Environ Geol (2009) 57:929–935 DOI 10.1007/s00254-008-1371-4
ORIGINAL ARTICLE
Geographical distribution of arsenic in sediments within the Rio Conchos Basin, Mexico Melida Gutierrez Æ M. Teresa Alarco´n-Herrera Æ Lucy M. Camacho
Received: 25 January 2008 / Accepted: 5 May 2008 / Published online: 22 May 2008 Ó Springer-Verlag 2008
Abstract Arsenic (As) content of sediments from the Rio Conchos and Rio San Pedro in northern Mexico were measured to determine if this toxic metalloid had accumulated to unsafe levels to humans and aquatic life. The spatial distribution of As in each of the six clusters of river and arroyo sediments was analyzed to determine variations with respect to background levels and to infer about potential As sources and sinks. In the northern part of the study area, background concentrations varied little throughout the area and concentrations in river sediments were close to background levels. In the southern part, however, the content of As in arroyo sediment contained a wider range of values and anomalous concentrations. The latter could be traced in part to the presence of mine tailings. As concentrations were below the limit in all studied river stretches and thus do not pose an immediate threat to the river environment, but As content in reservoir sediments exceeded the guideline values. Reservoirs seem to act as a sink for As, warranting closer observation and monitoring. Keywords Arroyo Arsenic Geochemical databases Chihuahua Rio Conchos River sediment M. Gutierrez (&) Department of Geography, Geology and Planning, Missouri State University, 901 S. National Ave., Springfield, MO 65897, USA e-mail:
[email protected] M. T. Alarco´n-Herrera Department of Environment and Energy, Advanced Materials Research Center CIMAV, Chihuahua, Mexico L. M. Camacho Department of Chemical Engineering, New Mexico State University, Las Cruces, NM 88003, USA
Introduction Arsenic (As) is a common element within the earth’s crust, widespread in nature in both living systems and geological materials. This metalloid is toxic at small concentrations, although its toxicity depends on the chemical form in which it is present (ATSDR (Agency for Toxic Substances and Disease Registry) 2007). The most toxic form is the inorganic reduced form, arsenite. Combined with carbon and hydrogen, As is called organic arsenic, while inorganic arsenic is formed from the combination of arsenic with oxygen, chlorine or sulfur. Organic arsenic is seldom a health concern because this form is less toxic. In water and soils, arsenic is found as arsenite (As+3) or arsenate (As+5). The World Health Organization guidelines set the limit for As in drinking water at 10 ppb (WHO (World Health Organization) 2004), and the Mexican guidelines at 25 ppb (Secretaria de Salud y Asistencia 1999). As has a strong affinity to solid materials and adsorbs onto clays and iron oxides that accumulate in soils and sediments (Welch et al. 1988). In rivers, As-rich materials present little harm to the environment as long as they remain in the sediment instead of in the water column. Arsenic can remain attached to river sediments during long intervals of time provided that alkaline pH and oxidized conditions prevail. However, sediments may receive pollutants or build up where their redox conditions change to a point at which arsenic is released from the sediments and into the water column (Lombi et al. 2000). The content of As in soils and sediments is commonly expressed in mg As/kg of dry solid. Permissible limits of As in soil and sediment vary from 0.39 mg/kg for residential soil to 1.6 mg/kg for industrial soil with direct contact exposure, while the limit for agriculture soils is 29 mg/L assuming a dilution attenuation factor of 20 (US EPA 2004). In Mexico,
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the permissible limit for agricultural soils is 22 mg/kg (DOF 2007). Based on the effects of contaminants on biological communities, Long and Morgan (1990) reported an informal guideline for As of effects range-low (ER-L) of 33 mg/kg and an effects range-median (ER-M) of 85 mg/kg. Sources of As can be natural or anthropogenic. Natural sources include As-rich sulfide minerals emplaced in volcanic rocks or hydrothermal mineral deposits (Welch et al. 1988; Flores-Tavizo´n et al. 2003). Oxidized sulfide minerals are able to release high concentrations of arsenic from their mine tailings (Smedley and Kinnigurgh 2002; Puga et al. 2006), while As may accumulate in irrigation areas where As-based pesticides have been applied (Duker et al. 2005). In an irrigated area in New Mexico, Norman and Dilley (2002) estimated that 80% of the arsenic present in irrigation waters remained in the irrigated fields. Sediment geochemistry Sediment composition is a widely utilized method of environmental assessment (Fo¨rstner et al. 1990; Long and Morgan 1990). In contrast to water, sediments integrate contaminant concentrations over time, rendering a longerterm picture of the effects of mining, agriculture and urbanization. In order to determine how strongly a contaminant is bound to the solid phase, a variety of procedures in the form of sequential extractions have been utilized. The number and type of extractions vary from study to study, but most times they follow a modified version of the method proposed by Tessier et al. (1979). Each extraction reveals the amount of contaminant bound to a particular solid phase and consequently the amount of contaminant that would be released under particular chemical conditions. The sum of contaminant content released in each of the solid phases is equal to the total content. The total content can also be obtained in a one-step procedure by utilizing an extractant strong enough to dissolve the contaminant in its entirety. A practical approximation to total content is obtained using concentrated nitric acid or aqua regia. Geochemical data bases Geochemical data bases originally intended for geochemical exploration purposes have been found to provide an inexpensive and accessible information source for environmental characterization. The Global Reference Network intends to compile geochemical data from all around the globe. Notable data sets are the National Uranium Resource Evaluation Data in the U.S. (Hoffmann et al. 1994) and the tri-national USGS Geochemical Landscapes Project for North America. As part of the latter, data for Chihuahua, Mexico were compiled in 2001 by the Servicio Geologico Mexicano (Servicio Geologico Mexicano 2001).
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SGM samples correspond to surface material from dry arroyos sieved to 80 mesh, which were further digested with aqua regia and analyzed for 64 elements using an induction coupled plasma mass spectrometer (ICP-MS). Previous studies A sequential extraction of As in different size fractions was conducted by Lombi et al. (2000) for Austrian soils. They found that clays retain As more strongly than sand and silt, in part because of iron oxides homogeneous dissemination in the clay fraction. Within the Rio Conchos basin, a sequential extraction of metals in river sediments was conducted (Gutierrez 2000) concurrent with water samples, but unfortunately this study did not include As. Closer to the study area, in New Mexico, Branvold and Branvold (1990) reported As content in bottom sediments of the Rio Grande and its tributaries of up to 30 mg/kg. Near their confluence with the Rio Grande, sediments of the Rio Conchos contained 8.1 mg/kg As (IBCW 1997), while the As content in sediment of three reservoirs in Chihuahua (two of which are within the study area) varied between 3.2 and 22.6 mg/kg (Hernandez-Garcia 2008). Higher As concentrations have been reported in sites affected by mine tailings. Puga et al. (2006) reported soils with an As content of up to 2,687 mg/kg in southern Chihuahua. Within the study area, total As content of 32 mg/kg has been reported for road dust (Benin et al. 1999) and 46 mg/kg As for soils in the vicinity of a hot spring (FloresTavizo´n et al. 2003).
Study area The area of study is part of the Chihuahuan desert and comprises the Rio Conchos and its tributaries Rio Chuvı´scar and Rio San Pedro (Fig. 1). Throughout the area, there are many ephemeral streams (arroyos) that drain the water collected from isolated short-lived rain events. These arroyos may remain dry for months and sometimes years at a time. Cities, agricultural areas and industry in the area are located near one of the rivers and discharge their wastes into it, polluting it, while arroyos can be considered unaffected by pollution, except for those arroyos located in the vicinity of mine tailings. Concern about high As content in the area has been sparked after high levels were found in wells within the study area (Vega-Gleason 2002). The As content in these wells has been associated with the presence of As-rich mineralization present in volcanic rocks (Rodriguez-Pineda et al. 2005) as well as in erosion products from these rocks that have accumulated in the lower elevation parts of closed sedimentary basins (Reyes et al. 2006). Studies reporting the As content of the waters in the Rio Conchos
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Fig. 1 Study area showing the location of sampling sites of river sediment (diamonds) and arroyo sediments (circles). Sample clusters are designated by name
include Gutierrez and Borrego (1999), Holguin et al., (2006) and Hernandez-Garcia et al., (2007). This study focused on the sediments instead of water, expecting that the distribution of As concentrations will show where As contamination sources may occur.
