A comparison of GIS-based approaches to environmental equity analysis

GIS-based analyses of environmental equity have often relied on circular buffer zones around hazardous sites to represent the area affected by an environmental hazard. This approach is questionable because physical processes rarely operate in a perfectly symmetrical manner. A new integrated approach, known as geographic plume analysis, accounts for directional biases in the distribution of hazards by using a chemical dispersion model to identify the area that is likely to be exposed to toxic releases. In this paper we implement, evaluate, and compare circular and plume-based approaches to environmental equity assessment in the city of Des Moines, Iowa. The analyses are based on locations of hazardous facilities listed in EPA’s 1994 Toxic Release Inventory (TRI) database. At each toxic site we generated: (i) circles of radii 0.5 and 1 mile, and (ii) a composite plume footprint based on the chemical released in the largest quantity at the facility, using a set of averaged weather conditions. Using the analytical capabilities of GIS software and 1990 Census data at the block group level of aggregation, we computed the racial and economic characteristics of the population residing within these two types of hazard zones. To determine the existence of inequity, we compared the results of each approach to the socio-demographic distribution for the entire city, and to populations residing outside each type of hazard zone in the city. Our results indicate that when a plume-based approach is used to determine the areas vulnerable to airborne toxic releases, a larger proportion of nonwhites and low-income households are found to reside within the high-risk regions.