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Article

Physicochemical and Sediment Characterization of El Conejo Lagoon in Altamira, Tamaulipas, Mexico

by
Sheila Genoveva Pérez-Bravo
1,*,
Jonathan Soriano-Mar
1,
Ulises Páramo-García
1,
Luciano Aguilera-Vázquez
1,
Leonardo Martínez-Cardenas
2,
Claudia Araceli Dávila-Camacho
3 and
María del Refugio Castañeda-Chávez
3,*
1
Tecnológico Nacional de México, Instituto Tecnológico de Ciudad Madero, División de Estudios de Posgrado e Investigación, Av. 1o. de Mayo esq. Sor Juana Inés de la Cruz s/n, Col. Los Mangos, Ciudad Madero 89440, Tamaulipas, Mexico
2
Secretaría de Investigación y Posgrado, Universidad Autónoma de Nayarit, Ciudad de la Cultura Amado Nervo s/n., Tepic 63190, Nayarit, Mexico
3
Tecnológico Nacional de México, Instituto Tecnológico de Boca del Rio, Departamento de Posgrado e Investigación, Carretera Veracruz-Córdoba, km 12, Boca del Rio 94290, Veracruz, Mexico
*
Authors to whom correspondence should be addressed.
Earth 2025, 6(3), 83; https://doi.org/10.3390/earth6030083
Submission received: 13 June 2025 / Revised: 19 July 2025 / Accepted: 22 July 2025 / Published: 25 July 2025
Browse Figures
Versions Notes

Abstract

Fresh water is vital for human activities; however, an increase in the contamination of water bodies has been observed, so it is necessary to monitor the degree of contamination and take measures to preserve it. In Altamira, Tamaulipas, the Guayalejo-Tamesí River basin has three estuaries and seven lagoons, including Laguna El Conejo, of which the National Water Commission only monitors one. The objective of this research is to determine water quality on the basis of the parameters COD, BOD5, T, pH, and sediment characteristics. The open reflux method was used according to NMX-AA-030-SCFI-2012 for COD, BOD Track II, HACH equipment for BOD5, and the granulometric characterization recommended by the Unified Soil Classification System ASTM D-2487-17. The water was found to be uniformly contaminated throughout its length in the range of 117.3–200 mg/L COD, and BOD5 ranged from 15.8–112.75 mg/L throughout the study period, with sediments dominated by poorly graded soil and fine clay. Comprehensive management is needed because the BOD5/COD ratio varies between 0.11and 0.56, indicating that it contains recalcitrant organic matter, which is difficult to biodegrade.

1. Introduction

Water covers 70% of the Earth’s surface, 97.5% of which is distributed in saltwater from seas and oceans; only 2.5% is fresh water and is distributed in glaciers, permafrost, groundwater, and surface water, accounting for 68.7%, 0.8%, 30.1%, and 0.4%, respectively. Water is a biochemical component of living organisms and is a living medium for various plants and animals [1]. Surface water from rivers and lakes supports human activities [2]. When water is used, it loses its quality and becomes wastewater (urban or industrial). The main problem is that it is discharged without prior treatment into receiving bodies. Other sources of anthropogenic pollution that reduce water quality are agrochemicals and natural pollution by the runoff of particulate and dissolved matter and by the degradation of animal and plant organic matter [3]. These emissions negatively affect aquatic ecosystems, especially primary producers such as aquatic plants, algae, and cyanobacteria [4].
The global water crisis is one of the major problems faced by several countries [5]. The monitoring and evaluation of eutrophication are critical points in protecting aquatic ecosystems, in addition to the design and monitoring programs that help control the quality of sources that supply freshwater, which is essential for achieving sustainable development [6,7]. Depending on the source of anthropogenic pollution, a water body may contain different constituents. Phosphorus and nitrogen alternate as limiting nutrients for phytoplankton proliferation; an excess of these nutrients triggers eutrophication processes [8]. In addition to organic pollution, algal growth, such as cyanobacteria, may occur [9]. An increase in the concentrations of metals such as cadmium, chromium, mercury, copper, lead, and zinc has been observed since the Industrial Revolution [10]. In addition, new contaminants classified as emerging, such as atrazine, simazine, terbuthylazine, adenosine, and ibuprofen, have been found in natural waters [11]. In Venice Lagoon, Italy, azithromycin, erythromycin, ciprofloxacin, clarithromycin, 17β-estradiol, and 17α-ethinylestradiol were found in water and sediments [12].
In Mexico, recent investigations into the quality of water bodies have been reported: in Guerrero, the trophic state of the Tres Palos Lagoon has also been studied [13]; the water quality of the Pie de la Cuesta Lagoon [14]; the water quality of the Bustillos and Mexicanos lagoons in Chihuahua has also been evaluated [15]; the water quality of the lagoons of Pueblo Viejo, Alvarado, and Mandinga, Veracruz [16], and La Pólvora Lagoon in Tabasco [17]; the presence of hydrocarbons in the Chajir and Media Luna lagoons in San Luis Potosi [18]; the accumulation of heavy metals in the sediments of the Palizada River in Campeche [19]; and also the physicochemical parameters and heavy metals in the water and sediments of the Las Ilusiones Lagoon in Tabasco [20]. Among the most commonly used parameters for water quality characterization are the chemical oxygen demand (COD), the flora and fauna observed in the study area [21], biological oxygen demand (BOD5), total nitrogen concentration (TN), nitrite nitrogen concentration (N-NO2), nitrate nitrogen concentration (N-NO3) [22], total phosphorus concentration (TP) [17], water conductivity, and pH and temperature, among others. These investigations reported various levels of contamination, which reflects the need to increase the number of wastewater treatment plants to avoid eutrophication of water bodies.
In Altamira, Tamaulipas, in 2013, the estimated daily load of wastewater was 35,126 m3; that is, 406.56 L/s of urban wastewater was generated in the Municipality of Altamira, Tamaulipas, Mexico. In that year, only two treatment plants were in operation. Roger Gómez had a capacity of 8 L/s, so its treatment capacity was 691 m3. Champayan Lagoon was the final receiving body of this plant. In addition, the Cuauhtémoc plant, with a capacity of 20 L/s, treated 1728 m3 and had the Tuna estuary as the final receiving body [13]. In 2016, it was reported that most of the urban wastewater was discharged to various water bodies in the Tamesí system and the Marismas area, in addition to the fact that there were no systematic studies to verify the state of water quality of these water bodies, sediments, and bioconcentration of substances in the biota [14,23]. On the other hand, the Municipal Development Plan 2022 reports that the Cuauhtémoc and Estación Colonias wastewater treatment plants have been in operation since 2021 [15]. In 2022, the National Water Information System reported that the municipality of Altamira treated wastewater at two treatment plants at 400 L/s, La Pedrera, with an installed capacity of 300 L/s, and Florida, with an installed capacity of 150 L/s [16].
In October 1994, high fish mortality was observed in Laguna El Conejo due to wastewater discharge from the Tampico–Altamira industrial corridor, which was attributed to contamination by heavy metals such as chromium, cadmium, and lead [24]. However, as of 2021, the discovery of Cu and Fe in lagoon sediments was reported, and both metals are not regulated by Mexican authorities [25]. This water body is directly connected to Las Marismas Lagoon, which has predominantly silty and sandy sediments containing Ni, Pb, and Cu [26], where copper is the most abundant. On the other hand, the cyanobacterium Planktothrix sp. [27], which can produce some toxins, including anatoxin-a, saxitoxins, and microsistin, has been reported [19,28].
This study is justified by the lack of environmental monitoring by the municipal authority, the interconnection of the lagoon with other water bodies, and its ecological importance. Specifically, this research aims to determine the physicochemical parameters of COD, BOD5, T, and pH to determine the water quality in addition to determining the characteristics of the sediment.

