North Skunk River IA 03-NSK-853
mouth (S5 T74N R10W Keokuk Co.) to confluence with Cedar Cr. in S15 T75N R12W Keokuk Co.
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trend
- Unknown
- Created
- 5/23/2019 2:46:21 PM
- Updated
- 8/2/2019 9:25:16 AM
The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" due to levels of indicator bacteria that slightly exceed state water quality criteria. The Class B(WW1) aquatic life uses remain assessed as "not supporting" due to violations of state water quality criteria for chromium during the 2004-06 assessment period. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment. The assessments of support of beneficial uses are based on results of DNR ambient monthly water quality monitoring conducted on the North Skunk River southwest of Sigourney in Keokuk Co. (STORET station 10540001) during the 2014-2016 assessment period. Data from this monitoring station were also used to assess support of the Class A1 and B(WW2) uses of the adjacent upstream segment of the North Skunk River (IA 03-NSK-854: from confluence with Cedar Creek near Sigourney to confluence with Middle Creek in Mahaska County).
The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 23 samples collected during the recreational seasons of 2014 through 2016 at DNR station 10540001 were as follows: the 2014 geometric mean was 225 orgs/100 ml, the 2015 geometric mean was 219 orgs/100 ml, and the 2016 geometric mean was 385 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Thirteen of the combined 23 samples (57%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as "partially supported." The Class B(WW2) aquatic life uses remain assessed (monitored) as “not supported" due to violations of the acute criterion for chromium. Monitoring showed no violations of Class B(WW2) water quality criteria for 34 Ammonia samples (maximum = 1.9 mg/L), 36 Dissolved Oxygen samples (minimum = 5.3 mg/L), 35 pH samples (range = 7.2 to 8.6), 36 Temperature samples (maximum = 25.6° c), 35 Chloride samples (maximum = 93 mg/L), or 35 Sulfate samples (maximum = 230 mg/L) occurred during monitoring from January 2014 to December 2016. Monitoring for toxic metals has not been conducted at this monitoring station since April 2006. During the 2004-2006 period, however, ten samples were analyzed for total chromium. Levels of chromium in two of these samples (collected on April 13, 2005 and April 5, 2006) contained 0.02 mg/l of total chromium and thus exceeded the Class B(WW1) aquatic life acute criterion of 0.016 mg/l. The remaining eight samples were reported as less than the detection level of 0.02 mg/l. Based on Iowa DNR’s assessment/listing methodology, more than one violation of an acute water quality criterion for a toxic parameter over a three-year period suggests impairment of aquatic life uses. Thus, the violations of the chromium criterion suggest that the Class B(WW1) aquatic life uses of this assessment segment should be assessed as impaired. [Note: Prior to EPA approval in 2009, Iowa’s Class B(WW1) acute criterion for chromium was 0.060 mg/l, and the chronic criterion was 0.040 mg/l.]