Because the samples were altered, as the tests were not performed “in situ”, we determined apparent porosity from the container volume and the volume of water required to achieve 100% saturation (Table 1). Because we did not control sediment-grain packing during container filling, we assumed the apparent porosity may differ slightly from the true porosity.

We calculated apparent porosity using the container volume and the volume of water added to the sediment collocated in the container. We also recorded the dry and 100% saturated sediment weights to obtain the water weight. To calculate the water volume, we used a water density of 0.99656 g/cm³ at 27.0˚C [25]. The apparent porosity values of samples ranged from 35.95 (S22) to 58.95 (S18) gr/cm³. (Table 1).

Apparent porosity values obtained from the 22 samples (including S19A and S19B) from Table 1 are, in general, consistent with porosity values from [26], which are as follows: Coarse sand with porosity values from 31% - 46%, fine sand with values from 25% - 53%, silt and loess with values from 35% - 50%, clay with porosities of 40% - 70%. From Table 1, S18 showed the highest porosity (58.95%), and S22 the lowest (35.95%).

Sediment samples were collected both within and outside the Coatzacoalcos paleolagoon area to quantify and record grain-size percentages for the geological formations at various points within the study area.

[24] presented the specifications of the sieves used, the sample sieving procedure, and the results. Following, we present the sediment volume percentages by grain size for each sample collected in the paleolagoon (Table 2).

Table 1.Apparent porosity (%) from the twenty-two sediment samples. * (19A) Upper layer; (19B) lower layer.

Table 2.Grain-size results for sediment samples collected in the Coatzacoalcos paleolagoon area. Values in bold represent most of the sample volume.

Based on the grain-size analysis results (Table 2), we generated graphs of volume (%) versus grain size passing through each sieve. Figure 3 shows that the bulk of the sample consists of coarse, medium, and fine sands. Additionally, this allowed us to observe slight differences in sediment grain sizes. Note that the percentage values in the graphs are cumulative.

Figure 3.Grain-size characteristics of the sandy sediments from the Coatzacoalcos paleolagoon. The graphs show the percentages of grain sizes passing through each sieve (cumulative values). Graph A shows the results for all samples. Graphs B, C, and D separately show each of the groups appearing in Graph A. Sediments in each graph exhibit similar characteristics.

Graph A in Figure 3 shows the results for all samples collected from the Coatzacoalcos paleolagoon. It reveals differences that allowed us to classify the sandy sediments into three groups. Later, we will correlate these differences with the hydrodynamic characteristics of the water flowing through these sediments.

Graph B in Figure 3 shows a similar grain size distribution across samples S1-S7. Most grains ranged from 0.180 to 0.355 mm. Additionally, samples S5, S6, and S7 contained between 8% and 10% of grains in the 0.150 - 0.180 mm range.

Graph C in Figure 3 shows that samples S8, S20, and S21 have a higher coarse-sand content than those in Graph B.

Finally, Graph D in Figure 3 shows that samples S9, S14, and S22 exhibit greater size diversity, with more coarse-grained sediment than in Graphs B and C.

Based on the grain-size distribution, we observe that the samples in Graph B are very well-sorted sediments. We classify the samples in Graph C as well-sorted sediments, and those in Graph D as moderately sorted sediments.

Likewise, we determined the grain-size distribution of sediments outcropping around the paleolagoon area (Table 3).

Table 3.Grain-size results for sediment samples collected outside the Coatzacoalcos paleolagoon area. Bold values represent the dominant sample volume. * Samples composed of a mixture of gravels, sand, and a small quantity of fine material collected in a transitional paleolagoon boundary.

Using the values from Table 2, we created graphs showing the volume (%) of grain size passing through each sieve (cumulative values). The graphs showed that sediments from samples collected outside the Coatzacoalcos paleolagoon area exhibit a wider grain-size range, from >2.0 mm to 0.090 mm (Figure 4(A)).

