Sedimentation Rate and Contamination Levels Profile of Potentially Toxic Elements in the Limoncocha Lagoon RAMSAR Wetland in the Ecuadorian Amazon

Abstract

The aim of this study is to analyze the recent sedimentation rate in the center of the Limoncocha lagoon, a Ramsar site in the Ecuadorian Amazon, using the ²¹⁰Pb dating method and identifying the potentially toxic elements along a 50 cm sediment core. A strategy based on the application of three single and four integrated indices is used to evaluate trace element contamination with depth. Single indices show mainly As and Mo, and Cu, Ba, Cd, Ni, and Pb to a lesser extent, as responsible elements of a minor enrichment between −10 and −40 cm. The multielement slight pollution shows a mixture of potential contamination sources, probably due to agricultural, oil activities, and urban wastewater discharges. However, integrated indices applied, classify the complete core as without potential risk. The ²¹⁰Pb_excess profile shows three differentiated sections. A surface section where new materials with lower concentrations have been found, probably due to the underground currents that connect the lagoon and the nearby Napo River; a central section where CF-CS model and mass accumulation rate calculations provide a sediment accumulation rate of 0.56 ± 0.03 cm y⁻¹; finally, a deeper section with a constant ²¹⁰Pb_exces profile, showing sediment reworking probably due to local flooding’s.

1. Introduction

Lake sediments provide an objective record of sedimentation history, and the study of heavy metals and trace elements in vertical sediment cores is an important method of tracing the pollution history of the study area. Several authors have used this study approach, some of them incorporating both the evolution of trace elements along the core [1,2,3,4,5,6,7,8,9,10], as well as the combined analysis of the elements using pollution indicators throughout the in-depth core, allowing the determination of historical evolutions of potential impacts [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25].

Pollution indices are widely considered to estimate the contamination level and the potential ecological risk of the potentially toxic elements in surface and core sediments, being a useful tool in monitoring and management of environmental programs [26,27,28]. Single indices, which refer to one single pollutant, are based on the concentration of the reference element or/and the background or baseline concentration. Integrated pollution indices, which refer to a group of pollutants, are based on background or baseline values, based on the reference element in combination with the background concentration, based on the toxic response, and based on the sediment quality guidelines. Approaches based on the application of a combination of single and integrated indices have been shown to be effective to assess the area under study [28,29,30].

Many sediment pollution indices require the assessment of a geochemical background or geochemical baseline and/or the selection of a conservative reference element. Several methods to determine the geochemical background values of elements in sediments have been used. In particular, direct methods use mean or median concentrations measured in sediments collected in pristine areas at a certain distance from any source of anthropogenic contamination [31], or concentrations in deep core sediment samples. In the first case, it is necessary that the entire study area have very similar lithological characteristics to avoid erroneous conclusions. In the second case, many authors have used the low stable values of element concentrations obtained in the lower, uncontaminated section of the depth sediment core as geochemical background concentrations [7,10,21,22,23,24,28,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46].

The use of sediment cores allows for establishing a “local baseline concentration” or “regional background concentration” considering the average of the values of lower concentration selected from the deepest layers, less affected by anthropic activities (industrial, agricultural and urban). However, authors such as Hernandez-Crespo and Martín [13] propose integrated methods based on direct geochemical analysis and statistical analysis, to determine the geochemical background of the sediments.

In addition to the use of sediment cores to determine the depth profile evolution of the studied trace elements and the geochemical background determination, the sediment cores allow estimation of sedimentation and dating rates through the analysis of radionuclides present in the different layers of sediment. The study of the isotopic variations with depth allows the historical record of the activities of the studied area.

Since the 1970s, ²¹⁰Pb concentrations have been used extensively to date sediment deposits in a wide variety of natural environments, such as lakes and wetlands, being able to obtain sedimentation rates to the last 100–150 years, a period in which could have produced the greatest anthropic influences [6,20,38,47,48,49,50,51,52,53,54].

²¹⁰Pb can be found in sediment samples both by its atmospheric deposition and by the disintegration of ²²⁶Ra present in the sedimentary material. This last fraction will be in radioactive equilibrium with ²²⁶Ra and will decay with its half-life (1600 years). This fraction is known as ²¹⁰Pb in equilibrium or supported (²¹⁰Pb_supported) while the deposited fraction is known as ²¹⁰Pb in excess (²¹⁰Pb_excess) or not supported by ²²⁶Ra. From the measured concentration of the depositional fraction (²¹⁰Pb_excess) in the different layers of a sediment core, it is possible to estimate the sedimentation rate and date the different sediment layers. The concentration of ²¹⁰Pb in equilibrium (²¹⁰Pb_supported), can be known through ²²⁶Ra and the concentration of total ²¹⁰Pb can be measured directly so that its difference provides the ²¹⁰Pb in excess, that is, ²¹⁰Pb_excess = ²¹⁰Pb_total − ²²⁶Ra. From the ²¹⁰Pb_excess concentrations profile with depth and under fulfilling certain hypotheses, different models have been developed which allow estimating a chronology for the sediment. Models such as the CF-CS (Constant Flux and Constant Sedimentation Rate), CF-CIC (Constant Flux and Constant Initial Concentration), and the CRS (Constant Rate of Supply) are widely used in the scientific literature for sediment dating [6,22,23,39,46,52,55,56,57,58,59,60,61,62,63,64]

The Limoncocha lagoon is the largest water body in the Limoncocha Biological Reserve identified as a Ramsar site since 1998 [65] The reserve has abundant biodiversity and is a source of natural human food base. Two small rivers and the runoff of the area reaches the lagoon. This area is influenced by oil well drilling and exploitation activities with an increasing population and human activities such as ecotourism, fisheries, and agriculture around the body of water [66,67]. The lagoon in particular is the main tourist attraction in the reserve area, and facilities are being developed for ecotourism as a driver of change in the Amazonian ecosystem [68]. In addition, storm rainfall events which lead to the rising water level of the nearby Napo River causing flood events on the lagoon, would be a long-term secondary source of sediment pollution. The monitoring of contaminants and the environmental risk assessment are essential for the environmental protection of the Limoncocha lagoon.

