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ARCADIS <br />Figures 12 and 13 present EC and SAR over time at upstream (PRS -1) and downstream (PRS -4) surface <br />water stations. As shown in Figure 12, the EC of upstream and downstream waters are typically similar in <br />magnitude, and with the exception of a few historical periods (e.g. 1994 through 1996, and 2004 through <br />2007) the temporal trends in EC at these two stations match well. Notably, in 2008 EC dropped <br />significantly at both stations. As shown in Figure 13, the magnitude of and temporal trends in SAR were <br />very similar at upstream and downstream stations until 2009, when the magnitude of the SAR at the <br />downstream station (PRS -4) increased. This result suggests an influence of discharge of sodium -rich <br />waters on downstream surface water quality. <br />5. Geochemical Mixing Model Results <br />A dilution model was used to assess the potential impact of mine water discharge on surface water quality <br />in terms of total recoverable iron, EC, and SAR. The following equation was used to estimate the resultant <br />concentrations of calcium, sodium, magnesium, bicarbonate and total recoverable iron as well as the EC <br />of river water downstream from mine water discharge. From the concentrations of the ionic species, the <br />adjusted SAR was calculated according to the guidance outlined in Lesch and Suarez (2008). This model <br />assumes complete mixing. <br />CRiverMixed = [CRiver x ( QRtver A + [CMine x ( QMtne A <br />[QRtver+QMtnei [QRtver+QMine] <br />Where: <br />CRivermwed = Estimated resultant concentration from mine discharge and river mixing (mg/L or Nmhos/cm) <br />CRiver = Average background concentration in river water (mg/L or Nmhos/cm) <br />CMine = Average concentration in mine water (mg/L or Nmhos/cm) <br />QMine = Estimated mine discharge rate (cfs) <br />QRiver = Estimated river discharge rate (cfs) <br />For each constituent, the average background concentration in river water was taken as the average <br />concentration at upstream station PRS -1. The average concentration of each constituent in mine water <br />was taken as the average of values measured at the two mine water monitoring stations NEW -2 and <br />NEW -4. As shown in the equation, the magnitude of dilution calculated from this model is based on the <br />relative proportions of mine water discharge and river discharge as components of the total discharge. <br />Maximum acceptable levels of EC and SAR in waters that may be used for irrigation are provided in the <br />WQCD (2008) guidance for discharge permits. A maximum EC value of 1,300 Nmhos/cm is protective of <br />Page <br />c WsersVwM mpsonlaWat3bcafvnitrosotmudowsltenWwy rbmetfilss%wnbntoutlook%k6uithihem&coalst0acewabwr&enceasseswnwtmm doa 6/8 <br />