Assessment Comments

Assessment is based on (1) an updated summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau in February 2011 and (2) results of water quality monitoring conducted in 2010 at Site DRC18 of a Section 319 water quality project.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(CW) aquatic life uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008, this segment is also now presumptively designated for Class A1 (primary contact recreation) and Class A2 (secondary contact recreation) uses.   This segment remains designated for coldwater aquatic life use (now termed Class B(CW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption) uses.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to high levels of indicator bacteria.   The presumptive Class A2 (secondary contact recreation) uses are assessed (monitored) as “fully supported.”  The impairment of the Class A1 uses is a new impairment for this assessment segment.   The Class B(CW1) coldwater aquatic life uses are assessed (monitored) as "fully supported" based on (1) the May 2006 summary of trout reproduction in Iowa streams as prepared by the IDNR Fisheries Bureau and (2) results of water quality monitoring conducted in 2010.   Fish consumption uses remain "not assessed."  The sources of data for this assessment include (1) results of water quality monitoring conducted in 2010 at Site DRC18 of a Section 319 water quality project and (2) a 2011 IDNR summary of trout reproduction in Iowa streams.

EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as not supported based on levels of indicator bacteria that exceeded state water quality criteria while the presumptive Class A2 (secondary contact recreation) uses are assessed (monitored) as "fully supported".   The geometric means of indicator bacteria (E.  coli) in the 13 samples collected during the recreational season of 2010 at DRC18 was 200 orgs/100 ml, thus slightly exceeding the Class A1 criterion of 126 orgs/100 ml.   This geometric means is well below the Class A2 geometric mean criterion of 630 orgs/100 ml.   Four of the 13 samples (31%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml, and three of the 13 samples (23%) exceeded the Class A2 single-sample maximum criterion of 2,880 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 are "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Thus, the Class A1 uses are assessed as “not supported” (impaired).

IDNR’s assessment methodology also states that if significantly more than 10% of the bacterial samples in any recreation seasons exceeds Iowa’s single-sample maximum value, the respective Class A uses should be assessed as “partially supported.”  This assessment approach is based on U.S.  EPA guidelines (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   According to this listing methodology, however, the percentage of samples that exceeded the Class A2 single-sample maximum criterion, however is not significantly greater than 10%; thus, the Class A2 uses are assessed (monitored) as “fully supported.  

The Class B(CW1) coldwater aquatic life uses are assessed as “fully supported” based on an updated summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau in February 2011.   According to the updated summary, Twin Springs is in the group of Iowa coldwater streams that supports natural reproduction of trout.   Based on results of surveys by the IDNR Fisheries Bureau, this stream is placed in the category of Iowa trout streams that exhibit recent, but inconsistent reproductive success and that are generally not capable of maintaining a viable population for the resident trout species at this time.  

Results of Section 319 monitoring from May to November 2010 at Site DRC18 also suggest that the Class B(CW1) coldwater aquatic life should be assessed (monitored) as “fully supported.”  No violations of Class B(CW1) water quality criteria for ammonia, dissolved oxygen, pH, or temperature occurred in the 13 samples collected in 2010.  

Fish consumption uses are "not assessed" due to lack of fish tissue monitoring in this stream reach.