Introduction

Industrialization produces effluents that contain toxic compounds such as metals, whose discharges into water bodies are the major source of pollution and various diseases (^{ATSDR, 2012}). The water and the sediments in lakes and lagoons become receivers of various pollutant, which enter the food chains (^{Theoneste, Gang, Jing, Xu, & Lei, 2013}; ^{Lemus, Castañeda, & Chung, 2014}). This poses a risk to biota because of bioaccumulation and biomagnification in the organs and tissues of many aquatic and terrestrial species (^{Sobrino, Cáceres, & Rosas, 2007}). Other equally important sources include municipal discharges and agricultural and particle emission. Forty-two percent of the substances discharged into surface water bodies are metals that end up in sediments. Sediments are matrices composed of organic matter in various stages of decomposition and organic and inorganic particulate material of biological and anthropogenic origin (^{Chen ( White, 2004}).

Heavy metals are pollutants that, due to their persistence and long-range transport, easily reach surface water bodies, posing a potential risk to biota. These pollutants can come from various sources such as: oil activities, agriculture, metallurgy, and sugar, as well as urban and sanitary (herbicides and insecticides). The high toxicity of metals bioaccumulate in biota and magnify up the food chain in higher organisms, including humans (^{Herrera, Rodríguez, Coto, Salgado, & Borbón, 2012}).

Rivers receive waste generated by human activities through effluents that introduce hazardous substances, especially metals such as lead, cadmium, chromium, mercury, and manganese, among others. These tend to settle in the sediment, become suspended, or dissolve in the water column (^{Rodríguez, Ruíz, & Vertiz, 1994}). These rivers carry pollutants along their courses, and when they overflow they may be deposited in water bodies such as lakes and springs. The Ilusiones Lagoon in Villahermosa is surrounded by two large rivers (Grijalva and Usumacinta) in an area with high rates of rainfall, which have caused these rivers to flood. Weather conditions also contribute to the potential transfer of pollutants from the atmosphere to the lagoon in particular, and other bodies of water in general (^{Palma, Cisneros, Moreno, & Rincón, 2007}).

Next are some examples that illustrate the extent of metal pollution in the country. Arsenic and lead concentrations in surface sediments in the Zahuapan-Atoyac hydrologic system ranged from 1.3 to 127.7 mg kg^-1 and 27.3 to 89.5 mg kg^-1, respectively (^{García-Nieto et al., 2011}). ^{Rosales-Hoz and Carranza-Edwards (1998}) detected levels of metals in sediment in the Coatzacoalcos River ranging from 21.2 - 53.5 mg kg^-1 for total lead, 2.1 - 2.7 mg kg^-1 for total cadmium and 21.6 - 152.7 mg kg^-1 for total chromium. ^{González-Mille et al. (2010}) found Pb (4 - 22 mg kg^-1), Cr⁺⁶ (9 - 93 mg kg^-1) and manganese (76.7 - 2129 µg kg^-1) in the same river.

Heavy metals bioaccumulate in aquatic species such as oysters, clams, shrimp, and some fish species. ^{Lango-Reynoso, Landeros-Sánchez and Castañeda-Chávez (2010}) determined the bioaccumulation of arsenic, cadmium, and lead in oysters in the Tamiahua Lagoon in Veracruz, with mean values of 4.02 mg kg^-1 for arsenic, 11.7 mg kg^-1 for cadmium and 0.48 mg kg^-1 for lead. The cadmium exceeded the maximum permissible limits stipulated by consumer health rules for bivalves and pose a risk to human health. The pollution in water and sediment in the Ilusiones Lagoon can change the balance of populations, especially those that are most susceptible. ^{Gamboa, Gamboa, Saldívar and Espejel (2007}) quantified lead in the blood of crocodile (Cocodrylus moreletti), finding values (6.55 - 15.34 µg dL^-1) that were higher than permissible limits for aquatic life.

