Rock Creek IA 01-MAQ-3

Assessment Comments

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

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses were assessed as "fully supported."  Results of water quality monitoring show generally good water quality in this segment of Rock Creek.   The assessment of support of the Class B(LR) 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 (one site) every two weeks.   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 Law Engineering of Wilmington, North Carolina, 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 Water Quality Bureau 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) and (2) immediately downstream from the wastewater lagoon at Low Moor (DNR/UHL Site 62).

EXPLANATION:  Results of water quality monitoring conducted by LTRMP staff and by UHL as part of the TMDL monitoring show no violations of water quality criteria designed to protect Class B(LR) aquatic life uses, thus suggesting "full support" of the Class B(LR) aquatic life uses.   Data for ammonia-nitrogen, dissolved oxygen, and pH were compared to Class B(LR) criteria.

AMMONIA-NITROGEN:  Levels of ammonia-nitrogen in all 71 samples collected by LTRMP staff at County Road Z40 between January 2000 and September 2002 were well-below Class B(LR) chronic criteria.   The maximum ammonia concentration in these samples was 0.65 mg/l; (median = 0.08 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 (see assessment for lower Rock Creek, Waterbody ID:  IA 01-MAQ-0010-1).   None of the combined 18 samples collected at the DNR/UHL 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 71 samples collected upstream from the PCS facility at County Road Z40 by LTRMP from January 2000 to September 2002 violated the Class B(LR) criterion for dissolved oxygen (5.0 mg/l).   In addition, none of the combined 18 samples collected 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 levels of dissolved oxygen in this segment of Rock Creek at adequate to fully support the Class B(LR) aquatic life uses.  

pH:  Similar to results for dissolved oxygen, none of the 71 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 state water quality criteria for pH.