When it rains in urban and suburban areas, rainwater washes pollutants such as nutrients, chemicals, and heavy metals off impervious surfaces, lawns, or bare soils into storm drains that lead to streams and rivers. With increased amount of impervious surfaces, larger quantities of rainwater reach the streams quickly causing flash flooding, stream bank scouring, and sedimentation of streambeds. Because of stream damage, litter, and pollution, stormwater has become a major concern in Pennsylvania impairing 4,170 miles of streams and accounting for one third of the problems facing our waterways.
Municipalities that are designated MS4 communities (Municipal Separate Storm Sewer Systems) by the EPA and DEP are tasked with finding ways to reduce stormwater runoff. Some municipalities face fines and enforcement for combined sewer overflows that cause untreated sewage to flow into waterways when pipes do not handle increased volumes of stormwater during rain events.
So how to do municipalities begin to reduce stormwater runoff and the pollution associated with it? Well, they can increase the size of underground pipes, holding tanks, and the capacity of their sewage treatment plants which would cost millions of dollars. Or, they can consider some greener and cheaper solutions in concert with the grey infrastructure of pipes and holding tanks.
One of those green and cheap solutions involves the planting of large canopy trees. A great deal of research by the USDA Forest Service and others has shown that trees and forests reduce stormwater runoff and pollution in several ways.
Trees work like large umbrellas intercepting and evaporating rainfall in their canopies. Average interception by deciduous trees can range from 700 to 1,000 gallons of rain water annually, while an evergreen can intercept more than 4,000 gallons. Large canopy trees such as London plane or oak planted over impervious surfaces provide more benefit than smaller trees like crabapple. A recent USDA Forest Service study found that New York City's street trees reduced stormwater runoff by 890.6 million gallons annually, with a value of $35.6 million in stormwater management costs. The average street tree in New York City intercepted 1,432 gallons of rainfall annually, while larger trees like London plane almost 3,000 gallons.
Infiltration and Groundwater Recharge
Trees and forests provide for infiltration of rain into the soil to recharge groundwater. In the soil, rainwater is filtered and slowly moves to streams as subsurface flows. In forest soils, infiltration rates can range from 18 inches per hour to 10 inches per hour depending on soil composition. In one study of a North Carolina watershed, the mean soil infiltration rate decreased from 12.4 in/hr to 4.4 in/hr when the site was converted from forest to suburban lawn.
Trees absorb and use tremendous amounts of water for photosynthesis and growth, moving it from the roots back into the atmosphere through leaf evapotranspiration. A single mature oak tree can transpire over 40,000 gallons of water per year. The cooling of air through evapotranspiration modifies summer temperatures in places surrounding trees. Hydrologic studies in Pennsylvania forests show that an average of 60% of rainfall is taken up by trees and transpired back into the atmosphere. When that forest is removed or harvested, evaporation declines while a stream receives additional water each year. The ever increasing conversion of forests to lawns and impervious surfaces (buildings, roads, and parking lots) continues to cause stream bank erosion and flooding that causes millions of dollars in damages.
Trees are very good at removing and using nitrates, phosphates, and other nutrients and contaminates such as heavy metals, pesticides, solvents, oils, and hydrocarbons from the soil and water. This process is called phytoremediation. In one study, a single maple growing road- side, removed 60 mg of cadmium, 140 mg of chromium, 820 mg of nickel, and 5200 mg of lead in a single growing season, storing those pollutants in their wood.
Riparian forest buffers filter sediments from streams during storm events, remove nitrogen and phosphorus leached from adjacent lands, provide stability to stream banks, shade and modify stream temperatures, provide aquatic and wildlife habitat for many species, reduce stream velocity, and reduce downstream flooding. Protected riparian buffer widths vary from 50 feet to provide some bank stability to 250 feet to provide flood mitigation, wildlife habitat, and recreation. While riparian areas continue to be preserved and restored across the state, mature wooded buffers are destroyed during new construction or by misguided landowners.
It's time we understand and talk about the role trees, forests, and healthy soils play in keeping our waters clean and reducing stormwater runoff and flooding instead of just mentioning how trees beautify a community. The Pennsylvania Department of Environmental Protection has recently included urban tree planting as one of the control measures that MS4 communities can use to meet the TMDL (total maximum daily load) requirements that have been established to meet their water quality improvement goals.