No recent studies have been published to assess the environmental and ecotoxicological conditions of the Ilusiones Lagoon. The few studies that have been reported include one from 1992 to 1994 of phosphorous (10.6 mg L^-1) and total nitrogen (4.1 mg L^-1) present in the lagoon, in which it was categorized as hypertrophic with a dendritic morphology and low hydraulic mixture, preventing water changes that favor the persistence of conservative and non-conservative substances (^{Rodríguez et al., 1994}). In general, the content of pollutants in water bodies increases over time. For example, in 1994, 30% of Mexican rivers were polluted by mercury and lead, whereas currently 95% are reported to be polluted. In 1992, the concentration of lead in surface sediments in the Ilusiones Lagoon was 158. 68 µg g^-1 (^{Villanueva & Botello, 1992}).

The Ilusiones Lagoon is important to the city of Villahermosa, Tabasco. Because of its great diversity of aquatic and terrestrial species, it was declared a nature reserve in 1995 (^{Semarnat-Conanp, 2014}). However, for several years it has been subjected to stress from human activities that have directly and indirectly deposited agricultural, urban, and industrial waste, degrading its ecological value (^{Semarnat-Conanp, 2014}). While there are practically no studies addressing pollution in the Ilusiones Lagoon, particularly of heavy metals, there are many examples of metal contamination in many rivers in Mexico, suggesting that a body of water with the conditions of the Ilusiones Lagoon can be considered as a potentially polluted site. The aim of this study was to determine the physical and chemical parameters and metals (Pb, Cd, Mn, Zn, Cr, Al and Ni) in water and surface sediment in the Ilusiones Lagoon, and whether they pose a risk of adverse effects to aquatic life.

Material and methods

Study location

The state of Tabasco is located in southeast Mexico, it measures 24 738 km² and has a warm humid climate with rains in the summer. The mean annual temperature ranges from 24 to 28 °C and annual precipitation ranges between 1500 to 2000 mm (^{Palma et al., 2007}). The Ilusiones Lagoon is located in hydrologic region RH30, in the Grijalva - Usumacinta Basin. It is located in the urban area of Villahermosa (17⁰ 59^´ 22^´´ to 18⁰ 01^´ 22^´´ N and 92⁰ 55^´ 20^´´to 92⁰ 56^´ 40^´´ O), in the physiographic province of the coastal plain of the Gulf of Mexico, and covers an area of 160 ha. It is surrounded by the Mezcalapa and Carrizal rivers at an average altitude of 10 masl (Figure 1). In 1995, the state proclaimed it to be protected area, as an ecological reserve for the preservation of the water, flora, and fauna. The area occupied by the lagoon has decreased an average of 30 ha over a period of 10 years because of the development of the city of Villahermosa (^{Semarnat-Conanp, 2014}; ^{Sernapam, 2014}).

Figure 1 The Ilusiones Lagoon study site. 

Collection of environmental arrays

The Ilusions Lagoon was divided into three zones, each defined based on the methodology proposed by Ilizaliturri et al. 2009, with six sampling sites (Figure 2). Six water samples were collected in each, in polyethylene containers. Surface sediments were collected in glass bottles. For the determination of metals in water, polyethylene bottles were used containing 1 mL of concentrated nitric acid to prevent volatilization losses. For the sediment sample, 2-liter amber containers were used. The samples were transported and stored at 4 °C until analysis (^{NOM-001-Semarnat-1996}). Two samplings of water were collected, one in April 2013 and the other in September 2014.

Figure 2 Ilusiones Lagoon sampling sites. 

Results and discussion

Figure 3 shows the physicochemical properties determined based on water samples collected from the three areas of the lake. The pH values in water, ranging between 8.9 and 9.6, were alkaline and above the limits (6.5 - 9.0) permitted by the Guidelines for Canadian Water Quality, and the lower limit (8.9) was greater than that which is allowed (5.5) by NOM-002-Semarnat-1996. The highest value (9.6) was present in Zone 3 and the lowest value (8.9) in Zone 1, presumably due to the presence of carbonate and hydroxide ions that can come from anthropogenic waste and soluble natural minerals.

Figure 3 Physicochemical properties determined in water samples. 

