Trichloroethylene and 1,1-dichloroethylene concentrations in ground water after temporary shutdown of the reclamation well field at Air Force Plant 44, Tucson, Arizona, 1999

Industrial activities beginning in the early 1940s resulted in extensive contamination of ground water near the Tucson International Airport, Tucson, Arizona, including an area around Air Force Plant 44, an industrial facility located on land owned by the U.S. Air Force and operated by a defense contractor. Principal ground-water contaminants are volatile organic compounds, primarily trichloroethylene (also called trichloroethene) and 1,1-dichloroethylene (also called 1,1-dichloroethene). A ground- water reclamation system was put into operation in 1987 to extract and treat contaminated ground water at Air Force Plant 44 and the downgradient area that is south of Los Reales Road. The ground- water reclamation system consists of 25 extraction wells, 22 recharge wells, and a water-treatment facility. Soil-vapor extraction techniques are being used to remove volatile organic compounds from the unsaturated zone. More than 120,000 pounds of volatile organic compounds have been removed from the regional aquifer and overlying unsaturated zone at Air Force Plant 44 and adjacent downgradient areas south of Los Reales Road. Air Force Plant 44 and adjacent areas being remediated by the ground-water reclamation system are about 7 square miles.

To assess ground-water cleanup progress at Air Force Plant 44 and surrounding areas south of Los Reales Road, and possibly to identify areas that are resistant to cleanup attempts, ground-water samples were collected and analyzed after water levels had returned to near-equilibrium conditions following a 3-week shutdown of extraction and recharge wells. Modifications of the standard ground-water sampling procedures used at the site also were tested. The modifications included tests of a reduced-flow purging and sampling method in six monitoring wells and vertical- profile sampling in five extraction wells at the reclamation well field.

The water treatment facility and all extraction and recharge wells at the reclamation well field were shut down on April 15, 1999, and water levels were allowed to recover for about 3 weeks before samples of ground water were obtained from 102 wells at Air Force Plant 44 and surrounding areas. Concentrations of trichloroethylene and 1,1-dichloroethylene were determined for samples obtained during the sitewide sampling effort. Data for 101 wells sampled in February 1999 before shutdown were compared with data obtained for wells sampled in May 1999 after shutdown. Concentrations of trichloroethylene increased in 36 wells, remained the same in 32 wells, and decreased in 33 wells. Increases in concentrations of trichloroethylene of as much as 1,476 micrograms per liter and decreases of as much as 2,292 micrograms per liter were reported after shutdown. Concentrations of trichloroethylene remained the same for the two sampling periods in wells that had concentrations that were at, or close to, the lower reporting limit (0.5 micrograms per liter) before shutdown. Net change in concentrations of trichloroethylene after shutdown on a percentage basis ranged from an increase of 1,300 percent to a decrease of 100 percent. Increases in concentrations of 1,1-dichloroethylene after shutdown of the reclamation well field of as much as 66 micrograms per liter and decreases of as much as 411.6 micro- grams per liter were reported. Concentrations of 1,1-dichloroethylene remained the same for the two sampling periods in wells that had concentrations that were at, or close to, the lower reporting limit (0.5 micrograms per liter) before shutdown. Net change in concentrations of 1,1-dichloroethylene after shutdown on a percentage basis ranged from an increase of 660 percent to a decrease of 100 percent.

Data obtained from the water samples indicate
that the largest changes in concentrations of
trichloroethylene and 1,1-dichloroethylene
occurred in samples collected from wells
completed in the upper zone of the regional
aquifer, along the axis of the contaminant plume,
in close proximity to previously identified
historical disposal areas. Changes in contaminant
concentrations observed after shutdown of the well
field probably were the result of changes in
ground-water flow directions under nonpumping
conditions compared with those present when the
extraction and recharge wells were operating.
Minimal changes occurred at the perimeter of the
plume, which suggests that operation of the
reclamation well field has been successful at containing the spread of the plume. New
contaminant-source areas were not identified
within the perimeter of the plume.

A modification of the standard sampling
technique used at Air Force Plant 44 was tested in
six wells. In these wells, greatly reduced flow rates
were used for well purging and sampling. Results
indicate no distinct pattern of change of
contaminant concentrations compared with
concentrations in samples subsequently obtained
using the standard technique, and no advantage
was evident for using this method in routine
sampling of the monitoring wells at Air Force
Plant 44.

Temperature profiles obtained before vertical-profile
sampling of selected wells indicate little
temperature variation with depth. The
temperature-profile information suggests that
under nonpumping conditions, most of the water
enters these wells near the top of the screened
interval and moves downward in response to a
hydraulic gradient in the regional aquifer. Samples
at depths below the top of the screened interval
probably do not accurately represent water from
the adjacent sediments.

Vertical-profile samples were obtained in five
wells and analyzed for concentrations of
trichloroethylene. None of the wells showed large
enough variation of contaminant concentrations
with depth to indicate that a major improvement in
extraction efficiency could be obtained by
pumping selectively from a restricted interval.
The largest variation in concentrations of
trichloroethylene with depth that was observed
ranged from 62 micrograms per liter near the top
of the screened interval to 42 micrograms per liter
near the bottom of the screened interval of one of
the wells. The lack of large variation is probably
the result of downward water flow in the casing of
these wells.