2018 assessment for Little Rock River

Little Rock River IA 06-BSR-1800

from confluence with Argo Slough (S17 T99N R43W Lyon Co.) to the Iowa/Minnesota state line

2018
Open Data Collection Period Modal

For purposes of developing stream/river water quality assessments for integrated reporting, three years of water quality data are typically used for both conventional pollutant parameters (e.g., indicator bacteria) and the less frequently monitored toxic parameters (e.g., toxic metals). Since the 2004 IR cycle, DNR has used a three-year data gathering period for streams, rivers, and beaches. For most assessments, the use of three years of data increases the number of samples upon which the decision on use support is based and helps address the problem of weather-related year-to-year fluctuations in water quality.

Due to the lower sampling frequency in Iowa’s ambient lake, biological, fish kill, and fish tissue monitoring programs, five years of data are used for developing IR assessments and for identifying Section 303(d) listings.

Data Type	Data Timeframe
Beach Sampling Data	CY 2014 - 2016
Stream/River Water Quality Data	CY 2014 - 2016
Lake Water Quality Data	CY 2012 - 2016
Biological Sampling Data	CY 2012 - 2016*
Fish Kill Data	CY 2012 - 2016
Fish Tissue Sampling Data	CY 2012 - 2016
Continuous Dissolved Oxygen Sampling Data	CY 2012 - 2016
*Biological sites sampled in both 2011 and 2015 were also assessed.

Open Segment Map
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Overall IR

5 - Water is impaired or threatened and a TMDL is needed.

Trend

Unknown

Created

7/30/2019 8:16:18 AM by

Updated

7/30/2019 8:20:04 AM by

Use Support

Class A1

Recreation - Primary contact

Not Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

303(d) Listing Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2008

Impairment Rationale

Geometric mean criterion exceeded

Data Source

TMDL monitoring: Iowa DNR

TMDL Priority

Tier III

Class BWW2

Aquatic Life - Warm Water Type 2

Fully Supported

General Use

General Use water -

Not Assessed

303(d) Delistings for this Cycle

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 (primary contact recreation) uses remain assessed as "not supported" due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW2) aquatic life uses remain assessed as “fully supported” based on results of ambient chemical/physical water quality monitoring conducted at the Iowa/Minnesota state line by DNR/SHL during the 2002-2004 assessment period as part of TMDL monitoring (TMDL station 1; STORET station 11720001).

Assessment Explanation

The Class A1 uses remain assessed (evaluated) as "not supported" based on results of ambient monitoring for indicator bacteria (E.coli). The geometric mean level of indicator bacteria (E.coli) in the 18 samples collected at the DNR/SHL TMDL monitoring station at the Iowa/Minnesota state line during summer recreational seasons of 2002-2003 (180 orgs/100ml) slightly exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml. Nine of the 18 samples (50%) exceed Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S.EPA guidelines for Section 305(b) reporting and according to DNR’s assessment/listing methodology, if the geometric mean level of E.coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35 of U.S.EPA 1997b).

Regarding support of the Class B(WW2) aquatic life uses, results of chemical/physical water quality monitoring conducted by DNR/SHL from March 2002 through December 2003 suggest relatively good water quality in this stream. None of the 21 samples collected had levels of dissolved oxygen, pH, or ammonia-nitrogen that violated state water quality standards (these samples were not analyzed for toxic metals or pesticides). These results, although somewhat limited in terms of parameter coverage, suggest "full support" of the Class B(WW2) aquatic life uses.

Monitoring and Methods

Assessment Key Dates

3/18/2002

Fixed Monitoring Start Date

12/15/2003

Fixed Monitoring End Date

Methods

150

Monitoring data more than 5 years old

240

Non-fixed station physical/chemical (conventional + toxicants)

420

Indicator bacteria monitoring