Silver Creek IA 01-UIA-282

The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria during the 2012-2014 period.  At site UIRW 8 (STORET station 191910002), the geometric means of indicator bacteria (E. coli) in the 19 samples collected during the recreational seasons of 2012 through 2014 were as follows:  the 2012 geometric mean was 432 orgs/100 ml, the 2013 geometric mean was 1,263 orgs/100 ml, and the 2014 geometric mean was 610 orgs/100 ml.  All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.  Sixteen of the 19 samples (84%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. 

At site 8 near Cresco (STORET station 15960010), the geometric means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2011 and 2012 were as follows:  the 2011 geometric mean (17 samples) was 962 orgs/100 ml, the 2012 geometric mean (five samples) was 1,007 orgs/100.  Both geometric means exceed the Class A1 criterion of 126 orgs/100 ml.  Eighteen of the 22 samples (82%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.  According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).  Thus, due to geometric mean violations at both monitoring sites in this assessment segment, the Class A1 uses are assessed (monitored) as “not supported”. 

Results of chemical/physical water quality monitoring from 2012 to 2014 suggest "full support" of the Class B(WW2) aquatic life uses.  Monitoring at station 191910002 showed no violations of Class B(WW2) water quality criteria for temperature in the 14  samples over the 2012-2014 period.  One of the 12 samples (8%) analyzed for ammonia appears to exceed the Class B(WW2) chronic criterion for ammonia (early life stages present).  The sample collected on September 4, 2012, contained 1.7 mg/l of ammonia.  Unfortunately, the supporting data are not available for that sample to calculate the temperature/pH-dependent ammonia criterion.  If, however, average pH and temperature values are used for this site (pH of 8.6 and temperature of 13.5C), the corresponding ammonia criterion is 0.92 mg/l.  Thus, the sample ammonia value of 1.7 mg/l would appear to be in clear violation of this criterion.  Other values of ammonia, however, were well below Class B(WW2) chronic criteria.  Levels of pH also violated the Class B(WW2) criteria.  Three of the 13 samples (23%) measured for pH violated the Class B(WW2) criterion of 9.0 pH units.  According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as ammonia and pH suggests impairment of aquatic life uses.  Based on IDNR’s assessment methodology, however, these results suggest that the frequency of violations is not significantly greater than 10 percent; thus, these results do not suggest impairment of the Class B(WW2) aquatic life uses.  Thus, the Class B(WW2) aquatic life uses are assessed (monitored) as “fully supported”.