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Well-known by the desert land, strong solar radiation, and cactuses, Mexico includes certainly such extreme arid condition in about two thirds of its territory – the central part and most of the north. A latitude at the world deserts belt as well as a horn-like contour toward the US border with a mountainous relief in the periphery altogether precludes the passage of humid clouds from the narrow coastal slopes toward the inner enclosed land. In contrast, the south and southeastern regions contain not only exuberant tropical forests and major river courses but also the recurrent threat of hurricanes and floods. Thus, the specific physical and chemical characteristics of surface and groundwater depend, firstly, on the water cycle. As mentioned before, the atmospheric precipitation falls in disproportion over the Mexican territory and eventually drains over the surface relief as runoff to then incorporate into lakes/reservoirs or permeates the soil into the underground for a last long storage. The ultimate interaction with the geologic substrate and anthropic by-products provides the particulate and dissolved composition of the hydric resources. Mexico’s southern portion undergoes a comparatively faster water renewal time than the rest of the country, and therefore the waters keep cleaner and diluted in this area; the central part of the country concentrates a maximum human/economic development around the Mexico Valley basin with consequent organic loadings and eutrophication for the nearby areas; while the north agonize in the scarcity of water, the availability depends mostly on fossil groundwater.