Study Area

The study was carried out in the Laguna La Vega Escondida, which is part of the lagoon system of the river Guayalejo - Tamesí, also known as Champayán (hydrological region RH26 Río Pánuco) located in the municipality of Tampico, at the southeast of the state of Tamaulipas, Mexico (^{Sánchez and Propin, 2005}), Figure 1. In the zone predominates the warm subhumid climate with rains in summer, with average humidity of 75%, temperatures of 22 to 26 ºC and 900 to 1100 mm per year of precipitation. The geology of the watershed that influences the lagoon is composed of sedimentary rocks, in the lower part can be found shale-sandstone, from the geological periods of Quaternary and Paleogene (INEGI, 2009) and in the middle and upper part there are metamorphic rock from the Precambrian Tertiary period, mostly limestone and shales and to a lesser extent gneises, which are occasionally covered by alluvial deposits of the Quaternary (^{Duma, 2010}).

Figure 1 Geographic location of La Vega Escondida lagoon in Tampico, Tamaulipas, México. 

Water sampling

Five points were established for water sampling (P1 = 22 ° 17 ' 40.9 "N - 97 ° 56 ' 5.62 O; P2 = 22 ° 19 ' 2.7 "N-97 ° 56 ' 5.9 O; P3 = 22 ° 18 ' 26.8 "N-97 ° 55 ' 1.1 O; P4 = 22 ° 18 ' 21.0 "N-97 ° 55 ' 53.6 O; P5 = 22 ° 17 ' 37.9 "N-97 ° 54 ' 58.0 O), Figure 2. These points covered the water inlet, outlet, and slowdown zones of the system. A grab sample was collected at each point from three depths (0.30; 0.70 and 1.2 m respectively) with an Alpha or Van Dorf bottle, following the methodology of the Mexican standard for sampling water bodies receptors NMX-AA-014-SCFI1980 (SCFI, 1980). Each sample consisted of 2 liters of water, which were placed in plastic bottles and stored in ice-coolers at 4 °C for preservation. The transport to the laboratory was carried out following the methodology stipulated at the APHA Standard Methods for the Examination of Water and Waste Water (^{Clesceri et al., 1998}).

Fuente: Google earth 2015.

Figure 2. Location of sampling points at Laguna La Vega Escondida, Tampico, Tamaulipas, México. 

Analytical methods

The concentration of the physicochemical parameters of the lagoon water was determined by direct measurement in the field and in the laboratory according to the Mexican norms stipulated for the analysis of each parameter. (Table 1). These parameters are part of the water quality ecological criteria (CE-CCA -001/89), which indicates the main components to establish water quality for potential uses as: source of drinking water supply, recreational use, agricultural irrigation and the livelihood of biological systems. These parameters were chosen for functioning as indicators of the general state of water quality and for technical and economic limitations for the development of this study.

Spatial distribution of compounds

It was carried out by applying the Geostatistical method of Kriging in the program ARC Gis 10.2, for which an exploratory analysis of the data was carried out to evaluate data normality and determination of tendencies and anisotropy. A Semivariogram was constructed for the determination of ranks, plateau and the value of the Pepita effect. Followed by adjustment of theoretical models to the variogram for analysis of the Pepita effect and evaluation of clusters and cross-validation.

Analysis of variance

The average pH value was 7.79 + 0.09, therefore, the water of the Laguna La Vega Escondida is classified as weakly basic (MayoClinic.com, 2013), similar pH values have been reported for the El Chairel lagoon (SEMARNAT, 2010), which is an adjacent water body. The average electrical conductivity in the La Vega Escondida lagoon was 872.7 + 40.6 ΜS cm^-1, this value differs with the average reported for an earlier monitoring period of 16 years of the Pánuco River water (1122.75 μS cm^-1) realized by ^{SEMARNAT (2010)} but is similar to the reported for the Chairel lagoon (650 μS cm^-1) by ^{Pérez-Arreaga (2012)}.

The comparison of the averages values of temperature, COD and nitrates (NO^-3) at La Vega Escondida Lagoon with those reported for a 15 years period from the water of the Chairel lagoon and the Pánuco River, showed that the temperature and the COD of these were higher (temperature = 27.0 and 27.2 °C; COD = 6.9 and 12.7 mg L^-1 respectively) than the water of La Vega Escondida lagoon. In the case of nitrates, the water of La Vega Escondida lagoon, has an average of 0.26 mg L^-1, this concentration is less than the reported for the El Chairel lagoon and the Pánuco river, where the content of NO^-3 was 0.75 and 0.5 mg L^-1 respectively (SEMARNAT, 2010).