Materials and methods River sediments were collected from eleven sites: nine along the Rio Conchos and two along the Rio San Pedro (Fig. 1). The samples were collected 1.5 m from the river’s edge and sieved at the sampling location through 200 and 270 mesh to isolate the 53–74 lm fine sand-silt fractions. Clays retain a large amount of metals (and As), but they are
also more mobile, and therefore they are not representative of the geographic location in which they are found. Thus, during the sieving process, clay was washed out from the sample in order to measure with more certainty the As fixed at this geographical site through time. The sediments were air-dried, disaggregated and sent to a commercial laboratory where they were pulverized and digested with aqua regia prior to being analyzed for As (and 51 other elements) using an ICP-MS, which has a detection limit of 0.1 ppm for As. The US-EPA 3050B method utilizing an ICP-MS and aqua regia extraction is the primary analysis method for determining As content in geological material (Hudson-Edwards et al. 2004). In addition to river sediment samples, sediment data acquired from SGM were utilized for comparison between
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Table 1 Arsenic content in Rio Conchos and Rio San Pedro sediment samples in mg/kg Location
River sediment As (SD)
Rio San Pedroa
7.3a (2.7)
Background concentrationb
Arroyo sediment No. of samples
As (SD)
No. of samples
As (SD)
No. of samples
2
35.3 (46.8)
12
16.1 (7.2)
10
Las Varas
24.9 (5.2)
2
20.8 (27.3)
14
14.4 (13.8)
13
Julimes
18.9 (10.5)
3
12.4 (5.4)
19
12.4 (5.4)
19
El Granero (reservoir)
12
33.0 (n/a)
1
20.3 (37.9)
13
10.2 (11.4)
San Pedro (Rio Conchos)
5.7 (0.3)
2
6.9 (1.7)
14
6.9 (1.7)
14
Cuchillo Parado
6.0 (n/a)
1
6.1 (1.9)
12
6.1 (1.9)
12
a b
Samples collected in the river right below the Las Virgenes reservoir Based on nearby arroyo sediment samples (see Fig. 1)
As content in the river and in dry arroyo sediments. The two 1° 9 2° quadrangles comprised 1,016 points scattered throughout the area (Fig. 1). Only arroyo data located in the surrounding river sampling sites were included in this study.
Results and discussion Based on their location, river sediment samples were grouped into six clusters of data points (Fig. 1), each cluster containing one to three river sediment samples and about a dozen arroyo sediment data surrounding the river sample(s). Each data cluster was named after an enclosed reservoir or town: Las Virgenes, Las Varas, Julimes, El Granero, San Pedro, and Cuchillo Parado. All of these are located along the Rio Conchos in upstream to downstream order, except Las Virgenes, which is located on the Rio San Pedro, a tributary of the Rio Conchos (Fig. 1). Table 1 summarizes the concentrations obtained for both river and arroyo sediments. With the exception of the Las Virgenes site, concentrations for river and arroyo sediment followed a close match (correlation coefficient 0.955, significant at P = 0.05). At the Las Virgenes site, the larger difference between the river and arroyo values may be the result of river samples taken downstream of the reservoir, while the surrounding arroyos discharged into the reservoir. These values imply an increase in As content in sediments of the Las Virgenes reservoir, which caused a subsequent depletion of As in the stream below the reservoir. Hernandez (2008) analyzed sediments of the three largest reservoirs in the Rio Conchos Basin and found the highest As content in Las Virgenes reservoir (22.6 mk/kg) as well as sporadic high As concentrations in the water in this reservoir (1–164 ppb), in agreement with a high As content contained by some of the arroyos discharging into the reservoir. For the El Granero reservoir, HernandezGarcia (2008) found an As content of 13.7 mg/kg in the
