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Miami-Dade
County, Florida
Source Number: 2.4.1 Source Description Also, water quality data from 30 monitoring wells associated with a Class I underground injection system operated by the MDWASD, and located at the MDWASD's South District wastewater treatment plant in southeastern Dade County, Florida. The data were submitted by MDWASD to the Florida Department of Environmental Protection, Tallahassee, Florida. Data cover the period January 1992 through September 2001. Data collection began in 1983. Data prior to 1995 are reported to be unreliable. Data are unpublished but available. Keywords Executive Summary The deep monitor zone, designated "lower" or "1,800-foot zone" (wells are designated "FAL-xx") monitors water quality between 1,790 and 1,890 feet bgs. This zone contains groundwater with an ambient chloride concentration of 19,000 milligrams per liter (mg/l), which is equivalent to seawater and also to the concentration in the Boulder Zone. The primary shallow monitor zone, designated "upper" or "1,500-foot" zone (wells designated "FAU-xx") monitors water quality between 1,490 and 1,590 feet bgs. It contains water with an ambient chloride concentration of 3,500 mg/l. The wastewater flow stream being injected into the Boulder Zone has a typical chloride concentration of about 100 mg/l. Monitored parameters include:
The monitoring data indicate the presence of wastewater (as high as 80 percent wastewater in a few samples) in 7 of 15 upper zone monitor wells and in 8 of 15 lower zone monitor wells. Primary indicators of the presence of wastewater are elevated nitrate, ammonia, and organic nitrogen concentration. An increase or decrease in chloride and total dissolved solids (TDS) from pre-injection background concentration also may be indicative of the presence of wastewater. Coliform bacteria, present in high numbers (104 to 105 col/100ml) in the wastewater effluent, which has received secondary wastewater treatment, are a potential indicator of wastewater. However, coliform bacteria were not detected in any of the monitor wells in either the upper or lower zones during the monitoring period reported by this source. Monitoring samples consist of three end-member water types having distinctive distinguishing characteristics. Mixing fractions of wastewater in the monitoring samples can be estimated from the end-member types. The effluent fraction, in samples clearly influenced by wastewater, ranges from 3 to over 80 percent. The age (time since injection) of monitoring samples can be estimated from the tritium content of the wastewater fraction in the samples. Ages of nine (of ten) 1,500-foot zone samples appear to be 1 year or less. Apparent ages of the wastewater fraction in the remaining four samples clearly affected monitoring wells range from three to ten years. Shorter travel times were evident in the 1,800-foot monitoring interval. Dilution factors and wastewater travel times can be estimated with reasonable accuracy from mixing fraction calculations. The data suggest that fecal coliform bacteria concentrations attenuate by at least five log cycles during a travel time of less than one year under temperature and salinity conditions prevailing at this site in South Florida. Graphics and
Tables Water Quality Analysis
from Monitoring Zone 15 FAL (1,790 - 1,890 feet) - Data
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