2. Study Area

The water body selected for this study is Laguna El Conejo, which is located at the following coordinates: N 22°25′37.945′′, W 97°52′55.938′′. Figure 1 shows the Mexican Republic, with the state of Tamaulipas marked in yellow, as well as the municipalities that comprise it, where the state of Altamira is marked in grey and Laguna El Conejo is delineated in green. This lagoon is part of the lagoon system of the eastern zone, which includes the lagoons El Chango, El Cañón, El Sauz, and Aguada Grande. In addition, it connects directly with the Marismas del Golfo Norte Lagoon [24], a lagoon system belonging to the Guayalejo-Tamesí river basin. These lagoons are shown in Figure 2. El Conejo Lagoon is a freshwater body with a perimeter of 5.4 km [17]. In general, lagoons play a crucial role in the ecological balance of the planet as they are a habitat for diverse flora and fauna, in addition to regulating the water cycle. In addition, fishing activities are carried out in El Conejo Lagoon.

3. Materials and Methods

3.1. Determination of Characteristics

The study area was investigated to photograph flora and fauna, analyze the water’s COD and BOD5, and determine the characteristics of the bottom sediment. Historical, photosynthetically active radiation (PAR) and temperature information were consulted in NASA’s data access viewer [29]; PAR and average ambient temperatures favor the growth of photosynthetic organisms. In the NASA data viewer, we searched for Laguna El Conejo, Tamaulipas, Mexico, which opened the page showing the geolocation of the area. In the right box of the regional tab, we selected agroclimatology in the User Community and monthly and annual in the Temporal Level tab. We introduced the data on the latitude and longitude of the study area. We selected the years 2012–2022 because we wanted information from these historical data. In the Parameters tab, we selected the options total PAR, and maximum and minimum temperatures. Finally, at the bottom, we selected download data. CSV, the web page provided us with a data file, with which the graphs were generated in the statistical software Minitab 22 without making any modifications to the data.

3.2. Water Sampling

Two liters of surface water were collected from each sampling point in polypropylene containers and stored in a cooler at 4 °C for transport to the laboratory following the recommendations of the reference standard NMX-AA-14-1980 [17,30]. For this study, five sampling points were selected, as specified in Figure 3, which cover the zones of inflow, outflow, and slowing of water in the lagoon [3]. In similar works, only 3 or 4 points have been considered [31,32]. The parameters that were determined were COD, BOD5, and in situ measurement of pH and water temperature [20] over three two-month periods; every two months, five samplings were carried out twelve days apart, from July to December 2019 [13,15]. The pH of the surface water sample was measured in situ with Civeq pH test strips, while an immersion thermometer from −20 to 150 °C, Brannan brand, was used to measure the temperature. Figure 3 shows the surface water sampling points, while Table 1 shows the corresponding geographic coordinates.
The evaluation period from July to December was selected because it is a transition period: September to October corresponds to the end of the rainy season and cyclonic events, whereas from November to December, there are cold fronts that cause the oxygenation of bodies of water. Generally, during the dry season, the concentration of pollutants is relatively high due to the reduction in water volume, which is why sampling was not considered during this season. The evaluation of water quality from July to December allows us to determine the impact of the water that is dragged into the lagoon and the winds that help in the movement of surface water.

3.3. Water Preparation and Analysis

The water sample was preserved with H2SO4 for COD determination, and the open reflux method recommended by NMX-AA-030-SCFI-2012 was used [20]. The water sample was analyzed within the first 5 days, even though the aforementioned standard stipulates that water preserved with sulfuric acid is suitable for analysis for only 30 days [33]. Each COD analysis was performed with three replicates. On the other hand, the determination of BOD5 was performed on a composite sample [14] made up of 5 simple samples from each of the sampling points, without preservatives, by NMX-AA-028-SCFI-2001 [34] and the respirometric method in the HACH brand BOD Track II equipment with HACH brand nutrient buffer solution sachets (Cat. 2962266); each analysis was performed with 5 replicates. This parameter is an estimate of the amount of oxygen required by a heterogeneous microbial population to oxidize the organic matter in a water sample over a period of 5 days.
The results of COD and BOD5 were analyzed via one-way analysis of variance (ANOVA), with a confidence level of 95%, after performing the Ryan–Joiner and Levene tests to verify that the data had a normal distribution and homogeneous variance via Minitab statistical software. The null hypothesis considers that the population means are equal to Ho: µ1 = µ2 = µ3 = µ4 = µ5, and the alternative hypothesis H1 states that at least one population mean is different.

