquifers characterized by rapid transport of microbial and
chemical contaminants from the surface to subsurface unconfined
and confined aquifers. Quaternary and Tertiary limestone
bedrock outcrops across this entire area, which exhibits less
than 3 m of local relief. Schlumberger array VES were made at
two locations.. One sounding revealed a 3-layered structure
consisting of a 177 ohm-m layer 2.1 m thick, (probably
weathered limestone), overlying a high resistivity layer 8.2 m
thick (massive limestone with some small caves), overlying
saturated limestone (45 ohm-m). The other sounding could not be
successfully inverted due to lateral resistivity variations.
Twenty-one GPR profiles were also made with 50- and 100-MHz
antennas along roads passing through the well field. In the
upper 5 m these profiles reveal cut-and-fill structures and a
myriad of diffractions that may represent collapsed and filled
sinkholes or solution-enlarged fractures. A major interface
delineated by GPR at about 6-8 m depth probably represents the
water table. An unusual transparent zone (absence of GPR
reflections) was also visible in one GPR profile made near a
surface conduit. This transparent zone was at least 1.5 m wide
and extended over several meters depth. SP measurements near
this conduit during a rainstorm revealed a peak-to-peak
variation of 16 mV, suggesting SP may also be a viable method
for mapping subsurface water movement in this well field. The
overall implication of this work is that geophysical methods
are valuable in delineating recharge points and shallow
contaminant pathways, and should be used more extensively in
this part of the Yucatn Peninsula to support groundwater
investigations.



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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 SPONTANEOUS POTENTIAL INVESTIGATIONS OF A Philip J. Carpenter, Ryan F. Adams, Melissa Lenczewski Dept. of Geology and Environmental Geosciences, Northern Illinois University, 312 Davis Hall, DeKalb, IL 60115 USA, pjcarpenter@niu.edu, radams1geology@niu.edu, lenczewski@niu.edu Rosa M. Leal-Bautista made near a surface conduit. This transparent zone was at least 1.5 m wide and extended over several meters depth. SP measurements near this conduit during a suggesting SP may also be a viable method for mapping overall implication of this work is that geophysical methods are valuable in delineating recharge points and shallow contaminant pathways, and should be used more extensively in this part of the Yucatn Peninsula to support groundwater investigations. Introduction The municipal water supply for Cancn, in the northeastern Yucatn Peninsula of Mexico, has been degraded often by fecal coliform bacteria and other contaminants. Water supplies for the Yucatn are largely derived from highly permeable fractured karstic limestone characterized by geophysical techniques that could be of use in identifying In early January 2012, a team from Northern Illinois University (NIU) traveled to Canc n, Quintana Roo, Mexico de Yucat n (CICY) to perform exploratory geophysical techniques were chosen based on instrumentation traveling that provide rapid recharge and contaminant pathways that lead from the land surface to the aquifer. VES were used to examine the overall vertical electrical structure of the well necessitate 2D resistivity surveys in the future. Abstract Mexico, approximately 20 km southwest of the city of Canc n, in order to identify karst conduits that channel contaminated surface waters into the main aquifer. electrical soundings (VES) and spontaneous potential (SP) surveys were employed to identify these conduits and detect water movement through them. Cancn s municipal water supply has frequently been affected by fecal coliform bacteria and other contaminants. Water supplies are largely derived from highly permeable fractured karst limestone aquifers characterized by rapid transport of microbial and chemical contaminants from the surface to subsurface Tertiary limestone bedrock outcrops across this entire Schlumberger array VES were made at two locations.. limestone), overlying a high resistivity layer 8.2 m thick (massive limestone with some small caves), overlying could not be successfully inverted due to lateral resistivity water table. An unusual transparent zone (absence of 231

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 232 and collapse features (generally less than 1 m across). visible at the surface. Thus no cave maps exist in the study area. Methodology Three areas were investigated in detail over the southwest were near Well (Pozo) 49A (Area 1) and at the intersection geophysically were essentially targets of opportunity: Well 49A was open and being serviced (Area 1), (2) a visible road, Well 40 and the Ruta de los Cenotes, and a service road intersection with nearby apparent depressions in the 2). In general geophysical surveys were made along roads. Road pavement consists of the limestone bedrock. resolution technique has the potential to identify conduits transmitting contaminants from the surface Geological Setting near the Canc n International Airport, approximately 20 km southwest of the city of Canc n. This area lies within the Yucatan northeastern coastal plain (Isphording, 1975) on a low upland between the eastern coastal ridge the west. Pliocene Upper Miocene limestone of the Carillo Puerto formation outcrops, or is covered by only turn, overlies carbonate Cretaceous and Jurassic units containing evaporites (Perry, 2002). Hydrogeology, Contaminants and Karst Features however, are under considerable strain from extensive pumping and development along the Caribbean coast, study area, discharging at the coast, about 25 km away The survey area is regarded as a contaminated part of the Cancn municipal well system, which consists of several fractures and conduits in the limestone that are directly connected to the shallow drinking water aquifer. There of contaminants. The most common contamination is related to feces pathogens, nutrients, etc. This area scale agriculture, tourism, or residential activities. solution openings. Numerous publications discuss the cave systems of northeast Yucatan, including Thomas (1999), Beddows Riviera Maya cave systems, consisting of several long caves 10s of km long and 10s of meters wide are the most extensively studied and mapped. Navigable caves Figure 1. Map of study areas showing roads and cenotes. Inset shows location of study area within the NE Yucatn (inset after Beddows, 2003).