The grain-size analysis of samples collected outside the Coatzacoalcos paleolagoon revealed greater grain-size diversity. Given this variability, we grouped samples with similar apparent groundwater flow velocity values (graphs B, C, D, and E in Figure 4). Below, we describe the procedure for the apparent groundwater flow velocity tests and the correlation between velocity and grain size in detail.

Figure 4.Grain-size characteristics of sediments collected outside the Coatzacoalcos paleolagoon. The graphs show the percentages of grain sizes passing through each sieve (cumulative values). Graph A shows the results for all samples, while graphs B to E show groups of samples with similar ranges of apparent groundwater flow velocity. We collected samples S10 and S15 on the transitional paleolagoon boundary.

To measure the apparent groundwater flow velocity through the sediments, we designed a device to measure the time required for water to pass through a sediment section of a collected sample. We used polymethyl methacrylate, commonly known as acrylic, to construct the device. We placed a fiber filter at the bottom to allow water to pass through with minimal resistance.

To standardize all tests, we measured the side lengths of the rectangular container holding the sediments and the water column container. This procedure ensured consistent measurements across tests. We placed sediments from each sample collected at various locations within the study area into the device (Figure 5) and reported apparent groundwater flow velocities in meters/day (m/d).

Figure 5.Schematic of the designed device to measure the time it takes for a water column to travel 4.4 cm through the sediments. The figure shows the device’s dimensions.

For the tests, we filled a 0.10 m sediment column to 100% saturation with water, then added water until we reached the 0.044 m mark on the device. The water column height was the same for every test. We used a stopwatch to measure the time it took for the water level to drop from 0.044 m to 0.00 m (Figure 5). Using the recorded time and distance traveled, we calculated the apparent hydraulic conductivity for each collected sample.

For each apparent groundwater flow velocity test, we took three measurements of the time required for the water column to drop by 0.044 m and averaged them. We replicated the three measurements using the same sediment in the receptacle to avoid altering the grain packing, which could change the results. We also determined the standard deviation of the measured times using Equation (1) (Table 4and Table 5).

Where: σ is the standard deviation, x i is the individual measurement value,µ is the mean, and n is the number of measurements.

Based on the grain-size analysis (Figure 3) and the apparent groundwater flow velocity tests on samples collected from the Coatzacoalcos paleolagoon area (Table 4), we observed three groups with similar grain-size distributions (Graphs 3B, 3C, and 3D).

The apparent groundwater flow velocity tests yielded values ranging from 13.68 to 29.19 meters per day (m/d) for the seven samples shown in Figure 3(B). For the three samples in Figure 3(C), the apparent groundwater flow velocities ranged from 38.45 to 55.77 m/d. For the three samples in Figure 3(D), the apparent groundwater flow velocities ranged from 16.18 to 20.01 m/d (Figure 3andTable 4).

To establish a correlation with the grain size distribution of the samples in each graph, we calculated the average apparent groundwater flow Velocity, yielding the following results: an average velocity of 23.31 m/d for the samples in Graph 3B, an average velocity of 45.49 m/d for those in Graph 3C, and an average velocity of 17.62 m/d for those in Graph 3D.

These results indicate that the samples in Graph 3C, which consist of well-sorted sediments, exhibit high hydraulic conductivity in the Coatzacoalcos paleolagoon. Samples in Graph 3B, which consist of very well-sorted sediments, exhibit intermediate hydraulic conductivity. However, sample 1 in Graph 3B had the lowest hydraulic conductivity (13.68 m/d). The grain-size analysis showed more fine material (<0.075 mm) in sample 1 (2.98%) than in the other samples. This factor could explain this difference.

Finally, the moderately sorted sediment samples from Graph 3D exhibit the lowest hydraulic conductivity in the Coatzacoalcos paleolagoon. These results show that sediment grain-size composition is one factor that determines how easily groundwater flows through rock pores.

Table 4.Results of apparent groundwater flow velocity tests on sediment samples collected at various points in the Coatzacoalcos paleolagoon.

