Aquatic systems and water security in the Metropolitan Valley of Mexico City
In megacities water quantity and quality are threatened by complex and interrelated processes caused by population growth, land use change, unsustainable agricultural practices, deforestation, erosion, destruction of ecosystems, lack of planning, laissez-faire policies, unsustainable water management, political conflicts, and increasingly also by the impacts of climate change. In the Metropolitan Valley of Mexico City (MVMC) located in the high plateau of Mexico City and on former lakes, the dry out policies during the last 300 years have been counterproductive, causing scarcity, pollution, health and environmental problems, subsidence and water conflicts, particularly affecting social vulnerable people. Megacities require an integrated management of water and aquatic systems — integral water resource management, rainwater harvesting, maintenance and replacement of infrastructure, restoration of ecosystems and urban planning. This implies investments and a water culture including a transparent administration and peaceful negotiation of conflicts regarding concessions, access, and reuse of water allocation. Socio-political and environmental processes are complex; besides technical factors and ecosystem recovery, socio-cultural changes must transform human settlements and power structures.
Water Quality in the State of Aguascalientes and its Effects on the Population’s Health
Aguascalientes is a state located in Central Mexico, with a surface area of 5,589 km2 (0.3 per cent of Mexican territory; CONAGUA, 2000). Most of the state of Aguascalientes (87 per cent) has a semi-dry climate, with annual average precipitation of 600 mm (rainy season in the summer) and an annual average temperature of 18°C. Some locations in Aguascalientes (13 per cent) have a warm sub-humid climate, with an annual average precipitation of 700 mm, and an annual average temperature of 16°C. The annual average precipitation in the state is 552 mm, with rainfall (80 per cent) predominant between June and September (CONAGUA, 2004).
Interactive Sorption of Metal Ions and Humic Acids onto Mineral Particles
The sorption of metal ions (Pb2+, Zn2+ and Cu2+) and soil humic acids (HA) from aqueous solutions onto mineral particles (sand, calcite and clay) was investigated using a batch equilibrium system. The sorption reactions in two- component systems (heavy metals-mineral particles and humic acids- mineral particles), as well as interactions in three-component system (heavy metals-humic acids-mineral particles) were examined. Results showed that the presence of humic acids, dissolved or bound onto mineral surfaces, considerably influenced the fixation of heavy metals. The various effects, depending on mineral type, humic concentration and specific metal-ion, were observed in three- component system. Sorption of Cu2+-ions on all minerals studied rapidly increased as the concentration of dissolved HA increased. The amount of Pb2+-ions sorbed on sand slightly decreased, while on kaolin increased between 15 and 20%. Sorption of Zn2+-ions on all minerals studied decreased at pH 4. At pH 5.5 the sorption of Zn2+-ions onto calcite decreased, while on kaolin and sand increased as a function of the humic acid concentration giving the curve with maximum at c(HA) = 2.5 mmol C L-1. At pH 6.5 sorption onto kaolin and sand increased. This effect occurs as a result of the conditional stability constant of Zn-HA complexes increasing at higher pH which in turn promotes the chelation of Zn2+-ions to mineral- bound humic substances. The enhanced sorption of metal ions from the aqueous phase in three-component systems is not only the result of mineral sorption of free metals but also the result of chelation with HA sorbed on the mineral surface.