3.4. Sediment Sampling and Analysis

Exploratory sampling, as well as the determination of the number of sampling points and the sample amount to be collected, was conducted on the basis of the recommendations of NMX-AA-132-SCFI-2016 [35]. In studies of lagoon sediments, 4 and 10 sampling points have been used [32]. To study a larger area, we chose to collect sediment samples at 15 points using an Ekman dredge with a 2–3 kg capacity, with a loop and plumb bob. The open dredge was dropped to the bottom and held with the loop. Once at the bottom, the metal piece called the plumb bob was dropped to activate the closing mechanism of the dredge, with which the sample was collected. On the other hand, the length of the loop necessary for the dredge to reach the bottom was the measurement that was recorded as the depth of the lagoon. The bottom sediment samples were transferred to the laboratory to be dehydrated at 110 °C for 16 h in a VWR 89511-404 oven. The pH of the bottom sediment was measured via ASTM D4972-19, method A [36], as a reference, with a pH meter with a glass-calomel electrode, model 913, Metrohm brand, periodically calibrated with buffer solutions pH 4, 7, and 9, Metrohm brand. The granulometric characterization was performed according to ASTM D-2487-17 [37], which outlines a methodology for soil classification for engineering purposes (Unified Soil Classification System). This approach involves the separation of coarse and fine fractions using sieves with varying mesh openings. The sediments at the ends of the lagoon were not sampled because of the shallow depth and accumulation of aquatic plants. Figure 4 shows the locations of the sediment sampling points, which are detailed in Table 2, along with the depth and weight of the samples obtained.
The methods used are represented in a flowchart for better understanding, which is shown in Figure 5.

4. Results

4.1. Radiative and Climatic Characteristics

Figure 6 shows an image of the lagoon. Point 1 indicates its junction with the Las Marismas Lagoon (N 22°25′51.727′′, W 97°52′49.635′′), and point 2 indicates the discharge of treated water from the company BASF Mexicana (N 22°25′30.925′′, W 97°53′5.582′′). The pipeline is marked on the perimeter of the lagoon, as shown in Figure 7. It is surrounded by companies with industrial and socioeconomic activities, such as the SMG fuel distributor, fuel service station, construction materials store, Depotmex intercontainer, TMM Logistics, two universities, ITCM campus III and Universidad Tecnológica de Altamira, in addition to human settlements of the ejido Armenta. On the other hand, in boulevards of rivers, there is a storm drainage channel that ends in the lagoon.
The flora and fauna present in the water body are mostly Tular (Typha dominguensis) [38]. In addition to observing aquatic plants such as water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), grey herons, and white herons in the field, as shown in Figure 8, fishing activity has also occurred in the area.
Solar radiation is a fundamental component of photosynthetic processes, with the potential for solar radiation varying depending on the geographic area and season of the year. Figure 9 shows the history of PAR in the study area, expressed in W/m2. This figure depicts a variation between 50 and 131 W/m2 from 2012 to 2022, with the highest radiation occurring during June and August. PAR is only a fraction of the solar radiation found in the visible range or color spectrum; however, solar radiation also occurs as ultraviolet and infrared radiation. Solar radiation in the infrared spectrum contributes to the greenhouse effect and global warming. In addition, temperature is another factor that affects biomass growth. Figure 10 shows the maximum temperatures recorded, ranging from 27 to 45 °C, and the minimum temperatures within the 3–25 °C range from 2012 to 2022. Therefore, the growth rate of photosynthetic organisms is influenced by PAR and ambient temperature, which are not proportional, which is why higher PAR is observed in July and August and why higher temperatures are observed in April.

4.2. Chemical Oxygen Demand of El Conejo Lagoon

The samples were taken 12 days apart over 6 months. The analyses were performed in triplicate; the results are shown in Figure 11. One-way ANOVA was used to determine whether the COD was the same at all sampling points bimonthly; the results are shown in Table A1 in the Appendix A.
The COD results at the sampling points were verified to have a normal distribution via the Ryan–Joiner test, with values of p > 0.05. Levene’s tests were performed to verify the homogeneity of variances, where coefficients of 0.212, 0.203, and 0.060 were obtained for the three bimesters evaluated. Then, one-way ANOVA was performed. According to the ANOVA results, Ho is accepted, and the COD parameter is the same at all sampling points during the three evaluation periods, i.e., the parameter is uniform throughout El Conejo Lagoon. The elevated COD values are attributed to the reception of contaminant loads, and the fluctuations among the different sampling points may be due to hydrodynamic phenomena, i.e., a mixture of recent and stored water [3].

4.3. Biological Oxygen Demand of El Conejo Lagoon

The single samples from each sampling point were mixed to form a composite sample representative of the entire lagoon. A composite sample was obtained every 12 days, i.e., 5 samples every 2 months, for a total of 15 samples during the entire evaluation period. The results are shown in Figure 12, showing a decrease–increase–decrease in this parameter, which may be due to the self-purification capacity of the water body. The results of Levene’s tests were 0.535, 0.154, and 0.758 for the three bimesters evaluated, respectively, demonstrating homogeneity of variances; subsequently, a one-way ANOVA test was performed, and the results are shown in Table A2 in the Appendix A. According to the results obtained, the Hos were rejected; therefore, the population means were unequal, implying that the BOD5 of the entire lagoon varied between each sampling during the evaluation period.
A Tukey post hoc test was performed with 95% confidence because it rejects Ho, i.e., at least one population mean is different; the results are shown in Table A3 in the Appendix A. In the first evaluation period, July–August, the first sampling, M1, presented the highest mean (115.20 mg/L) and was significantly different (p < 0.05) from all the other groups. The third sample, M3 (84.60 mg/L), was also significantly different from the other samples, except for M1. Samples M5 (64.20 mg/L), M2 (58.40 mg/L), and M4 (50.60 mg/L) presented letters in the clustering data, indicating that they do not differ significantly from each other. In particular, M5 does not differ significantly from M2, and M2 does not differ significantly from M4, but there is a significant difference between M5 and M4. The water quality of the lagoon is relatively high in all samples; however, the BOD5 values are relatively high in M1 and M2 and decrease in M3, M4, and M5, but still exceed 30 g/mL. These differences are attributed to the discharge of pollutants before the M1 and M3 samples.
In the second period, in September–October, samples M6, M8 and M7 do not present significant differences among them, with a range of means between 36.40 and 47.00 mg/L, which indicates polluted water; on the other hand, samples M9 and M10 do not present significant differences among them, with means of 22.40 and 21.80 g/mL, respectively, which indicates acceptable water quality.
In the third period, November–December, sample M11 presented the highest BOD5 (60.00 mg/L) and was significantly different from all the other samples, followed by M13 (42.60 mg/L), which was significantly greater than those of M12, M14, and M15. On the other hand, M12 and M15 do not present significant differences between them, while M14 has the lowest BOD5 (15.80 mg/L), which is not significantly different from that of M15, but it does differ from those of M11 and M13.
BOD5 indicates the amount of oxygen required by microorganisms to degrade organic matter in the water body. According to the CONAGUA standards, the water quality varies from polluted to acceptable; during the second and third two-month evaluation periods, samples M9, M10, M12, M14, and M15 reached acceptable levels. These results indicate that the lagoon is self-purifying and that the movement of aquatic plants caused by air currents, the positive influence of PAR on photosynthesis and oxygen generation, and the influence of ambient temperature on the solubility of dissolved oxygen and microbial activity favor the reduction in organic matter.

