Mélida Gutiérrez

Occurrence and treatment of arsenic in groundwater and soil in northern Mexico and southwestern USA

This review focuses on the occurrence and treatment of arsenic (As) in the arid region of northern Mexico (states of Chihuahua and Coahuila) and bordering states of the southwestern US (New Mexico, Arizona, and Texas), an area known for having high As concentrations. Information assembled and assessed includes the content and probable source of As in water, soil, and sediments and treatment methods that have been applied in the area. High As concentrations were found mainly in groundwater, their source being mostly from natural origin related to volcanic processes with significant anthropogenic contributions near mining and smelting of ores containing arsenic. The affinity of As for solid phases in alkaline conditions common to arid areas precludes it from being present in surface waters, accumulating instead in sediments and shifting its threat to its potential remobilization in reservoir sediments and irrigation waterways. Factors such as oxidation and pH that affect the mobility of As in the subsurface environment are mentioned. Independent of socio-demographic variables, nutritional status, and levels of blood lead, cognitive development in children is being affected when exposed to As. Treatments known to effectively reduce As content to safe drinking water levels as well as those that are capable of reducing As content in soils are discussed. Besides conventional methods, emergent technologies, such as phytoremediation, offer a viable solution to As contamination in drinking water.

Co-occurrence of arsenic and fluoride in groundwater of semi-arid regions in Latin America: Genesis, mobility and remediation

Several million people around the world are currently exposed to excessive amounts of arsenic (As) and fluoride (F) in their drinking water. Although the individual toxic effects of As and F have been analyzed, there are few studies addressing their co-occurrences and water treatment options. Several studies conducted in arid and semi-arid regions of Latin America show that the co-occurrences of As and F in drinking water are linked to the volcaniclastic particles in the loess or alluvium, alkaline pH, and limited recharge. The As and F contamination results from water-rock interactions and may be accelerated by geothermal and mining activities, as well as by aquifer over-exploitation. These types of contamination are particularly pronounced in arid and semi-arid regions, where high As concentrations often show a direct relationship with high F concentrations. Enrichment of F is generally related to fluorite dissolution and it is also associated with high Cl, Br, and V concentrations. The methods of As and F removal, such as chemical precipitation followed by filtration and reverse osmosis, are currently being used at different scales and scenarios in Latin America. Although such technologies are available in Latin America, it is still urgent to develop technologies and methods capable of monitoring and removing both of these contaminants simultaneously from drinking water, with a particular focus towards small-scale rural operations.

A mechanistic modeling of montmorillonite contamination by cesium sorption

Abstract The triple layer electrochemical adsorption model (TIM) was utilized to model adsorption of Cs+ onto Ca-montmorillonite. Input data for the model were obtained via batch adsorption experiments in which Cs+ and mixtures of Cs+ and Sr2+ were added to a Ca-montmorillonite suspension at various conditions of pH and background electrolytes. A best fit of the model was achieved after gradually adjusting the values of the equilibrium constants and adsorption site concentrations. Two types of adsorption sites were considered to be responsible for adsorption in the clay: interlayer and frayed edges. The dominant mechanism of adsorption was identified as specific adsorption in the frayed edge sites, which although composing only 5% of the adsorption sites, they accounted for about 94% of total Cs adsorption. The model predicted precipitation of CsOH at concentrations above 1 mmol Cs/1, this value depending strongly on pH. The TLM successfully represented the adsorption of Cs over a wide range of concentrations, pH, nature of background electrolytes, and presence of competing cations.

Water quality assessment of the Rio Conchos, Chihuahua, Mexico

A baseline study was conducted to evaluate the overall quality of the Rio Conchos (Chihuahua, Mexico) and to identify those chemical parameters that can best represent the water quality in different segments of the river. Chemical analyses included the measurement of 62 elements at more than 100 sampling stations along the river, in addition to conventional field analyses (e.g., pH, conductivity). Concentrations of these elements are reported and water quality indicators were identified. Based on the element concentration patterns, the segment of the river in which the water quality is most endangered corresponds to that receiving irrigation drain returns near the confluence of the Rio San Pedro. Self-cleaning and dilution processes account for the improvement in water quality observed as the Rio Conchos approaches the Rio Grande.

Modeling adsorption in multicomponent systems using a Freundlich-type isotherm

Abstract The suitability of a Freundlich-type isotherm, the Sheindorf-Rebuhn-Sheintuch (SRS) equation, to represent the competitive adsorption of Sr, Cs and Co in Ca-montmorillonite suspensions was investigated. Experimental adsorption data were obtained for systems containing these cations as single-component, binary and ternary mixtures. The competition coefficient α ij , which were obtained based on the experimental adsorption data for bicomponent systems, can be viewed as a way to quantify competitive interactions. The competition coefficients obtained for the cations under consideration indicate that their competitive interactions are of similar magnitude, with the cation least affected by competitive effects being Cs, while the adsorption of Co was more significantly affected by the presence of Cs, and Sr by the presence of Co. After α ij -values were substituted in the SRS equation, the adsorption of systems of three or more components could be predicted. To validate the SRS equation, the adsorption values predicted by this equation for the ternary mixture SrCs Co were compared to values determined experimentally. The SRS equation successfully modeled adsorption for the range of concentrations that followed Freundlich behavior.

Abandoned PbZn mining wastes and their mobility as proxy to toxicity: A review.