The water temperature ranged from 28.8 to 29.9 °C, which was below the limit allowed by Mexican regulations (40 °C). The ORP varied in the three areas, from 5 mV in zone 1 to as much as 80 mV in Zone 2. With regard to the DO, this ranged from 7 to 11 mg L^-1, similar to the limits proposed by the EPA (not less than 5 mg L^-1) and the Guidelines for Canadian Water Quality (9.5 mg L^-1). The EC values ranged from 225 to275 µS cm^-1 which were below (2500 µS cm^-1) the NOM- 069- ECOL- 1994 for freshwater bodies. The COD ranged from 105 to 155 mg O₂ L^-1, which was within the range classified as polluted by Conagua in 2012 (40 < CO < 200). TDS (114 - 137 mg L^-1) was similar to values reported by the Secretaria de Energía Recursos Naturales y Protección Ambiental (SERNAPAM) in 2012 and 2013 (Table 1). The physicochemical parameters are shown in (Figure 4).

Figure 4 Physicochemical parameters. 

In sediments, the pH ranged from 5.9 (mildly acidic) to 8.4 (mildly alkaline) (^{NOM- 021 Semarnat 2000}). Values of 1 to 11% of OM indicate volcanic origin, and the EC had negligible effects of salinity because it ranged from 4 to 28 µS cm^-1. The particle size was, on average, 67.2% sand, 17.2% clay and 15.4% silt for Zone 1; 75.4% sand, 14.5% clay and 10% silt for Zone 2; and 69.2% sand, 16.1% clay and 14.6% silt for Zone 3. In terms of the texture triangle used by the U.S. Department of Agriculture, the results from the three zones corresponded to a textural classification that ranges from sandy to moderately thick sandy loam.

Table 2 shows the concentrations of the metals quantified in the water (Cd, 2.2 - 5.6 µg L^-1), (Ni, 2.2 - 7.8 µg L^-1), (Cr, 28 - 64.4 µg L^-1), (Mn, 38.9 - 112.2 µg L^-1), (Zn, 12.2 - 79.2 µg L^-1), (Al, 218 - 1385 µg L^-1) and (Pb, ND a 0.1 µg L^-1), all below the ^{NOM-001-Semarnat-1996}. However, the Cr and Cd hazard ratio (reference concentration / mean) was greater than one. Table 3 shows the concentrations of metals found in sediments (Cd, 1.1 - 4.0 mg/kg), (Ni, 21.4 - 75.3 mg/kg), (Cr, 23.1 - 99.5 mg/kg), (Mn, 42.4 - 341.9 mg/kg), (Zn, 13.8 - 193.2 mg/kg), (Pb, ND - 204 mg/kg) and (Al, 4404.9 - 73593. 6 mg/kg). Here the hazard ratio was greater than one for all metals found except Zn and Mn. According to the legislation of Canada and the United States, when the hazard ratio is greater than unity, there is already a potential risk to aquatic life.

Conclusions

In this study site, the pH values in water ranged between 8.9 and 9.6, which were alkaline and above the limits (6.5 - 9.0) permitted by the Guidelines for Canadian Water Quality. The water temperature (28.8 - 29.9 °C) was below the limit allowed by Mexican regulations (40 °C). The dissolved oxygen was similar to values established by the EPA and the Guidelines for Canadian Water Quality. The COD value of the water body was classified as polluted by Conagua in 2012 (40 < COD < 200 mg L^-1). The EC and TDS values were within the parameters set by the SERNAPAM, while the heavy metals in the water (Cd, Ni, Cr, Mn, Zn, Al and Pb) were all below the ^{NOM-001-Semarnat 1996}.

In the sediment, the pH ranged from 5.9 (moderately acidic) to 8.4 (moderately alkaline), which was within permissible limits stipulated by ^{NOM-021 Semarnat 2000}. The percentage of organic matter indicated volcanic origin and the electric conductivity showed negligible effects from salinity. The particle size in the three sampling areas corresponded to a textural classification that ranged from sandy to moderately thick sandy loam. The concentrations of Cd, Ni, Cu, Mn, Zn, Pb and Al in sediment were within the Mexican norms, however when compared to the values corresponding to Canadian and United States standards, they exceeded the criteria established for heavy metals in freshwater bodies. This indicates that the biota was affected in terms of accumulating high concentrations of these metals in the system studied. Therefore, the authorities should be aware of the imminent dangers that to which nearby towns may be exposed.