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Well Design to Reduce Barometric Pressure Effects on Water Level Data in Unconfined Aquifers

^a Idaho National Engineering and Environmental Laboratory, Geosciences Research Department, P.O. Box 1625, MS 2107, Idaho Falls, ID 83415
^b Mining and Geological Engineering, University of Idaho, Moscow ID 83843
^c Idaho Department of Environmental Quality, 1410 North Hilton, Boise, ID 83642

View larger version (26K): [in a new window]   Fig. 1. Pressures relevant to measurement of water level in (a) a conventional (nonisobaric) well and in (b) an isobaric well. In both cases, water level is measured by using a pressure transducer to measure water pressure (Pobs) at a given elevation within the well. In the nonisobaric well, water levels in the well respond to changes in atmospheric pressure (Pbar) and the pressure transducer is referenced (Pref) to atmospheric pressure. The isobaric well is sealed from the atmosphere, and the pressure transducer is referenced (Pref) to gas pressure above the water table (Psg).

View larger version (23K): [in a new window]   Fig. 2. Location of the field implementation site. USGS-118 is located about 30 m south of the Subsurface Disposal Area at the Radioactive Waste Management Complex (RWMC), a low-level radioactive waste disposal site in the southwest quadrant of the INEEL.

View larger version (37K): [in a new window]   Fig. 3. Stratigraphy and well completion for Well USGS-118, which was used to evaluate the isobaric technique. The 5-cm casing was slotted with 1-mm saw cuts over the screened interval, and four vapor ports were added to the outside of the casing. Vapor ports were fabricated from lengths of 9.5-mm-diam. tubing. The bottom of each tube was crimped to prevent backfill from entering, and the next 1.5 m was perforated with 2-mm-diam. holes. Individual vapor ports were attached to the outside of the casing, then routed to the surface. The annular region was backfilled with coarse sand around the screened interval and vapor ports, the remainder of the borehole was sealed with bentonite and neat cement (Hubbell et al., 1998).

View larger version (19K): [in a new window]   Fig. 4. The airtight PVC well cap. The cap is sealed to the casing when the rubber gasket on the lower exterior of the cap is forced out against the 5-cm well casing by tightening the wing-nut. The one-holed, tapered rubber stopper is forced down into the tapered shaft by tightening the cap nut, thus sealing around the transducer leads. A stock rubber gasket from a mechanical test plug was used in this test.

View larger version (21K): [in a new window]   Fig. 5. Water level above the pressure transducer located at 183 m bls in Well USGS-118 measured for a 14-mo period. Data was collected from 25 Mar. 1997 to 29 May 1998 at 60-min intervals. Flow in the nearby Big Lost River (see Fig. 2) is shown for comparison. Boxes marked A and B are expanded in Fig. 6 to illustrated the difference between isobaric (Box A) and non-isobaric (Box B) data.

View larger version (32K): [in a new window]   Fig. 6. Comparison of water levels measured in Well USGS-118 for two 7-d periods. Barometric pressure in meters of water is shown for comparison. Data collected under isobaric conditions from 3 June 1997 to 10 June 1997 (Box A in Fig. 5). Linear regression of the data gives a slope of 0.0025 m h–1. Data collected from 26 June 1997 to 3 July 1997 (Box B in Fig. 5) begins and ends with isobaric data, while data in the center of the 7-d period (27 June 1997 to 3 July 1997) was collected under nonisobaric conditions. The isobaric portion of the data shows a slope similar to the earlier data (0.0026 m d–1), while the nonisobaric portion actually shows a declining trend (–0.0143 m d–1) of much larger magnitude.