Septic systems were discussed extensively at the Captiva Community Panel meeting on September 14th as the panel continues to seek research into whether the systems have an impact on coastal waters.
According to Doug Eckmann, chief engineer and project manager for Captiva’s wastewater project at Kimley-Horn, the main source of nutrients for coastal waters in an island community comes from rainwater runoff, which is a problem. He said there are septic tanks that rely on drain fields to defuse the liquid.
“Septic systems have done an excellent job of eliminating bacteria in the past. Sewage treatment plants are based on a treatment in dry soil and are generally ineffective at the biological nutrient reduction in wastewater. “ said Eckmann. “In sandy soils like Captiva, the water just flows through and then the high water table doesn’t leave much time for biological processes.”
Eckmann said rainwater makes a 64 percent contribution to nitrogen pollution, while sewage treatment plants contribute 34 percent.
“In the saturated sandy soil, very little nitrogen is removed”, he said.
CCP President Jay Brown said there must be at least 24 inches between the drainage field and the water table below. When all of the island’s septic tanks were sampled, it was found that at least 24 percent had no separation.
“If you realize that we have very sandy, porous soil, sewage treatment plants would really be a problem.” he said, adding that it will become a nitrogen loader due to the lack of compliance.
Brown, studying all of the permits for sewage treatment plants, said they shouldn’t be on much less than an acre and a half. He said the lots in the village area are less than half an acre to a quarter of an acre.
“We do not have enough vertical separation between the drainage field, groundwater and land are too small” he said.
The panel also heard from Mark Thompson, a research fellow with the Sanibel-Captiva Conservation Foundation, who shared his research on septic tanks. He explained that the runoff from a septic tank flows into a drainage field that is treated with nitrogen in the soil. Nitrogen occurs at the bottom of a drainage field and at the top of the water table.
“When talking about nitrogen load or nutrient load from a septic tank, the load at the bottom of the drainage field can be calculated after it has migrated through the ground, just before it reaches the water table.” he said.
Thompson said the average height of the water table on Captiva was 39 inches from the ground surface, based on the SCCF study. A minimum of 42 inches is required.
“On average, the average septic system on Captiva interacts with the water table”, He said, adding that the effects on the house will start as mechanical things can happen when the treatment plants have dropped to the water table.
Thompson said the concern is the older septic systems, especially with sea level rise.
He said about half of Captiva’s nitrogen pollution comes from sewage treatment plants, while the other comes from rainwater as the island is very lush. Thompson said Captiva has an impact within a kilometer of the coast, where there are fish habitats, invertebrate habitats and ecosystems.
“If you only start with 50 percent from sewage treatment plants and immediately go into the sewer system, you reduce the pollution coming from Captiva by 50 percent.” he said. “DEP will look at our calculations and show them what the numbers are. Since this was all done by Kimley-Horn and us, they will use this as a guide to get started. If you find problems, correct them. You do a good job at appraising. “
Thompson added that Pine Island Sound is nitrogen and chlorophyll-impaired water.
“In the next 10 to 15 years DEP will have a plan calculated for the maximum total daily load for this body of water. We have to reduce the nitrogen loads that enter this body of water. How do we do it? You will examine rainwater runoff, groundwater and sewage treatment plants. “ he said. “Septic systems are always an easy target when it comes to guilt.”