Recent studies conducted by Carrillo et al. [69,70] have shown the potentially toxic elements in the soils and surface sediments and estimated baseline values based on the statistical methods in the Limoncocha Biological Reserve. However, none of them deals with the study of sediment cores.

In order to contribute to improving the geochemical knowledge of the Limoncocha lagoon and to provide more comprehensive information about the potentially toxic trace elements, this study investigated the vertical distribution of trace elements As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, V, Zn and major elements Al, Fe, Mn, P and Si in a core sediment of Limoncocha lagoon, assessing the pollution degree and ecological risk using three single pollution indices, contamination factor (Cf), geo-accumulation index (Igeo), enrichment factor (EF) and four integrated indices as the modified degree of contamination (mCd), mean enrichment quotient (MEQ), potential ecological risk index (RI) and toxic-risk index (TRI). Another major objective was to determine the natural radionuclides ²¹⁰Pb, ²²⁶Ra, ²³²Th, and ⁴⁰K concentration profiles and estimate the sedimentation rate and deposition time of sediment by measuring the ²¹⁰Pb_excess in the core. The scarcity of geochronological data on sedimentary profiles in the lake and its characteristic ability to act as a potential sink of multiple contributions of potentially toxic elements motivated the development of this study.

2. Materials and Methods

2.1. Study Area: The Limoncocha Lagoon

The Limoncocha Biological Reserve (RBL), is a 4613.3 ha Ramsar site located on the northeastern side of the Ecuadorian Amazon, in the southwestern region of the province of Sucumbíos at a height of 220 m above sea level (Figure 1). The reserve is characterized by its abundant biodiversity with several ecosystems such as permanent wetlands, floodable forests, dry forests, and aquatic habitats. The Biological Reserve contains eight different plant formations, holding 144 bird species, 74 mammal species, 53 amphibian species, 39 reptile species, and 93 fish species. In 2006, four species of mammals and one species of bird were listed in the “red books” published by the World Union for Conservation of Nature in Ecuador [71,72]. The black caiman (Melanosuchus niger), the largest predator in the Neotropics, with individuals exceeding five meters, is considered the emblematic species of the Limoncocha lagoon [73].

The Limoncocha lagoon (2.3 km²) is the main body of water in the RBL. The Pishira and Playayacu small rivers flow into the Limoncocha lagoon, which is connected to the small, inaccessible body of water called Yanacocha. The lagoon is currently a “dead arm” of the nearby Napo River located south of the lagoon.

The Limoncocha village, which is located at the eastern end of the lagoon and had 6817 inhabitants in 2019, depends mainly on biodiversity for survival. Of the eight ancestral ethnic groups that make up the indigenous peoples of the Ecuadorian Amazon, the Kichwa population is the predominant one in the Limoncocha area [74]. A central production facility (CPF) of the Petroamazonas oil company is located to the northwest of the lagoon between the mouths of the Playayacu and Pishira rivers (Figure 1).

2.2. Sampling and Analytical Methods

Sediment core (0–50 cm) was collected in the subtidal central area (3.5 m depth) of the Limoncocha lagoon (0°23′58.5″ S; 76°36′45.4″ W) in September 2018 using a PVC tube of 50 cm long and 45 cm diameter. The core was sampled down by means of a percussion drill with various steel extensions. After driving the core tube into the sediment, it was extracted manually. On return to the laboratory, the core was opened, described, and photographed before being cut into 1-cm increments for analysis. Samples were placed into polyethylene bags previously washed with 1:1 HNO₃ and rinsed with distilled water, sealed, transported to the laboratory, and stored at 4 ºC until further analysis.

The density of the samples was determined on twenty-three composite 1-cm-thick samples by the pycnometer method [75]. Six selected sediment samples from 1 cm-thick intervals were used to determine major and trace elements. The analysis of the major elements Al, Fe, Mn, and Ti, was carried out at Activation Laboratories Ltd., in Canada using the Major Elements Fusion ICP package (Code WRA/ICPOES-4B). The rest of the metals (Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, V, and Zn) and metalloids (As) concentrations were determined by ICP-MS (Thermo iCAP-Q) in our laboratories. Metals were extracted with a microwave acid digestion system (CEM model Mars 5) according to the SW-846 EPA Method 3051 A. Complete procedural details are described in previous works of the authors [69,70].

Activity concentrations of radionuclides have been determined at the University of Cantabria (Spain) by gamma spectrometry, using a low-background high-purity HPGe detector. Thirteen composite sediment samples from 4 cm or 5 cm-thick intervals, in order to obtain a minimum mass of 50 g-dw in all the samples were homogenized, hermetically sealed, and stored for at least 25 days to ensure secular equilibrium between ²²⁶Ra, ²²²Rn and the short-lived daughter radionuclides of the latter. The analytical procedures used have been discussed in former contributions by some of the authors [48,49,76]

2.3. Contamination Assessment Methodology

A large number of contamination indices can be applied to assess the quality of sediments. However, the results of the evaluation have shown to be highly dependent on the reference values used [77,78]. Therefore, the use of a strategy found on the application of a combination of indices based on different and site-specific criteria has been widely used to obtain a more precise evaluation of the studied area [28,29,30].

