Natick Water Supply

Fact Sheet
(Presented to the SSCOM RAB in 1st Qtr 1996
by Robert Campbell, Mass. Dept. of Environmental Protection/NERO/BWSC)

• The Town of Natick's Springvale and Evergreen Municipal Drinking Water Wellfields have been impacted by chemical contaminants which include the primary contaminant, dissolved tetrachloroethylene (PCE), a secondary contaminant, trichloroethylene (TCE), along with their chemical breakdown products as tertiary contaminants.
  
  
• Dissolved PCE has persistently exceeded the drinking water standard or Maximum Contaminant Level (MCL) of 5.0 ug/L in the Evergreen # 1 Well, resulting in the removal of Evergreen # 1 from service. Dissolved PCE has occasionally exceeded the MCL in Evergreen # 2 in individual sampling events, but this usually occurred when Evergreen # 1 was offline. Since the average concentration for PCE in Evergreen # 2 has not exceeded the MCL for any 12-month period, it is not considered in exceedance.
  
  
• Evergreen Well # 1 is not in service for water consumption. However, it is actively pumped as an "interceptor well". In so doing, it creates a cone of depression which serves as a hydrological barrier so that contaminant levels in Evergreen # 2 remain below the MCL. This tactic has been successful; if it were not for the pumping of Evergreen # 1 to waste, Evergreen # 2 would be lost.
  
  
• Water is purged from Evergreen # 1 and delivered to an aeration spillway where it is agitated prior to discharge into Lake Cochituate. The process of aeration helps to breakdown and reduce the contaminant concentrations. Upon discharge into the lake, any residual contaminants are further degraded by natural processes (solar ultraviolet irradiation, volatilization/evaporation, and dilution). Monthly monitoring of the surface water at a location 100 feet from the spillway outfall indicates that contaminants are present in the surface water, but have been reduced to levels below the MCL. This practice is deemed to be most protective to the water supply until such time as the treatment plant is built.
  
  
• Concurrent with surface water sampling and analysis, water from both Evergreen wells is also sampled and analyzed for a full suite of chemical contaminants. In addition to this testing, two tap water samples are collected from households which receive water from the Evergreen system, and monitored on a monthly basis. Except for water obtained from Evergreen # 1, results have been below the MCLs.
  
  
• The MCL for dissolved TCE is 5.0 ug/L and for the breakdown product, cis-1,2-DCE, is 70.0 ug/1. An anomalous and unrepeated exceedance of the MCL for TCE occurred once in Evergreen # 1. No exceedances of the MCL for cis-1.2-DCE have been detected.
  
  
• In the Springvale Wellfield, contaminant levels for both PCE and TCE are below the MCLs. but recent quarterly monitoring has indicated that the contaminant levels may be on the rise at Springvale # 3.
  
  
• DEP has approved the use of additional water from the uncontaminated wellfield at Elm Bank to supplement the water supply at the Evergreen and Springvale wellfields, provided that the Town deal with the contamination at Evergreen and Springvale. The reason that the use of Elm Bank water is significant stems from its location in an adjoining drainage basin. The enabling legislation for use of Elm Bank said that no wells (from either Evergreen or Springvale wellfields) could be abandoned for the sake of using Elm Bank instead. DEP is enforcing this legislation. The Executive Office of Environmental Affairs, of which DEP is a part, would not approve Interbasin Transfer until after a Consent Order was signed.
  
  
• In response to the contamination present in the water supply, the Town of Natick has entered into a Consent Order with the Massachusetts Department of Environmental Protection to construct a wellhead treatment plant designed to remove volatile organic compounds from the groundwater. Plant design is expected to use a combination of greensand filtration and air stripping to remove the contaminants and to deal with indigenous iron and manganese mineralization.
  
  
• The configuration of the remedial system is expected to treat approximately 4 million gallons of groundwater per day from both wellfields. In order to meet the demand of 6 million gallons per day, the Town has proposed to DEP that it be allowed to pump Evergreen # 2 directly into the Town's water delivery system, by-passing the treatment system. Since the MCLs in Evergreen # 2 have not reached exceedances, which would require the well's removal from service and/or subsequent treatment of the well water, the Department cannot mandate that this water be included in the treatment stream. However, DEP's Division of Water Supply hasn't decided whether the Town will be allowed to have water from Evergreen # 2 by-pass the treatment system.