The SST showed significant statistical differences, both between depths and between sites (table 2). When making a comparison between the average SST (321.0 mg L^-1) of the Laguna La Vega Escondida, with those determined in the Tamesí river (50.0 mg L^-1) by ^{Batres (2012}) during a period of five years (2000 to 2005), it was observed that the average concentration of SST at La Vega Escondida was greater than in Tamesis river, with a difference of 290.0 mg L^-1. So the water from La Vega Escondida lagoon in relation to SST is classified as contaminated water (^{Nava, 2010}).

The average alkalinity found in this study (168 mg L^-1) differs with the average (70 mg L^-1) reported for the Tamesis river, so the study water has a good buffer capacity that can resist changes of pH (^{Riberos et al., 2008}).

The evaluation of analysis of variance of the results determined that the average values of temperature, SST, alkalinity, hardness, chlorides chemical oxygen demand, NO^-3 and SO₄ ^2-, had significant differences between the depths of each point, as well as between the different sampling points (Table 2), which could be due mainly to the hydrodynamics of the water within the lagoon that generates a complex mixture of the recent water with the already stored, involving processes of water residency (Calvo and Mora, 2007; ^{Pérez-Castillo & Rodríguez, 2008}; ^{Rendon-Dircio et al., 2012}). The average values of the physicochemical parameters analyzed (pH, Temperature, Cl^-, hardness, alkalinity, SO₄ ^2-, SST and OD) in La Vega Escondida lagoon, did not exceed the permissible maximum limits suggested in the ecological criteria of water quality (CE-CCA -001/89) except for SST, which exceeds the level of reference.

Spatial distribution

For the analysis of the spatial distribution of the physicochemical parameters of the study water, were established three concentration ranges (low, medium and high), which were distributed in each parameter between the lowest value and the highest value of all measurements.

The potential for hydrogen (PH)

Under the conditions of the present study, it was determined that the high concentration of pH was in the superficial and intermediate layers of inlet 2 and the outflow of water. The medium concentration was in the superficial and intermediate layer of the slowing zone and at Inlet 1, as well as in the deep layer of inlet 2 and the water outlet. The low pH concentration was in the three water layers of the slowing zone (Figure 3). The high concentration of pH at Inlet 2 is because it comes from a low-flow secondary channel, which may have potential sources of contamination in its route before reaching the lagoon, which also influences the area surrounding the exit of water since it is very close. The medium concentration is due to hydrodynamic properties, which allow a permanent mixture of water within the lagoon, making that the pH distribute evenly between the different depths. The low concentration of pH zones is formed because these receive water directly from the river Tamesí, which provides a high flow and provides damping to pH changes because there is a correlation between flow and pH (^{Calvo and Mora, 2007}).

Figure 3 Spatial distribution of pH in the superficial (a), intermediate (b) and deep (c) layers of water at La Vega Escondida lagoon, Tampico, Tamaulipas, México. 

Electrical conductivity (EC)

The spatial distribution of EC shows higher values in the intermediate and deep layers of inlet 2, the outflow of water and in the deep layer at the northeast of the slowing zone. The intermediate values of EC were in the central part from the north to the south of the intermediate and deep layers of the lagoon and in the same areas of the superficial layer the inlet 2. Finally, it was determined that the lowest values of EC are in the three water layers of inlet 1 of the lagoon (Figure 4). The heterogeneous behavior of the EC in space and time is conditioned by the fresh water mixture (^{Rendon-Dircio et al., 2012}) of the Tamesí River, which brings with it carbonates from the limestone rocks that make up the basin (^{Pérez-Arreaga, Garza-Flores, Canales-Caballero & Guevara-Guerrero, 2012}).

Figure 4 Spatial distribution of electrical conductivity (EC) expressed (S cm^-1, In the superficial (a), intermediate (b) and deep (c) water layers of Laguna La Vega Escondida, Tampico, Tamaulipas, México. 

Total suspended solids (SST)

It was determined that the highest concentration of SST was in the superficial layer of the Northeast area, in the intermediate layer of the entrances 1, 2 and the water outlet, as well as in the central part of the lagoon with relation: East-West (Figure 5). The intermediate concentration was in the three study layers of the slowing zone, in the area of influence of the inlet 2 and the outflow of water in the superficial and deep layers. The lowest concentration of SST was in the superficial layer of inlet 1 and in the intermediate layer of the northeastern area of the lagoon. The above is produced by the hydrodynamic characteristics of the lagoon, which allow the water to mix constantly.

After a comparison between SST and water COD of the study pond, was determined that the origin of SST was mineral and non-organic, because COD representing organic solids is at low levels, so the presence of SST is due to soil erosion occurring throughout the year in the middle and high portion of the basin from the river Guayalejo - Tamesí. Thus, when comparing the SST averages of this study with the ecological criteria of water quality, the vital fluid is within the range of contaminated water (^{Nava, 2010}).

