Laserfiche WebLink
ARCADIS <br />alfalfa, the most salt sensitive crop grown in the region. Waters with EC values below this threshold are <br />not expected to impact the 100 percent yield of alfalfa. The maximum allowable SAR is dependent on EC, <br />with higher SAR permissible at higher EC levels, and is defined by the Ayers-Westcot relationship (Ayers <br />and Westcot 1985). Figure 14 presents this relationship as it is presented in WQCD (2008), with a <br />maximum low-risk capacity value of SAR equal to nine, along with modeling results described below. As <br />illustrated in Figure 14, the maximum allowable SAR at an EC of 1,300 Nmhos/cm is 6.8, and at EC <br />values below this threshold, the threshold value of SAR that will result in "no reduction in infiltration" <br />decreases. Waters with EC values of approximately 350 Nmhos/cm are expected to result in some <br />reduction to infiltration regardless of the SAR. <br />Figure 14 shows the EC and SAR relationship of Purgatoire River water at PRS -1 (upstream of the mine) <br />as well as the relationship for five modeling scenarios. The average discharge rates at upstream (PRS -1) <br />and downstream (PRS -4) stations over the period of record are 45.9 cfs and 50.8 cfs, respectively. If the <br />mine dewatering rate is assumed to be within the range of calculated mine inflow rates (142 to 468 gallons <br />per minute; NECC 2012), the mine discharge rate is estimated to be between approximately 0.6 and 2 <br />percent of the river discharge rate. Modeling scenarios A and B represent results from modeling with mine <br />discharge equal to 0.6 percent and 2 percent of the total discharge. Model scenario C represents the <br />result from modeling with mine discharge equal to 10 percent of total discharge. Model scenario D <br />represents a back -calculation of the total proportion of mine discharge that would result in a SAR <br />approximately equal to the 6.8 threshold established at an EC value of 1,300 Nmhos/cm, and illustrates <br />that the EC of the resulting mixture remains too low to meet the criteria for "no reduction in infiltration". <br />Model scenario E represents a back -calculation of the total proportion of mine discharge that would result <br />in an EC of approximately 1,300 Nmhos/cm, and illustrates that the SAR of the resulting mixture is too <br />high to meet the criteria for "no reduction in infiltration". Modeling results are summarized in Table 3. <br />As illustrated in Figure 14, Purgatoire river water does not meet the criteria for "no reduction to infiltration" <br />at the upstream sampling location. In the dilution model, the initial characteristics of the river water and <br />mine water dictate the mixing line, and modeling results indicate that the high proportions of sodium and <br />bicarbonate in mine discharge water relative to the background EC of river water and the EC of mine <br />water preclude the resultant mixture from meeting the standard for "no reduction in infiltration". To meet <br />this standard, the EC of the mixture would need to be increased or the sodium concentration relative <br />calcium and magnesium concentrations would need to be decreased. As shown in Table 3, all modeled <br />scenarios meet the aquatic life standard for total recoverable iron (1 mg/L). <br />6. Conclusions <br />Surface water in the Purgatoire River and mine water from the New Elk Mine have distinct compositions, <br />and the composition of water in alluvial wells both upgradient and downgradient is an intermediate type. <br />Examination of temporal trends in total recoverable iron, EC, and SAR over time at upstream and <br />downstream stations on the Purgatoire River reveals a change in the SAR at the downstream station that <br />Page <br />c Wuwstmtw ws appdabWc*ncrosaNmndowsltemporary inwwt fiNs�conlentmitb Vc6ufiriiMewekcoakufacewatenn&mceassessmentnem d= 7/8 <br />