B. Pollution. With cities now being over populated, pollution is higher than ever especially in urban hubs like Mexico City, where the increase in cars and exhaust fumes and the lack of air flow in the Mexican valley, stagnant polluted air concentrates. Also, C. Over pumping of groundwater; in many landlocked desert cities, lack of access to water to supply the population results in groundwater pumping, which creates more pollution in the pumping process as well as damaging the infrastructure of cities.