Lead and zinc (PbZn) mines are a common occurrence worldwide; and while approximately 240 mines are active, the vast majority have been abandoned for decades. Abandoned mining wastes represent a serious environmental hazard, as Pb, Zn and associated metals are continuously released into the environment, threatening the health of humans and affecting ecosystems. Iron sulfide minerals, when present, can form acid mine drainage and increase the toxicity by mobilizing the metals into more bioavailable forms. Remediation of the metal waste is costly and, in the case of abandoned wastes, the responsible party(ies) for the cleanup can be difficult to determine, which makes remediation a complex and lengthy process. In this review, we provide a common ground from a wide variety of investigations about concentrations, chemical associations, and potential mobility of Pb, Zn and cadmium (Cd) near abandoned PbZn mines. Comparing mobility results is a challenging task, as instead of one standard methodology, there are 4-5 different methods reported. Results show that, as a general consensus, the metal content of soils and sediments vary roughly around 1000mg/kg for Zn, 100 for Pb and 10 for Cd, and mobilities of Cd>Zn>Pb. Also, mobility is a function of pH, particle size, and formation of secondary minerals. New and novel remediation techniques continue to be developed in laboratories but have seldom been applied to the field. Remediation at most of the sites has consisted of neutralization (e.g. lime,) for acid mine discharge, and leveling followed by phytostabilization. In the latter, amendments (e.g. biochar, fertilizers) are added to boost the efficiency of the treatment. Any remediation method has to be tested before being implemented as the best treatment is site-specific. Potential treatments are described and compared.

State of remediation and metal toxicity in the Tri-State Mining District, USA

Mining operations in the Tri-State Mining District of Kansas, Missouri and Oklahoma (TSMD), once one of the major lead and zinc mining areas in the world, had completely ceased by 1970. As mining companies moved out, the land was left with underground tunnels and mine shafts and the surface with abandoned tailings piles, which progressively contaminated groundwater and soil. Despite remedial actions undertaken in the 1980s, areas within the TSMD still contain Cd, Pb, and Zn concentrations exceeding safe levels. Because of the large area and highly dispersed occurrence of wastes, environmental studies generally have been confined either to a stream basin or to a single state. Studies also have differed in their approach and analytical methodologies. An overview of the totality of the TSMD and its present state of contamination is presented here. Data show that metal content in sediments have the following common features: (1) a wide range of Pb and Zn concentrations, up to three orders of magnitude, (2) median values for Cd, Pb and Zn content in sediments and soils were similar among studies, (3) median values for most studies were at or above the guidelines recommended for aquatic habitats, and (4) highest content of Pb and Zn were closely associated with the geographical location of former mining and smelting centers. The above observations imply that mine wastes remain a problem and further remediation is needed. Cost-effective remedial alternatives for this areas geology, climate, and land use, are discussed.

A Langmuir isotherm-based prediction of competitive sorption of Sr, Cs, and Co in Ca-montmorillonite

An accurate determination of parameters such as the distribution coefficient Kd (for low concentrations) and the maximum adsorption capacity ST (for high concentrations) is desirable, as these values can be used in contaminant transport modeling. Considering that contaminants would more likely be found as mixtures and not as single components, the effects on Kd and ST values resulting from competitive interactions between the target cation and other cations present in the system were estimated. This study focused on the determination of Kd and ST for suspensions of Ca-montmorillonite and mixtures of the solutes strontium (Sr), cesium (Cs), and cobalt (Co) as analogs of a possible release of contaminants from radioactive waste repositories. Batch experiments were conducted for mixtures of the above solutes at intermediate concentrations at varying pH values and for two background electrolytes, NaCl and CaCl2. The Langmuir isotherm adequately fitted the experimental data and was used to determine Kd and ST. Maximum Kd values were obtained for Sr at pH 6 and for Co at pH 8, while for Cs the pH did not affect the resulting Kd value. The presence of competing ions resulted in a decrease of Kd values depending on the type and concentration of ions in the original mixture. For clay preconditioned with NaCl electrolyte solution, the Na+ partially saturated surface attracted more solutes, leading to greater Kd values than for suspensions preconditioned with a CaCl2 solution. Although the total amount of adsorbed cations (Scum) in either electrolyte solution was nearly constant, a difference in the adsorption of individual cations was observed as an increased percentage of adsorbed Sr and Co in systems preconditioned with NaCl solution.

Watershed assessment along a segment of the Rio Conchos in Northern Mexico using satellite images

Satellite images were used to illustrate the usefulness of such data in evaluating the ecological impacts of precipitation and land use on selected segments of the lower Rio Conchos in northern Mexico. Variations in the size and turbidity of impounded reservoirs, riparian vegetation, soil salinity and land use within the Rio Conchos basin were analysed using four Landsat TM images over a period of 10 years. A variety of image enhancements were applied to determine subtle changes between the images. These, when combined with precipitation and historical land-use data as well as one time water quality and soil EC, provided useful interpretation of the images, and therefore, in the monitoring of the basin.

Alumni of Geology B.S. Program Express Strong Support for Field Geology and Related Field and Laboratory Experiences

In a recent survey, alumni of a Bachelor of Science program in geology expressed strong support for a required summer Field Geology course and for related field and laboratory learning experiences in the undergraduate geology curriculum. On a 4-point scale (4=essential; 1=unimportant and unnecessary), geology alumni rated “summer Field Geology course” 3.77, higher than any of the ancillary requirements (mathematics, chemistry, physics, biology, computer skills) or independent study. When asked about possible options for cutting the costs of the B.S. program in geology, only 1.8% of the alumni favored reducing the ratio of laboratory experiences to lecture contact hours and only 2.3% favored reducing the number of field trips and other out-of-classroom experiences.