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Foothill Landfill <br />Storm Water Pollution Prevention Plan <br /> <br /> <br /> <br />Page 65 <br />At this point, pending additional investigation, the County believes the cause of the iron NAL <br />exceedances to be solely attributable to the natural soils for the following reasons: <br />x The 2015/2016 iron NAL exceedance at ST-3 was also accompanied with a TSS NAL <br />exceedance. However, there were no active industrial activities occurring in this <br />drainage area other than the movement of dirt used for daily cover, therefore, the <br />concentration of iron in the sample does not indicate impact by landfill activity. <br />Vehicles and equipment are not stored or maintained in this area, and, therefore, not a <br />significant source of iron. There are no structures of any kind and no landfilling <br />activities occur in this drainage area. Because the TSS was elevated at 415 mg/L, it <br />can be concluded that sediment was a factor in the elevated iron concentration. The <br />iron concentration was 34.7 mg/L, which was significantly higher than the iron <br />concentrations observed during the 2016/2017 storm season when the TSS values <br />averaged much lower at 8.2 mg/L. <br />x The 2016/2017 iron NAL exceedances resulted from samples collected from non- <br />industrial areas ST-1 and ST-7. The concentration of iron was approximately the same <br />at ST-6, consistent with a concentration not caused by industrial activity. Therefore, <br />because these concentrations were basically the same for iron and TSS, we believe the <br />source of the iron to be the same for water discharged at all three sampling locations, <br />which is from the soils. <br />x Soil samples were collected at ST-1, ST-6, and ST-7 in April 2017. All nine soil <br />samples had significant concentrations of iron ranging from 15,900 to 27,800 mg/kg. <br />There were no indications of current or previous industrial activities in the areas where <br />the soil samples were collected. <br />x NRCS soil survey data shows that the site has soil types derived of volcanic activity. <br />Academic literature references iron to be a significant component of magna derived <br />igneous rocks. <br />x Field observations note plentiful igneous and/or metamorphic rocks having a reddish <br />hue which is indicative of iron oxide. <br />Even though all data is pointing to natural background as the source of the iron, WGR and the <br />County are proposing to make a few changes to the storm water management program: <br />1. Storm water discharge points and monitoring locations will be redefined. Discharge <br />points ST-1 and ST-7 should never have been brought into the monitoring program in <br />the first place. They are exclusively non-industrial and receive runoff from a vegetated <br />open space. They will be eliminated as monitoring locations. Discharge point ST-6 <br />will also be eliminated from future sampling because the County has decided to remove <br />all industrial activities from this small drainage area, making it non-industrial, and <br />discontinue sampling at ST-6. <br />2. Although no discharges occurred from ST-3 during the 2016/2017 storm season (which <br />was a very wet season), the County is will evaluate making changes to this drainage <br />area to prevent discharges and to better control sediment. Options being evaluated