URBANIZATION AND ITS EFFECTS ON THE HYDROLOGIC CYCLE IN THE SANTA ANA BASIN, CALIFORNIA, USA

Urbanization dominates the landscape in the Santa Ana Basin, which is home to more than 4 million people. Consequently, there has been a transformation of the hydrologic cycle including changes in the volume, routing, and timing of flow in surface and ground water; as well as the chemical composition.

From a mass-balance perspective, water is cycled twice in the inland and coastal parts of the Santa Ana Basin. In the first cycle, headwater tributaries in the Inland Basin are diverted to ground-water recharge facilities. In turn, ground water is extracted for use by consumers, routed to waste water treatment plants, and then discharged to the Santa Ana River. The volume of treated waste water released to the river is about equal to the volume of base flow exiting the Inland Basin and entering the Coastal Basin.

In the second cycle, the entire base flow and most of the stormflow of the Santa Ana River is retained in recharge facilities used to replenish the coastal aquifer system; base flow consists primarily of treated waste water, and stormflow consists primarily of urban runoff. The volume of water recharged in the Coastal Basin accounts for about three-quarters of the ground water pumped from the coastal aquifer system. After use, the water is routed through treatment plants before being released to the ocean.

The influence of urbanization can be seen in the presence of volatile organic compounds (VOCs) in surface and ground water. VOCs have been detected in all monthly and storm samples obtained at two surface-water monitoring sites, and in about two-thirds of the more than 200 municipal production wells sampled in the Santa Ana Basin and adjacent areas of the southern California coastal plain. The spatial distribution of VOCs in the coastal aquifer system, along with the distribution of tritium and the stable isotopes of water, indicates regional-scale replacement of native ground water in the coastal aquifer system by water recharged during the past 50 years.