Vertical migration of municipal wastewater in deep injection well systems, South Florida, USA

Abstract

Deep well injection is widely used in South Florida, USA for wastewater disposal largely because of the presence of an injection zone ("boulder zone" of Floridan Aquifer System) that is capable of accepting very large quantities of fluids, in some wells over 75,000m 3 / day. The greatest potential risk to public health associated with deep injection wells in South Florida is vertical migration of wastewater, containing pathogenic micro- organisms and pollutants, into brackish-water aquifer zones that are being used for alternative water-supply projects such as aquifer storage and recovery. Upwards migration of municipal wastewater has occurred in a minority of South Florida injection systems. The results of solute-transport modeling using the SEAWAT program indicate that the measured vertical hydraulic conductivi- ties of the rock matrix would allow for only minimal vertical migration. Fracturing at some sites increased the equivalent average vertical hydraulic conductivity of confining zone strata by approximately four orders of magnitude and allowed for vertical migration rates of up 80m/year. Even where vertical migration was rapid, the documented transit times are likely long enough for the inactivation of pathogenic microorganisms. Resume Les injections par puits profonds sont largement utilisees au sud de la Floride (Etats-Unis) pour stocker les eaux usees, essentiellement du fait de la presence d'une zone d'injection ("zone des blocs" du Systeme Aquifere de Floride) apte a accepter des quantites considerables de fluides : plus de 75,000m 3 /jour dans certains puits. Le plus