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sediment. Both of these sediment values and the river sediment values reported in Table 1 correspond to samples collected during the summer. To verify the validity of considering arroyo sediments as a background concentration, histograms were constructed (Fig. 2). Arsenic concentrations for Julimes, San Pedro and Cuchillo Parado plotted closer together while concentrations in Las Virgenes, Las Varas, and El Granero had several concentrations that seemed anomalously high. To determine which of these higher values were actual anomalies, sites that had a concentration higher than the mean plus or minus two standard deviations were identified. Two points at Las Virgenes and one point for the Las Varas and El Granero sites fell into this category. Once anomalies were removed from their cluster, the recalculated mean and standard deviation showed a more consistent pattern of gradual decrease in As content downstream. These values were labeled as background concentration (Table 1) and agree with reported control sediment values of 3 – 15 mg/kg reported elsewhere (Breckenridge and Crockett 1995). The anomalous concentrations can, in part, be traced to the presence of mine tailings after visual comparison between sampling sites and the location of sulfide-ore mines (Fig. 1). More intense irrigation and larger cities along the river in the southern part of the study area may account for the larger As content given that agricultural wastes (fertilizers, manure, pesticides) and sewage sludge are an important source of As contamination in soils (Breckenridge and Crockett 1995). Table 2 compiles the values of major ions within the sediment samples to show a more complete picture of the variation and trends in sediment geochemistry. This table shows that the As values reach a high value and then decrease to background levels, while the major elements remain relatively unaffected, except for Ca and Al. Ca content increases while Al decreases. This behavior may be associated to bedrock geology, as the Rio Conchos encounters more Ca-bearing material (limestone, gypsum) on its path downstream.
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Fig. 2 Histograms for each cluster of sediment data. For comparison purposes, average concentrations of river sediments were plotted as a solid line. Concentrations in mg/kg
Table 2 Water pH and major element content of (dry) river sediment. Sample no.
River/reservoir
pH
As mg/kg
Al (%)
1
San Pedro
8.24
5.4
1.99
2
San Pedro
8.06
9.2
1.65
3
Conchos
8.43
21.2
4
Conchos
8.31
5
Conchos
6
Conchos
7
Ca (%)
Fe (%)
Mg (%)
K (%)
Na (%)
2.61
5.74
3.23
0.08
0.07
6.16
2.18
0.58
0.26
0.02
1.80
9.93
2.07
0.79
0.32
0.02
28.6
1.47
3.84
1.87
0.40
0.21
0.05
8.02
30.9
1.26
7.10
1.65
0.40
0.24
0.10
7.90
11.1
0.72
4.01
1.89
0.23
0.14
0.07
Conchos
8.16
14.7
1.04
5.29
1.99
0.29
0.19
0.08
8
El Granero
7.74
33.0
0.73
18.15
1.19
0.39
0.15
0.05
9
Conchos
7.60
5.9
0.94
9.52
2.36
0.42
0.12
0.04
10
Conchos
7.70
5.5
0.95
9.34
2.25
0.42
0.12
0.04
11
Conchos
7.62
6.0
0.66
10.90
1.76
0.31
0.11
0.04
Particle size of sediment was 53–74 lm after sieving in situ. Samples 3–11 numbered downstream the Rio Conchos, sampling sites 1–2 from the Rio San Pedro (see Fig. 1)
Arsenic concentrations remained below permissible limits in all studied river stretches, but its content in reservoir sediments exceeded guideline values of 22 mg/kg for agricultural soils and were close to the 33 mg/kg limit for onset of impact to aquatic organisms. Although these values
do not pose an immediate threat to the river environment, reservoir sediments within the Rio Conchos basin seem to act as sinks for As, warranting closer observation and future studies that would consider more detailed analytical procedures, e.g., speciation and sequential extraction.
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Conclusions Arsenic concentrations that are part of a geochemical database were instrumental in determining background and anomalous values for As in this basin. These values, collected in arroyos, were compared to values of river sediment samples collected by the authors. In general, higher As concentrations were observed near tailings of sulfide-ore mines in dry arroyos, while the highest values within the rivers were found in the vicinity of the Irrigation District 005 and were assumed to be a result of agricultural and domestic wastes discharged into the river. All arsenic concentrations in river sediments fell within permissible limits. However, an enrichment of As in reservoirs within the study area suggests that these may be acting as a sink for As, raising the concern about a future release of As from the sediments and into the water column.
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