Controlling Nitrate-Laden Runoff in a Marine Environment

By Erik D.W. Greven, P.E., Principal, and Frank L. Crabtree, P.E., Associate, Harriman Associates

Abstract
Maquoit Bay near Brunswick, Maine, supports a productive shell fish industry. When a 50-acre site just a mile from the bay was selected for the town's new high school, there were concerns that pollutants in storm-water runoff would be carried to the bay. Controlling nitrates posed special concerns because the site is predominately sandy soil. It was feared that nitrates from fertilizers used on the school grounds might seep through the soil, eventually entering the bay -- a situation that some believed could create algae blooms that could harm the fishery.

The town enacted one of the strictest local codes in the nation to protect the bay, receiving national recognition for its work. Environmental engineers at Harriman Associates became the first to use the code, transforming the ideas in the code into reality with their site engineering at the high school.

The design employs a system of man-made ponds and wetlands as well as grassed swales to collect, divert, and slow the rate of storm-water runoff. Plants such as cattail, arrowhead, bulrush, sweet flag, and water lily help the cleansing process by assimilating nitrates in the water. In addition, a detailed maintenance plan spells out procedures that minimize the use of fertilizers and pesticides.

This paper will describe how the code differs from standard storm-water management practices, and how Harriman Associates interpreted and applied the principles in the code.
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Background
A three-story building in the heart of Brunswick had served as home to the town's high school since 1938. By the mid-1980s, however, its inadequacy for secondary education was clear, and town officials asked for and got state approval for a more modern, up-to- date facility.

After an initial evaluation of 125 possible sites, 30 were tagged for a more in-depth analysis. Sixteen of the 30 were evaluated in detail before town and school officials settled on a 50-acre site outside the center of town about a mile from Maquoit Bay.

Concerns raised Before construction began, however, some residents raised concerns about nitrates in the fertilizers that would be used on the school grounds. If the nitrates entered Maquoit Bay in storm-water runoff, some said, they might add to existing problems that could cause algae blooms that could endanger the bay's clam beds and commercial fishery.

Without a man-made, storm-water management system, run-off water on the site after a storm would follow a natural, gravity trail downward to lowlands and a stream on its way to the ocean. Along the way, pollutants and nutrients that had been dissolved into the water would be removed by the natural cleansing process of various plants.
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Initial design Harriman Associates had initially engineered the site to conform to the state code regulating storm water runoff, which essentially improves on this natural system.

To keep the rate of post-construction runoff no greater than it was before the development, a "dry pond" detention area was designed to provide a place for water to collect before entering a flow-control pipe. A dike would keep the water back until it could go into the pipe. At that point, Nature would take over, and the water would follow the gravity path to the sea, going through the natural cleansing process.

When the town decided that more stringent controls were needed for nitrate-laden runoff, however, a new section to the town zoning ordinance was enacted in 1992. Working closely with town and school officials, Harriman Associates re-engineered the site to comply with the new criteria.

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How the code is different
The new section established a Coastal Protection Zone that includes 36 acres of the 50-acre school grounds. It specifies uses permitted in the zone and mandates various performance standards such as requirements for density, lot size, landscaping, lighting, noise and odors -- all fairly typical for a zoning ordinance. But it also has unusually strict requirements for the management of storm water runoff into a marine environment - - requirements that few local governments in the country have enacted.

The code includes the requirements of the Maine state code, but goes further. Like the state code, Brunswick's requires that post-development peak runoff be equal to or less than pre-development rates. But it also specifies that a one-year frequency, 24-hour duration storm must be retained in the wet ponds. In addition, the Brunswick code prohibits storm water that flows directly off an impervious surface from being piped directly to any water body or wetland.

Acceptable methods for meeting the standards are described, including the construction of ponds and wetlands, and grassed buffer strips and swales. And the code requires, among other things, that a site maintenance plan be approved by the planning board as further insurance against nitrates reaching the bay in the future.
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Engineering to control nitrate-laden runoff
The new Brunswick High School was the first development in the new Coastal Protection Zone. Harriman Associates became the first to interpret and implement the standards established for the zone.

