Methane and Lake Fills in South Florida

If you fly over south Florida and look out the window, you are likely to observe man-made lakes dotting the landscape. Those lakes were often used for mining limestone aggregates. According to the South Florida Water Management District there is an approximately 89-square mile area in northwestern Miami-Dade County that historically provided a sizeable portion of the nation’s aggregate. Because the water table is shallow the mining results in water filling the depressions created, and viola there is a new lake.

In that past several years developable land in South Florida has grown increasingly scarce as property values have continued to climb. As a result, there has been an increased interest in developing areas that are further west, many times filling lakes to do so. Those areas are often closer to the turnpike, and thereby are uniquely positioned to support logistics and commerce in the region.

While traditional contaminants such as benzene and other hazardous waste are investigated as a routine matter in a typical ASTM-1527 Phase I ESA, another matter can sometimes be overlooked. I once read a report for a lakefill that described workers fishing a variety of miscellaneous materials, e.g., wood, scrap metal, rebar, and general refuse from of a lake that was supposed to be filled in with clean fill material under an issued permit. The anaerobic breakdown of organic matter leads to off-gassing of hydrogen sulfide and methane, which are common landfill gases. There is also the geotechnical hurdle that comes along with building on non-homogeneous material.

Methane is a significant concern because it isn’t something that you can smell or taste standing in a building. However, methane at concentrations of between 5 (its lower explosive limit [LEL]) and 17 (its upper explosive limit [UEL]) percent by volume are flammable conditions, i.e., you could blow yourself up lighting a cigarette in the bathroom. Unfortunately, there does not appear to be robust environmental scrutiny on this issue at this time. The FDEP guidance available regarding methane is primarily for old, closed landfills or known waste disposal areas. If you are interested in learning more about this issue, please reach out to Lion Point Engineering.