4.4. Water Quality

Table 3 lists the values established by the National Water Commission (CONAGUA) for the COD and BOD5 criteria used to indicate water quality [39]. Table 4 shows the summary of the results obtained from the sampling; the COD ranged between 117.3 and 200 mg/L, according to the scale of the quality indicators established by the National Water Commission. The water of Conejo Lagoon is contaminated mainly with municipal wastewater. On the other hand, the BOD5 varies from 15.8 to 112.75 mg/L, classifying it as polluted in the first evaluation period and acceptable in the following two periods, indicating the capacity for self-purification or reception of biologically treated water.
The lagoon’s pH remained at seven throughout the evaluation period, indicating no discernible change; additionally, the minimum and maximum temperatures aligned with the prevailing climatic conditions.

4.5. Sediment Characteristics

Table 5 shows the depth of the lagoon, which ranges from 1.15 to 3.2 m. The deepest zone is located in the central-west part of the lagoon. The pH of the surface water is 7, with slight acidification at a minimum of 6.4 and slight alkalization at 7.8. Four soil types were identified and classified according to the Unified Soil Classification System. In this context, SP-SM denotes poorly graded silty sand, CL represents low-plasticity clay, SP signifies poorly graded sand, and SM stands for silty sand.
The Unified Soil Classification System employs a two-category system for classifying soil texture and delineating coarse-grained and fine-grained soils. The designation of soil as fine-grained is contingent upon the observation that a minimum of 50% of the soil’s total mass passes through a 0.075 mm (#200) sieve. In contrast, soil is classified as coarse-grained if it does not meet this threshold. The classification of fine-grained soils is further subdivided into silts (M) and clays (C), whereas coarse-grained soils are classified into gravels (G) or sands (S). Among the four soil types identified in the lagoon, poorly graded sandy soil exhibited a uniform grain size, with intergranular spaces that allowed the seepage of other materials or smaller sediments. This soil type was observed at depths between 1.2 and 1.5 m. In contrast, fine clay sediments indicate low permeability or seepage and are found at depths of 2 m. The depth of the sediment samples ranged from 2 to 3.2 m. Notably, the samples contained organic matter because of the sedimentary environment from which they were obtained. This organic matter included vegetation remains, fish skeletons, snails, and tiny shells. Owing to the importance of sediments, future research on the concentrations of organic matter, metals, and nutrients in sediments is warranted.