Among the numerous indices available for ranking sediment samples for contamination by potentially toxic elements, a combination of individual and integrated risk assessment indices was used in this study to estimate the pollution degree. The same criteria have been followed in choosing the individual and integrated indices: indices that depend only on the reference values and indices that also consider a reference element. The contamination factor (Cf_i,j), the geoaccumulation index (Igeo_i,j), and the enrichment factor (Ef_i,j) are used as individual indices to calculate the degree of contamination of each potentially toxic element (i) in a particular site (j). The modified degree of pollution (mCd_j), the mean enrichment quotient (MEQ_j), and the potential ecological risk index (RI_j) are used as integrated indices to determine the degree of pollution of the set of pollutants in a given site (j). In addition, the assessment of sediment quality for potentially toxic elements can be determined by indices based on freshwater sediment quality guidelines (SQGs). In this sense, the toxic risk index, TRI_j, as an integral index is considered.

Table 1. Sediment contamination assessment index. Definition and classification criteria considered in this study.

Indexes	Equations	Criteria	Classification
Contamination factor, ${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ [79]	${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}=\frac{{\mathrm{C}}_{\mathrm{i},\mathrm{j}}}{{\mathrm{Cb}}_{\mathrm{i}}}$ ${\mathrm{C}}_{\mathrm{i},\mathrm{j}}$$\mathrm{is}\mathrm{the}\mathrm{concentration}\mathrm{of}\mathrm{element}\mathrm{i}\mathrm{in}\mathrm{sample}\mathrm{j}\mathrm{and}{\mathrm{Cb}}_{\mathrm{i}}$ contaminant baseline concentration i	${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ < 1	Low contamination
		1 < ${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ < 3	Moderate contamination
		3 < ${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ < 6	Considerable contamination
		6 < ${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ < 12	Very high contamination
		${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ > 12	Extreme contamination
Geoaccumul-ation index, ${\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}$ [80]	${\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}={\log }_2\frac{{\mathrm{C}}_{\mathrm{i},\mathrm{j}}}{1.5{\mathrm{Cb}}_{\mathrm{i}}}$ ${\mathrm{C}}_{\mathrm{i},\mathrm{j}}$$\mathrm{concentration}\mathrm{of}\mathrm{element}\mathrm{i}\mathrm{in}\mathrm{sample}\mathrm{j},{\mathrm{Cb}}_{\mathrm{i}}$ contaminant baseline concentration i and 1.5 is the correction factor of the bottom matrix due to lithogenic effects	${\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}\le 0$	Class 0, uncontaminated
		$0<{\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}<1$	Class 1, uncontaminated to moderately contaminated
		$1>{\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}>2$	Class 2, moderately polluted
		$2<{\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}<3$	Class 3, moderately polluting to heavily polluted
		$3>{\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}>4$	Class 4, heavily polluted
		$4<{\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}<5$	Class 5, heavily polluted to extremely polluted
		${\mathrm{Igeo}}_{\mathrm{i},\mathrm{j}}>5$	Class 6, extremely polluted
Enrichment factor, ${\mathrm{EF}}_{\mathrm{i},\mathrm{j}}$ [81]	${\mathrm{EF}}_{\mathrm{i},\mathrm{j}}=\frac{\raisebox{1ex}{${\mathrm{C}}_{\mathrm{i},\mathrm{j}}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{C}}_{\mathrm{R},\mathrm{j}}$}\right.}{\raisebox{1ex}{${\mathrm{Cb}}_{\mathrm{i}}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{Cb}}_{\mathrm{R}}$}\right.}$ ${\mathrm{C}}_{\mathrm{i},\mathrm{j}}$ $\mathrm{concentration}\mathrm{of}\mathrm{element}\mathrm{i}\mathrm{in}\mathrm{sample}\mathrm{j},{\mathrm{C}}_{\mathrm{R},\mathrm{j}}$concentration of the reference element (R) in sample j, ${\mathrm{Cb}}_{\mathrm{i}}$ $\mathrm{baseline}\mathrm{concentration}\mathrm{i},{\mathrm{Cb}}_{\mathrm{R}}$ baseline concentration of reference element	${\mathrm{EF}}_{\mathrm{i},\mathrm{j}}$ < 1	not enriched
		1 < ${\mathrm{EF}}_{\mathrm{i},\mathrm{j}}$ <3	slightly enriched
		3 < ${\mathrm{EF}}_{\mathrm{i},\mathrm{j}}$ <5	moderately enriched
