Douglas Smith: Septic tanks don't threaten springs
Published: Monday, March 3, 2014 at 6:01 a.m.
Last Modified: Friday, February 28, 2014 at 11:19 p.m.
Over the past several years, The Sun has been posting editorials on the degradation of Florida's springs. These editorials consistently point to "leaky" or otherwise malfunctioning septic tanks as being a significant source of the nitrate nitrogen found in spring waters.
The Sun supported legislation requiring the periodic inspection of the 2.7 million septic tanks in existence in Florida, supposedly in order to protect north Florida's springs from further nutrient related degradation.
A Dec. 20 editorial recommends stemming future septic tank construction, which presumes either the requirement that any new development be connected to a centralized wastewater treatment plant or that it be denied altogether.
A simplified definition of pollution is the concentration of harmful or undesirable constituents. Fertilizers, animal feed lots and centralized wastewater treatment plant effluents are all examples whereby nitrogen is concentrated. About 100 percent of the nitrogen in wastewater treatment plants is converted to nitrate-N.
Septic tanks are decentralized treatment units that do not actually discharge nitrate-nitrogen. A study posted on the Florida Department of Environmental Protection's web site entitled "Wakulla County Septic Tank Study" found that 100 percent of effluent nitrogen from conventional septic tanks is in the reduced forms of ammonia nitrogen plus organic nitrogen, and the study shows that greater than 90 percent of effluent nitrogen from septic tanks is in the form of ammonium.
The ammonium ion, having a positive valence, can be readily adsorbed by soils found throughout Florida and is probably not a significant threat to our springs. Dissolved nitrate-N is negatively charged and readily migrates with the groundwater.
The conversion from ammonia-N to nitrate-N in soils underlying septic tank drain fields requires an abundance of dissolved oxygen and the presence of nitrifying bacteria, neither of which is generally available below the root zone in Florida's soils.
The Florida Springs Initiative Monitoring Network Report, also found on the FDEP web site, states that ammonia nitrogen is seldom detected in spring water and that where ammonia-N does show up, no significant trends are found.
A U.S. Geological Survey investigation of nitrogen sources within the Suwannee River Basin contains graphs of nitrogen sources plotted for each of the basin's five counties. These graphs show septic tank contributions to be within a range of 0 to 3 percent of the total nitrogen input.
These levels of septic tank nitrogen were estimated by performing a mass-balance analysis based on population and not on actual monitoring data. This and other reports assume that the nitrogen from septic tank effluent is somehow oxidized to nitrate nitrogen, thus becoming a spring pollutant.
Septic tanks are not perfect systems and are not especially suited to areas where high water tables and artificial drainage exist. But these systems are, for the most part, self-regulating, in that clogged tanks and drain fields which lead to plumbing backups or effluent ponding are very likely to be repaired in rapid fashion.
In general, septic tanks, malfunctioning or not, do not have any measurable impact on Florida's springs. Such impacts can be attributed to fertilizer, feed lots, the atmosphere and centralized wastewater treatment systems, in that order.
Douglas Smith lives in Newberry.