EVIDENCE FOR COMPARTMENTALIZATION OF AQUIFER SYSTEMS: SOLUTE AND ISOTOPE GEOCHEMISTRY OF GROUNDWATERS IN THE MIDDLE SAN PEDRO BASIN, ARIZONA

Abstract

The Middle San Pedro Basin in southeastern Arizona is a typical alluvial basin in the semi-arid southwestern United States with a rapidly growing population that is dependent upon groundwater resources for water supply. This study investigated recharge areas, compartmentalization and potential mixing of water sources, and travel times of groundwater throughout the basin using variations in major ion chemistry (water type, Ca/Sr ratios, SO4/Cl ratios) and isotope ratios (18O, 2H, 3H, 34S, 13C, 14C) of groundwaters, surface waters and precipitation in conjunction with hydrogeologic data (e.g. hydraulic head and hydrostratigraphy). Recent recharge (<50 years) has occurred within mountain systems along the basin margins, and in shallow floodplain aquifers adjacent to the San Pedro River. Groundwaters in confined aquifers in the central basin were recharged at high elevation in the fractured bedrock and have been extensively modified by water-rock reactions over long timescales (up to 34,600 years). These results can be used to constrain physical assumptions of future groundwater flow models designed to help make improved water management decisions.