### 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

By: D. D. GrahamT.J. AllenM.L. BarackmanW.H. DiGuiseppiM.F. Wallace

                            https://doi.org/10.3133/wri014177

#### Links

-                                                                                                                             Document:                                                                                                                                                                                                                                                  **Report** 
- Download citation as:  RIS                         |                         Dublin                            Core

#### Abstract

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 indicatethat the largest changes in concentrations oftrichloroethylene and 1,1-dichloroethyleneoccurred in samples collected from wellscompleted in the upper zone of the regionalaquifer, along the axis of the contaminant plume,in close proximity to previously identifiedhistorical disposal areas. Changes in contaminantconcentrations observed after shutdown of the wellfield probably were the result of changes inground-water flow directions under nonpumpingconditions compared with those present when theextraction and recharge wells were operating.Minimal changes occurred at the perimeter of theplume, which suggests that operation of thereclamation well field has been successful at containing the spread of the plume. Newcontaminant-source areas were not identifiedwithin the perimeter of the plume.

A modification of the standard samplingtechnique used at Air Force Plant 44 was tested insix wells. In these wells, greatly reduced flow rateswere used for well purging and sampling. Resultsindicate no distinct pattern of change ofcontaminant concentrations compared withconcentrations in samples subsequently obtainedusing the standard technique, and no advantagewas evident for using this method in routinesampling of the monitoring wells at Air ForcePlant 44.

Temperature profiles obtained before vertical-profilesampling of selected wells indicate littletemperature variation with depth. Thetemperature-profile information suggests thatunder nonpumping conditions, most of the waterenters these wells near the top of the screenedinterval and moves downward in response to ahydraulic gradient in the regional aquifer. Samplesat depths below the top of the screened intervalprobably do not accurately represent water fromthe adjacent sediments.

Vertical-profile samples were obtained in fivewells and analyzed for concentrations oftrichloroethylene. None of the wells showed largeenough variation of contaminant concentrationswith depth to indicate that a major improvement inextraction efficiency could be obtained bypumping selectively from a restricted interval.The largest variation in concentrations oftrichloroethylene with depth that was observedranged from 62 micrograms per liter near the topof the screened interval to 42 micrograms per liternear the bottom of the screened interval of one ofthe wells. The lack of large variation is probablythe result of downward water flow in the casing ofthese wells.

#### Study Area

Part or all of this report is presented in Portable Document Format                        (PDF). For best results viewing and printing PDF documents, it is recommended that you download                        the documents to your computer and open them with Adobe Reader. PDF documents opened from your                        browser may not display or print as intended. Download the latest version of Adobe Reader, free of charge. More information                        about viewing, downloading, and printing report files can be found at the common download problems FAQ.