RESUMO

Since the middle of the 1500 s, mining has been active in central Mexico. Total estimates for low-grade piles and mine tailing materials in the Guanajuato mining district (GMD) are in the range of 150 million tons, covering an area of 15 to 20 km2. GMD is located in the Guanajuato River sub-basin (GRB), which is part of one of the largest basins in Mexico (Lerma-Santiago). Previous studies on the GRB found unusually high concentrations of heavy metals in mining tailings and sediments. Geochemical and statistical methods were used here to determine the sediment's origin, background values, degree of contamination, and toxicity through different contamination indices. This analysis shows that Cu, Co, As, Sb, and Hg are higher than they are in the upper continental crust (UCC) overbank sediments without human and mining influence, because of the ore deposits and rock weathering in GRB. Geochemistry results in stream sediments show anomalies, where Hg, Cu, Zn, As, and Pb are higher than UCC because those heavy metals and trace elements (HMT) have been influenced by human activities and mineral recovery (smelting, amalgamation, cyanidation). The distribution of high concentrations of HMTs and contamination indices occur in the main channel of the Guanajuato River and downstream of the city of Guanajuato. Statistical analyses (cluster and principal component analysis) reveal relationships between Cr, Ni, Cu, and Pb, which are primarily of natural origin, related to rocks of the upper basin. The middle and lower basins are distinctive in their associations between As, Sb, Zn, Pb, and Hg. Additionally, it is recognized that the origins of Pb, Zn, and Hg are geogenic and anthropogenic. This study demonstrates how crucial it is to understand the geochemistry of various HMT sources, with both natural and anthropogenic contributions (stream sediments and rocks), in order to calculate a more realistic background in a basin with both natural anomalies and anthropogenic contamination. The basin is a regional aquifer recharge area, so the new geochemical data are important for improving basin environmental management.

Assuntos

RESUMO

The thermal mineral water of Peñón de los Baños spa (Mexico City) has been used for over 500 years starting in pre-Hispanic times and is famous for the treatment of various pathologies. It has a temperature of 45 °C, which is rich in HCO3-, and its main trace elements are B, Li and Fe, which confers healing effects. Concerns about the sustainability of this important spa have motivated this study to understand the thermal system, possible hydraulic and hydrochemical changes over time and its implications. Stable water isotope data indicate that the thermal water sources originate from local precipitation at Sierra de las Cruces with a recharge elevation of approximately 2770 m above sea level. The recharged water percolates through volcanic and carbonate rock formations and ascends via fault structure conduits, where it eventually is extracted 25 km downstream in Peñon de los Baños. During the gravity-driven deep circulation of up to 4.9 km, the groundwater is heated up to 136-160 °C. A comparison of past and current water levels and water chemical analyses indicates a water table drop and few variations in the chemical composition, confirming the presence of anthropic impact on water quality. Due to the heavy groundwater extractions in Mexico City, the spring water flow has ceased, and water must be pumped now from a 203-m deep well. In addition, the concentration of bicarbonate, sodium and chloride has been reduced by half since the onset of groundwater development. The therapeutic effects of this thermal mineral water are at risk due to the alteration of the chemical signature. However, new and different therapeutical uses may prevent a future deterioration or closure of this historically important thermal spa. It is crucial to establish a monitoring program of the thermal mineral water and reducing or minimizing nearby urban extractions which tap the regional flow component to preserve the properties of the thermal water.

Assuntos

RESUMO

The Mapocho River's upper basin (Chilean Central Andes) was studied as a proxy of a high-mountainous hydrothermally-altered (HMHA) system comprised by three sub-basins developed over very different rocks and submitted to different anthropic pressure: 1) a natural acid rock drainage (i.e., Yerba Loca), 2) a creek with mining activity in its headwaters and hydrochemically classified as non-affected by acid mine drainage (i.e., San Francisco), and 3) a low metal concentration creek (i.e., Molina). In general terms, the geochemical composition of the clastic sediments was consistent with the geochemistry inferred from the mineralogical study. However, sediments with a smaller grain size showed higher concentrations than the bigger grain size counterparts for elements such as Fe, S, Cu and As. This behavior was particularly evident in the Yerba Loca basin and it was attributed to the seasonal appearance of Fe- and Al-rich precipitates as constituents of the finer sediments. Different methodologies for the calculation of geochemical backgrounds (Tukey's inner fence, TIF; Median + 2*Median Absolute Deviation, MAD; and 95th percentile) were tested. Results suggest that the 95th percentile-method was the most appropriate for this type of mountainous systems. Using the selected methodology, three different geochemical backgrounds were calculated: 1) Yerba Loca basin, 2) Molina basin, and 3) Mapocho Upper basin. When the generated background levels were compared with the Consensus-Based (CB) Sediment Quality Guidelines; Fe, Mn, Zn, Pb, Cu, Cr, Ni and As showed background values that were consistently higher than the values set by the CB Threshold Effect Concentration and, even higher than the CB Probable Effect Concentration for Fe (MUBBackground: 6.78 wt% vs CB PEC: 4.00 wt%; and Cu (MUBBacground: 3387 mg kg-1 vs CB PEC: 149 mg kg-1). The present study clearly states the paramount importance of having a solid geochemical background before any attempt of a sediment risk assessment is made at HMHA regions.

RESUMO

Karst environments have an inherent complexity that interferes with their hydrogeology comprehension. Hence, isotope hydrology can be a valuable tool to assess trajectory of subsurface flows in an unexplored setting. The study area is located in the Lagoa Santa Karst, an environmental protection area of great economic, cultural and ecological importance, where Neoproterozoic metalimestones accommodate karst-fractured aquifers, characterized by complex water dynamics, essential vulnerability and high productivity. The purpose of this study was to investigate groundwater flow origins of springs using principally environmental stable isotopes 2H and 18O. Rainwater and spring water were sampled and analysed. The LMWL presents angular and linear coefficients strongly similar to those of the GMWL. Spring isotopic signatures, which represent the base flow and present wide-ranging of 2H and 18O, were separated into two groups. The first group can be associated with recent rainwater major contributions, while the second group shows significant evaporated water contributions, largely represented by resurgences. Tritium concentration and physico-chemical parameter data supported this interpretation, pointing that waters of the second group remained more time on the surface and subsurface. Therefore, using isotope tracers to evaluate upper groundwater zone in this tropical karst system is a powerful instrument for water resources management.

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