		$5<{\mathrm{EF}}_{\mathrm{i},\mathrm{j}}<10$	Moderately to severely enriched
		$10<{\mathrm{EF}}_{\mathrm{i},\mathrm{j}}<25$	severely enriched
		$25<{\mathrm{EF}}_{\mathrm{i},\mathrm{j}}<50$	very severely spiked
		${\mathrm{EF}}_{\mathrm{i},\mathrm{j}}<50$	extremely enriched
Modified pollution degree, ${\mathrm{mCd}}_{\mathrm{j}}$ [34]	${\mathrm{mCd}}_{\mathrm{j}}=\frac{{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}}{\mathrm{n}}$ ${\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ is the contamination factor i at site j and n is the number of contaminants studied	${\mathrm{mCd}}_{\mathrm{j}}<1.5$	Without pollution
		$1.5\le {\mathrm{mCd}}_{\mathrm{j}}<2$	Low pollution
		$2\le {\mathrm{mCd}}_{\mathrm{j}}<4$	Moderate pollution
		$4\le {\mathrm{mCd}}_{\mathrm{j}}<8$	High pollution
		$8\le {\mathrm{mCd}}_{\mathrm{j}}<16$	Very high pollution
		$16\le {\mathrm{mCd}}_{\mathrm{j}}<32$	Extremely high pollution
		${\mathrm{mCd}}_{\mathrm{j}}\ge 32$	Ultra-high pollution
Average enrichment ratio, ${\mathrm{MEQ}}_{\mathrm{j}}$ [82,83]	${\mathrm{MEQ}}_{\mathrm{j}}=\frac{{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{EF}}_{\mathrm{i},\mathrm{j}}}{\mathrm{n}}$ ${\mathrm{EF}}_{\mathrm{i},\mathrm{j}}$ is the enrichment factor of element i at site j and n the number of contaminants.	${\mathrm{MEQ}}_{\mathrm{j}}<1.5$	Category 1 (No enrichment)
		$1.5<{\mathrm{MEQ}}_{\mathrm{j}}<3$	Category 2 (Minor Enrichment)
		$3<{\mathrm{MEQ}}_{\mathrm{j}}<5$	Category 3 (Moderate Enrichment)
		$5<{\mathrm{MEQ}}_{\mathrm{j}}<10$	Category 4 (Extensive Enrichment)
		${\mathrm{MEQ}}_{\mathrm{j}}>10$	Category 5 (Severe Enrichment)
Potential ecological risk index, ${\mathrm{RI}}_{\mathrm{j}}$ [79]	${\mathrm{RI}}_{\mathrm{j}}={\displaystyle {\displaystyle \sum }_{\mathrm{i}=1}^{\mathrm{n}}}{\mathrm{Er}}_{\mathrm{i},\mathrm{j}}$ ${\mathrm{Er}}_{\mathrm{i},\mathrm{j}}={\mathrm{Tr}}_{\mathrm{i}}·{\mathrm{Cf}}_{\mathrm{i},\mathrm{j}}$ ${\mathrm{Er}}_{\mathrm{i},\mathrm{j}}$ is the ecological risk factor of element i at site j, n number of elements and ${\mathrm{Tr}}_{\mathrm{i}}$ the toxic response factor for element i. Tri has a value of 30 for Cd, 10 for As, 5 for Cu, Ni, and Pb, 2 for Cr and 1 for Zn.	${\mathrm{RI}}_{\mathrm{j}}$<150 ${\mathrm{Er}}_{\mathrm{i},\mathrm{j}}$<40	Low potential risk
		150<${\mathrm{RI}}_{\mathrm{j}}$<300 40<${\mathrm{Er}}_{\mathrm{i},\mathrm{j}}$<80	Moderate potential risk
		300<${\mathrm{RI}}_{\mathrm{j}}$<600 80<${\mathrm{Er}}_{\mathrm{i},\mathrm{j}}$<160	Considerable potential risk
		${\mathrm{RI}}_{\mathrm{j}}$>600 160<${\mathrm{Er}}_{\mathrm{i},\mathrm{j}}$<320	High potential risk
		${\mathrm{Er}}_{\mathrm{i},\mathrm{j}}$>320	Very high potential risk
Toxic risk index (${\mathrm{TRI}}_{\mathrm{j}}$). [84,85]	${\mathrm{TRI}}_{\mathrm{j}}={\displaystyle {\displaystyle \sum }_{\mathrm{i}=1}^{\mathrm{n}}}{\mathrm{TRI}}_{\mathrm{i},\mathrm{j}}=$ $={\displaystyle {\displaystyle \sum }_{1=1}^n}\sqrt{\frac{{\left(\frac{{\mathrm{C}}_{\mathrm{i},\mathrm{j}}}{{\mathrm{TEL}}_{\mathrm{i}}}\right)}^2+{\left(\frac{{\mathrm{C}}_{\mathrm{i},\mathrm{j}}}{{\mathrm{PEL}}_{\mathrm{i}}}\right)}^2}{2}}$ ${\mathrm{C}}_{\mathrm{i},\mathrm{j}}$ is the concentration of elements i at site j, ${\mathrm{PEL}}_{\mathrm{i}}$ is the likely effect level of item i, ${\mathrm{TEL}}_{\mathrm{i}}$ is the threshold effect level of element i and${\mathrm{TRI}}_{\mathrm{i},\mathrm{j}}$ is the toxic risk index of element i at site j.	${\mathrm{TRI}}_{\mathrm{j}}<5$	No toxic risk
		$5\le {\mathrm{TRI}}_{\mathrm{j}}<10$	Low toxic risk
		$10\le {\mathrm{TRI}}_{\mathrm{j}}<15$	Moderate toxic risk
		$15\le {\mathrm{TRI}}_{\mathrm{j}}<20$	Considerable toxic risk
		${\mathrm{TRI}}_{\mathrm{j}}\ge 20$	Very high toxic risk

shows a detailed description of the indices considered in this study to estimate the level of contamination and the potential ecological risk of potentially toxic elements in the sediments of the Limoncocha lagoon.

3. Results and Discussion

3.1. Density Profile

The geological formations present in the surface sediments around the Limoncocha lagoon are mainly shale and, to a lesser degree, sandy shale. As surface soil deposits, there are fluvial sandstones, especially in the southern part of the lagoon (corresponding to old flood areas associated with the Napo River) [86,87].