According to [27] and [28], the hydraulic conductivity of fine sand ranges from 1 to 5 m/d, medium sand from 5 to 20 m/d, and coarse sand from 20 to 100 m/d. The apparent hydraulic conductivity values measured for the Coatzacoalcos paleolagoon sediments intercorrelate with the sediment type (coarse and medium sand). However, grain size does not clearly correlate with apparent hydraulic conductivity because the sediments are mixed sand with small quantities of silt and clay.

For samples collected outside the Coatzacoalcos paleolagoon area, the results showed greater diversity in grain-size distribution (Figure 4), preventing their classification into groups based on similar affinities. For that reason, we classified the groups based on the apparent groundwater flow velocity range measured during the tests (Table 5).

Likewise, Figure 4(A) shows the grain-size distribution of all samples collected outside the paleolagoon area. Figure 4(B) shows sediments from samples S18, S11, and S16, which exhibited low apparent velocities (0.58 - 1.03 m/d). Figure 4(C) shows sediments from samples S19B, S17, S13, and S19A, which exhibited higher apparent velocities than the previous group (2.44 - 4.32 m/d).

Sediments from samples S12 (5.16 m/d) and S10 (8.14 m/d) in Figure 4(D) exhibited a notable difference in their velocity. Finally, in Figure 4(E), sediment from sample S15 (13.88 m/d) showed the highest apparent velocity. These values suggest that samples S10 and S15 are from a transitional zone at the boundary of the Coatzacoalcos paleolagoon (Figure 1).

Table 5.Results of apparent groundwater flow velocity tests on sediment samples collected at various points outside the Coatzacoalcos paleolagoon. * Samples located in the transitional paleolagoon boundary. ** Sample 19 with two strata. The upper strata are sand with organic matter; the lower strata are sand with silt and clay.

According to [27] and [28], the hydraulic conductivity of clay soil ranges from 0.01 to 0.2 m/d, that of Loam soil from 0.1 to 1.0 m/d, and that of fine sand with different mixes (silt and clay), from 1.0 to 5.0 m/d. The low apparent hydraulic conductivity measured outside the Coatzacoalcos paleolagoon intercorrelates with volcanoclastic sediment types (clayed and silty sands, sandy silts, loam soils, and some with fine gravels). However, samples S10 and S15 had higher apparent hydraulic conductivity (8.14 and 13.88 m/d); These samples are from the transitional paleolagoon boundary.

We observed that samples with high apparent groundwater flow velocities showed low uncertainty (green circle); samples with medium apparent groundwater flow velocities showed moderate uncertainty (blue circle); and samples with low apparent groundwater flow velocities showed high uncertainty (red circle) (Figure 6).

Figure 6.Apparent groundwater flow velocity for the 22 samples collected during this work. Samples with high groundwater flow velocity had a small standard deviation; samples with a moderate groundwater flow velocity had a medium standard deviation; and samples with low groundwater flow velocity had a high standard deviation.

Based on the apparent velocity test results, we observed that sediments collected outside the Coatzacoalcos paleolagoon exhibited lower apparent groundwater flow velocity values than those collected within the paleolagoon area, except for samples S10 and S15 that are located on the transitional boundary of the paleolagoon (Figure 7). This parameter (hydraulic conductivity) correlates with rock type and sediment grain size in both the paleolagoon and its surroundings.

Samples S8 (38.45 m/d), S20 (55.77 m/d), and S21 (42.26 m/d) recorded high apparent groundwater flow velocities in the coastal area (Figure 1) and, according to the grain-size analysis, showed higher coarse-sand content.

This coarser grain-size distribution indicates that sediments in samples S8, S20, and S21 originated from former freshwater outflow channels connecting the paleolagoon to the sea, where high-energy fluvial currents drove deposition, distinct from wind-transported (aeolian) sediments. Indeed, we collected sample S20 at the present-day mouth of the Coatzacoalcos River, where high-energy dynamics driven by the river current are observed; these value differences show grain-size variability within the same geological formation (Figure 7).

