Scientific Investigations Report 2006–5232

Scientific Investigations Report 2006–5232

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Qualitative Evaluation of Well-Purging Effects on Water-Quality Results

A qualitative evaluation was initiated to determine if water-quality data collected during October-November 2003 from 30 wells (in the RCRA-listed waste polygon) were consistent with water-quality data collected from the same wells since 1990. October-November 2003 water-quality data were collected at each of the 30 wells for each constituent included in the USGS’s routine sample-collection program and plotted on graphs (figs. 3-32, at back of report) along with water-quality data collected since 1990. Water-quality data used to plot the constituent graphs are available at Graphic water-quality data for the 30 wells (figs. 3-32) were visually inspected to determine if apparent water-quality data trends met one of four criteria: (1) increasing concentrations continued to increase, or (2) decreasing concentrations continued to decrease, or (3) constant concentrations remained constant, or (4) October-November 2003 data were consistent with long-term ranges of water-quality data when no apparent concentration trends existed. If constituents met one of these criteria, the conclusion was that purging either 1 or 3 wellbore volumes from the well did not affect data comparability.

Constituent data examples that met one of the above criteria include:

The visual inspection of 322 constituent plots in figs. 3-32 indicated that nearly all plots met one or two of the four criteria identified for data comparability. Six constituent plots and three field-measurement plots did not meet any of the criteria. These included sodium concentrations for wells USGS 57 and 115 (figs. 22, 30), sulfate concentrations for wells USGS 42, 43, and 57 (figs. 12, 13, 22), pH measurements for wells USGS 47 and 112 (figs. 17, 27), temperature measurements for well USGS 111 (fig. 26), and cesium-137 concentrations for well USGS 113 (fig. 28). In each instance, concentrations or measurements for the October-November 2003 samples were 14-year minimums and did not follow any trend. All cesium-137 concentrations at well USGS 113 for the 10-year period of record were less than the radionuclide reporting level of three times the standard deviation and were within two standard deviations of each other (fig. 28). As a result, cesium-137 for the October-November sample is considered comparable to historical data. When cesium-137 is included with the 313 constituent plots or measurements that met one or more of the acceptance criteria for data comparability, 97.5 percent were comparable.

The remaining eight constituent or measurement plots did not follow any trend or fall within the historical range of water-quality data and were not considered comparable; however, concentrations or measurements did plot near the mean values for the period of record. The October-November 2003 pH measurements (7.6 pH units at well USGS 47 and 7.5 pH units at well USGS 112) (figs. 17, 27) were within 0.1 and 0.2 pH units of the previous minimums (7.7 and 7.7 pH units in 2001 and 1994, respectively) and within 0.3 and 0.4 pH units of the respective mean concentrations (7.9 and 7.9 pH units). The October-November 2003 temperature measurement of 12.5oC at well USGS 111 (fig. 26) was within 0.2oC of the previous minimum (12.7oC in 1995) and within 0.8oC of the 13-year mean (13.3oC). The October-November 2003 sodium concentrations (30 mg/L at well USGS 57 and 10 mg/L at well USGS 115) (figs. 22, 30) were within 8 and 3 mg/L of the previous minimums (38 and 13 mg/L in 2001 and 1991, respectively) and within 27 and 4 mg/L of the respective mean concentrations (57 and 14 mg/L). The October-November 2003 sulfate concentrations (22 mg/L at well USGS 42, 22 mg/L at well USGS 43, and 27 mg/L at well USGS 57) (figs. 12, 13, 22) were within 2, 1, and 2 mg/L of the previous minimums (24, 23, and 29 mg/L in 2001, 1998, and 2000, respectively) and within 3, 2, and 4 mg/L of the respective mean concentrations (25, 24, and 31 mg/L).

Because of the good qualitative agreement between the October-November 2003 water-quality data and the corresponding historical water-quality data, there is no indication that the change from purging 3 wellbore volumes of water to purging 1 wellbore volume of water (for the 30 wells in the RCRA listed-waste polygon) has an effect on the comparability of water-quality data.

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