The ⁴⁰K and ²³²Th are mostly found in clays and shales, while shale-free sandstones and carbonates (of lower density) have low concentrations of radioactive material. Therefore, in the core under study, the last layer deposited, the most recent, may have incorporated soil material rich in sandstone with lower concentrations of radioactive isotopes due to increased runoff as a consequence of the increased deforestation and human activity (urbanization, agricultural land, oil exploitation) as well as by the contribution of fluvial sandstones from the Napo River during flooding events (Figure 2a,b). The evolution of the densities (Figure 2c) shows an increase in density at greater depths, starting at 38 cm, which remains at high values (2.1 ± 0.01 g cm⁻³) until the deepest cm, compared to the first 10 cm with values of (2.0 ± 0.03 g cm⁻³).

3.2. Trace Elements Concentration Profiles

Figure 3 shows the concentration profiles of studied minor elements (As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, V, Zn) and major elements (Al, Fe, Mn, P, and Si) of interest. Six samples of layers (0–1 cm, 10–12 cm, 20–21 cm, 30–31 cm, 40–41 cm, 47–50 cm) of different size, from 1 to 3 cm in length depending on the availability of material, have been analyzed.

The first analysis of Figure 3, shows a generalized behavior in which the concentrations decrease between −10 cm and the core surface. Figure 4 shows the concentration profiles of each element studied together with the evolution of the ²¹⁰Pb_excess value, allowing establish potential correlations between elements’ concentrations and chronology of human actions and natural events. The dashed lines envelop the values with differences of ±10% related to the 50 cm depth value, which include the lower values obtained at the bottom of the core. Two different trends can be considered in the evolution of the studied elements.

The majority of studied elements (Figure 4-Type A) show their highest values in the depth range of −10 cm and −30 cm, with sharp declines at −40 cm of Al, As, Fe, Mn, and Mo but moderate decreases of Ba, Cd, Cr, Ni, Pb and Zn at the same depth. From this depth (−40 cm) the concentrations remain practically constant at low values with a similar behavior to the observed ²¹⁰Pb_excess concentration profile. Although Ni shows this trend, the entire profile concentrations differ only by ±23.3%. In the case of Co, Cu, P, Si, and V, no defined trend is observed in the concentration profiles with maximum variation in concentrations throughout the entire profile between ±12% for Cu and ±22.5% for V (Figure 4-Type B).

In the upper scale of Figure 4, the concentration values of the surface sediments analyzed in previous works [69,70] have been included, highlighting in red the one corresponding to the surface sample called “Centre of the Lagoon” (SE2). Some of the surface sediments analyzed exhibit element concentration values lower than the minimum values reached at a core depth of 50 cm as it is summarized in

Table 2. Minimum concentrations of minor and major analyzed elements in the sediment core of the center of the lagoon in comparison to surface sediment concentrations.

Element	Minimum Concentration (mg kg−1)		Sampling Point of Surface Sediment
	Core in the Centre of the Lagoon	Surface Sediment Samples [69,70]
As	3.12	1.92	S3-Mouth of the Playayacu river
Ba	263	188	S5-Caño
Cd	0.253	0.118	S5-Caño
Co	11.3	10.7	S5-Caño
Cr	15.5	11.4	S5-Caño
Cu	70.8	50.6	S3-Mouth of the Playayacu river
Mo	2.5	1.65	S3-Mouth of the Playayacu river
Ni	16.0	13.2	S5-Caño
Pb	8.7	4.62	S3-Mouth of the Playayacu river
V	71.2	61.8	S5-Caño
Zn	51.6	48.4	S5-Caño
Al	71887	89600	S4-Mouth of the Pishira river
Fe	28990	34800	S5-Caño
Mn	532	643	S7-Pishira River
P	1221	961	S6-Playayacu River

. This fact indicates that, in this case, with data obtained from a single core, the concentrations of trace elements in the deeper samples cannot be considered as values to establish geochemical background values.

3.3. Pollution Indices Profiles

In order to determine the level of contamination and evaluate the ecological risk of the sediment core, both individual and integrated pollution indices are applied to each of the sections analyzed in the core.

3.3.1. Selection of Geochemical Baseline and Reference Element

To determine the pollution indices, it is necessary to establish both a geochemical baseline and a reference element. On the one hand, Carrillo et al. [70] established the geochemical baseline of As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, V, and Zn in the surface sediments of the Limoncocha lagoon, using four different statistical methods as relative cumulative frequency method, iterative 2σ-technique, 4σ-outlier-technique and normalization method to a “conservative” element. This work concludes that the relative cumulative frequency method leads to the best performance of the geochemical baselines. Given the impossibility of using the deep layers concentrations of the studied sediment core as the geochemical baseline, the values in mg kg⁻¹ of As: 3.09; Ba: 311; Cd: 0.352; Co: 18.8; Cr: 36.0; Cu: 80.4; Mo: 2.36; Ni: 21.6; Pb: 13.4; V: 129 and Zn: 102 provided by Carrillo et al. [70], will be considered as the geochemical baseline for the determination of the pollution indices in the present work.

On the other hand, the previous work of Carrillo et al. [69] considers Al as the conservative element with a value of 99,078 mg kg⁻¹ that serves to normalize the concentration of the studied elements in the core. The selection was based on the low values (6.75%) of the robust nonparametric estimator CV*(%) = (MAD/Median) × 100, where MAD = Median [|Ci-median (Ci)|] with Ci the concentration of element i obtained from surface sediments samples in the Limoncocha lagoon.

