USGS_GeophysicalLoggingInSLV_Report

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Alamosa County ALA.2 is about 80 ft from ALA.3, and water levels in both wells fluctuate in a similar manner CAWN wells that are in Alamosa County are (fig. 5). The water level in ALA.3 decreased about 20 named with the ?ALA? prefix and a sequence number. ft in a short period in 1989. Since mid-1989, the water Geophysical logs were recorded in 12 of the 16 level measured in ALA.3 typically has been several CAWN wells in Alamosa County. feet lower than the water level in ALA.2, whereas prior to mid-1989 water levels were higher in ALA.3 Well ALA.2 than in ALA.2. Possibly nearby pumping or a change in casing integrity caused the water level in ALA.3 to Well ALA.2 was constructed by the USGS in decline while water levels in ALA.2 were not similarly 1969 to measure the water level in the confined- affected. The location and length of the open interval aquifer system. A lithologic log is not available for this in ALA.3 are unknown, so it is assumed that inflow flowing well. Construction is reported to be 2.0-inch- comes through possible casing imperfections at 110 diameter steel pipe to a depth of 415 ft. Partial logging and 160 ft and from the entire unlogged section from with the caliper tool showed a reduction in the casing 881 to 970 ft. The water level measured in ALA.3 is a from 2.0-inch diameter to 1.0-inch or 0.75-inch diam- composite of the heads in the aquifer at depths of eter at a depth of 350 ft. Logs could not be obtained about 110 and 160 ft and in the open part of the below the casing reduction because the diameter of the interval between 881 and 970 ft. smallest logging tools is approximately 1.6 inches. Flowmeter logs were not recorded in the 2.0-inch- Well ALA.4 diameter section of the casing. Inflow is assumed to come from below a depth of 350 ft because both the Well ALA.4 was constructed in 1957 for irriga- specific-conductance and temperature logs do not tion use and is a flowing well. The casing is 20.0-inch show significant inflections that would indicate inflow diameter from land surface to a depth of 38 ft, between the surface and 350 ft (fig. 3). The location 12.5-inch diameter from 38 to 1,422 ft, and 8.5-inch and length of the open interval in ALA.2 are unknown, diameter from 1,422 to 1,973 ft. The intervals so it is assumed that flow comes from the entire containing torch-cut slots (540 to 690 ft; 885 to unlogged section from 350 to 415 ft. The water level 1,973 ft) are open to lithologic layers containing measured in ALA.2 represents a composite measure- (1) sand, gravel, or cobbles; (2) clay with some sand or ment of the heads in the confined-aquifer system in the gravel; or (3) sand or gravel with some clay (fig. 6). 65-ft interval from 350 to 415 ft. The lithologic layers contributing water to the well are below a thick clay layer that is present from a depth of Well ALA.3 about 290 to 442 ft. Specific-conductance and temperature logs and Well ALA.3 was constructed by the USGS in the free-flowing stationary flowmeter measurements 1969 to measure the water level in the confined- indicate two inflow zones in this well from a depth of aquifer system. A lithologic log was not available for 1,200 to 1,600 ft and 540 to 640 ft. Specific-conduc- this flowing well. Construction is reported to be tance and temperature logs also indicate inflow occurs 2.0-inch-diameter steel pipe to a depth of 970 ft. below a depth of 1,600 ft that is not confirmed by the Partial logging with the caliper tool showed a reduc- flowmeter data. One possible zone of outflow is indi- tion in the casing from 2.0-inch-diameter to 1.0-inch- cated from a depth of 970 to 1,000 ft. The small or 0.75-inch diameter at a depth of 881 ft. Logs could amount of outflow measured is either outflow into a not be obtained below the casing reduction because the permeable layer or outflow into the annular space logging tools are approximately 1.6-inch diameter. around the casing that eventually becomes inflow to Flowmeter logs were not obtained in this well. the well at a shallower depth. The decrease in flow at a A plot of the specific-conductance and tempera- depth of 1,422 ft is caused by the change in casing ture logs indicates that flow enters the casing at depths diameter there. of 110 and 160 ft (fig. 4). The video log shows that a break in the casing may exist at 110 ft and the casing is Under shut-in conditions, one inflow zone exists slightly corroded near 160 ft, but no obvious perfora- from a depth of 930 to 1,520 ft, and two outflow zones tions were seen at either depth. exist from a depth of 885 to 930 ft and 540 to 690 ft. 10Geophysical Logging to Determine Construction, Contributing Zones, and Appropriate Use of Water Levels Measured in Confined-Aquifer Network Wells, San Luis Valley, Colorado, 1998?2000