By Sarah Boser, Sauk River Watershed District Water Resource Manager
The Judicial Ditch 2 Sediment Pond project is a two-pond system that was installed off of the main JD2 ditch channel near Osakis in 2002-2003 (small highlighted area in the included map).
Due to a massive rain event shortly after the original construction was completed, the ponds were reconstructed in 2004-2005.
The two pond system has a smaller primary pond (approximately 0.75 acres) which is narrow and deep as well as a larger secondary pond (approximately 10 acres) which is wide and shallow. Water is diverted from JD2 ditch to the primary pond which captures larger sediment particles. Water flows from the primary pond to the secondary pond where the finer, smaller particles are captured. The water is then routed back into the ditch through a culvert (see image).
As the sediment is captured in the ponds, the pond capacity is diminished. When about 25-30% of the pond capacity has been reached, maintenance is needed for proper pond function to continue. Based on the rate of sediment buildup, cleanouts for the primary pond are needed every three to five years and for the secondary pond every seven to 10 years.
The main purpose of the JD2 Sediment Pond project is to reduce sediment and phosphorus loading to Lake Osakis from JD2. Both ponds were cleaned out early in 2019, which was the third cleanout for the primary pond and the first cleanout for the secondary pond. The cleanout resulted in the removal of 17,600 cubic yards of sediment and 13,736 pounds of phosphorus which equates to 1,057 pounds of phosphorus being captured per year.
To provide some context regarding this information, Lake Osakis is impaired for excess nutrients. When a water body is listed as impaired, a Total Maximum Daily Load (TMDL) study must be completed. The TMDL for Lake Osakis was completed in 2013 and revised in 2016. The TMDL requires a phosphorus reduction of about 3,330 pounds per year to Lake Osakis from all water flowing into the lake. JD2 ditch contributes about 3,700 pounds of phosphorus to Lake Osakis per year. The JD2 Sediment Ponds are removing about 1,057 pounds of phosphorus each year, which is a 32% reduction of the TMDL requirement. It is important to note that Lake Osakis has a five year residence time, which means that it takes about five years to see changes in water quality within the lake.
The average total assessment (to be paid over 10 years) for the maintenance completed in 2019 and other accrued project expenses is $322.70 for residents within the City of Osakis and $325.56 for lake residents. The average annual assessment (to be paid each year for 10 years) is $38.03 for residents within the City of Osakis and $38.24 for lake residents.
For comparison, to evaluate the effectiveness and cost of the JD2 Sediment Ponds compared to individual landowner practices, a modeling exercise was completed using the Hydrologic Simulation Program – FORTRAN (HSPF) Scenario Application Manager (HSFP-SAM). In order to achieve a reduction of 1,000 pounds of phosphorus using a range of best management practices (BMPs) on individual landowner properties, it would require 100% landowner participation. It would also require that multiple BMPs are installed on each available acre. While the chances of 100% participation are unrealistic, the cost of implementation is very high, with an estimated cost of $769.64 per year per pound of phosphorus removed. The cost per pound of phosphorus removed from the JD2 Sediment Pond project in 2019 was approximately $94 per pound.
Further questions regarding this article or the JD2 Sediment Pond project may be directed to the SRWD at email@example.com or 320-352-2231.
Osakis Voices is a rotating column written by community leaders who share their thoughts in their field of expertise.