2007-02-02_HYDROLOGY - M2004078 - Laserfiche WebLink

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3.1.1 Arsenic The arsenic concentrations in seven of the sampled wells exceeded the PDWS in either the April or July 2006 sampling events. The soils in the area are known to have higher natural concentrations of arsenic. All of the samples collected except in November and December of 2005 were reported as total arsenic. With the heavier sediment loading in the samples, the arsenic levels that exceeded PDWS are likely caused from the sediment in the samples. November and December 2005 samples were reported as dissolved arsenic samples. There was no exceedance of the PDWS for arsenic in the November or December of 2005 sampling events. . Arsenic • We lls (CRM W) Site/ 01 02 04 05 06 07 09 16 40 41 42 Grou Oct-05 U 0.007 0.003 ----- 0.003 0.006 ----- ----- 0.005 0.004 ----- 0.0047 Nov-05 U ---- U ---- U ---- ---- ---- ---- U ---- ---- Dec-05 U ---- U ---- U ---- ----- ----- ---- U ----- ---- Apr-06 0.014 ---- 0.13 ---- 0.02 ---- ----- ----- ---- 0.0093 ----- 0.0433 Jul-06 0.0043 0.014 0.0032 ---- 0.01 0.015 ----- ----- 0.013 0.0033 ----- 0.0090 Oct-06 0.0047 ---- 0.0057 ---- 0.0075 ---- ---- ---- ---- U ---- 0.0060 Average 0.0077 0.0105 0.0355 ---- 0.0101 0.0105 ---- ---- 0.0090 0.0055 ---- 0.0094 Stnd Dev. 0.0055 0.0049 0.0630 ---- 0.0072 0.0064 ---- ---- 0.0057 0.0033 ---- 0.0231 Baseline 0.013 0.015 0.099 0.000 0.017 0.017 0.000 0.000 0.015 0.009 0.000 0.032 The average arsenic level for the entire site is 0.01 mg/L with a standard deviation of 0.02. Based on the groundwater sampling plan approved by the DMG, a site wide average may be used plus 1 standard deviation to establish background levels. This will establish a site wide baseline arsenic level of 0.03 mg/L. 3.1.2 Nitrate Nitrate concentrations exceeded PDWS consistently from October 2005 through October 2006 sampling events. The nitrate concentrations ranged from non-detection in one sample to a maximum of 70.3 mg/L. The majority of the samples were over the PDWS of 10 mg/L. Nitrate concentration in the outlying wells (CRMW01, CRMW02, CRMW07, and CRMW40) show increased concentration levels in the Fall months and the lowest concentration levels of nitrate in the Spring months. See Appendix A3 for a graphical representation of the nitrate concentrations. There is a noticeable increase in nitrate levels from the Fall of 2005 to the Fall of 2006. All four of the above mentioned wells are directly downgradient of areas that were actively farmed with vegetable crops in 2006. However, CRMW06 is located approximately'/a of a mile downgradient of CRMW40 and CRMW07 and from any cultivated crops shows a nitrate spike in July. The average nitrate concentration of these four wells from October 2005 to October 2006 is 23.0 mg/L. The monitoring wells (CRMW04, CRMW06, and CRMW42) all show a slight decline in nitrate concentrations over the sampling period. The nitrate concentrations of these three wells are relatively lower over the past year than the outlying wells mentioned above. CRMW04 and CRMW42 are located at the far west edge of the mining area with predominately non-cultivated land located immediately upgradient of them. The average nitrate concentration of these two wells is 8.9 mg/L. CRMW06 is located downgradient of the former Yokooji property that has been partially mined. This site had cultivated crops prior to October 2005 and includes the mining area south of Weld County Road 6'/a, north of Weld County Road 6, west of Weld County Road 27, and east of Highway 85. The highest nitrate level in CRMW06 was recorded in October 2005. The concentration levels decreased until March of 2006 when they began to rise and peak around July of 2006. Between July and October 2006 the concentration level of nitrate dropped to the lowest concentration in this well. The Annual 2006 Sampling Event -Lupton Lakes -2- January 2007