Increasing development disrupts the natural hydrologic cycle by increasing runoff and decreasing evapo-transpiration and infiltration. At their core, the goal of all BMPs is to restore the hydrologic cycle and therefore reduce flash flooding, pollutants and erosion in streams, while increasing the amount of groundwater.
Although the infiltration of stormwater is currently garnering much attention, it is not a new idea. Several seepage pits located on the southern side of Tolentine Hall, located at Villanova University in Villanova, Pennsylvania were discovered in July 2005. These seepage pits presented a unique opportunity to study the long-term impacts of infiltration. The history of these pits is still somewhat uncertain because of their age and a fire in Tolentine Hall in 1923. It is understood that the structures were built at the turn of the 20th century around the time Tolentine Hall, at the time called College Hall, was built. Originally, four pits existed in the area to collect and infiltrate stormwater off of Tolentine Hall, St. Thomas Monastery, and St. Thomas of Villanova Church which are located on the southwestern corner of the main campus. At the time of this research, two of the pits have been replaced by large box culverts and are no longer functioning as seepage pits. The pits are brick cylinders with sand bottoms. The pits were given names based on their location to certain buildings (i.e. “Monastery” and “Tolentine”).
The goal of this study was to determine the functionality of the pits, the impact of infiltrating stormwater for 100 years, and what would be required to restore the pits. Our analysis showed that although elevated copper concentrations were found in the soils beneath the pits, these levels were not above Pennsylvania standards for clean fill. The infiltration capacity of the Tolentine pit was quite high; however, the infiltration capacity of the Monastery pit had been compromised by the inadvertent addition of organic matter. The organic matter entered the pit when a sewer line was connected to the seepage pit. A design using earthquake drains was used to alleviate the Monastery pit infiltration capacity problems. Our study into the efficacy of this improvement technique was inconclusive.
The seepage pit site is located in the northern edge of the large open field area south of Tolentine Hall and St. Thomas Monastery. The figure below shows an aerial photo of the area with the seepage pits indicated by red dots. The Tolentine pit is to the left and the Monastery pit is to the right.
The Tolentine pit is currently disconnected from all stormwater sources. We are currently investigating a use for this pit. The Monastery pit accepts runoff from the following impervious surfaces:
All of the St. Thomas Monastery roof: 1720 m2
Part of the Tolentine Hall roof: 900 m2
An average rainstorm event of 12.7 mm (0.5 in) for the Philadelphia, PA area will produce 33,500 L (8,850 gallons) of water and a 50.8 mm (2.0 in) rainstorm event will produce 122,120 L (32,260 gallons) of water. The pits are circular in shape with brick walls and a sand layer bottom
Our historical research indicates that the pits were always intended for the storage and infiltration of stormwater. The connection of the sewer line in the 80's was completely accidental.
This work was supported, in part, by a grant from PADEP through their Growing Greener program. This support does not imply endorsement of this project by the PADEP. Furthermore we would like to offer a special thanks to:
Robert Morro, Executive Director of Facilities
Steve DiValerio, Director of Facilities Services
Leo Kob, Senior Project Manager of Villanova’s Facility Management Office
Their insight into the seepage pit area helped make this project a success.
These pits were discovered when a foul odor indicated that a sewage line had accidentally been connected to the Monastery pit. The sewer line was disconnected. The next step was to map out all lines entering and exiting the site as seen in the figure below.
After finding all connections between the pits, three borings were drilled to obtained soil. The soil was then transported back to the lab for testing and classification. An auger is seen below completing one of the bore holes.
Soil samples for all the testing described below were obtained from holes inside each pit and one hole located in between the pits (null). The soil beneath the pits is a silty sand (SM) according to the Unified Soil Classification System. In addition to classifying the soils, the following tests were performed:
Only the results of the bacterial, copper, and infiltration testing are presented here. Complete results can be found in Matthew Gore's thesis, which will be posted on the VUSP website soon.
Understanding that a sewer line had been connected to the Monastery seepage pit, data needed to be gathered to determine whether the system would be a safe environment for a study. The results for the two pits, plus a control site located in between the two pits, are shown below at a depth of 6 inches. Total coliforms and some e-coli coliforms were found. Because bacteria were also found in the null site, it can be concluded that the bacteria occur naturally in the soil and that the site presented no health and safety problems.
Copper entered the pits from the copper gutters, downspouts, and sheathing used on the rooftops. While elevated levels of copper were found in the pits as compared to the null site, the soil in the pits would still be considered clean fill by the PADEP.
The infiltration rate was determined for both the Tolentine and Monastery pits. The infiltration rate of the Tolentine pit was excellent; unfortunately, there are no easily accessible lines to connect to this pit. The infiltration rate of the Monastery pit was low and we investigated a plan to restore the infiltration rate using earthquake drains.
Unfortunately, there was not much improvement in the infiltration rate after the earthquake drain installation as can be seen in the figure below which compares the infiltration rates at a head of 0.76 m.
Although we are unable to say definitively why there was no improvement in the infiltration rate, we believe there are two possible explanations. First, the most logical explanation is that the deeper soil is not any more permeable than the upper soils. Second, we were only able to install one out of four drains because of the size of the entryway into the pit. It is our opinion that more would be required to see a change in the infiltration rate. We still believe that an earthquake drain can be effective if the infiltration rate is being compromised by a low hydraulic conductivity layer at the surface.
Q: If you infiltrate stormwater over a long period of time, won't the soil become contaminated?
A: It is true that stormwater does contain contaminants. That is one of the reasons we want to reduce the direct discharge of stormwater into streams. The infiltration of stormwater is effective in reducing contaminants for several reasons. One reason is that soil acts as a natural filter because many contaminants "stick" to the sediments carried along with stormwater. The sediments that carry the contaminants are trapped in an infiltration BMP.
BMPs must be sited and selected very carefully. Obviously, a heavily contaminated site would not be an ideal location for an infiltration BMP. In some cases, a pretreatment system can be used in conjunction with an infiltration BMP.
At our particular site, we found that after 100 years of infiltration the soil under the infiltration pits would still be considered clean fill according to PADEP standards.
Q: Could the pits have originally been cesspools?
A: 1928 records show the existing sewer lines of the two buildings do not connect or even go near the pits. So, the idea that the pits were originally designed as septic tanks was ruled out. There was also a line from the buildings for grease from the Monastery kitchen, but did not travel to or near the pits. Thus, the pits could only have one function: rain water storage and infiltration.
Q: How did you determine the copper concentrations?
A: The particular method used for extraction and testing was the DTPA extraction method described in the “Recommended Chemical Soil Test Procedures for the North Central Region” by the Missouri Agricultural Experiment Station.