3.3.2. Contamination and Potential Risk Assessment Based on Pollution Indices

Figure 5 shows the profiles of the individual indices, Cf_i,j, Igeo_i,j and Ef_i,j, for all the studied elements. A bar graph is shown with the values of the individual indices obtained at the different depths of 1 cm, 10 cm, 20 cm, 30 cm, 40 cm, and 50 cm. Obtained values are compared with each index criterion (Table 1) with the easy capturing of the pollution level through a color code. Figure S1 of the supplementary information shows the numerical values of each index for all potential contaminants.

A general trend of Cf_i,j values is observed (Figure 5 and Figure S1), with the highest values between −10 cm and −30 cm, and a slight decrease at the surficial layer. Cf_i,j values are lower than 2 for all elements and depths, showing moderate contamination (1< Cf_i,j <2) due to As and Mo at all depths. Cu up to −30 cm; Ba, Cd and Pb at −10 cm and −20 cm and Ni at −10 cm exceed the value of Cf_i,j = 1, providing a classification of “moderate contamination” at these depths. The Igeo_i,j index only classifies as pollution class 1, “between polluted and moderately polluted” the depths −10, −20, and −30 cm due to As, −20 cm due to Cd and −10 cm due to Mo. The remaining potential contaminants are classified in class 0 as “uncontaminated”. Obtained Ef_i,j values are lower than 2 for all elements and depths, except for As at −10 cm and −30 cm with Ef_i,j = 2.04 and Ef_i,j = 2.10 respectively, showing “low enrichment”. As, Cu and Mo at all depths, Ba at −10 to −50 cm depth, Ni at −10, −20, −40, and −50 cm depth, Cd at −10, −20 and −40 cm depth and Pb at −20 and −40 cm depth show Ef_i,j > 1 (“less enrichment”). In addition, it is observed that Pb and Cd exhibit irregular behaviour in relation to the Ef_i,j index.

The individual indices show As and Mo and to a lesser extent Cu, Ba, Cd, Ni, and Pb, as elements with concentrations greater than the reference values, classifying some depths with a certain degree of contamination and enrichment with respect to the geochemical baseline and reference element; however, in all cases, only the lowest values indicative of certain contamination and enrichment is reached. Such contamination is multi-element due to several trace elements that probably come from potential wastewater discharges from oil exploration and exploitation activities (Ba, Ni, Mo) [2,15,54,88,89,90,91,92] and domestic, agricultural, and industrial activities as aquaculture, all of them potential sources of As, Cu, Cd and Pb [93,94,95,96,97]. Additionally, there are keep in mind that this sampling point, the center of the Limoncocha lagoon, collects all the runoff from the area which is influenced by human activity.

Since the Limoncocha lagoon is an important source of food for the community, its health and wellness may be compromised by toxic substances in the lagoon’s sediments. Additionally, sediment pollution will have a negative effect on the environmental conservation and the preservation of the area as a tourist center, generating elements associated with the community identity and its link with the Kichwa culture. The meanings and relationships established with the biological reserve in general and with the lagoon in particular, are being strengthened through community tourism activities as well as conservation and care of the environment, especially among children and young people. Mestanza et al. [98], found that among the community there are different appreciation regarding the importance of the lagoon: while some value the reserve as a source of food, others value it as a source of work through tourism and recreation. In both cases, the incidence of toxic substances in sediments will jeopardize the sustainability of the area and, in any case, will harm the native inhabitants and visitors of Limoncocha.

As a complement to the individual indices, the following integrated pollution indices have been applied to the sediment core: (i) Modified contamination degree, mCd_j, based on background values since it integrates Cf_i,j; (ii) Mean Enrichment Quotient, MEQ_j, based on Ef_i,j which considers a reference element; (iii) Potential ecological risk index, RI_j, which takes into account the toxic response of each element studied, and (iv) Toxic risk index, TRI_j based on sediment quality guidelines (SQGs). Figure 6 shows the vertical distribution and the values obtained for each of the integrated indices at the different depths of the sediment core. The four applied indices indicate “zero or very low degree of contamination” considering the limit values of each indices (Table 1). In all cases, mCd_j < 1.5 so it is classified as “Without pollution”, MEQ_j < 1.5 corresponds to category 1 (“No enrichment”), RI_j < 150 corresponding to “no toxic risk” and TRI_j < 5 classified as “no toxic risk”. The indices’ s depth profiles are similar, showing the highest values at depths between −20 cm and −30 cm and minimum values at depths of −40 cm.

3.4. Radionuclides and Sedimentation Rate in the Sediment Core

The activity concentration in sediment core slice samples is shown in

Table 3. Activity concentration in Bq kg⁻¹ in sediment core slices with uncertainties given as 1σ and global average and ranges of natural background radiation concentration in soils.