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 233 geophysical surveys one wellhead had been removed for maintenance (Pozo 49A); the water level in this well was measured at 6.2 m beneath the surface. It is not or not. Most likely it represents the water table, since personnel showed up, and the well was left open. The nearest vertical outcrop is at the Calica quarry, near this area of the Yucatn make this a plausible comparison to the study area. water and small ponds, suggesting the water table is at the base of this massive unit, placing the water table Results GPR zone between depths of 4 and 12 m containing perhaps surveys were along straight lines, although some lines were run parallel to each other to examine the areal 4 to 40 m. Antenna separation depended on antenna frequency and antennas were manually moved along arrival and shifting traces, if necessary, to account for area and color displays were employed, generally using automatic gain control with a maximum gain of wave velocity measured in the CMP surveys or through The SP and VES surveys were both run from the same information about SP and VES surveys in karst areas see Ford and Williams [2007]). The VES utilized four stainless steel electrodes pounded into the thin topsoil SP surveys were made over several intersecting lines near the pumping wells, as well as over apparent karstic conduits (holes visible at the surface) during rainfall. The target was small voltages induced by moving water (streaming potentials) (Reynolds, 2011). Two porous electrode while the other acted as a roving electrode and the unit recorded the difference in potential (in mV) least 10 m from the roving electrode. Ground truth calibration area. The municipal wells for this area were not logged, and any records made during drilling (during the 1970s and 80s) were not immediately available from the Cancn Municipal Water Department. During the Figure 2. Cross-section of limestone at the Calica quarry, near Playa del Carmen, scale bar is in upper left.

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 234 resistivity model consisting of a 2.1 m upper layer space layer, which probably represents saturated limestone. This structure is consistent with what was observed at Calica (Figure 2), although thicknesses are different. The lower resistivity upper layer is probably highly fractured and weathered limestone, relatively unweathered limestone and may contain as shown in Figure 2, resulting in its elevated resistivity. The lowermost layer probably represents diffractions, possibly generated by conduits, small caves, or other sharp heterogeneities. Figures 5 and 6, directly below a surface conduit (a hole at the surface VES Schlumberger resistivity arrays were used for VES in Areas 1 and 2 (Figure 7). Electrodes were inserted into the thin soil (zero to 5 cm thick) covering bedrock. No electrode conditioning was employed. Figure 3. GPR section across part of Area 1. Antenna frequency was 50 MHz, separation 2 m and the step size between traces 0.1 m. Figure 4. GPR section across part of Area 2, showing disrupted reflections and possible collapse features. Antenna frequency was 100 MHz, separation 1 m and the step size between traces 0.1 m.

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 235 in the potential was noted. This could be due to the conduit flow nature of the aquifer, i.e. the SP lines might not have passed near the hydraulically active fractures those wells were drawing from (streaming potentials that generate SP are discussed in Reynolds [2011] as well as other geophysical texts). The SP surveys, however, recorded significant changes in potential (about 16 mV) over a conduit where rainwater was infiltrating, as shown in Figure 8. 5 and 6. The VES in Area 2 was severely affected by lateral resistivity variations and could not be interpreted as a layered model with high confidence. This suggests 2D resistivity should be employed in future surveys in Area 2. SP Several profiles were collected along cross shaped, intersecting lines near the pumping wells. While the lines were not very long, the wells were pumping at Figure 5. GPR section across part of Area 3. Antenna frequency was 100 MHz, separation 1 m and the step size between traces 0.1 m. Figure 6. Same GPR section as in Figure 5, but plotted with variable density and in color, to denote areas of signal loss.

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 236 methods should also be employed that would allow for more expansive and contiguous data sets such as towed antennas, employing very low frequency (VLF) and other systems. This would allow the extent of the conduits to be better characterized and help to understand possible contaminant routes. Acknowledgements The authors would like to thank two anonymous reviewers and the Editors whose comments and corrections greatly improved the quality of this manuscript. Travel expenses were paid for, in part by the Mobil Oil Foundation at Northern Illinois University. References Juan A. 2009. Mapping subsurface karst features Analysis of the karst aquifer structure of the Review: The Yucatan Peninsula karst aquifer, (published online) Beddows PA. 2002a. Where does the sewage go? The karst groundwater system of the Municipalidad de Solidaridad, Quintana Roo, Mexico. Association for Mexican Cave Beddows PA, Smart PL, Whitaker FF, Smith SL. 2002b. Density stratified groundwater circulation on the Caribbean coast of the Yucatan Peninsula, Mexico. In Martin JB, Paleozoic carbonate aquifers, Karst Waters Yucatan coast. Bulletin 11, Association of Mexican and Sons. Conclusions and Future Work This study evaluates the feasibility of using geophysical conduits in a karstic aquifer utilized as a water source by the City of Cancn. Three techniques were evaluated: model with a moderate resistivity upper weathered layer overlying a high resistivity layer perhaps representing lowermost layer was much lower resistivity, suggesting it is surface, where an anomaly of about 16 mV was recorded, presumably due to streaming potentials. Future work should concentrate surveys over known voids or high permeability zones, so that geophysical Figure 8. SP profile across a flowing conduit in Area 3. Figure 7. Resistivity sounding made with a Schlumberger array in Area 1, along with the layered model inverted from the sounding curve.

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 237 Isphording WC. 1975. The physical geology of Yucatan. hydrogeochemistry of the karst aquifer system of the northern Yucatan Peninsula, Mexico. Reynolds JM. 2011. An Introduction to Applied and Smart, PL, Beddows PA, Coke J, Doerr S, Smith S, Whitaker FF. 2006. Cave development on the Caribbean coast of the Yucatan Peninsula, C, editors. Perspectives on karst geomorphology, hydrology and geochemistry a tribute volume to

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 238