Minnewashta Lake IA 06-LSR-1650

Dickinson County S29T99NR36W 2 mi. NNE of Milford.

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 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.

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Cycle: 2020
Release Status: Final
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trophic: Eutrophic
Trend: Stable
Created: 8/31/2020 8:47:56 AM
Updated: 10/15/2020 8:16:04 AM

Use Support

Class A1

Recreation - Primary contact

Partially Supported

Algal Growth: Chlorophyll a

Support Level

Partially Supported

Impairment Code

5* - 303(d)-impaired last cycle; fully supporting this cycle; potential de-listing.

Cause Magnitude

Slight

Status

Continuing

Source

Agriculture

Source Confidence

Moderate

Cycle Added

2018

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier I

Class BLW

Aquatic Life - Lakes and wetlands

WINOFI

Exotic/Invasive Species: Exotic Species

Support Level

Further Investigation Needed

Impairment Code

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

Cause Magnitude

Slight

Status

Continuing

Source

Other: Exotic Species

Source Confidence

High

Cycle Added

2016

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Biological monitoring: Iowa DNR Fisheries

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 use is assessed (monitored) as “partially supported” due to levels of indicator bacteria that exceed Iowa’s water quality standard and due to poor water transparency caused by algae blooms. The Class BLW use is assessed (evaluated) as “partially supported” due to an invasive species introduction (Zebra Mussels). All other designated uses are “not assessed.”

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 2014-2018	22300011	Lake Minnewashta

Class A1 - Indicator Bacteria: No Data

lass A1 - pH:

Site ID	Data Source ID	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Violations	% Violations	Significantly >10% Violations	Assessment Type	Support Level
22300011	6	15 / 5	8.8	8.4	8.4	0	0%	NO	Monitored	Full

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
22300011	6	Chlorophyll a TSI	15 / 5	36	74	59	NO	Monitored	Full
22300011	6	Secchi TSI	15 / 5	41	70	57	NO	Monitored	Full
22300011	6	Total Phosphorus TSI	15 / 5	50	76	65	NA	NA	NA
22300011	6	Inorganic suspended solids	15 / 5	0.3	22	2.5	NA	NA	NA

Although the index value for Chlorophyll a is below the impairment trigger of 65 for this assessment cycle, Lake Minnewashta was listed as partially supporting its Class A1 uses due to aesthetically objectionable conditions. Based on DNR's methodology, the median TSI value for Chlorophyll a must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5). Therefore, Lake Minnewashta will remain listed as "not supported" for the 2020 assessment/listing cycle.

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
22300011	6	Ammonia	15 / 5	0.167	0.03	0.02	0 / 0	0% / 0%	NO	Monitored	Full
22300011	6	Dissolved oxygen	15 / 5	14.1	9.3	8.5	NA / 0	NA / 0%	NO	Monitored	Full
22300011	6	pH	15 / 5	8.8	8.4	8.4	0 / NA	0% / NA	NO	Monitored	Full
22300011	6	Temperature	15 / 5	28	23	23	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
22300011	6	11	107.323	99.8 %	10.543	79.1 %	NO	Evaluated	NA

Class BLW - Additional Information

The Class BLW uses are assessed (evaluated) as “partially supported” based on information from the DNR Fisheries Bureau and results from the ISU lake surveys. Based on information from the DNR Fisheries Bureau, this lake was infested with Zebra mussels in 2012 and the population exploded in 2015. This invasive species will likely cause water quality to degrade in the future. This potential impairment is not appropriate for Iowa’s Section 303(d) list of impaired waters (Integrated Report Category 5) but is appropriate for IR Category 3b of Iowa’s Integrated Report and Iowa’s list of waters in need of further investigation.

Class HH - No Data

Monitoring and Methods

Assessment Key Dates

6/3/2014

Fixed Monitoring Start Date

9/3/2018

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)