Depth cm	210PbTotal	226Ra	210Pbexcess	40K	232Th
			Bq kg−1
1	49.8 ± 10.4	25.1 ± 4.1	24.7 ± 10.4	293.9 ± 23.8	20.9 ± 1.8
5	61.9 ± 13.5	31.3 ± 3.6	30.5 ± 13.5	445.1 ± 35.9	26.0 ± 2.2
9	69.3 ± 9.9	32.7 ± 3.6	36.5 ± 9.9	525.0 ± 42.5	30.2 ± 2.7
13	69.0 ± 15.1	35.4 ± 4.4	33.6 ± 15.1	442.4 ± 35.7	29.4 ± 2.5
17	59.8 ±12.6	32.3 ± 3.8	27.6 ± 12.6	402.7 ± 32.4	25.7 ± 2.2
21	55.8 ± 12.9	34.6 ± 4.0	21.2 ± 12.9	426.8 ± 34.4	25.0 ± 2.2
25	45.6 ± 9.8	33.3 ± 3.9	12.3 ± 9.8	395.7 ± 31.9	22.9 ± 2.0
33	44.5 ± 9.6	30.8 ± 3.6	13.7 ± 9.6	408.8 ± 32.9	20.9 ± 1.8
37	42.7 ± 9.6	33.8 ± 3.8	8.9 ± 9.6	403.8 ± 32.5	25.1 ± 2.1
39	41.0 ± 10.1	34.8 ± 3.9	6.3 ± 9.6	398.6 ± 32.1	22.1 ± 1.9
41	47.3 ± 10.3	36.9 ± 4.0	10.4 ± 9.	399.9 ± 32.2	25.0 ± 2.1
42	41.9 ± 10.9	32.6 ± 3.7	9.4 ± 10.3	407.7 ± 32.8	21.5 ± 1.9
44	---	32.0 ± 3.6	---	386.9 ± 31.2	20.2 ± 1.8
46	38.4 ± 8.6	26.8 ± 3.3	11.6 ± 8.6	397.3 ± 32.0	19.7 ± 1.7
50	42.0 ± 9.4	32.5 ± 3.4	9.6 ± 9.4	379.1 ± 30.5	22.9 ± 2.0
Range this work	38.4–69	25–37	---	294–525	20–30
Average this work	47.8	32.3	---	407.6	23.8
World average [99,100]	---	32 (17–60)	---	420 (140–850)	45 (11–64)
World soil [101]	---	30 (7–180)	---	440 (0.2–1200)	37 (4–78)
UCC [102]	---	33	---	720	43

. The activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K are within the ranges given by UNSCEAR [99,100], World Soil [101], and UCC [102].

The results of ²¹⁰Pb_total, ²²⁶Ra, and ²¹⁰Pb_excess concentrations (Figure 7) show slight variations of ²²⁶Ra (32.2 ± 3 Bq kg⁻¹) and that secular equilibrium has not been reached at the studied depth. The ²¹⁰Pb_excess profile does not show an exponential decrease with depth in the 50 cm sampled that would be expected in a simple radioactive decay. The ²¹⁰Pb_excess profile obtained in the present work can be subdivided into three sections: one up to 9 cm in which the concentration decreases towards the surface, another central section, in which the concentration decreases with depth following an exponential curve, and a third section in which the concentration remains practically constant.

In the superficial section of the core (0–9 cm), there is an increase in ²¹⁰Pb_excess concentration to −9 cm. This corresponds to the incorporation of new material rich in sandstone with lower concentrations of radioactive isotopes, due to either punctual or periodic floods with different materials. It might think that from that depth to the surface, a material with lower concentrations of all analyzed elements has been dragged. ⁴⁰K is present in terrestrial material in a fixed ratio of 0.0115% of total K depending on the type of material.

Therefore, the first 9 cm can be made of a different material than the rest of the core. Most of the elements have the same evolution as the radionuclides with a decrease in concentration from −10 cm to the surface of As, Ba, Co, Cr, Cu, Mo, Ni, Pb, V and a decrease from −20 cm of Cd and Zn (Figure 4). There may have been a continuous phenomenon that has taken place cyclically with other material (the profile is descending and not flat), caused probably by the groundwater connections between the Napo River and the Limoncocha lagoon. After relevant flooding events, every approximately 2 years, the waters of the lagoon rise about three meters, but without flooding the lagoon as in previous periods, recovering the level of the water to its normal state. Several reasons can be put forward for that behavior. On the one hand, the oil infrastructures, mainly highways, have been able to act as screens against possible avenues of the river. On the other hand, the water of the Napo River has decreased in flow, and there have been sandbanks making the river in certain places no longer navigable. As a consequence, when the river flow rises, it drags the sandbanks that accumulate on the banks, acting as containment barriers for the overflow of the river water. Finally, the Lagoon has underground currents that keep the river and Lagoon connected, so that in recent decades, the lagoon’s water mirror rises to 3 m above normal, without there being a visible direct connection between the two bodies of water. Jarrin et al. [86] show that in the entire southern part of the lagoon, up to the left bank of the Napo River, the groundwater is less than 5 m deep and may constitute the connection between the river and the lagoon in times of great rains and avenues. These cyclical flooding events, together with bioturbation processes, may have caused remobilization and resuspension of sediments, providing the radionuclides and elements profiles obtained. Several authors such as [103,104,105,106,107,108] among others, have discarded the data of the most superficial layers of the cores to obtain the graphical solution of the sediment mass flow. The main arguments put forward by those researchers to justify the presentation and treatment of their data included the invalidity of steady-state accumulation caused by bioturbation, turbulent flow, and sediment core sampling,

In the deeper section (39 cm−50 cm), the ²¹⁰Pb_excess is constant and everything measured for radionuclides and elements is constant, but not zero. The constant ²¹⁰Pb_excess would indicate a process of mixing and/or resuspension of the same material so that in those centimeters it is always the same. This final section seems mixed and the balance between ²²⁶Ra and ²¹⁰Pb is not reached. Authors such as Mabit et al. [58], San Miguel et al. [109], and Harb [110] indicate similar situations, either because the core is too short to reach the equilibrium depth (depth at which the total ²¹⁰Pb is in equilibrium with the supported ²²⁶Ra) or because the analytical uncertainties are very large. Although there are only two element concentration data in this core section (−39 cm and −50 cm), a slight increase in the elements Cr, Ni, Zn, Co, P and V is observed at −50 cm, as well as the conservative elements Al and Fe which may indicate an entry of terrigenous material; additionally, these last layers present a higher density with respect to the more superficial layers.

