Hewett Creek IA 01-VOL-307

The presumptive Class A1 (primary contact recreation) uses remain assessed (monitored) as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2012 through 2014 at the station HEW-20 (STORET station 15220013) were as follows: the 2012 geometric mean was 655 orgs/100 ml, the 2013 geometric mean was 1,569 orgs/100 ml, and the 2014 geometric mean was 544 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Seventeen of the combined 24 samples (71%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. The 2012 and 2013 geometric means also exceed the Class A2 criterion of 630 orgs/100 ml, and five of the 24 samples (21%) exceed 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 DNR’s assessment/listing methodology, if a geometric mean is greater than respective Class A1 or A2 criteria, the presumptive primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).

The Class B(CW1) aquatic life uses are assessed (monitored) as “partially supported.” Five of the 24 samples measured for water temperature during the April 2012 to November 2014 period at Station Cox 20 violated the Class B(CW1) criterion of 20C: According to the DNR assessment/listing methodology, the percentage of violations (21%) is significantly greater than 10% and thus suggests “partial support” of the Class B(CW1) aquatic life uses. Monitoring results from April 2012 to November 2014, however, show no violations of the Class B(CW1) criteria for ammonia (16 samples), pH (24 samples), or for chloride (16 samples). Two of the 23 samples violated the Class B(CW1) criterion for dissolved oxygen of 7.0 mg/l (the samples collected on July 2 and August 1 of 2012 both had 6 mg/l of dissolved oxygen) (note: all dissolved oxygen values during the 2012-2014 monitoring period were reported to the nearest mg/l). According to the DNR assessment/listing methodology, however, the percentage of violation (9%) is not greater than 10% and thus these violations do not suggest impairment of the Class B(CW1) uses.

Additional continuous monitoring data collected in 2013 and 2014 for the DNR/SHL coldwater stream sampling project were examined for DO and stream temperature criteria violations. The lowest DO concentration recorded in 126 days of continuous monitoring was 6.7 mg/L. The daily DO minimum concentration failed to meet the 24-hour DO criterion (5 mg/L) and the 16-hour DO criterion (7 mg/L) in 0 of 126 days each. The percentage exceeding the 24-hour and 16-hour criteria (0% for each) was less than (passing) the 10% impairment threshold used to assess conventional water quality parameters. The associated statistical analysis confidence level result was high (>90%). The maximum stream temperature recorded during the sensor deployment period was 23.8 degrees (C). The B(CW1) temperature criterion of 20 (C) was exceeded in 40 of 126 days. The percentage exceeding (31.7%) was greater than (failing) the 10% impairment threshold with high statistical confidence (>90%). Based on these data, the B(CW1) aquatic life use is assessed as impaired due to elevated stream temperature levels. The assessment type is “monitored” because the data meet the minimum requirements of the DNR’s continuous monitoring assessment methodology. This assessment agrees with the temperature assessment completed using grab water sampling data from watershed project monitoring in 2012-2014, which also indicated a thermal impairment.

The assessment of the Class B(CW1) coldwater aquatic life uses is also based on results of biological monitoring in 2013 and 2014 conducted as part of the DNR/SHL coldwater stream sampling project. The results of this monitoring suggested (monitored) “partial support” of the Class B(CW1) uses. A series of biological metrics which reflect coldwater stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa that were collected in the stream sampling reach. The biological metrics were combined to make a coldwater benthic index (CBI). The index ranks the biological integrity of a coldwater stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2013 CBI scores were 53 and 66. The 2014 CBI scores were 53 and 55. The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the CBI scores with biological impairment criteria (BIC) established for the 2012 Section 305(b) report. The biological impairment criteria were determined from a statistical analysis of data collected at coldwater stream reference sites from 1994-2011. The CW BIC is 60 and this segment passed the CW BIC 1/4 times in the last five years (2012-2016). This aquatic life assessment is now considered "monitored" based on a change in the 2010 DNR assessment methodology.

In contrast to the water quality and benthic macroinvertebate aquatic life uses assessments, the assessment based on an updated summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau in January 2019 suggests that the Class B(CW1) uses should be assessed as "fully supported". According to the updated summary, Hewett is in the group of Iowa coldwater streams that supports natural reproduction of trout. Based on results of surveys by the DNR Fisheries Bureau, this stream is placed in the category of Iowa trout streams that exhibit fairly consistent natural and maintain a viable population of Brown Trout without any stocking (a Category I trout stream).

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