5. Discussion

El Conejo Lagoon has a depth of 1.15–3.20 m, and the pH of the surface water was 7 during the sampling period. The minimum ambient temperature in the zone ranged from 3 to 25 °C, and the maximum temperature ranged from 27 to 45 °C. The water temperature ranged from 18 to 33 °C, and the PAR varied from 50 to 131 W/m2.
The depth of the lagoon is similar to that reported for La Pólvora Lagoon in Tabasco, from 0.6 to 3.5 m, and it presented a pH of 7.0–7.8 in 2011 [17]. Other Mexican lagoons presented similar pH values and water temperatures: Pie de la Cuesta Lagoon in Acapulco, Guerrero, presented pH values ranging from 8.0 to 8.4 and temperatures ranging from 18 to 20 °C in 2017 [14]. The pH in water bodies fluctuates depending on photosynthetic and respiratory processes and is generally alkaline during the day photosynthesis and more acidic at night because of respiration. In natural waters, pH is influenced by the characteristics of the drainage basin, photosynthesis, oxidation of organic matter, and addition of pollutants; among other factors, the lethal effects on the ecosystem appear at pH values below 4.5 and above 9.5 [40].
On the other hand, the presence of water hyacinth (E. crassipes) and water lettuce (P. stratiotes) aquatic plants was observed, which causes a decrease in the oxygen exchange capacity at the air–water interface, impedes oxygen transfer, and reduces the infiltration of sunlight, thus reducing phytoplankton growth, causing a reduction in the population density of zooplankton and impacting the food chain, increasing water loss through transpiration, reducing water quality by increasing sedimentation, reducing dissolved oxygen levels, and negatively affecting both fish populations and native fauna [41,42]. The excessive presence of aquatic plants indicates a high concentration of nitrates (NO3), as they are vital compounds for the development of aquatic plants and algae [43]. The excessive presence of these compounds is attributed to the favorable environmental conditions of PAR, pH, and temperature for the development of photosynthetic organisms.
The temperature of water bodies is influenced by solar radiation, which affects the kinetics of microbial reactions and oxygen solubility; high temperatures increase the metabolic activity of heterotrophic bacteria and decrease oxygen solubility, whereas lower temperatures reduce bacterial activity and increase oxygen solubility. Villagómez-Ibarra et al. reported that in December, dissolved oxygen increased notably, decreasing BOD5 in Laguna Tecocomulco in Hidalgo, Mexico, due to increased oxygen solubility and decreased biological oxygen demand caused by low bacterial activity [44]. The samples collected from July to December 2019 showed a decreasing trend; as the environmental and water body temperatures decreased, the BOD5 also decreased due to low bacterial activity, and the solubility of dissolved oxygen increased. The BOD5 spikes can be attributed to discharges received in the water body. In the months of January–March and October–December, the minimum ambient temperatures were below 15 °C, whereas maximum temperatures above 35 °C were observed in the months of March–August from 2012 to 2022.
In water bodies, PAR incident on the surface is partially reflected, and the remaining PAR penetrates the water, varies throughout the day according to the solar angle, penetrates deeper in the morning, and decreases at dusk [45], enhancing algal photosynthesis and increasing dissolved oxygen. Aquatic producers such as phytoplankton, macrophytes, and periphyton are influenced by PAR, and phytoplankton receive more illumination closer to the surface. However, their photosynthetic rate is not linear; it is limited to the maximum saturation rate, and higher irradiances can cause photoinhibition. On the other hand, PAR determines the photic zone; submerged macrophytes are restricted to the depth of this zone, whereas periphyton also depends on the light reaching the bottom or the macrophyte leaves. Factors that modulate PAR penetration include turbidity, suspended solids, water color, chromophoric dissolved organic matter, chlorophyll concentration, and nutrients. Fine sediments and suspended particles scatter light, limiting their depth; chromophoric dissolved organic matter is optically active and absorbs light; and high levels of nitrates and phosphorus favor algal blooms that increase biological turbidity. Therefore, the balance between light and nutrients determines the ecology of phytoplankton, macrophytes, and periphyton in lagoons [46,47]. The incident photosynthetically active radiation in El Conejo Lagoon ranged between 50 and 131 W/m2 between 2012 and 2022, with irradiation exceeding 100 W/m2 in the May–August period.
The results of the COD measurements revealed a random variation at sampling points 1 and 2, which were located at the northern end of the lagoon, a shallow area where a greater amount of contaminants accumulated between July 27 and August 30, which was attributed to the washout of contaminants due to rainfall. However, the trends in sampling points 3, 4, and 5 revealed little variability in the range between 125 and 150 mg/L in the period from July to November, with a decrease in the month of December between 100 and 125 mg/L, which was probably due to the oxygenation caused by cold fronts in that season. According to the COD parameters, the water quality is classified as polluted, and according to BOD5, it was found to be between polluted and acceptable, according to the CONAGUA standards. Chemical and biological contamination are associated with nonmunicipal and municipal water discharges, respectively. In general, socioeconomic activities such as domestic and industrial wastewater discharges, in addition to adjacent land use and agricultural runoff, are responsible for the variability in reservoir water quality, which contributes to the high levels of COD and BOD5 [48,49]. By NOM-003-ECOL-1997, which establishes the maximum permissible limits for contaminants in treated wastewater for reuse in public services, only 20 and 30 mg/L BOD5 are allowed for use in public services with indirect and direct contact with the public, respectively; in other words, the water from the lagoon is not suitable for use in public services [50]. According to the results, this lagoon is part of a lagoon system connected to the adjacent lagoons of El Chango and Las Marismas, which receive municipal water discharges. In addition, signs were found indicating the reception of industrial wastewater and a storm drain. Therefore, the pollutant load comes mainly from wastewater, since the area has no agricultural activity.
