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Background

Previous studies have identified 49 landfills located in Hillsborough County, Florida that were used for solid waste disposal between 1947 and 1978. It is believed that many of the landfills were used and/or operated by the City of Tampa (the City). These landfills actually served as a secondary means of waste disposal, since the primary means of solid waste processing, during this period of time, was incineration. Originally, most of these landfills were located near the outer extent of the City boundaries as they existed at that time. However, later expansion has resulted in many of these landfills now being located well within developed areas of the City. In many cases, these areas have been subsequently developed for industrial, residential, commercial, and recreational uses.

During the early 1980s, the State of Florida Department of Environmental Regulation, now known as the Florida Department of Environmental Protection (FDEP), requested that the City develop a database that incorporated all of the known landfills previously used by the City for waste disposal. The landfills would then be the subject of investigations to assist in an evaluation of their impact on the surrounding community. The original landfill inventory was compiled and reported by Florida Land Design & Engineering, Inc. (FLD&E) in a November, 1984 document entitled: Landfill Site Inventory, Prioritization, and Water Quality Monitoring Plan (FLD&E, 1984). This work was contracted by the City of Tampa.

During the 1980s and early 1990s, FLD&E and Florida Groundwater Services, Inc. conducted a number of landfill investigations. The sites selected for these early investigations were generally based on the 1984 site priority schedule. However, the scope of these investigations was limited and typically varied from site to site. According to the FDEP, the early work does not adequately characterize the environmental condition of these sites as required by standard Preliminary Contamination Assessment Actions (FDEP, 1992).

HSA was contracted by the City in 1992 to develop and implement a Preliminary Contamination Assessment Plan (PCAP) for those landfills that had not been previously investigated. The PCAP was submitted to the FDEP on 8 April 1993. The PCAP was implemented at a number of sites between 1993 and 1995. HSA was also assigned the task of addressing regulatory comments, and undertaking additional site investigations at previously investigated landfills.

As work progressed and new information was complied, it became clear that certain recognized environmental conditions (RECs) were of greater immediate concern to the public and the FDEP than others. In December of 1995, FDEP and the City agreed that available resources should be temporarily refocused away from the assessment of a limited number of sites each year, towards the near-term identification of potential imminent threats to the public health and welfare at all of the landfill sites. The Historic Landfill Assessment Program was initiated to accomplish this task. The Historic Landfill Assessment Program is designed to identify exposed solid wastes, sensitive potential receptors (i.e., groundwater users), and hazardous concentrations of landfill gases. The information collected during this Historic Landfill Assessment can be used to assist in 1) an evaluation of the potential risk each landfill may pose to the community, and 2) prioritizing future investigative efforts in accordance with the City’s ongoing Landfill Investigation Program.

For the purposes of this investigation, all landfills are referenced according to the "Identification Numbers" (Table 1). However, it should be noted that the various consultants and regulatory agents working on these and other investigations at the landfill sites, have frequently used prioritization numbers (which are subject to change) and property names of individual parcels located within the landfills, for correspondence items and reports. The Hillsborough County Environmental Protection Commission (EPC) also maintains its own unique numbering system for these sites. Therefore, caution should be exercised when referencing landfill numbers and site descriptions.

Table 1 also lists the "Prioritization Ranking Number" based on a 1994 revised ranking scheme that incorporated additional criteria. The table includes additional general information characterizing the landfill in terms of the type of debris thought to be contained in the landfill, its ownership, the size of the property, the year it was closed, the year and by whom the site has been investigated, and the present condition of the property above the landfill.

Objectives of the Historic Landfill Asessment

The objectives of the Historic Landfill Assessment are to:

  1. Identify areas of exposed solid waste;
  2. Identify users and uses of groundwater in close proximity to each of the historic City landfill sites; and
  3. Identify the presence and concentrations of landfill gases which may exist within the landfill.

Areas of exposed solid waste at some landfill sites may be subject to the Rules of the EPC. Solid Waste Management Rule 1-7.045(5)f requires that a minimum cover be maintained over buried solid waste in sanitary landfills. Exposed solid waste may also present physical hazards, and development constraints.

The identification of groundwater users and uses is important because water soluble substances that are dumped, spilled, spread, or stored on the ground may, over time, infiltrate the subsurface. Furthermore, because these historic landfill sites are not covered or lined with impermeable material, rainwater has infiltrated and facilitated the decomposition of landfill materials. The liquid leachate formed, can enter the underlying groundwater and/or nearby surface water bodies. In sufficient concentrations, the leachate may be of concern to the neighboring groundwater users and the environment.

Landfill Gas (LFG) is mainly composed of carbon dioxide (CO2), hydrogen sulfide (H2S), and methane (CH4). Methane generation is the result of biological decomposition of organic material under anaerobic conditions. Methane is ignitable at concentrations between 5 percent and 15 percent by volume in air. Due to its volatility and low density, methane can displace oxygen and may act as an asphyxiating agent in confined spaces. The gas may migrate within the subsurface, and accumulate under and/or within structures or subsurface voids.

The U.S. Environmental Protection Agency (U.S.EPA) has promulgated minimum criteria for municipal solid waste landfill (MSWLF) units under the Resource Conservation and Recovery Act (RCRA). These criteria include provisions for the control of explosive gases (Title 40 CFR 258.23) at regulated landfill sites. According to these regulations, the concentration of methane gas should not exceed 25% of the Lower Explosive Limit (LEL – 1.25 percent of total gas volume) within facility structures, and should not exceed the LEL at the facility property boundary. If the methane concentrations in the building exceed 25% of the LEL, operators or owners of the units are required to: (a) undertake immediate measures for protecting human health and notify the State Director of the Solid Waste Program; (b) record gas levels and the measures taken within 7 days of detection; and (c) implement a remediation plan, maintain the plan on file, and notify the State Director of its implementation within 60 days.

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