The school is a 175,000-sq.-ft. building situated on a site that is primarily of sandy soil, with about 75 feet of sand above ledge. All but 10 acres of the 50-acre site were originally blueberry fields. Fifteen acres are preserved in their natural state: five acres of blueberry fields serve as a transition between the developed site and the natural landscape; another 10-acre wooded ravine containing the perennial brook that leads to Maquoit Bay has also been preserved.

A 1,700-foot-long access road off Maquoit Rd. leads to the school. Three parking lots at the sides and rear of the school can handle a combined total of about 400 cars. The few berms that existed on the relatively flat site were preserved, and several manmade berms were added to shield and soften the expanse of pavement. Playing fields account for about 16 acres, and include an eight-lane, 400-meter track that surrounds the football field.

Harriman Associates used a multipronged approach to accomplish the town's goals for keeping nitrates used on the site from entering the bay. Strategies were devised to reduce the amount of nitrates used on the grounds and to provide ways to enhance the natural process that cleanses runoff.

Low-fertilizer grasses To limit the amount of nitrates used, a species of grass was planted that requires a minimum of fertilizer for good growth. In addition, the selected species is disease- and pest-resistant so that fewer and weaker herbicides and pesticides will be needed.
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Slow-release fertilizers Use of slow-release fertilizers with low nitrogen levels will also cut down on the volume of nitrates on site. The fertilizers provide just enough nutrients to maintain the sports fields properly, and since they last for a long time, fewer applications are required. Even lesser amounts of fertilizer will be used on other grassy areas on the site.
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Site maintenance plan A site maintenance plan, approved by the town planning board, clearly spells out the routines and procedures that must be followed to keep the volume of pollutants low. It includes procedures for mowing, testing soils, applying fertilizer, irrigating, using herbicides and pesticides, removing thatch, and overseeding high-use areas.
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Soil composition The composition of the soil was also changed to reduce the volume of nutrients that run off the site. Native, sandy top soil was replaced with carefully constructed silt loam soil to promote quality turf for playing fields, yet minimize leaching of nutrients to the ground water. Clay mixed in with the sandy soil makes the surface less porous, and prevents runoff from seeping through upper layers.
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Swales and wet ponds To provide natural cleansing of the runoff, two similar systems of storm-water treatment swales and wet ponds were constructed.One system collects runoff from 5 acres of paved parking lots and access drives, and the other collects runoff from 16 acres of grass sports fields. One-half of the runoff from the building's roof is piped into each system. This increases the likelihood that the ponds will stay filled during dry summer months.

Run-off water flows through vegetated buffer strips and grassed swales into a grassed, dry, detention ponding area. Regrading helps direct runoff to those areas. The grasses in the swales are deliberately under-fertilized so they will make use of nutrients in the runoff.

An earth dike holds the large volume of water during a storm, while an outlet structure and pipe control the flow of water to an 8-foot diameter concrete sediment trap. The sediment trap doubles as an overflow device which controls the water level in the downstream wet pond.

When the pond fills with the nutrient-laden "first flush" of water from the pavements or sports fields, the water level backs up in the structure and causes the large volume of relatively clean storm water to by-pass the wet pond. This prevents the flushing out of captured pollutants from the wet pond.

Each man-made wet pond is approximately one-half acre in top area and consists of a shallow 18-inch deep wetland and an adjacent ponding area up to 5 feet deep. The shallow wetland provides an environment where nitrate-laden water can come in contact with a variety of wetland plant species. Plants such as cattail, arrowhead, bulrush, sweet flag, and water lily provide cleansing by absorbing nitrates from the water.

To overcome the permeability of the deep, sandy soil, clay mat liners were installed beneath each wet pond to help keep the ponds wet in the dry summer months. The liner also keeps the water level in the pond at about 20 feet above the ground water level of the surrounding area.

A pond-outlet pipe "throttles" the flow of water as it overflows to a level spreader, where cleaned water is dispersed over a wide area of natural vegetation as it flows through the woods to the brook that leads to Maquoit Bay. In addition, procedures to monitor water quality determine how efficiently the system is working. At various intervals, pollutant levels exiting the school site are measured.

In a nutshell, the system slows down the release of water collected on the site after major storms, and allows the natural cleansing process to work.

Written for Technical Seminar, American Society of Civil Engineers, Maine Section, 1996
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