Rock Creek IA 01-MAQ-3

Assessment Comments

Assessment is based on monitoring upstream from PCS Nitrogen Co. by (1) LTRMP staff (Bellevue Office) from 2008-2010 and (2) IDNR and UHL in support of TMDL development in 2001.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) 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 now presumptively designated for Class A1 (primary contact recreation) uses.   The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses).   Thus, for the current (2012) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses remain not assessed due to the lack of information upon which to base an assessment.   None of the monitoring conducted in this segment has included analysis for indicator bacteria.   The Class B(WW2) aquatic life uses are assessed (monitored) as "fully supported."  Results of water quality monitoring from 2008 through 2010 continue to show generally good water quality in this segment of Rock Creek.   The assessment of support of the Class B(WW2) uses as "fully supported" is based on comparison of monitoring data to Iowa water quality standards for ammonia-nitrogen, dissolved oxygen, and pH.  

SOURCES OF DATA:  Staff of the Upper Mississippi River "Long Term Resource Monitoring Program" at Bellevue, Iowa, continue to monitor the water quality of Rock Creek (two sites) and Shrickers Slough.   The LTRMP monitoring station on Rock Creek above PCS-Nitrogen (Site RK03.7M) is located at County Road Z40, approximately 1.5 miles upstream from the former PCS-Nitrogen facility.   Although surface water monitoring is also conducted on a weekly basis at seven locations in the Rock Creek watershed by QC Analytical Services of Le Claire, IA, none of these sites is useful for characterizing water quality upstream from the PCS-Nitrogen facility.   As part of monitoring in support of development of the total maximum daily load (TMDL) for Rock Creek, the Iowa DNR sponsored monthly water quality monitoring from March to November 2001 at five locations on Rock Creek; sample sites in this reach of Rock Creek include (1) County Road Z40 (DNR/UHL Site 61, STORET station 11230007) and (2) immediately above the outfall of the wastewater lagoon at Low Moor (DNR/UHL Site 62; STORET station 11230008).  

EXPLANATION:  Results of water quality monitoring conducted by LTRMP staff from January 2008 to November 2010 show no violations of water quality criteria designed to protect Class B(WW2) aquatic life uses, thus suggesting "full support" of these uses.   Data for ammonia-nitrogen, dissolved oxygen, and pH were compared to Class B(WW2) criteria.  

AMMONIA-NITROGEN:  Levels of ammonia-nitrogen in all 42 samples collected by LTRMP staff at County Road Z40 between January 2008 and November 2010 were well-below Class B(WW2) chronic criteria.   The maximum ammonia concentration in the LTRMP samples was 0.55 mg/l; (median = 0.03 mg/l).   In contrast to the lower segment of Rock Creek, these results suggest that levels of ammonia are not a water quality concern in this segment of the stream (for comparison, see assessment for lower Rock Creek, Waterbody ID:  IA 01-MAQ-0010_1).   None of the combined 18 samples collected in 2001 at the DNR/UHL TMDL sites (Site 61 at bridge on County Road Z40 and Site 62 downstream from the Low Moor wastewater lagoon) contained a level of ammonia above state water quality criteria (maximum concentration = 0.7 mg/l).  

DISSOLVED OXYGEN:  None of the 42 samples collected upstream from the PCS facility at County Road Z40 by LTRMP from January 2008 to November 2010 violated the Class B(WW2) criterion for dissolved oxygen (5.0 mg/l).   In addition, none of the combined 18 samples collected in 2001 as part of IDNR TMDL monitoring at County Road Z40 (Site 61) and downstream from the Low Moor wastewater lagoon outfall (Site 62) violated this criterion.   These results suggest that levels of dissolved oxygen in this segment of Rock Creek are adequate to fully support the Class B(WW2) aquatic life uses.  

pH:  Similar to results for dissolved oxygen, none of the 42 samples collected by the LTRMP at County Road Z40, and none of the 18 samples collected as part of IDNR TMDL monitoring at County Road Z40 and downstream from the Low Moor wastewater lagoon, violated Class B(WW2) water quality criteria for pH.