Research

There were two major storms during the course of the data collection period that produced large amounts of rainfall.  Hurricane Ophelia passed near the site on September 13 – 14, 2005 producing 250 mm of rain.  Tropical Storm Tammy, October 5 – 8, 2005 produced 216 mm of rain.  Malfunctions in the automated rain gauge prevented rainfall data from being collected during the period of October 19 to November 16, 2005 and December 20, 2005 to January 18, 2006.  There was 25 mm and 71 mm of rainfall respectively during those periods.  Preliminary results show that the septic tanks exceeded capacity on two occasions in tank 1 and 2 and possibly 3 occasions in tank 3 (Figure 1).  The point where the level exceeds capacity and passes into the drain field is approximately 55 inches.  No runoff from the tank or overflow in the “T” connectors was observed.

Only tank 3 was able to empty completely.  Tank 2 was lower in the landscape and is tidally influenced preventing all of the water from infiltrating into the soil below the tank.  The irrigation demand was low preventing tank 1 from emptying.  There was a crack in tank 1 at 63.5 cm above the bottom of the tank that allowed the water to infiltrate into the surrounding soil until the water level in the tank was reduced to 63.5 cm.  This improved the performance of Tank 1, however, allowing more runoff to be captured due to the increase in storage volume.

Figure 1                                                                     
 

Each of the septic tanks, and their associated drain fields, were able to capture all of the roof top runoff that was routed into them with no observed overflow.  Because of the potential to connect roof top runoff to 2,300 abandoned septic tanks, further research will determine the island wide run off reduction associated with septic tank conversion.

For more information please contact Jason Wright at jdwright4@ncsu.edu