EFFECTS OF ROOFTOP RUNOFF ON TRACE METAL CONTENT IN SOILS ON AN URBAN COLLEGE CAMPUS

St. Mary’s University is a private Catholic 4-year university located in a mixed-use urban area on the west side of San Antonio, Texas. The 135-acre campus is largely developed with slightly more than 50% impervious cover consisting of buildings, parking lots, and walkways. Although much of the storm water runoff from buildings and impervious cover quickly enters the municipal storm drain system, some of the runoff empties through building downspouts and overland flow onto open soils, potentially introducing a variety of different compounds into the campus environment. Many studies in the soil chemistry literature have previously indicated that soil minerals can adsorb transition and other metals, resulting in locally elevated concentrations. The purpose of this study is to measure trace metal concentrations in soils around the campus to examine the influence of rooftop runoff on soil chemistry, and evaluate the water quality effects from different roofing materials of differing ages. Soils were collected from 3- to 12-in depths near downspouts from eight buildings across campus, as well as from undeveloped areas of the campus. These buildings were selected to represent a variety of roofing materials, roofing styles, and ages. Trace metal concentrations for the samples were determined using a portable X-ray Fluorescence analyzer. Preliminary results suggest some geochemical fingerprinting is possible. For example, buildings with galvanized roofing showed higher soil-zinc concentration, while soils near other buildings showed a higher concentration in copper due to copper guttering. Although elevated above background, observed concentrations were generally well below regulatory limits established by the Texas Commission on Environmental Quality. Preliminary data also suggested that metal concentrations dropped off quickly with distance from the downspout. The results of this work could be used to evaluate the potential effects of roofing materials on long-term rooftop runoff water quality on the St. Mary’s campus, a potentially important consideration in the ongoing implementation of rainwater harvesting as a source of non-potable water for the university.