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Red Creek Quarry Baseline Water Investigation Section 6 6-9 DRAFT for review purposes only. Use of contents on this sheet is subject to the limitations specified at the end of this document. 6.2 Constituents with Naturally Occurring Elevated Concentrations One objective of the RCQ baseline water quality project was to establish the background level <br />concentration of regulated constituents. Background level is defined as “the level of any parameter in the groundwater within a specified area as determined by representative measurements of the groundwater quality unaffected by activity” (WQCC 5 CCR 1002-41.3 (3)). A comparison of established surface and groundwater water quality standards was conducted to identify analytes with naturally occurring elevated concentrations and to serve as a useful frame of reference for understanding background level water quality. The use of water quality standards and criteria in this report is for technical analytic purposes; it should not be interpreted to include any legal or regulatory compliance conclusions, or to suggest any sampling point or other location is a discharge or other regulatory point of compliance. Highly turbid sample data may be overestimating the concentrations of both the total and dissolved metals because filtering does not remove all suspended particulates (Puls and Barcelona 1989). Because of the occurrence of suspended particulate matter in numerous groundwater samples and upstream surface water samples, assessments of baseline water quality at the Site based only on total metals/metalloids concentrations alone would likely overstate the extent of mobile constituents. Turbidity field measurements, TDS, and total suspended solids concentrations were reviewed when evaluating the metal/metalloid concentrations for surface and groundwater data. 6.2.1 Elevated Constituents Concentrations in Groundwater Groundwater data were compared to the drinking and agricultural water quality standards listed under WQCC 5 CCR 1002-41 to identify constituents with naturally occurring elevated concentrations (Appendix H). Results of the comparison to water quality standards are summarized in Table 6-1. Scatter plots of groundwater data were generated to evaluate the spatial variability of elevated constituents (Figure 6-8). The red lines on the plots indicate the groundwater standard concentration for each constituent. Sulfate concentrations for all ground water samples exceed the drinking water quality standard of 250 mg/L; therefore, there is no line to indicate the sulfate drinking water standard on the plots. Naturally occurring elevated concentrations of selenium, uranium and sulfate have been identified within lithological members of the Niobrara Formation in the Arkansas River valley near the Site (Arnold et al. 2016; Bern and Stogner 2017; Miller et al. 2010). Elevated concentrations of iron and manganese have been measured in shallow alluvial groundwater samples collected along the Fountain Creek drainage (Arnold et al. 2016). The groundwater data in Figure 6-8 show a range in concentrations across the Site: • Well 1 has the highest concentrations dissolved selenium and dissolved uranium. • Well 3 has the highest concentrations in dissolved manganese and sulfate. • Well 4 has the highest concentrations of gross alpha and dissolved iron.