The values found are lower than those reported in Tres Palos Lagoon in Guerrero, Mexico, with 96–476 mg/L COD and 43.6–116 mg/L BOD5 in 2007; both lagoons present COD values that double or triple the BOD5 values, indicating the presence of nonbiodegradable material [13]. The BOD5/COD ratio is used to calculate the level of biodegradability of organic wastes. Laguna Tecocomulco presented values lower than 0.1 in 2009; BOD5/COD values > 0.6 indicate highly biodegradable organic matter, whereas BOD5/COD < 0.2 indicate little degradable organic matter [44]. Among the three bimesters evaluated in El Conejo Lagoon, the BOD5/COD ratios were 0.38–0.56, 0.18–0.32, and 0.11–0.39, respectively, indicating that the water body contains organic matter that is difficult to degrade, possibly from the discharge of industrial wastewater from a polymer production company, as well as runoff from the nearby fuel distribution and service station businesses.
The COD remained in the contaminated water grade, with values of 117.3–200 mg/L, which are well above those reported for the Vega Escondida Lagoon in Tampico, Tamaulipas, with values of 3.85 ± 4.9 mg/L [3], and for the Pie de la Cuesta Lagoon in Acapulco, Guerrero, with values of 22–24 mg/L [14]. The variability of the BOD5 parameter during the evaluation period shows a decreasing trend, with a slight increase on October 19 and November 11, which decreased again in December, indicating that the aerobic microorganisms present in the lagoon had the dissolved oxygen necessary to degrade the organic matter polluting the lagoon. The BOD5 concentration during the evaluation period ranged from 15.8 to 112.75 mg/L, which was much higher than that reported in the Pie de la Cuesta Lagoon in Acapulco, Guerrero, which ranged from 55 to 62 mg/L in low water and 59–70 mg/L in the rainy season [14]. Water of acceptable quality can be used in various activities; with a BOD5 between 6 and 30 mg/L, it presents medium quality for fish development and restricted agricultural irrigation; that is, it can be used for sowing, cultivation, and harvesting of agricultural products, except legumes and vegetables that are consumed raw [49]. Only La Pólvora Lagoon presented values as low as 2–3 mg/L BOD5 [17].
In the dynamics of lagoon systems, PAR increases photosynthesis and the dissolved oxygen concentration, favoring the degradation of organic matter COD and BOD5, whereas temperature modulates bacterial kinetics and oxygen solubility, allowing self-purification; however, they continuously receive external inputs from discharges, runoff, and natural phenomena.
During this investigation, the presence of contaminant ions was not quantified; only the COD and BOD5 were measured; however, in subsequent investigations, the nitrate and phosphate concentrations were measured. Water was extracted from El Conejo Lagoon for use as a culture medium for the Tetradesmus dimorhus strain under ambient conditions between 7 April and 2 September 2022. The water presented values of 11.2 to 44.0 mg/L nitrate nitrogen and 79.04 to 383.04 mg/L COD, of which 74.24 ± 27.99% N-NO3−1 and 43.37 ± 20.75% COD were removed [51]. In another investigation, water from El Conejo Lagoon was also used to grow the same strain. In a flat-layer photobioreactor with a settler in 2024, the water content ranged from 22.7 to 41.6 mg/L for nitrate and was 0.9 mg/L for phosphate; at the end of cultivation, 23.6% nitrate, 7.2% COD, and 0% phosphate removal [52] was observed for 4 to 124.8 ppm COD. According to CONAGUA, concentrations between 40 and 200 mg/L indicate contaminated water; i.e., Laguna el Conejo had contaminated water in 2019, 2022, and 2024. CONAGUA does not use nitrate and phosphate parameters as direct indicators of pollution, but these parameters are considered important because they are essential nutrients for plants and algae and cause eutrophication of natural water bodies. Laguna Yuriria, in Guanajuato, Mexico, presented nitrate concentrations between 2.1 and 3.7 mg/L in 2005 and between 1.7 and 3.9 mg/L between 2009 and 2010, whereas the phosphate concentration varied between 0.6 and 4.7 mg/L in 2005 and between 0.6 and 6.3 mg/L between 2009 and 2010 [53]. On the other hand, Laguna Vega Escondida in Tampico, Tamaulipas, presented average nitrate concentrations of 0.26 mg/L in 2019 [3]. The nitrate concentration in the water of Laguna El Conejo was greater than that in the water of Lagunas Yuriria and Vega Escondida; however, the phosphate concentration was greater in Laguna Yuriria. A 2013 study of lagoon systems in Sinaloa, Mexico, reported undetectable nitrate concentrations of up to 0.59 mg/L, with a mean of 0.06 mg/L, while phosphate levels of 0.01 mg/L to 0.54 mg/L were detected, with a mean of 0.06 mg/L [40]. The agreement that establishes the ecological criteria for water quality in Mexico does not establish limit concentrations of nitrates in freshwater bodies, only in coastal zones, with a maximum of 0.04 mg/L. Permitted concentrations of total phosphorus, measured as phosphorus, must not exceed 0.025 mg/L in freshwater lakes and reservoirs to protect aquatic life, prevent the development of undesirable biological species, and control accelerated eutrophication [54]. The nitrate and phosphate values found in El Conejo Lagoon are much higher than those recommended to protect aquatic life.
Sediments constitute a reservoir of organic matter and trace elements such as heavy metals and nutrients, which can be fixed or released into the water, depending on pH, salinity, flow, etc. [32,55]. The sediment presented a pH of 6.6 to 7.8, similar to that presented by the Tampamachopo Lagoon in Veracruz, which varied from 6.5 to 7.65 in the period from January 2009 to March 2010 [32]. The Las Ilusiones Lagoon in Tabasco presented more acidic values of 5.9 until it reached an alkalinity of 8.4 [20]. Generally, an alkaline or slightly alkaline pH in sediments favor the sedimentation of heavy metals or toxic substances [32].
A research report on the coastal lagoons of Sinaloa reported sediment textures according to Shepard’s triangle. In the El Colorado lagoon system, sands and clays predominated, whereas in the Aguiabampo-Bacorehuis, Topolobampo-Ohuíra-Santa María, Navachiste-San Ignacio, and Navachiste-San Ignacio-San Ignacio lagoon systems, sands and clays predominated. In Navachiste-San Ignacio-Macapule, and Huizache-Caimanero, sands, silts, and clays predominated; and in the lagoon systems Playa Colorada-Santa María-La Reforma, Altata-Ensenada-El Pabellón, Ceuta, and Chametla-Teacapán, sands predominated, followed by clays and silts [40]. Sediments act as contaminant sinks; suspended particles with heavy metals bind to water to form complex compounds that gradually precipitate and accumulate [19]. The sediment types found in Laguna El Conejo are silty sand, low-plasticity sand, and poorly graded sand, whereas those in Laguna Las Ilusiones in Tabasco are coarse-grained sandy sediments composed of sand, clay, and silt [56].

