Groundwater samples are collected from an array of monitoring wells and multilevel samples to monitor the natural restoration of water quality in the treated-wastewater plume. The array includes about 187 monitoring wells at 43 sites and 52 multilevel samplers at 38 sites. Samples have been collected at least annually since 1994 from a subset of these sites to monitor the natural restoration.

The monitoring wells were installed with a hollow-stem-auger drilling method. The wells are constructed with 2.0-inch-diameter polyvinyl chloride (PVC) well casing and 2-ft-long slotted PVC screens. The aquifer sediments are allowed to collapse naturally around the well screens after the augers are removed. The PVC casings are protected at land surface by lockable steel casings.

The multilevel samplers (MLS) also were installed with a hollow-stem-auger drilling method and natural collapse of the aquifer material. The MLS are constructed with 15 color-coded polyethylene tubes (0.25-inch outside diameter) within a 1.25-inch-diameter PVC casing. The color-coded tubes extend from land suface, down the PVC casing, and out into the aquifer through holes drilled in the PVC at various depths. The open bottom end of each polyethylene tube, referred to as the sampling port, is screened with a fine nylon fabric.

Groundwater samples are collected from the monitoring wells using a submersible pump fitted with Teflon tubing and an isolation packer. Groundwater samples are collected from the MLS by using a peristaltic suction pump fitted with Norprene tubing that is connected directly to the MLS tubing. Water-quality samples are collected after field water-quality parameters (specific conductance, pH, temperature, dissolved oxygen, and turbidity) measured in the discharge have stabilized.

See Savoie and others (2006) for additional information about the methods used to monitor groundwater quality in the treated-wastewater plume.

The 2006 field season marked the eleventh year of natural restoration since the land disposal of treated wastewater on the Massachusetts Military Reservation ended in 1995. Groundwater samples were collected in the summer of 2006 from 46 monitoring wells and 25 multilevel samplers (MLS) as part of a continuing effort to monitor the natural restoration of the aquifer. At each well cluster and MLS, water samples were collected for analysis of cations and anions, including nitrate, ammonium, and dissolved organic carbon by the laboratories of the USGS National Research Program. The pH, specific conductance, and concentrations of dissolved oxygen and phosphate were measured in the field during sample collection.

Water-Quality Observations in 2006
	Map showing the locations of monitoring wells and multilevel samplers that were sampled in 2006. The sites were sampled to monitor the natural restoration of the aquifer following the cessation in 1995 of disposal of treated wastewater onto infiltration beds at the Massachusetts Military Reservation.
			Vertical profiles of water-quality parameters in groundwater sampled annually from 2003 to 2006 at MLS S436 and MLS S469. MLS 469 is 150 feet downgradient from MLS S436 in the direction of groundwater flow. At both sites, specific conductance increased temporarily in 2004 and 2005 to more than 200 microsiemens per centimeter in the zone between 0 and -30 feet altitude. The thickness of the zone of low dissolved oxygen (D.O.) concentrations at MLS S436 decreased from more than 20 feet to less than 10 feet between 2003 and 2006. During the same period, the thickness of the zone of low D.O. at MLS S469 remained more than 40 feet thick. The pH at MLS S436 decreased in the zone where D.O. increased and increased at greater depths. At MLS S469, the pH changed little from 2003 to 2006. Phosphate concentrations were approximately unchanged at both sites during the 4-year period.
	Longitudinal vertical sections showing dissolved oxygen, pH, specific conductance, and phosphate in groundwater along the A-A' transect in 2006. Dots show the positions of well screens and multilevel-sampler ports.