High Water Table problems...
High water tables can be a problem for an onsite septic system. Many homeowners or land developers do not realize where the water table might be.
The picture above shows a great example. During the site evaluation, we inspected the lot, test pits, as well as the basement excavation that was recently completed. We noticed groundwater at around 9.5', We had a chat with the basement excavator and confirmed this. Although there were no indications in our test pits (which only went down to 9'), we knew ahead of time what to expect around the 9.5' - 10' level.
I was onsite to supervise and help out with the installation of my design. We trenched in to connect to the house's main sewer line, which came out of the house under the footings, around 9.5'. We encountered only moist soil during the dig, but we quickly made the connections from the house to the lift station.
While waiting for the inspector, the trench slowly filled up over a number of hours. We made sure to take pictures of the connections before the trench filled with water, in case the inspector didn't make it out in time.
High water tables can cause problems for both an onsite septic design as well as the septic installation. The design must give at least 5' separation between effluent soil loading zone and any groundwater to avoid contamination (3' separation for secondary treated effluent).
During installation, high water tables can cause tanks to float, rotate, turn, even cause damage. There are several "tricks" of the trade to deal with these issues, but the homeowner needs to be aware that high water tables can create extra costs for some projects.
I sincerely enjoy the opportunity to work onsite with installers that install my designs. Each installer has different ways to accomplish the same task. I learn something every time, and try to incorporate new ideas and concepts into each of my designs if they make it an easier, cleaner install.