Careful planning tempered by a site's unique conditions can result in ecological productivity, beauty and functionality when restoring a landfill to a productive end use.
Transforming a closed landfill into Mayor Thomas W. Danehy Park, for example, has increased open space in Cambridge, Mass., by 20 percent.
Opened to the public in 1990, the 50-acre park includes multisport fields, children's play areas, horse-shoe pits, boccie courts and more than 2.5 miles of jogging, walking and biking trails.
Beginning in the early 1980s, Camp Dresser & McKee Inc. (CDM) and Haley & Aldrich, both of Cambridge, monitored the site, investigating issues like settlement, combustible gas migration and generation, landfill cover thickness, air and groundwater quality, radioactivity, storm water drainage and reveg-etation.
Between 1977 and 1982, the Massachusetts Bay Transportation Authority covered the entire site with four to 40 feet of dense fill from a local subway project, as part of an agreement with the City of Cambridge.
During final grading, the designers added a six- to 18-inch sand-gravel drainage and leveling layer, and a four- to six-inch sandy loam top soil layer. Because of the cover materials' unusual depth and densi-ty, these materials were allowed as a substitute for the two-foot cap required by the state's Department of Environmental Protection.
The deep cover also allowed gra-ding, which provides more gradual transitions than normally are available on closed landfills. To prevent erosion and provide seasonal variation, slopes were hydroseeded with a wildflower mix, which proved difficult to establish and maintain.
Slopes were replanted with a grass mixture including fescue (50 to 55 percent), perennial ryegrass (20 percent), bluegrass (15 percent) and legumes such as clover (10 to 15 percent), which reportedly have been successful in controlling erosion. After hydroseeding the grasses, designers used straw mulch to create a moistureretaining thatch to aid germination and increase stability.
Playing fields were covered with approximately 18 acres of sod, which included a bluegrass/fescue mix to increase drought tolerance. Drought turned out, however, not to be the problem; in fact, flooding has occurred due to the flat surface required for sports (1 percent) and the sod's poor drainage capability. "If I could do it over again," said John Kissida, CDM's project manager, "I would use washed sod, which drains better than ordinary sod, even though it's more expensive."
To solve a storm water drainage problem, runoff from normal rainfall is directed to the park's six- acre detention area, designed as a wet-land. To prevent surface water contamination, this area was sealed with a bentonite (clay) liner.
The marsh above the liner is 12 inches deep with nine to 12 inches of loam cover. Agrosoke crystals, which reportedly absorb water equal to 40 times their weight, were added to the loam to sustain the marsh in dry periods. Planted with wetland species, the marsh has become one of the park's most popular features and supports a functioning ecosystem including fish, frogs and a range of wetland birds.
Because of the cover depth, Danehy Park has trees uncommon to other revegetated landfills. The original planting included 800 trees, which have had a 95 percent survival rate. Kissida attributes this success to careful tree siting in deep cover areas, enlarged tree pits and Agrosoke in the backfill.
CDM continues to maintain the park. Regular fertilization and frequent aeration with a deep plow are necessary to maintain nutrient levels and to prevent compaction and soil anaerobism.
Kissida stresses that a park built on a landfill can only succeed with careful maintenance and monitoring. This is especially true where the park includes recreational areas, such as sports fields. Cam-bridge spends approximately $150,000 per year in maintenance, plus monitoring and full-time park staff expenses.
Today, success is Danehy Park's biggest problem. "When we have to tell soccer players who show up for a game the day after a storm that they can't use the field, they get angry," said Kissida. " don't remember that the park used to be a landfill, and that it has certain limitations."
A partial solution to this problem has been to retire one sports field a year. Kissida feels it would be preferable to educate visitors by revealing the site's history through the park's design.