The Ditch 20 Feasibility Study aims to identify projects that can reduce phosphorus export from the wetlands drained by Ditch 20. This phosphorus contributes to algae blooms in Typo and Martin Lakes downstream. The feasibility study is located in northeastern Anoka County and southeastern Isanti County, primarily within Oxford Township west of Typo Lake (see public meeting presentation link below for more detailed map).
Ditch 20 has higher phosphorus than surrounding streams. The earliest monitoring in 2001 found phosphorus levels in Ditch 20 were as much as 7 times higher than the state water quality standard of 100 ug/L. In more recent years, phosphorus levels have been near the state standard. The reason for the apparent improvement is unknown. A graph of Ditch 20 phosphorus measurements over time is found in the September 2017 presentation link below.
Previous study has determined that the dominant source of this phosphorus is the peaty, organic wetland soils surrounding the ditch. The ditch facilitates drying and periodic re-wetting of these soils. During dry periods aerobic decomposition occurs. During wet periods, the soluble products of that decomposition, including phosphorus, move into the ditch. The ditch provides a pathway for nutrients to escape this large wetland that historically was probably a more closed system.
Based on the 1855 land survey, it appears there was no defined creek or channel through these wetlands prior to European settlement of the area. In 1922 Ditch 20 was dug and tamarack trees were largely cleared for agricultural production. 1938 aerial photos show agriculture, mostly haying, in the wetlands on either side of the ditch during this dry “dust bowl” period. Today, there is no agricultural production in the wide wetlands through which the ditch flows, and only modest agriculture on adjacent uplands. The wetlands around the ditch are dominated by reed canary grass.
To reduce phosphorus export through the wetlands the feasibility study is examining a variety of possible projects. These projects may aim to restore some characteristics of the pre-settlement hydrology, provide additional filtering of the water, settle particles that carry much of this phosphorus, or others. Some possible project types include water control devices to maintain more consistent water levels, blocking certain lateral ditches to reduce the area of wetland drained by the ditch system and settling ponds.
Any project is dependent upon a cost-effectiveness analysis, the approvals of affected landowners and funding. Because some changes to the ditch system could affect the hydrology, the Anoka Conservation District is communicating directly with landowners whose properties might be affected by proposed projects. Again, only projects with landowner support will be pursued. Determining landowner support is a critical component of the feasibility of any study.
Components of the feasibility study include:
Monitoring – Soil and water quality testing to understand chemical mechanisms related to phosphorus release and any ditch sections with particularly high phosphorus.
Modeling – An XP-SWMM hydrologic computer model of the watershed has been created to understand the current hydrology and evaluate the feasibility and effect of potential projects. The model includes LiDAR (airplane-collected) elevation data for the entire area that is accurate to within a few centimeters. Other models, such as P8, and literature research are being used to estimate water quality benefits of each proposed project.
Flow measurements – Flow volumes are being measured to ensure the hydrologic model is well calibrated to on-the-ground conditions.
Potential project identification – Develop concepts of projects to be further evaluated. Projects are eliminated when the landowner(s) are not supportive or they are not feasible from a construction or legal standpoint.
Cost-effectiveness analysis – Determining which projects will result in the greatest phosphorus reduction per dollar spent. and also have landowner support.
Concept designs – Prepare concept designs sufficient to pursue installation funding.
This project began in spring 2017. Preliminary findings were completed in September 2017. Final reporting is expected before the end of 2017. Thereafter, the findings will be presenting to the Isanti County Water Plan Committee and Sunrise River Watershed Management Organization Board for possible inclusion in their 10-year water management plans. If included, local and grant funding would still need to be secured for construction.
Civil Methods, Inc.
Presentation from April 24, 2017 public informational meeting (click to download)
Summary Report of Preliminary Findings October 11, 2017 (click to download)