Flow in the unsaturated zone, Tucson, Arizona

Field studies in Tucson, Arizona were conducted based on the discovery of contaminants in groundwater from a substantial number of municipal and private wells, and evidence that the presence of a thick unsaturated zone does not prevent the eventual migration of contaminants to regional groundwater systems. A pulse of water containing a tracer was monitored as it passed through the unsaturated zone by using six soil-moisture samplers (lysimeters) that were installed at depths of 11 to 45 ft, 10 ft apart, beneath a shallow, manmade, 3/4-acre recharge basin. The tracer was allowed to infiltrate into the ground for seven days, and water was collected from samplers at 12-hr intervals for about 30 days. Well defined tracer peaks, presented graphically as tracer breakthrough curves, were observed for all but one of the lysimeters installed at the site. Maximum tracer concentration showed no consistent relation with depth; tracer breakthrough sometimes occurred earlier in deep sampling locations than in shallow ones. Rather than moving straight down, water movement occurred along preferential flow paths, referred to as bypass or macropore flow, probably at low soil tension which occurs when the soil is near saturation. Under such conditions, contaminant arrival times can occur sooner than would be expected if flow was assumed to occur as a uniform wetting front that pushes ahead of it the water previously stored in the pores of the unsaturated sediments. Much of the water stored in the soil profile is not involved under conditions of bypass flow, and less interaction occurs between the recharge water and the solid matrix of the unsaturated zone. Therefore certain substances such as chlorinated hydrocarbons (trichloroethylene, pesticides) , other refractory organic compounds (detergents, humic acid), and microorganisms (bacteria) could pass into the underlying groundwater. Results of this study have applicability throughout the southwestern United States as well as in other parts of the country where the practice of artificial recharge is being considered. (Lantz-PTT)