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Models of the Transport of Deep-Well Injectate at the North District Wastewater Treatment Plant, Miami-Dade County, Florida

Project Chief: Jeffrey N. King
Cooperator: Miami-Dade Water and Sewer Department
Period of Project: May 2007 - September 2012

Problem Statement

Figure 1. North District Wastewater Treatment Plant, Miami-Dade County, Florida; injection wells IW-1, IW-2, IW-3, and IW-4; Floridan aquifer monitoring wells FA-1, FA-2, FA-3, and FA-4

The Florida Legislature found that "elimination of ocean outfalls as a primary means of domestic wastewater discharge are in the public interest." In response to regulatory requirements associated with this finding, the Miami-Dade Water and Sewer Department (MDWASD) is transitioning the primary means of treated, domestic wastewater discharge at the North District Wastewater Treatment Plant (NDWWTP) (fig. 1) from the existing ocean outfall to reuse and deep-well injection. The NDWWTP is in Miami-Dade County, Florida, near the City of North Miami Beach.

MDWASD began construction of injection wells IW-2 and IW-3 in 1994, and injection wells IW-1 and IW-4 in 1997. MDWASD injects secondarily treated domestic wastewater (effluent) from the NDWWTP through these four Class I injection wells, into a hydrogeologic unit at the base of the Floridan aquifer system known as the Boulder zone (fig. 2). Class I injection wells are industrial or municipal disposal wells that inject fluids beneath an underground source of drinking water (USDW). A USDW is an aquifer that contains a sufficient quantity of groundwater to supply a public water system, where the groundwater contains fewer than 10,000 milligrams per liter total dissolved solids. The Boulder zone is a transmissive hydrogeologic unit spanning an interval from approximately 900 to 1,000 meters below ground surface at the NDWWTP. The Boulder zone is beneath USDWs that are within both the Floridan and surficial aquifer systems.

MDWASD began a 28-month test of injection wells IW-2 and IW-3 on June 17, 1997. This test was the first instance of the underground injection of effluent at the NDWWTP. Dual-zone monitoring wells exist within the Floridan aquifer system at the NDWWTP, above the Boulder zone. The lower monitoring zone is primarily within the Avon Park permeable zone and the upper monitoring zone is primarily within the Upper Floridan aquifer (fig. 2). In May 1998 (11 months after the June 1997 commencement of the injection test), MDWASD began to observe elevated concentrations of effluent constituent NH₄⁺ in the Avon Park permeable zone (lower monitoring zone). In April 1999 (22 months after the June 1997 commencement of the injection test), MDWASD began to observe a decrease in total dissolved solids, or freshening, of the Avon Park permeable zone (lower monitoring zone). MDWASD did not observe effluent constituent NH₄⁺ with an associated freshening in the Upper Floridan aquifer (upper monitoring zone) until May 2009. The Upper Floridan aquifer (upper monitoring zone) is a USDW; the Avon Park permeable zone (lower monitoring zone) and Boulder zone (injection zone) are not USDWs.

Figure 2. Counter-clockwise from upper left: Location map in the x-y plane for hydrogeologic sections A-A’ and B-B’ at the North District Wastewater Treatment Plant, Miami-Dade County, Florida. Axes are distance in meters from the origin of the Albers conical equal area projection. Location map is oriented 90 degrees clockwise, such that north is oriented to the right, along the y-axis. Vertical, south-north oriented section A-A’ in the y-z plane. Vertical, west-east oriented section B-B’ in the x-z plane. Hydrogeologic units are shown as horizontal colored regions, where the light blue unit labeled UFA is the Upper Floridan aquifer, the light green unit labeled APPZ is the Avon Park permeable zone, and the dark orange unit labeled BZ is the Boulder zone. Vertical sections show injection well cased intervals as vertical black lines, injection well open interval as vertical red lines, Floridan aquifer monitoring well cased intervals as vertical grey lines, and Floridan aquifer monitoring well open intervals as vertical red lines. Position within each section is expressed as elevation in meters relative to NGVD 1929, versus distance in meters from the origin of the Albers conical equal area projection.

Objectives

The major objective of this project is to characterize flow paths that permit effluent to be transported from the Boulder zone (injection zone) to the Avon Park permeable zone (lower monitoring zone).

Approach

Study objectives are met by building models that show how effluent can be transported from the Boulder zone (injection zone) to the overlying Avon Park permeable zone (lower monitoring zone).

Results

Not yet available.

Information Product

A U.S. Geological Survey Scientific Investigations Report will describe project results.