There are two TMDLs (Total Maximum Daily Loads) critically important to Lake Pepin sedimentation and water quality concerns:
- Addresses the accelerated sedimentation in Lake Pepin
- Examines the reach of the Mississippi River impaired for turbidity upstream of Lake Pepin
- Determines that the Minnesota River Basin (MRB) contributes 75% of the sediment
- Requires the following sediment load reductions:
- 50% reduction from Minnesota and Cannon Rivers
- 20% reduction from the Upper Mississippi River, St. Croix and Vermillion Rivers
- 50% reduction in internal sediment loading within Lake Pepin
- Addresses the Lake Pepin water quality impairment making it unsuitable for swimming and fishing
- Examines the sources of phosphorus and nitrogen that contribute eutrophication
- Relates closely to Lake Pepin sedimentation because phosphorus binds to soil particles
- Project Underway
Read on to learn about these TMDL's in greater detail.
South Metro Mississippi Turbidity TMDL
In 2004, the US EPA declared the South Metro Mississippi -- from Fort Snelling to upper Lake Pepin at Red Wing -- to be impaired for turbidity (i.e. excessively cloudy due to elevated sediment loading that results in harm to aquatic habitat.) Consequently by law the MPCA, with input from the WI DNR and several stakeholders, began the process of determining a Total Maximum Daily Load (TMDL), or maximum amount of total suspended solids (TSS) that this stretch of river can accept yet still meet water quality standards and support designated uses.
The South Metro Mississippi TMDL is one of nearly 50,000 TMDLs that have been developed for water bodies throughout the country and the TMDL process has become a key tool for protecting and restoring waters nationwide.
As part of the South Metro Mississippi TMDL process, the MPCA first developed a site-specific TSS standard. This was necessary because scientists had determined that the state-wide turbidity standard was not adequate to protect this particular stretch of river, given its unique hydrologic features and designated uses. Among other features, the Mississippi at Red Wing has more than twice the flow volume of the next largest tributaries of the Minnesota and St. Croix Rivers.
Following a multi-year development process, including a public comment phase, the South Metro Mississippi River Total Suspended Solids (TSS) TMDL received final approval by the US EPA in June 2016. It calls for a 50% reduction in current TSS conditions in the Minnesota and Cannon Rivers (60% at high flow), in order to achieve a summer average (Jun – Sept) of 32 mg/L total suspended solids (TSS) in this stretch of the Mississippi River in at least 5 years over a 10-year period.
The TMDL also calls for 20% reductions in current TSS conditions in several other watersheds including the upper Mississippi River (above Lock & Dam #1), the St. Croix, and Vermillion Rivers.
In addition, the TMDL calls for a 50% reduction in internal sediment loading within the water body through strategies that consolidate riverbed sediment, such as targeted island construction, and strategic drawdowns in certain locations so as to promote the growth of rooted vegetation.
The targeting of upstream watersheds within the TMDL reflects the fact that the major sources of sediment are external to the South Metro Mississippi, specifically 75% of the sediment load to the South Metro Mississippi comes from the Minnesota River. While sediment occurs naturally to some degree in rivers, the excess load in this case comes predominantly from nonpoint sources in the Minnesota River Basin including fields, ravines, bluffs, and streambanks, and to a lesser degree from point sources such as municipal separate storm sewer systems (MS4). Within the nonpoint source category, scientists have determined that the major sediment source has shifted over time from field to non-field sources (i.e. the ravines, bluffs, and streambanks).
When a TMDL is developed for waters impaired by both point and nonpoint sources, the state agency allocates the target pollutant load limits among the sources. The National Pollution Discharge Elimination System (NPDES) permitting process is utilized to establish appropriate limits for point sources and these limits are based on assumptions that nonpoint sources load reductions will also occur. The EPA’s 1992 TMDL Guidance states that the TMDL should provide “reasonable assurances” that nonpoint source control measures will achieve expected load reductions.
According to the South Metro Mississippi TSS TMDL report, reasonable assurance of nonpoint source sediment reductions can be provided through, “commitments of funding, watershed planning, and use of existing regulatory authorities” (MPCA 2015).
According to the MPCA report, the funding commitments are demonstrated through the Clean Water, Land and Legacy Amendment funding, approximately $5.5 billion of which will be channeled into protecting water and land over the next 25 years. The MPCA will work with local land-use authorities to support and implement non-regulatory measures and best practices that will result in reduced sediment loads. The MPCA report also states the agency will pursue the following policies:
- Comply with 50-foot buffer required for the shore impact zone of streams classified as protected waters (Minn. Stat. 103F.201) for agricultural land uses.
- Comply with requirements to buffer highly erodible land within the 300-foot shoreland district, as described in the state shoreland rule.
- Establish a process and timeline to ensure compliance with the requirement for a 16.5-foot buffer on agricultural drainage ditches as defined in Minn. Stat. 103E.021.
- Review the use of excessive soil loss ordinances by counties (described in Minn. Stat. 103F.415) and the potential benefits of applying soil loss ordinances specifying a maximum rate of “T” (the tolerable rate of soil erosion which the NRCS defines as the rate at which soil can replenish itself) to areas contributing high amounts of sediment to the South Metro Mississippi and tributary watersheds.
- Review the MPCA’s authorities on the prohibition of nuisance nonpoint source pollution (Minn.R.7050.0210, subp.2) (MPCA 2015).
The South Metro Mississippi TSS TMDL also entails a monitoring plan – monitoring both for sediment loading rates and aquatic vegetation within the focal river reach. Finally, in conjunction with this TMDL process, a separate, parallel, effort was launched to create an implementation strategy designed to help local watershed planning efforts in reaching sediment reduction goals. The Sediment Strategy document can be found here.
The Lake Pepin Eutrophication TMDL
The MPCA and partners continue to work on this complex project. As a first step, the MPCA conducted a study to calculate site-specific water quality standards. The report for the study was completed in 2011, but is still waiting for EPA approval before it can go through a public comment period. The suggested water quality standards for Lake Pepin, as it relates to eutrophication are:
- 100 µg/L total phosphorus
- 28 µg/L for chlorophyll-a
The MPCA believes these criteria will provide sufficient protection of submerged vegetation and aquatic recreational uses, and should be applicable over the range of flows for which the criteria were developed. Achieving these standards will require a large reduction from current levels, given that Lake Pepin had an average total phosphorus of 171 µg/L and an average chlorophyll-a of 30 µg/L between 2000-2009.
LPLA will be assisting with public engagment and comments throughout the TMDL process. Stay tuned for more information on the next public meeting to be involved directly!