Figure 7.Apparent groundwater flow velocities in the study area. The lowest apparent velocities are outside the paleolagoon area, and the highest apparent velocities are in the aeolian, palustrine, and alluvial sediments filling the paleolagoon.

In other locations, apparent groundwater flow velocity varied considerably between points separated by short distances (Figure 8). We observed this behavior in the samples collected at Universidad Veracruzana. Samples S1, S2, S3, and S4 exhibited very similar grain-size distributions; therefore, we consider that grain arrangement can also influence the apparent groundwater flow Velocity. This behavior demonstrates the heterogeneity of sediment distribution in the Coatzacoalcos paleolagoon.

Figure 8.Location of samples S1 - S4 collected at the Universidad Veracruzana, Coatzacoalcos Campus. The four sampling points are close to one another; however, the apparent groundwater flow velocities differed at each one (13.68, 17.23, 28.80, and 23.90 m/d).

Simulating an estimation process requires considering many variables, making it highly complex and multifactorial. For this reason, we selected only four scenarios with constrained parameters to calculate the recharge volume, discharge volume, effective recharge, and apparent groundwater flow Velocity from the Coatzacoalcos paleolagoon to determine changes in the aquifer’s water table following a rainfall event in the area.

We constrained certain variables to simplify the conceptual model and calculate groundwater flow through the unconfined aquifer of the Coatzacoalcos paleolagoon. We treated rainfall as the only recharge source, omitting runoff, evapotranspiration, moisture, river-stage effects, tidal effects, and spatially variable recharge (Figure 9).

Figure 9.Conceptual model. (a) South-to-north view and (b) north-to-south view of the Coatzacoalcos paleolagoon. (c) Conceptual model of the paleolagoon area with a variable aquifer thickness of 10 to 30 m (no scale).

The stratigraphy of the paleolagoon is complex due to sedimentation processes in this type of geological environment, where fluvial currents, Gulf of Mexico coastal currents, and winds distribute sediments according to the energy of each one at different times.

In [29], geotechnical borehole results (Table 6) showed that the stratigraphy within the Coatzacoalcos paleolagoon is complex and heterogeneous. The unconfined aquifer consists of unconsolidated sediments, primarily medium to fine sand, with small amounts of silt and clay (>5%). However, he identified thin silt and clay lenses (1 - 3 m) in some geotechnical boreholes.

In this type of geological environment, sediment distribution is irregular, resulting in localized deposits with varying grain sizes. Consequently, the deposits consist primarily of sand, silt, and clay layers. However, we observed thin horizons containing angular fragments of older rock alongside younger sand, silt, and clay that filled the paleolagoon, reflecting distinct sedimentation and erosion events over multiple cycles.

Based on geotechnical borehole data (Table 6), the maximum aquifer thickness is about 30 m in some areas of the paleolagoon and about 10 m in others.

Additionally, the geotechnical soundings found the water table at depths ranging from 0.5 to 5.8 m below ground level in the geotechnical boreholes.

[29] did not include the ground-level elevation (masl) for each geotechnical sounding site, and the soundings were also conducted intermittently over several years.

In May 2015, during the dry season, a vertical electrical sounding (VES) was conducted on the grounds of the Universidad Veracruzana in Coatzacoalcos, Veracruz. The results indicated that the water table was at a depth of 1.41 m.

In June 2024, after a dry period, manual drilling was carried out on land belonging to the Universidad Veracruzana in Coatzacoalcos, Veracruz, to measure the water table level of the free aquifer. The result showed a depth of 1.30 m. Two weeks later, heavy rainfall occurred, and two days after that, another manual borehole was drilled at the site to determine the groundwater depth; the result was 0.80 m. These two measurements show a 0.50 m variation in the water table.

Table 6.Geotechnical boreholes drilled along the coastline of the Coatzacoalcos paleolagoon. The table shows borehole coordinates, total depth, water table level, and depth intervals for the various sediment strata [29].