Hydrological and climatological local events of natural origin as flooding may be related to the observed remobilization which leads flat ²¹⁰Pb_excess profiles (see [111,112]). Since there are no written records of processes related to hydrological changes in the lagoon, interviews were conducted with people from the Limoncocha Community. Apart to the episodes of El Niño in Ecuador [113,114] verbal contacts with the inhabitants of the area confirm the cyclic flooding of the lagoon in ancient times.

Considering the intermediate section [9–39 cm] of the studied core, the ²¹⁰Pb_excess concentrations decrease at a constant rate (See Figure 8a). The application of the constant flux/constant sedimentation rate (CF-CS) model provides a good fitting (R² = 0.95) of the ²¹⁰Pb_excess exponential decrease with depth and a sedimentation accumulation rate (SAR) of 0.56 ± 0.03 cm y⁻¹ (Figure 8b,c). In this section, trace elements reach the highest values along the core (Figure 4) with a minor enrichment of As, Mo, Cu, Ba, Cd, Ni and Pb respect to the geochemical baseline (Figure 5).

In order to validate the sedimentation rate obtained and reduce the uncertainty introduced when applying the CF-CS model to a single section of the core, the evolution of the mass accumulation with depth has been verified, obtaining a linear profile throughout the entire core (Figure 9a). The profile of ln (²¹⁰Pb_excess) respect to the accumulated mass (g cm^–2) shows a good linear fit (Figure 9b) allowing to obtain a value of accumulated mass sediment rate (MAR) of 1.17 ± 0.04 g cm⁻² y⁻¹ in the intermediate section (9–39 cm) of the core. Assuming this MAR value constant in the initial and final sections of the core, that is, throughout the entire core, their application to the three differentiated sections allows to estimate the ages of the layers and the sediment rate in each of them, as a function of the mass depth reached in each layer, as shown in Figure 9c. The sediment accumulation rates (SAR) obtained from the accumulated mass sediment rate (MAR) match the sedimentation rate obtained by the CF-CS model in the intermediate section of the core.

As a summary of the three studied sections, Figure 10 shows the most plausible dating with the possible events that occurred in the sections under study. Values of the individual and integrated pollution indices compared with the indicative limits of contamination in each case are included in Figure 10. As has been mentioned, the uncertainty in the radionuclide data, as well as the mixed compositions of the studied samples, makes the uncertainty dating high. However, the values obtained show referenced and observed phenomena in the area that allow to estimate the events that occurred in the Limoncocha lagoon.

4. Conclusions

Trace elements concentration profiles in the core of the center of the Limoncocha lagoon allows us to differentiate two trends. On the one hand, Al, As, Ba, Cd, Cr, Fe, Mn, Mo, Ni, Pb and Zn with minimum concentrations at −40 cm which remain constant down to −50 cm. On the other hand, Co, Cu, P, Si and V with a non-defined concentration trend with depth and concentration variation between ±12% and ±22.5% throughout the profile. However, in all of the studied elements, concentrations decrease between −10 cm and the surface and maximum values are between −10 cm and −30 cm. The minimum values reached at a depth of −50 cm are greater than the concentration values of superficial sediments analyzed previously, so the concentrations of trace elements in the deeper samples cannot be considered as the geochemical background of the lake.

Individual pollution indices Ef, Cf and Igeo show As and Mo as well as Cu, Ba, Cd, Ni and Pb to a lesser extent, as elements with concentrations greater than the reference values. This implies a certain degree of contamination and enrichment with respect to the baseline, although in all cases they only reach the lowest values indicative of certain contamination. Detected anthropogenic contamination can come from potential wastewater discharges from petroleum, domestic, agricultural and industrial aquaculture activities. The four integrated indices applied mCd, MEQ, RI, and TRI indicate “zero or very low degree of contamination”, “category 1 non-enrichment”, “low-risk potential” and “no toxic risk” respectively.

The ²¹⁰Pb_excess profile obtained can be subdivided into three sections that may be due to different natural and/or anthropogenic processes. Superficial section (0–9 cm) with an increase in ²¹⁰Pb_excess up to −9 cm where a contribution of material of less density could have been produced due to the underground avenues of the nearby Napo River. A deeper section (39–50 cm) with a ²¹⁰Pb_excess profile, radionuclides and studied trace elements constant, where reworking and resuspension due to floods could have taken place. The intermediate section (9–39 cm) with a ²¹⁰Pb_excess exponential decrease with depth allows to apply CF-CS model to obtain an average sediment ratio of 0.56 ± 0.03 cm y⁻¹. Analysis of the accumulated mass sediment rate confirms this ratio and allows a dating estimate of the sediment profile.

The results obtained will be useful data to feed mechanistic models applied to the mobility and transport of metals in water masses; they may also improve estimations of the contribution to sediment pollution and provide insight into mechanistic explanations of sediment-water interactions.

A sampling of sediment profiles in different locations of the Limoncocha lagoon is proposed, both near and far from the possible sources of contaminant input such as tributary rivers or near the population in order to corroborate the obtained results.