2010-09-14_ENFORCEMENT - M1977300 (2)

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September 13, 20 10 4 of 6 PRIMARY LOCATION OF GROUNDWATEXIMPACTS TO CREEK Recent water quality data collected from the historic "local" surface water stations in Ralston Creek provides important new information about the location and source of loading to Ralston Creek. Monitoring was reinitiated at five "local" surface water stations adjacent to the alluvium and fill (SW-BDIS, SW-PL, SW-OS, SW-BOS, and SW-GS) on June 9, 2010 to meet the requirements of DRMS's EPP Adequacy Review #2 (DRMS, May 19, 2010) and final results were received in July. Several sample rounds were necessary before any conclusions could be drawn, and samples collected during low stream flow (August 2010) were received in early September. Based on an assessment of the new data for the "local" surface water stations, combined with comparative evaluations with results of the 2008 electrical conductivity (EC) survey conducted by Whetstone, and comparative assessments with previous site groundwater data, it is now clear that a relatively discrete portion of the alluvial fill functions as the primary source of uranium loading to Ralston Creek. Uranium concentrations at stream monitoring locations from the August 10, 2010 sampling event, along with EC values from the November 2008 stream survey, are mapped and annotated in Figure 1. The uranium and EC results shown in Figure 1 were obtained during low stream flow conditions, minimizing potential confounding effects of dilution during high seasonal stream flows. Based on these data, contaminated alluvial groundwater clearly begins to impact Ralston Creek in the vicinity of monitoring station SW-BOS. Additional stream water monitoring data collected since June 2010 consistently indicate the same location for initial impacts to the creek. It is clear from this evidence that as stream water flows from upstream to downstream locations adjacent to the alluvial fill, the stream is not gaining significant inputs of flow or measurable contaminants from alluvial groundwater until it reaches the vicinity of monitoring station SW BOS. Historical groundwater uranium concentrations in alluvial monitoring wells indicate that the highest concentrations are found in MW-6, followed by MW-7 and MW-9. Note from the figure that the onset of water quality impacts to the creek near stream monitoring station SW-BOS occurs in the vicinity between groundwater monitoring wells MW-9 and MW 6. It is clear from uranium and EC data for the creek that contaminated alluvial groundwater flows most strongly into the creek between monitoring wells MW-9 and MW-6. Flows and contaminants within the alluvium appear to preferentially converge or concentrate in this vicinity. Furthermore, stream concentrations remain relatively consistent further downstream from this initial zone of loading to the creek. There appears to be only minor additional loading to the creek from alluvial groundwater further down gradient from this zone. Below station SW-BPL, years of historic data indicate that all stations downstream of the alluvial fill have essentially had constant or slightly decreasing concentrations at any given point in time, indicating that there are no other locations along Ralston Creek downstream of the alluvial fill where loading to the creek occurs. The primary significance of this finding is that the data provide concrete evidence of exactly where loading to the creek is occurring, and that it occurs solely in the alluvial fill where it can be intercepted and treated before reaching the creek or alternatively, where it can be effectively