The public water supply on the Island is provided by seven (7) groundwater sources located in two systems, Wannacomet Water Company and Siasconset Water Department. The Wannacomet Water Company and Sconset Water Department well sampling results have been below the Maximum Contaminant Level (MCL) Massachusetts Department of Environmental Protection (MassDEP) set as safe for PFAS in drinking water, with most wells showing no PFAS.
The majority of the Town's public water supply is provided by the Wannacomet Water Company that is served by five wells (Well 12, 13, 14, 15, and 16) located in Nantucket's Sole Source Aquifer. Two wells (Well 15 and 16) pump groundwater from a shallow aquifer at about 75 feet, while the remaining three wells (Well 12, 13, 14) pump groundwater from a deeper aquifer at about 150 feet with a confining unit separating the two. The wells are located in the center of the Island in the proximity of the intersection between Milestone Road and Polpis Road. Wells 12, 15, and 16 are manifolded together at the Wyers Valley Pumping Station. Wells 13 and 14 enter the distribution system individually. The groundwater does not require any physical or chemical treatment before pumping to distribution.
The Siasconset Water Department serves a much smaller distribution area on the east side of the Island with two wells (Wells 6 and 7). Well 7 is the primary well while Well 6 acts as the backup well. The two wells are located on Milestone Road, also in Nantucket's Sole Source Aquifer. The wells are screened at a depth of 70 feet below the confining unit.
Wannacomet public water supply wells draw water from a deep aquifer below confining units. A confining unit is a layer of low permeable material (such as clay and fine silt) may help prevent PFAS from moving to deeper aquifer layers. For comparison, most private residential (domestic) wells are screened in the upper Unconfined Aquifer, thus withdrawing groundwater from above the confining unit.
Note that groundwater sample results have been reported by Nantucket Memorial Airport for on-airport shallow monitoring wells and adjacent residential drinking water wells and with private well testing conducted by MassDEP and UMass (see FAQ no.8). This data is not a complete picture of PFAS in the Island's aquifer. The Airport is continuing to advance its investigation through ongoing sampling and source identification activities. The Town with consultant CDM Smith also continues to advance its investigation through on-going sampling program at the WWTP and the landfill. The Water company continues to take samples as mandated by MassDEP.
An aquifer is underground layer of water bearing rock and/or granular materials from which water can be extracted using a well.
A confining unit is relatively dense and contains impermeable materials, such as clay, that may prevent transport of groundwater and pollutants deeper into the subsurface.