6. Conclusions

El Conejo Lagoon is a warm, shallow lagoon with a neutral pH, high COD pollution levels ranging from 117.3 to 200 mg/L, and insufficient degradation of biodegradable material, with BOD5 values ranging from 21.8 to 112.75 mg/L during the study period. Ambient temperatures and photosynthetically active radiation, in conjunction with pH and contaminant loading, favor the excessive growth of E. crassipes and P. stratiotes. The bottom sediment is sandy in areas up to 1.5 m, whereas it is clayey in deeper areas up to 3.2 m. The BOD5/COD ratio varies between 0.11 and 0.56, indicating that organic matter is difficult to biodegrade; therefore, integrated management, including improving water treatment processes before discharge into the lagoon, controlling the growth of macrophytes, especially the invasive species E. crassipes, conducting environmental education and citizen participation campaigns, and continuous monitoring, is recommended. To understand more about this ecosystem, the presence of heavy metals in sediments, as well as pathogenic microorganisms and emerging contaminants in the water, can be investigated.

Author Contributions

Conceptualization, S.G.P.-B. and J.S.-M.; data curation, L.M.-C. and C.A.D.-C.; formal analysis, L.M.-C. and C.A.D.-C.; methodology, S.G.P.-B. and J.S.-M.; resources, U.P.-G., L.A.-V. and M.d.R.C.-C.; supervision, U.P.-G., L.A.-V. and M.d.R.C.-C.; validation, L.M.-C., C.A.D.-C. and M.d.R.C.-C.; writing—original draft, J.S.-M.; writing—review and editing, S.G.P.-B., U.P.-G. and L.A.-V. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We would like to thank Consejo Nacional de Humanidades, Ciencias y Tecnología for the national scholarships 729401 and 961796 awarded to the first and second authors to pursue a postgraduate degree in Engineering Sciences at the Tecnológico Nacional de México, campus of the Instituto Tecnológico de Ciudad Madero.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Results of the one-way ANOVA of COD in the three evaluation periods.
Table A1. Results of the one-way ANOVA of COD in the three evaluation periods.
July–August 2019
SourceDFSS Ajust.MS Ajust.Value FValue p
Factor439,75999402.100.089>0.05 Ho is accepted
Error70330,7064724
Total74370,465
September–October 2019
SourceDFSS Ajust.MS Ajust.Value FValue p
Factor4804520111.920.116>0.05 Ho is accepted
Error7073,2651047
Total7481,310
November–December 2019
SourceDFSS Ajust.MS Ajust.Value FValue p
Factor42185546.20.420.795>0.05 Ho is accepted
Error7091,3181304.5
Total7493,503
Table A2. Results of the one-way ANOVA of COD in the three evaluation periods.
Table A2. Results of the one-way ANOVA of COD in the three evaluation periods.
July–August 2019
SourceDFSS Ajust.MS Ajust.Value FValue p
Factor413,474.83368.7081.060.000<0.05 Ho is rejected
Error20831.241.56
Total2414,306.0
September–October 2019
SourceDFSS Ajust.MS Ajust.Value FValue p
Factor42788.2697.0222.020.000<0.05 Ho is rejected
Error20633.231.66
Total243421.4
November–December 2019
SourceDFSS Ajust.MS Ajust.Value FValue p
Factor46855.81713.96131.640.000<0.05 Ho is rejected
Error20260.413.02
Total247116.2
Table A3. Results of Tukey’s simultaneous tests for differences in BOD5 means in the three evaluation periods.
Table A3. Results of Tukey’s simultaneous tests for differences in BOD5 means in the three evaluation periods.
July–August 2019
Difference in LevelsDifference in MeansConfidence IntervalAdjusted p Value
M2-M1−56.80(−69.00, −44.60)0.000
M3-M1−30.60(−42.80, −18.40)0.000
M4-M1−64.60(−76.80, −52.40)0.000
M5-M1−51.00(−63.20, −38.80)0.000
M3-M226.20(14.00, 38.40)0.000
M4-M2−7.80(−20.00, 4.40)0.343
M5-M25.80(−6.40, 18.00)0.621
M4-M3−34.00(−46.20, −21.80)0.000
M5-M3−20.40(−32.60, −8.20)0.001
M5-M413.60(1.40, 25.80)0.024
September–October 2019
Difference in LevelsDifference in MeansConfidence IntervalAdjusted p Value
M7-M6−10.60(−21.40, −0.04)0.051
M8-M6−3.00(−13.64, 7.64)0.914
M9-M6−24.60(−35.24, −13.96)0.000
M10-M6−25.20(−35.84, −14.56)0.000
M8-M77.60(−3.04, 18.24)0.244
M9-M7−14.00(−24.64, −3.36)0.007
M10-M7−14.60(−25.24, −3.96)0.004
M9-M8−21.60(−32.24, −10.96)0.000
M10-M8−22.20(−32.84, −11.56)0.000
M10-M9−0.60(−11.24, 10.04)1.000
November–December 2019
Difference in LevelsDifference in MeansConfidence IntervalAdjusted p Value
M12-M11−35.40(−42.23, −28.57)0.000
M13-M11−17.40(−24.23, −10.57)0.000
M14-M11−44.20(−51.03, −37.37)0.000
M15-M11−40.60(−47.43, −33.77)0.000
M13-M1218.00(11.17, 24.87)0.000
M14-M12−8.80(−15.63, −1.97)0.008
M15-M12−5.20(−12.03, 1.63)0.193
M14-M13−26.80(−33.63, −19.97)0.000
M15-M13−23.20(−30.03, −16.37)0.000
M15-M143.60(−3.23, 10.43)0.528

