South Prairie Lake IA 02-CED-962

Black Hawk Co.; west of Waterloo in S35 T89N R14W

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.

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Cycle: 2018
Release Status: Final
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trophic: Eutrophic
Trend: Declining
Created: 5/7/2019 2:35:18 PM
Updated: 5/7/2019 2:39:46 PM

Use Support

Class BLW

Aquatic Life - Lakes and wetlands

Not Supported

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

Status

New

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2018

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier IV

Class HH

Human Health -

Not Assessed

General Use

General Use water -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class B(LW) aquatic life uses are assessed (monitored) as assessed (monitored) as "not supported" due to violations of the Class B(LW) criterion for pH. [Note: This lake is not designated for Class A1 (primary contact recreation) uses.] Fish consumption uses are “not assessed” due to the lack of fish contaminant monitoring at this lake. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2012 through 2016 by Iowa State University (ISU), and (2) information from the IDNR Fisheries Bureau.

Assessment Explanation

Results from the ISU lake survey show water quality is stableat South Prairie Lake.Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 63, and 73 respectively for South Prairie Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place South Prairie Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and very high levels of phosphorus in the water column.

The level of inorganic suspended solids was relatively low at South Prairie Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in South Prairie Lake (2 mg/L) was ranked 45th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a moderately large population of cyanobacteria exists at South Prairie Lake. These data show that cyanobacteria comprised 65% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (17.5 mg/L) was ranked 75th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "not supported" due to violations of the Class B(LW) criterion for pH. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 4 violations of the criterion for pH in 15 samples(27%). Based on DNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of South Prairie Lake.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.

Monitoring and Methods

Assessment Key Dates

6/4/2012

Fixed Monitoring Start Date

9/7/2016

Fixed Monitoring End Date

Methods

120

Surveys of fish and game biologists/other professionals

222

Non-fixed-station monitoring (conventional during key seasons and flows)

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)