In conjunction with the April 2003 conservative tracer test, fluid-temperature and fluid-conductivity borehole measurements were collected in the G-3772 observation well, located ~66 m from injection well G-3773 and ~34 m from the S-3164 production well (Fig. 1D). Borehole temperature and conductivity profiles were collected within the open-hole section of the G-3772 observation well at 102-322 min intervals as the tracer plume moved toward the production well (Fig. 8). An anomalous temperature change of ~0.8 °C was observed 3 h and 22 min (12:52 p.m. local time) after the completion of the tracer injection (Fig. 8). We assumed, for purposes of this discussion, that the liquid tracer (~210 L) had equilibrated to the average ambient air temperature (26 °C) on the day of the injection. Groundwater temperature on 22 April 2003 ranged from 22.5 to 23.5 °C. The observed 0.8 °C increase in fluid temperature at the observation well G-3772 was attributable to movement of the tracer pulse as it passed the G-3772 well bore. This temperature anomaly was recorded ~3 h prior to peak breakthrough of the tracers at pumping well S-3164 and at about the same time the leading edge of the tracer pulse was first detected at well S-3164 (Fig. 9). The change in fluid temperature appears to have been greatest at a depth interval of ~12.2-12.8 m below land surface (Fig. 8). This depth corresponds to an apparent high-permeability flow zone characterized by touching-vug porosity. This zone is located at the base of high-frequency cycle HFC2e2 (Fig. 7), which is just above the flooding surface bounding the top of high-frequency cycle HFCd2 (Fig. 4). This observation corroborates our conceptual karst aquifer model, which links most high-permeability zones to the lower part of high-frequency cycles. A fluid-temperature anomaly in the G-3772 observation well (Fig. 1D) strongly suggests that a substantial part of the tracer moved through a relatively thin (0.6 m) flow zone at the base of high-frequency cycle HFC2e2 (Fig. 7 and Fig. 8). A comparison of fluid-temperature data with fluid-conductivity profiles is less persuasive. Conductivity of the Rhodamine tracer was greater than ambient groundwater at 12:52 and 14:33 p.m. local time, and appeared to be dispersed within the borehole section that includes high-frequency cycles HFC2a to HFC2g1 (Fig. 8), but it is unclear what controlled the measured changes in conductivity during the tracer test.

Figure 8. Correlation of cyclostratigraphy, lithofacies, pore classes, and borehole geophysical logs for injection well G-3773, observation well G-3772, and open hole interval of production well (S-3164) at Northwest Well Field (Fig. 1D). Major flow zone at the high-frequency cycle HFC2e2 base is shown as a horizontal blue stripe inferred from digital borehole images, flow-meter measurements, caliper, and fluid-temperature logs. Pumping flow-meter measurements were accomplished by pumping groundwater out of a cased borehole for well G-3773. Positive and negative flow-meter rates indicate upward and downward flow in the borehole, respectively. For additional information about this figure, please contact Kevin Cunningham at kcunning@usgs.gov. [larger image]

Stationary and trolling electromagnetic flow-meter data obtained during ambient and pumping measurements at injection well G-3773 (Fig. 8) were collected in October 2003. Results suggest that under stressed conditions, significant movement of groundwater occurs at the base of high-frequency cycle HFC2e2, which is consistent with the borehole fluid temperatures collected during the tracer test. Uncertainty of the amount of fluid flow bypassing the flexible-disk diverter on the flow meter limits the accuracy of the measurements (Paillet, 2004).

Figure 9. Graph showing breakthrough curves for two tracers, Rhodamine WT and deuterated water (2H), based on groundwater samples collected from pumping well S-3164 during a tracer test conducted at the Northwest Well Field in April 2003 (Fig. 1D). The leading edge of the tracer pulse was detected ~3 h prior to peak breakthrough of the two tracers, which occurred 6.5 h after they were introduced into the G-3773 injection borehole (Fig. 1D). VSMOW-Vienna standard mean ocean water. [larger image]

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