Lemon Lane Final Report
June, 2001
1.0 Introduction | 2.0 Site Description and History | 3.0 Remediation Planning | 4.0 Changes to Original Work Plan | 5.0 Sampling Activities | 6.0 Preparation of Iterim Cap Surface | 7.0 Intallation of the Geosynthetic Clay Barrier | 8.0 Placement of Geomembrane and Geonet | 9.0 Placement of Vegetative Support Zone | 10.0 Site Surface Water Controls | 11.0 Site Restoration | 12.0 Data Validation | Photos
10.0 SITE SURFACE WATER CONTROLS
10.1 Perimeter Channels
A perimeter channel was constructed beyond the LOC to control water flow
associated with the cap. As mentioned previously, water that collects in the
geocomposite drainage layer discharges into the perimeter channel. Also, surface
run-on and run-off water has been directed to the perimeter channel. Surfaces
outside the channels were graded so that a majority of the surface water would drain
into the channel. Soil from the grading was used as cover soil on the cap or as fill to
shape areas outside the cap. All areas on the cap and outside the perimeter
channel were covered with 6 to 10 inches of topsoil after final grading.
Soil excavated from the channels was used as backfill outside the LOC and as fill to
shape the western bench between the LOC and the retention pond. Bedrock was
encountered while excavating a portion of the southeast channel and in the west at
the entrance to the retention pond. A hydraulic ram was used to remove the
bedrock. Due to this bedrock, the southeast channel was moved slightly further
south than the Final Cap Design Report drawings show. This helped to limit the
quantity of bedrock that needed to be moved. Backfill was placed over the bedrock
and compacted. During construction, the soil associated with the perimeter channel
was compaction tested along the length of the channel by the Thelen Associates
representative. The results of the tests are provided in the PSARA portion of
Appendix G. After excavation the channels were lined with a non-woven geotextile.
The geomembrane lining from the cap was extended through the bottom of the
channel and anchored at the outside perimeter of the channel. The surface of the
geomebrane was covered with a layer of non-woven geotextile prior to placing the
riprap in the channel. The perimeter channels were constructed as presented in the
Colder Final Cap Design Report. The high point of the channel is located on the
east side of the site. From that point water flows around the south or north of the
landfill to the retention pond located on the southwest corner of the site. The final
as-built configuration of the surface water control system around the RCRA cap
covering the LL consolidation material is shown on Figure 8.
10.2 Retention Pond and Subsurface Drainage
Water collected in the perimeter channel is directed to the retention pond at the
southwest of the site. The retention pond was excavated into the native clay at this
location. This clay was used as backfill along the south of the site between the
railroad tracks and the LOC. The slope of the west bank of the retention pond was
changed from 5:1 to 3:1, limiting the quantity of soil to be excavated but still
providing a maintainable slope.
Construction of the pond is similar to the perimeter channels, with the bottom of the
pond covered first with non-woven geotextile, then seamed geomebrane and
another layer of non-woven geotextile. The geosynthetics are covered by riprap
down the center and vegetative cover soil over the remainder of the bottom and side
slopes. The geomembrane for the pond was double fusion welded to the
geomembrane coming off the cap on the east side of the pond. This essentially
made one continuous piece from the cap across the bench on the east through the
pond and up the west side of the pond. The geomebrane was placed in an anchor
trench along the south and west perimeter of the pond. The geosynthetics were
placed over the clay bottom to limit the possibility of water piping into the soil and
eventually causing macro-pores in this area.
Water can drain from the pond by way of a drainage structure located at the north
end of the pond. The structure outlet is to a 24 inch reinforced concrete pipe that
was placed underground and drains to Sargent's Pond. Water can enter the
structure in two ways. Typically water will enter the three 3 inch diameter holes near
the base of the structure. These holes will drain the water from the pond at a
controlled rate, thus limiting the water flow rate to Sargent's pond. The other method
to drain the retention pond is through the 3 ft. x 4 ft. opening at the top of the
structure. In the event of heavy rains or if the 3 inch diameter holes get blocked and
the level of the retention pond reaches the top of the structure, the opening at the
top will quickly drain water. The arrangement of the drainage structure that was
installed varied from the proposed structure shown in the Colder Final Cap Design
Report. The change reduced the amount of standing water that would remain in the
pond by raising the elevation of the bottom of the structure. Drawings 6, 9 and 10 of
the Design Report were revised to represent the final arrangement. These drawings
are presented in Appendix J.
As mentioned above, the outlet of the 24 inch concrete pipe was at the edge of
Sargent's Pond. An outlet structure was constructed to minimize erosion at the end
of the pipe and to drain the water to the pond. The structure consisted of a flared
end to the 24 inch concrete pipe, a Reno mattress and riprap.
The pond is designed to retain approximately 82% of a 10 year, 24 hour storm while
the three 3 inch diameter holes are functioning to drain water during the storm event.
The base of the drainage structure where the three drainage holes are located is
covered with riprap. The riprap will help keep debris from closing off the holes. The
function of the three holes is to limit the rate at which water drains to Sargent's
Pond. Limiting the rate to Sargent's Pond will allow the natural drainage of
Sargent's Pond to function.
10.3 Surface Drainage Outside Perimeter Channels
Prior to the remediation activities during heavy rains water ponded on Lemon Lane
road at the northeast corner of the site. As part of the final grading beyond the
perimeter channel, a swale was cut from the edge of the road to the eastern
perimeter channel. The cold and snowy conditions in December, at the time of this
work, limited what could be done to construct the swale. The swale and the area
adjacent to the swale were covered with hydromulch that included seed and
fertilizer. The swale was also covered with a layer of erosion mat. Once spring
comes the area will be reworked as needed to improve the integrity of the swale.
As shown on the drawings in the Final Cap Design Report, a V-ditch was to be
constructed in the southeast along the old fence line. The V-ditch was not
constructed. Originally the V-ditch was to collect run on water from the eastern side
of the site and from the railroad embankment on the south. Several factors
influenced the decision to eliminate the ditch. The slope of the drainage area
collected by the V-ditch along the railroad tracks was minimal. The volume of water
expected from the east was also minimal. In the final cap design drawings the slope
paralleled the perimeter channel for a distance before water flowed into the
perimeter channel. A gradual slope was developed to flow directly toward the
perimeter ditch during the final overall grading of the surface in the southeast.
|
