Lake Darling IA 03-SKU-924

Washington County S21T74NR9W 3 mi W of Brighton.

Assessment Cycle

2020

Release Status

Final

Data Collection Period

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 2016 - 2018
Stream/River Water Quality Data	CY 2016 - 2018
Lake Water Quality Data	CY 2014 - 2018
Biological Sampling Data	CY 2014 - 2018*
Fish Kill Data	CY 2014 - 2018
Fish Tissue Sampling Data	CY 2014 - 2018
Continuous Dissolved Oxygen Sampling Data	CY 2014 - 2018
*Biological sites sampled in both 2013 and 2017 were also assessed.

Overall IR Category: 5 - Water is impaired or threatened and a TMDL is needed.
Trophic: Hypereutrophic
Trend: Unknown

Created: 7/30/2020 10:01:19 AM
Updated: 10/14/2020 10:27:34 AM

Assessment conducted in accordance with Iowa's 2020 IR methodology

Use Support

Class A1

Recreation

Primary contact

Partially Supported

Impairments

Support Level

Partially Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

High

Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

Moderate

Cycle Added

2002

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

Support Level

Partially Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Slight

Status

Continuing

Source

Natural: Internal Nutrient Recycling (Primarily Lakes)

Source Confidence

Low

Cycle Added

2008

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Ambient monitoring: Iowa DNR-lakes

Support Level

Partially Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2018

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Beach monitoring: Iowa DNR WQMA

TMDL Priority

Tier II

Support Level

Partially Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Slight

Status

New

Source

Agriculture

Source Confidence

Low

Cycle Added

2020

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

Class BLW

Aquatic Life

Lakes and wetlands

Partially Supported

Impairments

Support Level

Partially Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Slight

Status

Continuing

Source

Natural: Internal Nutrient Recycling (Primarily Lakes)

Source Confidence

Low

Cycle Added

2008

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Ambient monitoring: Iowa DNR-lakes

Further Investigation Needed

Support Level

Water in Need of Further Investigation (WINOFI)

Impairment Code

3b - Use potentially impaired based on an evaluated assessment.

Cause Magnitude

Moderate

Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

High

Cycle Added

2002

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Ambient monitoring: Iowa DNR-lakes

Support Level

Water in Need of Further Investigation (WINOFI)

Impairment Code

3b - Use potentially impaired based on an evaluated assessment.

Cause Magnitude

Moderate

Status

Continuing

Source

Agriculture

Source Confidence

Moderate

Cycle Added

1998

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Biological monitoring: Iowa DNR Fisheries

Class C

Drinking Water

Not Assessed

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 A1 uses are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by algae blooms, by poor water clarity due mainly to non-algal turbidity, and by violations of the state's criteria for indicator bacteria. Violations of the Class A1,B(LW) criterion for pH also contribute to the impairment of Class A1 uses at this lake. The Class BLW uses are assessed (monitored) as “partially supported” due to violations of the state’s pH criterion and impacts of nutrients and sediment at this lake. The Class C uses are "not assessed" due to a lack of enough information upon which to base an assessment. All other designated uses are “not assessed”. Note: A TMDL for siltation and nutrients at Lake Darling was prepared by IDNR and approved by EPA in 2003. Because, however, the Section 303(d) impairment due to indicator bacteria (aka, pathogens) was not addressed in the TMDL, this waterbody is placed in IR Category 5a (impaired; TMDL required) for the current assessment/listing cycle.

Assessment Explanation

Sources of Data:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2016-2018	21920001	Lake Darling Beach (Lake Darling State Park)
Iowa DNR	6	WQ	CY 2014-2018	22920004	Lake Darling Max Depth

Class A1 - Indicator Bacteria:

Site ID	Data Source ID	# Samples / # Years	2016 Geometric Mean	2017 Geometric Mean	2018 Geometric Mean	Annual Geometric Mean Violation	# Violations	% Violations	Significantly >10% Violations	Assessment Type	Support Level
21920001	6	20 / 3	221.48	20	302.21	Yes	8	40%	YES	Monitored	Partial

Class A1 - pH:

Site ID	Data Source ID	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Violations	% Violations	Significantly >10% Violations	Assessment Type	Support Level
22920004	6	14 / 5	9.4	8.4	8.4	4	29%	YES	Monitored	Not

Class A1 - Lake TSI's

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Minimum Value	Maximum Value	Median Value	Median TSI Violation	Assessment Type	Support Level
22920004	6	Chlorophyll a TSI	14 / 5	44	75	66	YES	Monitored	Not
22920004	6	Secchi TSI	14 / 5	50	77	66	YES	Monitored	Not
22920004	6	Total Phosphorus TSI	14 / 5	63	90	75	NA	NA	NA
22920004	6	Inorganic suspended solids	14 / 5	0.3	13	3.6	NA	NA	NA

Class BLW - Conventional Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Acute / Chronic Violations	% Acute/ Chronic Violations	Significantly >10% Violations	Assessment Type	Support Level
22920004	6	Ammonia	14 / 5	0.624	0.09	0.03	0 / 1	0% / 7%	NO	Monitored	Full
22920004	6	Dissolved oxygen	14 / 5	16.8	9.2	9.5	NA / 2	NA / 14%	NO	Monitored	Full
22920004	6	pH	14 / 5	9.4	8.4	8.4	4 / NA	29% / NA	YES	Monitored	Not
22920004	6	Temperature	14 / 5	29.4	24.7	24	0 / 0	0% / 0%	NO	Monitored	Full

Class BLW - Lake Planktonic Cyanobacteria:

Site ID	Data Source ID	# Samples	Maximum Cyanobacteria Wet Mass	Maximum Cyanobacteria % Composition	Median Cyanobacteria Wet Mass	Median Cyanobacteria % Composition	Median Cyanobacteria Wet Mass = 75th Percentile	Assessment Type	Support Level
22920004	6	10	563.134	100 %	20.937	85.6 %	NO	Evaluated	NA

Class C - Conventional Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Violations	% Violations	Significantly >10% Violations	Assessment Type	Support Level
22920004	6	Inorganic nitrogen (nitrate and nitrite)	14 / 5	4.4	0.87	0.2	0	0%	NO	Monitored	Full

The Class C uses are not assessed due to the lack of data sufficient for developing a valid assessment of support of the Class C uses.

Class HH - No Data

Note: Lake Darling was drawn down in 2009 as part of a restoration project at this lake. It remained drawn down in 2010.

Monitoring and Methods

Assessment Key Dates

7/16/2014

Fixed Monitoring Start Date

9/9/2018

Fixed Monitoring End Date

1/30/2003

TMDL Completed

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)

420

Indicator bacteria monitoring