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Figure 1. The state of Tamaulipas is in northwestern Mexico and is divided into 43 municipalities: Altamira is in the southeast, and El Conejo Lagoon is outlined in green.
Figure 1. The state of Tamaulipas is in northwestern Mexico and is divided into 43 municipalities: Altamira is in the southeast, and El Conejo Lagoon is outlined in green.
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Figure 2. Lagoons in Altamira, Tamaulipas, Mexico.
Figure 2. Lagoons in Altamira, Tamaulipas, Mexico.
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Figure 3. The map above illustrates the geographical location of the water sampling points in El Conejo Lagoon.
Figure 3. The map above illustrates the geographical location of the water sampling points in El Conejo Lagoon.
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Figure 4. The map above illustrates the geographical location of the sediment sampling points in El Conejo Lagoon.
Figure 4. The map above illustrates the geographical location of the sediment sampling points in El Conejo Lagoon.
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Figure 5. Sequence of methods used in this research.
Figure 5. Sequence of methods used in this research.
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Figure 6. At point 1, Laguna El Conejo connects with Laguna Las Marismas (22°25′51.3′′ N 97°52′48.5′′ W), and a treated water discharge is located at point 2 (22°25′31.6′′ N 97°53′03.3′′ W).
Figure 6. At point 1, Laguna El Conejo connects with Laguna Las Marismas (22°25′51.3′′ N 97°52′48.5′′ W), and a treated water discharge is located at point 2 (22°25′31.6′′ N 97°53′03.3′′ W).
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Figure 7. Wastewater discharge from the BASF company.
Figure 7. Wastewater discharge from the BASF company.
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Figure 8. Some examples of flora and fauna were found during visits to El Conejo Lagoon: (A) water hyacinth (E. crassipes), (B) water lettuce (P. stratiotes), and (C) white heron.
Figure 8. Some examples of flora and fauna were found during visits to El Conejo Lagoon: (A) water hyacinth (E. crassipes), (B) water lettuce (P. stratiotes), and (C) white heron.
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Figure 9. Annual photosynthetically active radiation in the study area.
Figure 9. Annual photosynthetically active radiation in the study area.
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Figure 10. Ambient temperatures of the study area; the legend indicates the year, the + symbols indicate the maximum temperature, and the—symbols indicate the minimum temperature.
Figure 10. Ambient temperatures of the study area; the legend indicates the year, the + symbols indicate the maximum temperature, and the—symbols indicate the minimum temperature.
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Figure 11. COD results at the five sampling points (PMs) in the five bimonthly samples during the July–December 2019 assessment period.
Figure 11. COD results at the five sampling points (PMs) in the five bimonthly samples during the July–December 2019 assessment period.
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Figure 12. Results of BOD5 sampling during the evaluation period.
Figure 12. Results of BOD5 sampling during the evaluation period.
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Table 1. Geographic coordinates of the water sampling points.
Table 1. Geographic coordinates of the water sampling points.
Sample PointCoordinates GMS
122°26′2.926′′ N, 97°53′16.22′′ W
222°25′58.656′′ N, 97°53′6.514′′ W
322°25′46.204′′ N, 97°53′1.003′′ W
422°25′21.422′′ N, 97°52′51.034′′ W
522°25′9.872′′ N, 97°52′48.313′′ W
Table 2. Geographic coordinates of the sediment sampling points.
Table 2. Geographic coordinates of the sediment sampling points.
Sampling PointCoordinates GMS
122°25′41.699′′ N, 97°52′46.714′′ W
222°25′44.1′′ N, 97°52′49.5′′ W
322°25′49.3′′ N, 97°52′49.799′′ W
422°25′52.7′′ N, 97°52′50.199′′ W
522°25′53.6′′ N, 97°52’53.0′′ W
622°25’54.4′′ N, 97°52’55.999′′ W
722°25’55.3′′ N, 97°52’58.0′′ W
822°25’54.9′′ N, 97°53’4.999′′ W
922°25′55.6′′ N, 97°53′4.999′′ W
1022°25′50.2′′ N, 97°53′6.299′′ W
1122°25′47.9′′ N, 97°53′6.6′′ W
1222°25′43.9′′ N, 97°53′7.0′′ W
1322°25′41.2′′ N, 97°53′8.0′′ W
1422°25′39.5′′ N, 97°53′6.6′′ W
1522°25′36.0′′ N, 97°53′1.413′′ W
Table 3. Water quality indicators according to the COD and BOD5 parameters established by CONAGUA in Mexico.
Table 3. Water quality indicators according to the COD and BOD5 parameters established by CONAGUA in Mexico.
RatingCriteria (mg/L)
Excellent 1COD ≤ 10BOD5 ≤ 3
Good quality 210 < COD ≤ 203 < BOD5 ≤ 6
Acceptable 320 < COD ≤ 406 < BOD5 ≤ 30
Polluted 440 < COD ≤ 20030 < BOD5 ≤ 120
Heavily polluted 5COD > 200BOD5 > 120
1 Not polluted. 2 Surface water with low organic matter content. 3 Evidence of contamination. Surface waters with self-purification capacity or with biologically treated wastewater discharge. 4 Surface waters with raw sewage discharges, which are mainly of municipal origin. 5 Surface waters strongly impact municipal and nonmunicipal raw sewage discharges.
Table 4. Physicochemical parameters of El Conejo Lagoon water.
Table 4. Physicochemical parameters of El Conejo Lagoon water.
PeriodpHTemperature (°C)Parameter (mg/L)Quality
July–August732–33COD = 130.8–200Polluted
BOD5 = 50.6–112.75Contaminated
September–October726–33COD = 117.3–143.7Polluted
BOD5 = 21.8–47Acceptable–Polluted
November–December718–24COD = 137.19–151.6Polluted
BOD5 = 15.8–60Acceptable–Polluted
Table 5. Sediment characteristics and classification.
Table 5. Sediment characteristics and classification.
Sampling
Point		Sediment
Sample (g)		Depth
(m)		pHSediment
-Retained (g)
Mesh #4
(4.75 mm)		Sediment
-Retained (g)
Mesh #40
(0.425 mm)		Sediment
-Retained (g)
Mesh #200
(0.075 mm)		Bottom% Gravels%
Sands		% FinosType of Sediment
11751.37.803113014092.08.0SP 6-SM 7
21051.36.9002184020.080.0CL 8
3861.36.909698090.79.3SP-SM
41951.36.40018312093.86.2SP-SM
52891.156.6717251142.492.74.8SP
65431.26.3924492181.795.03.3SP
73601.36.901432917095.34.7SP
83451.36.906027114095.94.1SP
9421.67.2001923045.254.8CL
102202.27.30013882062.737.3SM
11903.07.0002070022.277.8CL
12603.27.1001347021.778.3CL
131253.26.6002699020.879.2CL
142672.56.6412237141.593.35.2SP-SM
153031.57.00182796098.02.0SP
6 SP: Poorly graded sand, 7 SM: Silty sand, 8 CL: Low-plasticity sand.
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Pérez-Bravo, S.G.; Soriano-Mar, J.; Páramo-García, U.; Aguilera-Vázquez, L.; Martínez-Cardenas, L.; Dávila-Camacho, C.A.; Castañeda-Chávez, M.d.R. Physicochemical and Sediment Characterization of El Conejo Lagoon in Altamira, Tamaulipas, Mexico. Earth 2025, 6, 83. https://doi.org/10.3390/earth6030083

AMA Style

Pérez-Bravo SG, Soriano-Mar J, Páramo-García U, Aguilera-Vázquez L, Martínez-Cardenas L, Dávila-Camacho CA, Castañeda-Chávez MdR. Physicochemical and Sediment Characterization of El Conejo Lagoon in Altamira, Tamaulipas, Mexico. Earth. 2025; 6(3):83. https://doi.org/10.3390/earth6030083

Chicago/Turabian Style

Pérez-Bravo, Sheila Genoveva, Jonathan Soriano-Mar, Ulises Páramo-García, Luciano Aguilera-Vázquez, Leonardo Martínez-Cardenas, Claudia Araceli Dávila-Camacho, and María del Refugio Castañeda-Chávez. 2025. "Physicochemical and Sediment Characterization of El Conejo Lagoon in Altamira, Tamaulipas, Mexico" Earth 6, no. 3: 83. https://doi.org/10.3390/earth6030083

APA Style

Pérez-Bravo, S. G., Soriano-Mar, J., Páramo-García, U., Aguilera-Vázquez, L., Martínez-Cardenas, L., Dávila-Camacho, C. A., & Castañeda-Chávez, M. d. R. (2025). Physicochemical and Sediment Characterization of El Conejo Lagoon in Altamira, Tamaulipas, Mexico. Earth, 6(3), 83. https://doi.org/10.3390/earth6030083

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