Mexico City

Jing Yan (MLA ‘20)

Heavy metals such as Cr, Ni, Cu, Zn, Pb and V are naturally occurring elements all around us. According to the World Health Organization (WHO), using a calculation called Significance of intervals of contamination/pollution index (C/P) , concentration of heavy metals over 0.51( mg /kg) is considered to reach the level of  severe contamination (stl). Heavy metal contaminants are polluting air, water and soil by industrial productions, burned fuel, fertilizers, pesticides and even from rocks in Earth’s crust. Plants and crops will absorb these contaminants from air and soil where eventually these heavy metals elements will transmit the elements into animals and humans. Mexico City specifically suffers from heavy metal pollution. The main culprits are leaded petroleum for old car combustors, wastewater irrigation canals and industrial refineries. Not only industrialization causes heavy metal pollutants but traditional mexican pottery also could be hazardous for accidental heavy metal poisoning from food and drink consumption.

Pb (lead) is one of the major pollutants affecting local people and young children are especially vulnerable. Lead poisoning is very dangerous for young children. They absorb lead 4 or 5 times more than adults due to their still developing nerve system. It causes permanent brain damage. Pb comes from a variety of sources, from contaminated food and water or from pregnant mothers to their babies. To trace the sources of heavy metal, we have to draw a picture of the whole system of how heavy metals contaminate people’s digestive system and inter to the bloodstream. One study conducted for a 7-year-old child of a US Embassy official in Mexico City revealed that lead from traditional pottery continanimate drinks and food.3 Traditional pottery is safe unless the glaze – which may contain lead to facilitate the melting of glaze particles – when the fire process of pottery making is not properly done.

Another form of tradition also causes heavy consumption unexpectedly by the local people since they have done it for thousands of years. Historically, during pre-colonial time, chinampa agriculture was a very effective way of wetland farming. The method itself is relatively safe but in1980s Mexico City became more industrialized and urbanized, the economic boom brought the side effect of environmental problems one is higher heavy metals concentration in soil. The emissions of 2.4 million vehicles and 35,000 factories, by the smoke from burning garbage dumps and the dust thrown up by a dry lake bed, a huge blanket of brownish smog covers Mexico City most days. The stories spawned by the city's pollution are graphic - the 11,000 tons of waste material pumped into the atmosphere daily, the doctors who come from all over the world to study new respiratory diseases, the canary that was left in a cage on a downtown street corner and was dead within an hour.1 Car emissions are from leaded gasoline by Mexico’s state owned oil company Pemex. In 1974, although EPA announced to phase out the lead content in gasoline due to environmental hazards. In 1993, 80% of the gas sold by Pemex was still contained Pb. Study shows that there is a higher concentration of Pb along the road and highway infrastructure compared to other parts of Mexico City. Another problem is that there are few vapor recovery nozzles at Pemex station resulting in the escape of noxious fumes into the atmosphere. Fe, Cu, Zn and Pb are detected and higher bacterial counts and heavy metal concentrations reported during the rainy season due to all the pollutants being washed down the drainage system into the irrigation system.2 Rainfall brings all the hazards heavy metal elements into the wastewater canal system and irrigated agriculture areas in Mezquital. Sewage effluent contaminated by industrial pollution eventually is absorbed by crops and animals which ends up into the human digestive system. The matter didn't get better but worse over the years as a result of the NAFTA agreement with the US and Canada. Mexico City became the main manufacturer for steel. Steel production processes and refineries contain Fe, Zn, Ca and Si in the form of simple or mixed oxides, and also Cu, Mn, Ni, Cr, Cd, Pb etc., present in the raw material or introduced as additives. The study of heavy metals in soils around the steel production facility in Smederevo proves that heavy metals are accumulating due to the steel industry.

A consequence of urbanization and growth of  population also causes environmental concerns about waste treatment. According to studies in Newcastle, waste incinerators are likely to be one of a number of sources of contamination that have caused urban soils to have generally greater concentrations of metals than those in rural areas. Cd, Cu, Hg, Pb, and Zn are found in soil that is related to waste incinerators. In Mexico City, a new billion-dollar waste incinerator brewing from 2018 to generate new energy has been under public scrutiny. Growthing job opportunities in Mexico City attract millions of people to migrate to the city which puts more people to expose under the heavy metal contaminations.

