INTRODUCTION
The Fresh Kills
Landfill located on Staten Island, New York, is approximately 3,000
acres in size and is the largest landfill in the United States (refer
to Figure 1 for a map showing the regional location of the project
site). Stone & Webster Engineering Corporation (SWEC) is currently
(March 1996) supervising and managing the construction of a Leachate
Mitigation System at the Fresh Kills Landfill, on behalf of the
City of New York Department of Sanitation. The Leachate Mitigation
System at the landfill is to consist of a leachate (groundwater)
collection, distribution and treatment system. A “relatively”
high elevation of the water-table aquifer at the project site hindered
the construction of a leachate treatment plant; hence, a de-watering
system was installed to lower the elevation of the water-table aquifer.
The Multi-Flow Drainage System was installed by SWEC as a primary
component of this de-watering system. The following text includes
a summary of the Leachate Mitigation System, a description of the
installation of the Multi-Flow Drainage System, performance results
of the Multi-Flow, and conclusions regarding the use of the Multi-Flow
Drainage System for de-watering purposes.
LEACHATE
MITIGATION SYSTEM
A Leachate Mitigation
System (LMS) is currently being installed at the Fresh Kills Landfill
site. The LMS consists of a system to collect, distribute and remediate
contaminated groundwater (leachate). Construction of the LMS will
be completed in phases: Phase I, a leachate treatment plant has
been completed; Phase II, the expansion of an existing leachate
treatment plant, is currently under construction. Expansion of the
existing leachate treatment plant had been hindered by a “relatively”
high elevation of the water-table aquifer. The elevation of groundwater
at the construction area was approximately two (2) + feet above
land surface. To lower the elevation of the water-table aquifer
at the construction area, SWEC proposed a de-watering system. The
de-watering system was designed to reduce the elevation of groundwater
at the area of construction to below land surface; and hence enable
the construction of the new treatment plant. Stone & Webster
Engineering Corporation proposed a traditional crushed stone drainage
for the de-watering system; however, upon learning of the Multi-Flow
Drainage System, SWEC chose to install Multi-Flow. Refer to Exhibit
A for correspondence between SWEC and Multi-Flow Systems, Inc (dated
October 7, 1994) regarding the decision to install the Multi-Flow
Drainage System.
INSTALLATION
OF THE MULTI-FLOW DRAINAGE SYSTEM
In March, 1995,
Stone & Webster Engineering Corporation installed the Multi-Flow
Drainage System along the perimeter of the treatment plant construction
area. The Multi-Flow was installed to a depth of five (5) + feet
below land surface (refer to Figure2 for a schematic showing the
location of the Multi-Flow and other pertinent features). Two (2)
parallel 1,800 foot lengths of 18 inch Multi-Flow were installed
at the north and east perimeter of the construction area (refer
to Figure 2). During installation, the groundwater elevation was
approximately two (2) + feet above land surface; the Multi-Flow
was placed within a wire structure and submerged within a trench
to a depth of approximately five (5) + feet below land surface.
The trench was subsequently infilled to land surface with crushed
stone. (*note: Multi-Flow Systems, Inc recommends the use of coarse
sand as backfill material; crushed stone was used by SWEC at the
project site due to difficulties of infilling the trench with sand
through seven feet of water). The gradient of the drainage system
trends from west to east, and after a 90 degree turn, from north
to south to the discharge point (refer to Figure 2 for groundwater
flow directions). Groundwater which enters the Multi-Flow discharges
to a concrete sump; when the water level within the sump approaches
the elevation of the discharge point, two (2) pumps automatically
activate to distribute the groundwater to the existing treatment
plant for remediation.
PERFORMANCE
RESULTS OF THE MULTI-FLOW DRAINAGE SYSTEM
The Multi-Flow
Drainage System has been operating at the Fresh Kills Landfill for
approximately one (1) year (between March 1995 and March 1996).
Flow rates within the Multi-Flow range between 1,260 gallons per
day (GPD) and 39,000 GPD; the average flow rate within the drainage
system is between 12,000 GPD and 24,000 GPD. Refer to Exhibit B
for flow rate data provided by SWEC. According to SWEC personnel,
the efficiency of the system has not decreased since startup of
the de-watering system. The flow rate of the drainage system decreases
as the elevation of the water-table aquifer stabilizes; as the elevation
of the water-table aquifer rises, flow rates through the system
increase. As evidence of the continued efficiency of the Multi-Flow
Drainage System, flow rates through the system increased dramatically
(from approximately 3,600 GPD to 23,700 GPD in a 48 hour period)
after heavy rains in late January 1996. This suggests that groundwater
flow through the system is limited only to the elevation of groundwater.
CONCLUSIONS
