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

restrict communication to the atmosphere. As a result, gas pressures in the vadose zone will change more slowly Barometric pressure fluctuations may influence measured water and to a lesser extent than barometric pressure. Conlevels in wells where thick vadose zones or low permeability materials versely, gas pressure in an unsealed well will equilibrate overlie unconfined aquifers. Here, we present a well completion method designed to reduce the effects of barometric pressure fluctuaalmost instantaneously to changes in barometric prestions on measured water levels. This well configuration, called the sure. Where gas pressure in the vadose zone exceeds isobaric well, seals the interior of the well from atmospheric pressure, atmospheric pressure, water will move from the aquifer and vents the reference side of the water level pressure transducer into an unsealed well bore, creating an unnaturally high to the gas phase pressure above the water table. We tested the isobaric water level, and vice versa. design on a well completed about 180 m below land surface in the Unsealed wells penetrating deep unconfined aquifers Eastern Snake River Plain aquifer at the Idaho National Engineering frequently exhibit barometric induced fluctuations in and Environmental Laboratory. Water level data collected during water levels on the order of 6 cm d 1, and up to 30 14 mo show more than an order of magnitude decrease in diurnal cm over several months (Weeks, 1979; Rasmussen and fluctuations when the well was operated in the isobaric mode. DecreasCrawford, 1997). Data collected at the INEEL (Wylie ing the noise level allowed clear definition of water level trends that would otherwise have been at least partially obscured by barometric and Hubbell, 1994) and the Savannah River Site (Benfluctuations. This well configuration allows direct monitoring of water nett et al., 1997) show barometric pressure induced fluclevel changes, without the need to rely on postprocessing to mitigate tuations at depths to water of 180 m and 15 to 30 m, barometric influences. respectively. Similar water potential fluctuations are observed from advanced tensiometers measuring sediments and basalt in the vadose zone (Sisson and HubT fluctuations in barometric pressure can bell, 1999). At sites with closely spaced wells and/or significantly complicate measurement of water level small gradients, barometric induced fluctuations in meain unconfined aquifers, particularly where the vadose sured water level may significantly impact estimates for zone is thick or contains low permeability zones. First, the magnitude and direction of hydraulic gradient. water levels are commonly measured using differential Spane (2002) presented several examples representative pressure transducers referenced to barometric pressure of the Hanford Site, where barometric induced fluctuaat the wellhead. Second, unsealed observation wells protions could lead to estimation errors of up to 180 for vide a direct connection to atmospheric pressure changes, groundwater flow direction, and a factor of four for the while the surrounding aquifer is partially buffered by hydraulic gradient. the intervening vadose zone materials. Thus, water levA number of authors have suggested numerical methels in the well may not be at equilibrium with the aquifer, ods for correcting water level data by first estimating, leading to inaccurate measurements. Accurate water level then removing the influence of barometric pressure flucmeasurements are particularly important at sites with tuations (e.g., Clark, 1967; Weeks, 1979; Rojstaczer, closely spaced wells, small hydraulic gradients, high trans1988a, 1988b; Rojstaczer and Agnew, 1989; Furbish, missivity, and/or low storage coefficients, all of which 1991; Rasmussen and Crawford, 1997; Spane, 2002). To are issues of concern at the Idaho National Engineering do so, one must have both an accurate record of baroand Environmental Laboratory (INEEL). metric pressure at the wellhead and adequate models for The effects of barometric fluctuations on water table the relevant transfer functions. However, most existing measurements have been documented by numerous auapproaches employ a single constant (barometric effithors (e.g., Jacob, 1940; Weeks, 1979; Rojstaczer, 1988a; ciency) to describe system response to barometric presRasmussen and Crawford, 1997; Hare and Morse, 1997). sure changes (see summary in Rasmussen and Crawford, Low air permeability materials in the vadose zone can 1997). This is a significant assumption, as it has been suggested that the barometric efficiency can vary seasonally and with depth (e.g., Rojstaczer, 1988a). J.M. Hubbell and J.B. Sisson, Idaho National Engineering and EnviAs an alternative to cleaning up the data in postproronmental Laboratory, Geosciences Research Department, P.O. Box cessing, we present a well completion technique de1625, MS 2107, Idaho Falls, ID 83415; M.J. Nicholl, Mining and Geological Engineering, University of Idaho, Moscow ID 83843; and R.G. signed to reduce the impact of barometric pressure flucTaylor, Idaho Department of Environmental Quality, 1410 North tuations on water level measurements in unconfined Hilton, Boise, ID 83642. Received 19 Feb. 2003. Special Section: aquifers. We begin by presenting a conceptual model Understanding Subsurface Flow and Transport Processes at the Idaho for an isobaric well (Hubbell and Sisson, 1999) design National Engineering & Environmental Laboratory (INEEL) Site. *Corresponding author ([email protected]). Abbreviations: bls, below land surface; ESRP, Eastern Snake River Published in Vadose Zone Journal 3:183–189 (2004).  Soil Science Society of America Plain; INEEL, Idaho National Engineering and Environmental Laboratory. 677 S. Segoe Rd., Madison, WI 53711 USA