Assessment Comments

Assessment is based on (1) an updated summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau in February 2012 and (2) results of water quality monitoring conducted in 2010 and 2011 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 “partially supported” (IR 5a) due to high levels of indicator bacteria.   The presumptive Class A2 (secondary contact recreation) uses are assessed (monitored) as “fully supported” (IR 2a).   The Class B(CW1) coldwater aquatic life uses are assessed (monitored) as "fully supported" (IR 2a) based on (1) the February 2012 summary of trout reproduction in Iowa streams as prepared by the IDNR Fisheries Bureau and (2) results of water quality monitoring conducted in 2010 and 2011.   Fish consumption uses remain "not assessed."  The sources of data for this assessment include (1) results of water quality monitoring conducted from April 2010 to October 2011 at Site DRC18 of a Section 319 water quality project and (2) a 2012 IDNR summary of trout reproduction in Iowa streams.

EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “partially 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, and the geometric mean of E.  coli in the 16 samples collected during the recreational season of 2011 was 361 orgs/100 ml.   These geometric means are slightly greater than the Class A1 criterion of 126 orgs/100 ml.   These geometric means, however, are well below the Class A2 geometric mean criterion of 630 orgs/100 ml.   Thirteen of the 29 samples (45%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml, and five of the 29 samples (17%) 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 should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Thus, the Class A1 uses are assessed as “partially 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 2012.   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 (a Category II stream).  

The Class B(CW1) coldwater aquatic life uses are assessed (monitored) as “fully supported” based on results of water quality monitoring conducted from April 2010 to October 2011 as part of a Section 319 water quality project.   No violations of Class B(CW1) water quality criteria for ammonia, pH, or temperature occurred in the approximately 30 samples collected in 2010 and 2011.   Two of 32 samples (6%), however, violated the Class B(CW1) criterion for dissolved oxygen of 7.0 mg/l.   The sample collected on July 22, 2011 contained 6.7 mg/l of dissolve oxygen, and the sample collected on August 10, 2011 contained 4.2 mg/l; the samples were collected at 0945 and 0830, respectively, on those dates.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), "partial support" of beneficial uses is indicated if criteria are exceeded in more than 10% of the samples for conventional parameters (e.g., pH, temperature, or dissolved oxygen).   Based on IDNR’s assessment methodology, the results from Site DRC18 of the Section 319 project do not suggest that the violation frequency for dissolved oxygen is significantly greater than 10% and thus do not suggest impairment of Class B(CW1) uses of this assessment segment.

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