To simulate water recharge and discharge in the Coatzacoalcos paleolagoon and their influence on aquifer water table fluctuations, we calculated the paleolagoon’s area using Google Earth; the result was 348,014,937.00 m². Based on the paleolagoon’s surface area and precipitation (mm), we calculated the volume of rainwater (m³/d) falling on it for four precipitation scenarios.

To calculate the daily volume of water flowing from the aquifer into the sea along the coast (discharge), we determined the aquifer’s cross-sectional area. We did this by considering the length of the coast and the aquifer’s average thickness. In this case, the aquifer thickness ranged from 10 to 30 m; however, most of the range was at 10 m. Consequently, we adopted a general thickness of 15 m. The aquifer cross-sectional area was 487,500.00 m².

We then calculated the average apparent groundwater velocity toward the sea from tests on sediment samples collected near the coastline (Table 7). The average velocity was 29.95 m/d. The daily discharge volume was 13,279,500.00 m³/d. This volume remains constant across the four simulated scenarios.

For simplicity, we used precipitation/day from the Coatzacoalcos and Cangrejera weather station data, assuming 100% infiltration as recharge and effective recharge as recharge minus discharge to the Gulf of Mexico, using an average apparent groundwater velocity along the coastline.

Using data from the Coatzacoalcos and La Cangrejera weather stations, we considered four possible precipitation scenarios for the Coatzacoalcos paleolagoon area, excluding precipitation and runoff in areas located outside its surface.

Table 7.Apparent groundwater flow velocities in sediment samples collected near the shoreline in the discharge zone of the Coatzacoalcos paleolagoon.

Average apparent Velocity: 29.95 m/d.

Scenario 1. Coatzacoalcos station, date: March 29, 2026, precipitation: 26.0 mm/d.

Based on the precipitation on March 29, 2026 (0.026 m) and the paleolagoon area, we calculated a rainwater recharge volume of 9,048,388.36 m³.

We subtracted the water discharge volume across the coastline (14,600,625.00 m³/d) from the paleolagoon’s water recharge volume (9,048,388.36 m³/d), resulting in −5,552,236.64 m³/d. In other words, discharge exceeds aquifer recharge. This result could reflect a 0.016 m/d drop in the water table.

Scenario 2. Coatzacoalcos Station, date: December 30, 2025, precipitation: 123.0 mm/d.

Based on the precipitation recorded on December 30, 2025 (0.123 m), the rainwater volume was 42,805,837.25 m³, recharging the paleolagoon aquifer.

Effective recharge volume equals the recharge volume (42,805,837.25 m³) minus the aquifer discharge volume (14,600,625.00 m³) across the coastline, resulting in 28,205,212.25 m³/day. Effective aquifer recharge could raise the water table by 0.081 m/d.

Scenario 3. Coatzacoalcos Station, date: October 2, 2024, precipitation: 258.0 mm/d.

Based on the precipitation recorded on October 2, 2024 (0.258 m), we obtained a rainwater recharge volume of 89,787,853.75 m³/d.

That is, we calculated the effective recharge volume as the recharge (89,787,853.75 m³/day) minus the discharge through the aquifer’s coastline (14,600,625.00 m³), resulting in 75,187,228.75 m³/d. Therefore, the effective recharge volume could represent a water-table rise of 0.216 m/d.

Scenario 4. La Cangrejera Station; date: November 20, 2000; precipitation: 390.4 mm/d.

With a precipitation of 0.3904 m on November 20, 2000, we obtained a recharge volume of 135,865,031.40 m³/day for the paleolagoon.

The effective rainwater recharge volume equals the recharge (135,865,031.40 m³) minus the aquifer discharge (14,600,625.00 m³), resulting in 121,264,406.40 m³/day. Therefore, this effective recharge volume corresponds to a water-table rise of 0.348 m/d.

If we consider a daily rainfall event with this volume of precipitation over three days, we could observe a water-table rise of 1.044 m. This fluctuation is significant and could help prevent flooding in vulnerable areas.