Delimitation of a hydrogeological reserve for a city within a karstic aquifer: the Merida, Yucatan example

Merida is the largest city in southeastern Mexico, where the urban supply is obtained mainly from three well®elds outside of the urban limits in a karst aquifer that is particularly vulnerable to contamination. Merida, Yucatan has a population greater than 600 000 inhabitants. The city obtains its freshwater from a thin (<50 m thick)sole-source aquifer. This aquifer also supplies water to satisfy the industrial and agricultural needs of the region. In order to help in maintaining an adequate water supply for the city, a detailed study has been conducted to determine the best area to establish a hydrogeological reserve zone (HRZ). A polygon of 900 km2 was chosen southwest of the city. This area is upgradient from Merida, no industrial activity is present, and there are two natural boundaries that bound the proposed HRZ. These are no-¯ow boundaries that have developed as a result of the second boundary: a high permeability zone locally known as the ring of cenotes (ring of sinkholes). The major current obstacle to designate this area as a hydrogeological reserve is an industrial land®ll that is present in within the proposed area. As part of the establishment of the HRZ, the land®ll should be relocated, and a comprehensive ground water quality study should be conducted to determine the actual water quality of the aquifer.