Figure 5 Spatial distribution of total suspended solids (SST) expressed in mg L^-1, in the superficial (a), intermediate (b) and deep (c) water layers of Laguna La Vega Escondida, Tampico, Tamaulipas - México. 

Temperature (T)

The warmest temperatures (24.5-25.5 º C) were in the superficial and intermediate layers of inlet 2, the water outlet, in the central part and northeast of the lagoon. The intermediate temperature (24.3-24.5 º C), it is in the superficial and intermediate layers of the slowing zone, the Inlet 1 and in the deep layer of inlet 2 and its area of influence. The lower temperature range (24.0-24.3 º C) was found in the entire water column (three layers of study) of inlet 1 from where it is distributed towards the slowing zone and the water outlet through the deep layer (Figure 6). The behavior of the high temperature range of (24.5-25.5 º C) is because the water of inlet 2, comes from a small secondary channel, where it has more time of residence and there is a positive correlation between the air temperature with the thermocline (^{Zadereev et al. 2014}), this has the tendency to be warmer, so it is maintained and always moves by the superficial layer of the lagoon.

The tendencies of the intermediate water temperature (24.3 - 24.5 °C) are due to that this is a mixture of the warm water from the inlet 2 with the cold water of the inlet 1, product of the hydrodynamics and the water body size. The behavior of the low temperature range (24.0-24.3 º C) is because at entry 1, the water comes from the main channel (Tamesí River), which has a large flow that remains in constant movement (^{Batres, 2012}), this allows the water to maintain a stratified thermal pattern (^{Montoya-M, 2008}), with lower temperature and density in relation to the water that is inside the lagoon, so when the cold water enters the lagoon, it is located and spreads through the bottom of the lagoon (Figure 6).

Figure 6 Spatial distribution of temperature (T) expressed in º C in the superficial (a), intermediate (b) and deep (c) layers of the water of the Laguna La Vega Escondida. Tampico, Tamaulipas, México. 

Chlorides (Cl^-)

The higher concentrations (45.4-46.4 mg L^-1) of Cl^- was in the superficial and deep layers of the area of inlet 2 and in the intermediate layer of the area of inlet 1 and the slowdown zone. Intermediate concentration (44.7-45.4 mg L^-1), were in the superficial layer of the central part (from the north to the south) of the lagoon, in the intermediate layer of the inlet 2, exit of water, the deep layer of the inlet 1 and the slowing zone. The lowest concentration of Cl^-, was located at inlet 1 of the lagoon (Figure 7). This distribution is due to the hydrodynamic conditions of the water (^{Wetzel, 2001}), which are influenced by the area and the average depth of the lagoon (^{Cardoso et al., 2003}), as well by the orientation of the water inlets. These conditions allow the water body to have horizontal and vertical homogeneity (^{Branco et al., 2000}; ^{Briand et al., 2002}; ^{Figueredo and Giani, 2009}; Nogueira and Ramírez, 1998; ^{Coutinho and Mello, 2011}), because there is a constant and homogeneous mixture of water that enters with the one located inside the lagoon.

Figure 7 Spatial distribution of chlorides (Cl^-) expressed in mg L^-1, in the superficial (a), intermediate (b) and Deep (c) water layers of the Laguna La Vega Escondida, Tampico, Tamaulipas - México. 

Hardness

The spatial distribution of the study water, determined that the highest concentration of hardness (282.0-290.0 mg L^-1), was in the intermediate layer of inlet 1 and 2 and in the water outlet and in the deep layer north of inlet 2. Intermediate concentration (276.5-282.0 mg L^-1) of hardness, covered approximately 90% of the surface layer and the intermediate layer of the central zone (East-west relation). Low concentration (267.0-276.5 mg L^-1), was in the surface layer of inlet 2, in the intermediate and deep layer of the slowing zone as well as in the deep layer of the water outlet (Figure 8). The variation in the concentration of hardness in the water column is mainly due to the hydrodynamics of the water, which keeps in constant mixture and movement the carbonate that comes from the meteorization of the limestone rocks that are forming the Sierra Madre Oriental of Mexico, through which flows the water that is in the system lagoon of the river Guayalejo-Tamesí and thus within the lagoon of study (^{Vera, 2004}).

Figure 8 Spatial distribution of the hardness expressed in mg L^-1, in the superficial (a), intermediate (b) and deep (c) water layer of Laguna La Vega Escondida, Tampico, Tamaulipas, México. 

