Contaminant Impact Investigations

Treatment Options for Atypical Groundwater Constituents

Weston & Sampson evaluated the treatment of the MacIntosh well in Newmarket for removal of arsenic, manganese, chloride, sodium, and total dissolved solids (TDS). A five-day pump test was conducted to determine the well’s baseline water quality after an extended period of inactivity. The well exhibited high levels of manganese and arsenic, which are typical constituents found in New England groundwater. However, the elevated chloride, sodium, and TDS levels were atypical and needed to be considered when developing treatment alternatives to determine the best approach for using the well as a public water supply source. The treatment alternatives examined during the pilot test were:

  • Developing the MacIntosh well as a sole source and pumping directly into the town’s distribution system.
  • Mixing the water from the well with the current distribution system water before allowing it to enter the system.
  • Using an electrodialysis reversal (EDR) membrane treatment for the well

The well produces water with an arsenic concentration slightly above the primary maximum contaminant level (MCL). Primary MCLs set by the Environmental Protection Agency and are enforceable by law, eliminating the option of pumping the MacIntosh well water directly into the distribution system.

Mixing the well water with the existing distribution system water was investigated as a less expensive alternative to EDR treatment. Distribution water was collected from a hydrant close to the proposed location for a treatment facility, and mixed with water collected from the well. Water quality results showed that mixing the well water with the distribution water reduced arsenic, sodium, chloride, TDS, and manganese to below the arsenic primary MCL level and below most respective secondary MCL levels.  The resulting water quality has higher constituent levels compared to the water from the town’s two gravel pack water supplies.

The EDR membrane treatment was found to reduce the target constituents below the primary and secondary MCL limits, and to produce a treated product with a water quality similar to that being produced from its two gravel pack water supplies.

After evaluating all three treatment alternatives, Weston & Sampson recommended use of EDR treatment to allow the town to maintain the exceptional water quality being provided to its customers.  Implementation of EDR treatment would allow the MacIntosh well to serve as a stand-alone well and greatly reduce the observed levels of the target constituents identified during the pilot study.


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