Additional Case Study Simulations of Dry Well Drainage in the Tucson Basin

Abstract

Executive Summary: Three case study simulations of dry well drainage were performed using the saturated-unsaturated groundwater flow model UNSAT 2. Each case simulated injection of storm water runoff water into a dry well from two five-year, one-hour storm events, separated by a 24-hour lag period. The first case assumed subsurface conditions of a uniform gravelly sand material from land surface to the water table at 100 feet below land surface. The second case assumed the same gravelly sand, underlain by a uniform sandy-clay loam material beginning at 30 feet below land surface and extending to the water table. The third case assumed the same conditions as in Case 2, except for a sandy loam soil replacing the sandy-clay loam material. Simulated subsurface flow of injection water for the first case was primarily vertical. The cross-sectional radius of the 95% saturated portion of the drainage plume reached a maximum of about nine feet during stormwater injection. In the second and third cases, horizontal flow took place at the layer boundary between the gravelly sand and underlying fine material. As a result, the cross-sectional radius of the 95% saturated portion of the drainage plume reached a maximum of about 27 feet for Case 2, and about 21.5 feet for Case 3. Arrival times of injection water at the water table varied from between 0.25 and 0.75 hours (Case 1), and between 130 and 150 hours (Case 2). Attenuation of water-borne pollutants in the vadose zone is related to the degree of exposure of drainage water to soil particle surfaces. The specific surface area of soil particles to which drainage water was exposed was used as an indicator of the relative degree of attenuation that may take place among the three cases. The ratio of specific surface area of soil matrix exposed to the portion of the subsurface reaching a state of 80% saturation was approximately 1 : 16.2 : 5.6 (Case 1 : Case 2 : Case 3).