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1.2 Data Retrieval from SPDSS Monitoring Wells <br />As part of the SPDSS groundwater level data collection effort, groundwater level data were <br />retrieved from the data loggers in SPDSS monitoring wells. There are 37 wells in the alluvial <br />aquifer and one well in the Upper Arapahoe Aquifer (locations shown in Figure 3). Three of the <br />SPDSS monitoring wells are owned by other entities, including the United States Geological <br />Survey, the City of Sterling, and a private owner. The remaining SPDSS monitoring wells were <br />drilled and installed by the SPDSS. The data loggers were installed at these wells during Phase <br />1 and 2 and have been continually collecting water level measurements on an hourly interval. <br />The Phase 3 data were retrieved in the fall of 2005 and the spring of 2006. <br />The data loggers are suspended in each well by a data cable that vents the loggers to <br />atmospheric pressure. The water level data are downloaded from the data logger by connecting <br />a laptop to the data cable using an RS-232 serial port. The downloaded data are saved to the <br />laptop and transported back to the office for processing. <br />During the download of water level data field activities, a field book is used to record the <br />manual water level measurements taken with a conductivity meter (described below), the <br />battery and free storage capacity of the data logger, the serial number of the data logger, and <br />the interval of data collection by the data logger. <br />Standard data download field protocol includes collecting a manual field measurement before <br />the download of the water level data and comparing it to the instantaneous water level <br />measurement of the data logger. This is done to verify the accuracy of the data logger's <br />measurements since initiating the data logging. It is common for the manual and data logger <br />measurements to differ by a couple of tenths of a foot. This difference is commonly referred to <br />as data logger drift. This may be the result of the data logger's accuracy limitations, elapsed <br />time since its calibration, cable stretch, and/or subtle changes in the data logger's resting height. <br />A data logger's drift error can increase from year to year, which may indicate the data logger <br />should be recalibrated. <br />The significance of drift error can be evaluated by comparing the drift error to the magnitude of <br />water level change observed over a given time period. For example, if the drift error is 0.5 foot <br />and the maximum water level change is one foot, then the drift error is a significant portion of <br />the water level change observed. However, if the maximum water level change is ten feet, then <br />the drift error is a small percentage of the overall change. Additionally drift error can be <br />evaluated by reviewing the water level data for the period of record. If there is a sudden offset <br />in the water level data, the drift error may be caused by a sudden change in the height of the <br />data logger. This sort of drift error can often be corrected in the water level data. Each data <br />logger should be evaluated and potentially sent to the manufacturer for recalibration when <br />significant drift error is observed. <br />To account for data logger drift error, a linear correction was applied to all the water level data <br />measurements to average the drift error over the duration of the test. After the linear <br />corrections, the measurements were normalized to elevations and converted to an average daily <br />value. <br />SPDSS Phase 3 Task 39 Technical Memarandum -Final <br />09/27/2006 <br />