Most of the residential buildings in the central neighbourhoods of Boston — the South End, the Back Bay, Beacon Hill, Bay Village, the Fenway, and Chinatown [view aerial photo map]— were built upon landfill poured onto wet, marsh-like land. To provide a stable base for this construction, wooden piles were driven deep into the mud to sit on the more solid clay beneath the sand/gravel fill. Building foundations were then contructed atop these piles. For the most part, the tops of these piles were cut off below the water table at the time of construction with the expectation that they would be preserved if permanently immersed below the groundwater table.

With construction of sewers, drains, MBTA subways and the basements of buildings below the water table, some of which leak, the groundwater level has dropped. Where wood piles have been exposed to air for some time, the piles have rotted when attacked by fungi, borers and other organisms. A few buildings have settled and cracked, requiring owners to underpin their structures at great cost in order to restore the foundations. Current estimates for repairing rotting piles start at $250,000 per townhouse.

Deterioration or decay of wood piles is the most serious potential problem associated with lowered water levels. As long as the water table remains above the tops of the piles, and the wood and surrounding soil remain saturated, the wood will not rot. Under these conditions, untreated wood piles can be considered to be permanent.

However, if the groundwater level drops below the tops of the piles, favorable conditions may be present for plant growth and insect attack. A greatly increased supply of oxygen, combined with moisture and moderate temperatures, facilitate the growth of fungi. Grubs or wood borers, termites and other insects may also attack the "exposed" wood.

The butts of piles that are surrounded by fill, in particular sand and gravel as well as ashes and cinders, are more prone to rotting than are piles that are embedded in organic silt, peat and other relatively impervious soils. When the water table drops, the fine-grained soils remain saturated for a time, thus protecting the piles from immediate deterioration.

The time required for significant deterioration to occur, following a drop in groundwater level below the tops of wood piles, is highly variable. It depends on the species of wood, the type of soil in which the piles are embedded, the amount of moisture, temperature and other factors. Exposure for a few months is not considered serious. However, serious deterioration will probably occur after a drawdown period of 3 to 10 years.

The Boston Groundwater Trust is the entity charged with monitoring groundwater status throughout the City. Its mission is to take regular, quarterly readings of its wells, reporting the findings on the website, www.GroundwaterTrust.org. With the installation of its final well in October 2006, the Trust now monitors a network of 800 wells across the city.