Ground-Water Recharge from Urban Runoff and Irrigation Returns

Abstract

Preliminary information on urban runoff from selected small watersheds in the Tucson area indicates that average annual runoff from the urbanized areas is more than four times as much as that of a comparable undeveloped desert area, and may be ten times as much in some individual years. The urban runoff contains relatively high concentrations of bacterial loading and dissolved organics; although it is not now known to be a seriously hazardous source of pollutants in ground water, urban runoff should be monitored with increasing urban growth, especially for content of organics, microorganisms, and trace metals. Additional study also should be given to the travel-time regime of runoff from the small tributary urban watershed to the major stream channels where recharge mainly occurs. Deep percolation from irrigation return flows was evaluated during a one -year study for the U.S. Geological Survey's "Southwest Alluvial Basin, Regional Aquifer System Assessment Program". Objectives of the study included (1) identifying sources of recharge information, (2) collecting and summarizing available recharge information, (3) identifying methods for interbasin transference of recharge values, (4) characterizing deep percolation models, and (5) itemizing methods for overcoming data gaps. Apparently there is a difference in opinion among irrigation experts on the extent to which recharge from deep percolation occurs. One reason for the difference of opinion is that field measurements of the flux and velocity components of deep percolation through the vadose zone are scarce, particularly for deep alluvial basins. Similarly, there is a need for a simple, theoretically-based model of deep percolation /recharge. Many of the data deficiencies could be overcome by conducting lumped and site-specific field studies. Such studies, although expensive, would be timely in light of the current interest in ground-water management.