Alkalinity

The spatial distribution (Figure 9), determined that the high range (168.0-176.0 mg L^-1) of alkalinity is in the three layers of input 2, in the deep layer of inlet 1 and the intermediate layer of the slow zone. This behavior is because the channel of Inlet 2 has a higher concentration of carbonates (CaCO₃), the same that comes from the weathering of limestone rocks that conform the geology of the basin (^{Vera, 2004}) and possibly also by discharges of contaminants that may be in the path of the Tamesí river that passes very close to the city of Tampico. The middle range (162.0-168.0 mg L^-1), was found in the superficial and deep layers of the slowing zone and in the intermediate layer of the central part (North-South relation) of the lagoon, which is due to the mixture of water coming from Inlet 2 and the inlet 1 where the low range of alkalinity (15 6.0-162.0 mg L^-1) is located. There is a positive relationship between alkalinity with pH, the alkalinity is in greater concentration where there are higher pH values. According to the average of alkalinity found, it can be said that the water has good capacity to maintain the adequate pH for the development of aquatic life.

Figure 9 Spatial distribution of alkalinity in the surface layer expressed in mg CaCO₃ L^-1 in the superficial (a), intermediate (b) and deep (c), water layers of Laguna La Vega Escondida, Tampico, Tamaulipas - México. 

Sulfates (SO₄ ^2-)

The spatial distribution analysis showed that due to the hydrographic conditions of the body of water under study, the SO₄ ^2-, were in Higher concentration (16.0-19.0 mg L^-1) into the surface layer of the inlet 1. Intermediate concentration (14.8-16.0 mg L^-1), was in the superficial and deep layers of the slowing zone and in the intermediate layer of inlet 1 and the water outlet (Figure 10). This is due to the constant mixing of the sulfate in the Lagoon's water system. The concentrations found of SO₄ ^2- do not represent risk to the development of the life in the water system in study, since these levels, are not toxic for the plants and animals.

Figure 10 Spatial distribution of sulphates (SO₄ ^2-) expressed in mg L^-1, in the superficial (a), intermediate (b) and deep (c) layers of water of Laguna La Vega Escondida, Tampico, Tamaulipas, México. 

Chemical Oxygen Demand (COD)

The spatial distribution of the COD in the water column of the Laguna La Vega Escondida, showed that most part of study area had low concentration of COD (0.0-3.0 mg L^-1) in the three water layers of the laguna. The medium and high concentration was found in the superficial and intermediate layers of the slowing zone, as well as in the deep layer of the inlet 2 and its area of influence and at the outflow of water (Figure 11). It should be noted that the levels of COD found would correspond to natural levels of organic matter that come from the biological activity.

Figure 11 Spatial distribution of the chemical demand for oxygen (COD) expressed in mg L^-1, in the superficial (a), intermediate (b) and deep (c) water layers of Laguna La Vega Escondida, Tampico, Tamaulipas, México. 

Dissolved oxygen (DO)

The spatial distribution determined that the highest DO concentration was in the superficial layer of all sampling points and in the central part (relation: East-west) of the lagoon. The intermediate concentration was in the intermediate layer of the slowing zone and at outflow of water. The lowest concentration was in the deep layer of the entire lagoon (Figure 12). The above, it happens because OD has a close relationship with COD, so in the areas of the lagoon where this parameter are at high level, DO decreases, because there is a high consumption of oxygen from the microorganisms that are degrading the organic matter present in the water. The levels found, do not present risk of producing hypoxia in aquatic species of this body of water (^{Montalvo et al., 2008}).

Figure 12 Spatial distribution of dissolved oxygen OD mg O₂ L^-1, in the superficial (a), intermediate (b) and deep (c) water layers of Laguna La Vega Escondida, Tampico, Tamaulipas, México. 

Nitrates NO₃

The spatial distribution determined that NO₃ ^- were in greater concentration in the superficial layer of the central area of the lagoon and the intermediate and low concentrations were in most of the superficial and intermediate layer of the slowdown zone and at outflow of water (Figure 13). It should be indicated that the analysis of NO₃ ^- did not determine a concentration in the deep layer, so no distribution map is Presented. The levels of nitrates found in this study correspond to natural concentrations, product of the breakdown of organic materials, therefore when the NO₃ ^- at a low level (0.26 mg L^-1), you can tell the study water is not contaminated by this parameter, because it is below the nitrate levels of natural waters which is 0.45 mg L^-1 (^{Chapman, 1996}).

Figure 13 Spatial distribution of nitrates (NO₃ ^-) expressed in mg L^-1, in the superficial (a), intermediate (b) and deep (c) water layers of Laguna La Vega Escondida, Tampico, Tamaulipas, México.