Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings ofthe  Groundwater Quality 2001 Conference held at Sheffield. UK, June 2001). IAHS Publ. no. 275. 2002.  561 Arsenic and lead pollution of the Salamanca aquifer, Mexico: origin, mobilization and restoration alternatives RAMIRO RODRIGUEZ, AURORA ARMIENTA Geophysics Institute, UNAM, Cd. Universitaria, 04510 Mexico City, Mexico e-mail: rrdz@tonatiuh.igeofcu.unarri.rrix JOEL BERLIN Ecology Direction, Salamanca Municipality, CIA, Salamanca Glo, Mexico J. ANGEL MEJIA COTAS, Parque Ecolôgico, Salamanca Glo, Mexico Abstract Arsenic and lead concentrations, around and over Mexican standards for drinking water, were detected in the Salamanca aquifer system. Groundwater is the only water supply source for the more than 140 000 Salamanca inhabitants. A refinery, a thermoelectric plant and agrochemical industries are established in the urban area. Three aquifer units integrate the aquifer system: a shallow one, a semi-confined intermediate one and a deep formation. The highest As and Pb values correspond to the non-exploited shallow formation. The deep aquifer contains As but not Pb. A monitoring programme is being earned out in wells, shallow dug wells and piezometers. Aquifer vulnerability mapping, using the AVI and DRASTIC methods, revealed the influence of a subsidence fault on the solute transport. Variations in the concen- trations are related to the abstraction regime, the fault extension and also to the irregular distribution of clay and sand layers. Key words aquifer pollution; arsenic; hydrocarbons; lead; Mexico; restoration; subsidence faults THE SALAMANCA CASE Salamanca, in Guanajuato State, central Mexico, is an important industrial centre (Fig. 1). A Pemex (Mexican oil company) refinery, a thermoelectric plant and pesticide manufacturing industries are located in the urban area. Agriculture is also an important activity. Groundwater is the only water supply source for the more than 140 000 inhabitants. In the basin there are more than 1900 wells; 1600 of them are active. 85% of the abstraction is used for agriculture. An active subsidence fault associated with the intense abstraction regime crosses the urban area and the refinery lands. It has been recorded since 1982. The accumulated displacement is about 60 cm in the urban area and 1.5 m in "J. Rosas", a rural community located 7 km from Salamanca (Garduno et al, 2001). The fault provoked a pipeline breakage and a hydrocarbon spill. Free phase was detected in one urban well. The urban wells have 40 m of close casing preventing flows from the shallow