Coastal pollutant Loading from On-Site Treatment & Disposal Systems

Household wastewater contains a variety of constituents, including 1) oxygen-demanding organics, which can deplete dissolved oxygen in receiving waters; 2) pathogenic microorganisms, which can spread disease; and 3) nutrients, which can stimulate the growth of aquatic plants. Domestic wastewater may also contain potentially toxic, mutagenic, or carcinogenic compounds. For these reasons, proper disposal of this waste stream is necessary to protect the public health and the environment. As opposed to a centralized treatment facility, which collects, treats, and discharges wastewater on a regional basis, onsite treatment and disposal systems (OSTDS) treat and discharge sewage within the site where the wastewater originates. The goal of an OSTDS is to adequately treat a relatively small waste flow to prevent the surrounding surface water, groundwater, and soil from being contaminated. OSTDS are generally most applicable to residential use. The most common type of OSTDS is the septic tank system (Kaplan 1991). Septic tanks are buried, watertight receptacles designed and constructed to receive wastewater, separate and store solids, provide limited digestion of organic matter, and allow partially clarified effluent to discharge to a gravel drainfield through a series of perforated underground pipes (USEPA 1999) for soil absorption. When operated under normal conditions, a typical septic tank system can be expected to remove about 50 to 70 percent of suspended material. Andreadakis (1987) noted that septic tanks (when properly operated and simulated in the laboratory) can remove over 90 percent of BOD and TSS, with ammonia nitrogen removals of up to 70 percent through nitrification, but when OSTDS are not properly sited, installed or maintained, the amount of treatment is reduced substantially, and the partially treated effluent may pose a higher potential risk to public health and the environment. As of 2000, onsite treatment and disposal systems serve approximately 25 percent of the population of the United States. The Bureau of the Census indicated that 10 percent of those systems have stopped working (USEPA 2000), a figure that may exceed 70 percent in some communities. A substantial portion of the over 5,200 water bodies in the United States do not meet ambient water-quality standards for their designated uses as a result of pathogens, and nearly 4,800 are impaired as a result of nutrients, causally linked to failed OSTDS (USEPA 2000). DeBorde and coworkers (1998) found that 42 percent of all waterborne disease outbreaks were associated with untreated groundwater impacted by septic tanks. It was also reported that viruses exhibited long survival times and low sorption and therefore could travel well over 100 feet in the subsurface, while remaining infectious and in quantities that could readily cause infection. In the year 2000, an estimated 2.3 million OSTDS were in operation in Florida, serving approximately 4.5 million people (FDEP 2001). These systems discharge over 426 million gallons per day into the subsurface soil environment. Nearly 40 percent of those systems are found along Florida’s southeastern Atlantic coastline, and according to the Florida Department of Health (FDoH), the number of septic tanks installed by registered contractors has doubled over the last 30 years (see Figure 1). Throughout the state of Florida where the water table is high (< four feet below ground surface), OSTDS have proven to be problematic from a groundwater protection/water resource perspective (Bloetscher and van Cott 1999). Impacts are traced to a lack of regulations before 1980 and to high densities of septic tanks on relatively small lots—a problem that is particularly acute in summer when the groundwater table is highest. Lack of proper maintenance is also believed to be a contributing factor. An added complication is that many of the high-density developments, historically inhabited only in the winter months when the water table was low and performance was optimal, are now inhabited by full-time residents. D. E. Meeroff, Ph.D., and Frederick Bloetscher, Ph.D., P.E. , are assistant professors in the Department of Civil Engineering at Florida Atlantic University. At the time this article was written, F.J. Morin, MSCE, was a graduate student in the department.