Rock Creek IA 01-MAQ-4

Assessment Comments

1991 SUAs: habscrs/fshscrs=>22/10, 18/9 (both seine). Monitoring upstream and downstream from PCS Nitrogen Co. by LTRMP staff.

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses were assessed as "partially supported."  EXPLANATION:  The assessment of support of the Class B(LR) uses remains based on high levels of ammonia in Rock Creek contributed by contaminated groundwater at the PCS Nitrogen facility.   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 Shricker Slough (one site) every two weeks.   Results of this monitoring show general poor water quality at the lower Rock Creek (Highway 67) station.   Of the 93 samples collected by LTRMP staff between June 1996 and December 1999, levels of ammonia-nitrogen in 8 samples (8.6%) exceeded Class B(LR) chronic criteria; no samples exceed Class B(LR) acute criteria for ammonia-nitrogen.   The highest three readings for ammonia nitrogen during the three-year period (6.7, 5.9, and 5.8 mg/l) did not approach the temperature/pH-dependent acute criteria of 22.2, 36.1, and 45.0 mg/l, respectively.   Seven of 93 samples (7.5%) exceeded the Class B(LR) criterion for dissolved oxygen (5.0 mg/l), with violations ranging from 2.1 to 4.7 mg/l.   One of 88 samples violated the Class B(LR) criterion for pH (the sample collected on December 16, 1996 had a pH of 6.0 units).   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), if chronic criteria for a toxic pollutant (for example, ammonia) are exceeded in more than one sample, but in < 10% of the samples collected within a three-year period, the aquatic life uses are "partially supported."  Thus, because ammonia criteria were exceeded in 8.6% of the samples from lower Rock Creek, the support of the Class B(LR) uses of this stream reach was assessed as "partially supported.   According to these EPA guidelines (U.S.  EPA 1997b, page 3-17), however, the percentages of violations of the dissolved oxygen criterion at this station (7.5%) and pH (1.1%) do not suggest a water quality impairment (these guidelines allow up to 10% violations for conventional parameters such as pH, dissolved oxygen, and temperature before impairment of water quality is indicated).   Water quality at the upper Rock Creek station was generally good, with none of the 93 samples collected violating criteria for either dissolved oxygen, pH, or ammonia nitrogen.   Several attempts at correcting the problem of high levels of ammonia and other nitrogen compounds in Rock Creek were made during the 1998-1999 biennial period.   Beginning in 1999, hybrid poplar trees were planted along the unnamed tributary of Rock Creek (aka Ammonia Creek) that flows through the PCS property and that delivers high levels of ammonia to Rock Creek.   These trees have dense root masses that penetrate deep into the soil profile; research has demonstrated the ability of these trees to remove nitrogen compounds from contaminated groundwater (see Gatliff 1994).   A containment trench with a sump system was constructed by PCS Nitrogen to collect and remove the top ten feet of ammonia-rich groundwater feeding Ammonia Creek; this system began operation in April 1999.   PCS also plans to convert approximately 120 acres of cropland to native prairie plantings.   Through the cooperative efforts of DNR, a private landowner, and PCS Nitrogen, an 80-acre wetland was created in early 2000 through the impoundment of Rock Creek downstream from PCS.   This wetland is expected to help process and assimilate high levels of nitrogen compounds leaving the PCS facility and improve the water quality in lower Rock Creek and in Shrickers Slough.   In September 1999, PCS Nitrogen announced the closure of its Clinton plant.   After closure, the plant was to be dismantled; the site will no longer be used for the storage or production of products produced by PCS Nitrogen.   PCS Nitrogen agreed to continue it's activities to recover and remove/treat nitrogen compounds (for example, planting of hybrid poplar trees, conversion of cropland to prairie, and construction/operation of a groundwater recovery system); PCS will continue to monitor groundwater at the PCS site.   PCS also assisted with the development of the 80-acre wetland constructed on lower Rock Creek.   Surveys of fish and macroinvertebrates were conducted in September 1999 by the DNR Fisheries Bureau to document pre-remediation conditions (see Gritters 1999).   Fish sampling locations included (1) the vicinity of the Highway 67 bridge (LTRMP downstream water quality site), (2) upstream and downstream from Ammonia Creek, (3) in Ammonia Creek, and (4) an upstream site above County Road Z-40 (LTRMP upstream water quality site).   Results from the sites and Highway 67 and County Road Z-40 were compared to an August 1997 survey also by the DNR Fisheries Bureau.   Despite the documented water quality problems, results of the September 1999 fish survey showed a relatively healthy and diverse fish community in Rock Creek, even in the reach with high levels of ammonia downstream from the confluence with Ammonia Creek.   The aquatic community in Ammonia Creek, however, was limited to a few tolerant species.   These results indicate the need to include both chemical as well as biological monitoring when assessing stream quality.   Although sampling for macroinvertebrates was limited, results showed both pollution tolerant and sensitive species at all sample sites except Ammonia Creek.

Continued monitoring by Gritters and Gould (DNR, LTRMP, Bellevue) strongly suggests that a local industry is the source of much of the high levels on nitrate and ammonia in the middle and lower reaches of Rock Creek, including Shricker Slough.   DNR Water Quality, Land Quality, and Field Office 6 met with industry representatives in June 1998 and discussed alternatives to reduce nitrogen levels in Rock Cr.  and Shricker Slough.   Industry will begin implementation of corrective measures and monitoring of surface and groundwater in the near future and report back to DNR in one year on progress.   Due to results of continued upstream/downstream monitoring by LTRMP/Bellevue staff that show excessively high levels of nitrogen in Rock Creek and excessively high levels of chlorophyll-a in Shricker Slough, continue to assess Rock Creek as partially supporting its Class B(LR) aquatic life uses.  A review of the field sheets from the October 1991 DNR stream use assessments supports the assessment of the Class B(LR) aquatic life uses as PS.   That is, results of the stream use assessment near Low Moor show the presence of less than a majority of the expected fish taxa (4 of 9) for streams in the Southern Iowa Rolling Loess Prairies subecoregion and show a relatively low fish community diversity of 8 cyprinid species.  Fish community diversity (11 species from 3 families) is somewhat better at the second SUA site SW (upstream) from Quantum Chemical but is below that suggested by the above average habitat quality.   However, an August 1997 DNR Fisheries survey 2 mi E of Low Moor suggests full support of the Class B(LR) uses (i.e., 10 spp; 3 fams, with 6 of 9 expected fish taxa present).

[NOTE: The 2000 assessment refers to the Rock Creek IA 01-MAQ-0010 waterbody as a whole.  For the 2002 report, Rock Creek was subdivided into three segments:  IA 01-MAQ-0010-1 (lower), IA 01-MAQ-0010-2 (above PCS-Nitrogen), and IA 01-MAQ-0010-3 (headwaters).]