2020-06-08_REVISION - C1982057 (20)

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Phase III Hydrology A.) Basic Standards Interim Narrative Standard for Ground Water(CWQCC Regulation 41.5(c)(6). The Groundwater Point of Compliance (GWPOC)for the Seneca II West Mine was established in 2009 under Technical Revision 63 (TR-63).The GWPOC well at the Seneca II West Mine is monitoring well DCAL-02 which is screened within the Dry Creek Alluvium downgradient (north) of the mines permit boundary(see Map 1). A GWPOC for the Sage Creek Alluvium was deemed unnecessary due to the small portion of the ridgeline mined within the Sage Creek watershed.The bedrock within the permit area dips to the west and the spoil water generated at the ridge top drains to the west along the dipping bedrock, away from the Sage Creek tributaries. GWPOC bedrock wells were also deemed unnecessary due to the limited potential for the mine to negatively impact the quality of bedrock groundwater.The Wadge and Wolf Creek Coal exhibit low hydraulic conductivity(3.5E-8 to 3.5E-5 cm/sec) and attenuation and dilution should further limit water quality impacts. Low permeability confining layers isolate mine- impacted water bearing units from the nearest useable aquifer,the Trout Creek Sandstone.The Trout Creek Sandston lies approximately 300 to 400 feet below the Wadge Coal seam and approximately 60 to 100 feet below the Wolf Creek Coal Seem. See TR-63 in Appendix 15-313 of the Seneca II West Mine permit package for additional justification for the Groundwater Point of Compliance. Table 16 includes a summary of the water quality collected at well DCAL-02 over the last five years (2015 —2019) and provides a comparison against the Dry Creek alluvial GWPOC water quality standards established in TR-63 in 2009. The groundwater quality at DCAL-02 consistently meets the water quality standards except for dissolved iron. Over the last five years dissolved iron has ranged from 0.14—10.8 mg/L(mean: 6.74 mg/L). DCAL-02 was installed in the Dry Creek alluvium in 1998 after mining had already started in the watershed. Since pre-mining background water quality data is not available for the Dry Creek alluvium the ambient groundwater standards were established from wells located within the adjacent Hubberson and Watering Trough Gulch alluvium.The dissolved iron at these locations was variable with concentrations as high as 61 mg/L. Potential outliers were excluded from the dataset and a standard of 8.06 mg/L was established for the alluvium in this area. Dissolved iron ranged from 6.96- 11.1 mg/L(mean: 8.59 mg/L) in the three initial samples collected in July 1998, shortly after the well's installation. A statistical comparison of the dissolved iron measured in the 1998 samples to the dissolved iron present in the samples collected over the last five years indicates that there has not been a statistically significant change (95%Confidence Limit) in the dissolved iron concentration at DCAL-02 (Attachment 1). Considering that no other groundwater quality standard has been exceeded and that there has not been a statistically significant change in dissolved iron at the well it is probable that the dissolved iron in the post mining samples is reflective of the Dry Creek Alluvium's ambient iron concentration.Therefore, it is not believed that the elevated dissolved iron is indicative of an offsite impact to the local groundwater system and may be the result of organic matter and plant debris present in the alluvium (Hem 1985, pg. 77). B.) Instream Numeric Standards(CWQCC Regulation 33) The Colorado Water Quality Control Commission (CWQCC) has established segment specific water quality standards for upper Dry Creek(Yampa River Segment 13d) and Sage Creek(Yampa River Segment 13e).Tables 1—4 includes the analytical results for the samples collected from the four stream points within Hubberson Gulch and Dry Creek during the last five years (2015—2019) and a comparison against the Segment 13d water quality standards. Table 5 includes the analytical results for the samples