Water Potential Response in Fractured Basalt from Infiltration Events

Abstract

The detection of moisture movement in the vadose zone beneath hazardous waste disposal sites is important for monitoring and predicting contaminant transport in the subsurface. This study was conducted to determine if moisture movement could be detected from natural water infiltration in a dual porosity, fractured basalt at the Idaho National Engineering and Environmental Laboratory (INEEL). Episodic infiltration events were determined by examining long-term water potential over three and a half years. Water potential measurements were collected from 2 to over 30 m below land surface. Instruments were placed in both fractured and unfractured basalt media. Water potential measurements were within the tensiometric range, from approximately +100 to −250 cm of water. Typically, water potentials within the fractured basalt exhibited a near steady-state unit-gradient downward flux. However, episodic snowmelt infiltration events at land surface produced detectable changes in water potentials at depths to 15.5 m, in some cases within a few days of the infiltration events. Smaller infiltration events were difficult to distinguish due to fluctuations in water potential resulting from changes in barometric pressure. Water potential responses in boreholes varied both temporally and spatially during episodic infiltration events indicating preferential flow pathways through the fractured basalt. The results of this study indicate that water potential measurements can be used to detect and monitor deep infiltration events at waste disposal sites using tensiometers.

Application of a Darcian Approach to Estimate Liquid Flux in a Deep Vadose Zone

“…3). One well (76‐5) was installed in 1995 (details in Hubbell et al, 2002), while the remainder (I and O prefixes) were installed during 1999 and 2000 (details in Dooley and Higgs, 2003). All wells were drilled by air‐rotary, using either a downhole hammer bit (25.1‐cm diam.)…”

Evaluation of the Conceptual Flow Model for a Deep Vadose Zone System Using Advanced Tensiometers

“…Wells 77‐2 and 78‐1 were instrumented with advanced tensiometers in December 1995, with the addition of a portable tensiometer (Hubbell and Sisson, 1996) at the bottom of 77‐2 in December 1999. Previous tensiometer data from Wells 76‐5, 77‐2, and 78‐1 are presented in Hubbell et al (2002) and McElroy and Hubbell (2003)…”

“…Local run‐off from late winter and early spring snowmelt has the greatest potential for extensive infiltration because these events occur when evapotranspiration rates are low. Hubbell et al (2002) monitored a nested series of advanced tensiometers from 6.7 to 31.4 m bls, in a single borehole at the SDA. Based on 4 yr of water potential data, they found water potentials at this location were generally constant, but were interrupted by episodic recharge that was tracked to the 17‐m depth, during the 1999 spring snowmelt.…”

“…Downward movement through the basalts is assumed to occur primarily through open or sediment‐filled fractures or joints, rather than the basalt matrix (Holdren et al, 2002). Hubbell et al (2002) monitored the advance of a wetting front in the SDA that moved through basalt at 0.1 m d −1 With a continuously ponded water source, such as during the Large‐Scale Infiltration Test performed near the RWMC (Wood and Norrell, 1996), the advance of the wetting front through the basalt was on the order of 5 m d −1…”

“…Sully et al (1994) noted higher water potentials (−1000 cm of water) at Hanford, WA than at the Nevada Test Site (−6000 cm of water) in the southwestern United States. In Idaho, temporal water potentials ranged from +100 to −250 cm of water at the 2‐ to 30‐m depths (Hubbell et al, 2002).…”