The presence of heavy metals in soils is a serious issue due to its residence in food chains, thus destroying the entire ecosystem. As much as organic pollutants can be biodegradable, their biodegradation rate, however, is decreased by the presence of heavy metals in the environment, and this in turn doubles the environmental pollution, that is, organic pollutants and heavy metals thus present.4 There are various ways through which heavy metals present risks to humans, animals, plants and ecosystems as a whole. Such ways include direct ingestion, absorption by plants, food chains, consumption of contaminated water and alteration of soil pH, porosity, colour and its natural chemistry which in turn impact on the soil quality.


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Mezquital

Michele Turrini

The mezquital valley is an agricultural region located north of Mexico City which has been irrigated with the urban wastewater since 1890. The use of wastewater for agricultural purposes has been contributing to the fertility of the soil within the region, with a generous amount of organic matter spreading over the fields through each irrigation. On the other hand, irrigation with wastewater is responsible for the accumulation of heavy metals within soil, and it has been a cause of concern for the risk of  entering the food chain with all the associated health problems for animals, plants, bacteria and humans.

Although the presence of metals in soil is a natural occurrence and can be traced back to volcanic activities, some metals are considered harmful in high proportion and non necessary to living organisms for performing their life processes, those include heavy metals such As, Ag, Cd, Hg, Pb. Heavy metals like Pb rarely occur as a consequence of volcanic activity, they are most commonly found in soil due to industrial human activities such as mining and smelting of metalli- ferrous ores, electroplating, gas exhaust, energy and fuel production, fertilizer and pesticide application, and generation of municipal waste and are brought to the Mezquital by wastewater irrigation practices.

The characteristic properties of the soil of the Mezquital are due to past volcanic activity and the consequent accumulation of alluvial pyroclastic material. Its form  is given by the normal faulting of the crust where tensional forces stretched the crust and caused a downwards shift of land establishing the Mezquital graben. A series of layered volcaniclastic tuff deposits (known as Tarango Formation) lay at the base of the Mezquital and are topped by a layer of colluvio-alluvial sediment deposition. This layer can be classified  within the valley as either haplic soil or vertic Phaeozem soil.

The topsoil within the mezquital is mostly composed of clayey soil, with a high proportion of smectite minerals and  with originally little organic matter content which has then been constantly added through  years of wastewater irrigation . In fact the addition of organic matter changed dramatically the soil composition whilst also changing the soil classification,  in the region are classified as Calcaric Phaeozem (50 years irrigation) and Leptic Calcaric Phaeozem (100 years irrigation).

Heavy metals availability in soil is dependent on characteristics such as organic matter, clay, silt, Phosphorus and pH, the latter being the most important factors in the retention and solubility of heavy metals. In fact heavy metals are less available in soil containing high clay percentage (11-60% within the Mezquital) and with a basic pH  which in the Mezquital varies from 6.7 to 8.4, depending on the time of irrigation. Throughout irrigation the heavy metals particles having positive charge, bind with the negatively charged clay particles. Heavy metals are not subject to degradation, this means that less and less binding sites will be available to heavy metals in the future, with possible health risks due to plant intake or heavy metals leakage to underground aquifers.

In addition soils which have been irrigated for more than 100 years tend to have the lowest pH values (slightly acidic),this is due to the loss of the active limestone in the clay caused by intensive irrigation, therefore resulting in higher metal bioavailability. Whilst increasing organic matter content, long term irrigation has also an impact on the quality of organic matter available, which by becoming more hydrophilic tends to reduce its capacity of restricting heavy metals movement.

All those factors play a fundamental role in the future of the Mezquital, essentially it is a matter of time before the clay particles will exhaust all the available binding sites making the heavy metals available for plant intake and with the reduction in organic matter quality the possibility to restrict heavy metals movement will thin out, making possible the pollution of underground aquifer due to the leakage of heavy metals from the topsoil.




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