Minnesota Local Road Research Board Local Operational Research Assistance (OPERA) Program

Project Name: Improvement of Water Quality in Storm Ponds Proposal Sponsor: Jim Foldesi, P.E. Principal Investigator: Tom Tri County Engineer/Public Works Director: Marcus J. Hall, P.E. Date of Proposal: April 5, 2006 Agency: St. Louis County Public Works Department Contact Phone#: 218-625-3876 Fax: 218-625-3888 Background: When St. Louis County constructed the Richard H. Hansen Transportation Complex, it was the County Engineer’s vision to leave a better environmental footprint on the landscape. With the increased impervious surface of buildings and bituminous surfaces, it was necessary to construct stormwater ponds as part of the overall landscaping plan. Problem: The Minnesota Pollution Control Agency requires public road authorities and others to construct stormwater ponds as part of its NPDES permit. Many of these ponds detain runoff for long periods of time and later discharge the water to protected and or impaired waters of the state. With longer detention periods, the stormwater may undergo deterioration of water quality in terms of dissolved oxygen, and temperature. Preliminary measurements of dissolved oxygen (DO) in the pond indicate that DO concentrations are very particularly low during winter months, and turbidity is high in the ponds following storm events. It has been documented that nutrients trapped in the bottom sediment of lakes or ponds can become redistributed in the water column under conditions of little or no dissolved oxygen, so dissolved oxygen levels in the pond can become critical. Further, temperatures discharged to streams from ponds can be critical to fisheries habitat. The Richard H. Hansen Transportation Complex located about 10 miles northwest of Duluth, Minnesota, has a stormwater pond system to treat the 25 acre facility. The system, created in 2002,has a large pond (1.8 acres), a grassy swale (0.2 acres) and a small pond (0.3 acres). Both ponds are unique in that they are fed by cooler water from underground springs in addition to storm runoff from the buildings and pavement. Since the stormwater system discharges to a ditch that drains to a stream that flows into Pike Lake, there was a concern for temperature and dissolved oxygen levels and how they might affect the stream.

Richard H. Hansen Transportation Complex

Solution: The proposed solution to the problem was to install two windmills to aerate the large pond. This method would use wind energy (no cost) and a direct drive compressor to provide the air flow/oxygen necessary to aerate the pond. Two 23’ windmills were easily assembled and installed by volunteers on a berm adjacent to the large pond for optimum wind conditions. The cost for both of the 23’ tall windmills (including shipping) was $3,360.

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Evaluation Procedure: Dissolved oxygen (DO) and temperature (temp) levels were measured several times during the winter 2005 and during most of 2006 and 2007 using the Department’s equipment. Monitoring would continue at two locations in the pond and at the outlet. In addition, measurements were taken within the pond at depths of 3’, 6’, 9’ and 12’ (bottom). Turbidity levels were taken at the two locations in the pond but not at the outlet because the outlet occasionally receives backwater from the road ditch and would mix and contaminate the outlet’s results. The two windmills were installed in the middle of October of 2006 and sampling continued. During 2007, more DO and Temp sampling points were added in the smaller pond (no aeration) and its outlet, and in the road ditch and in the stream that the ditch empties into. Results:

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NTU's Station 1

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Turbidity results: Overall turbidity has declined to less than 10 NTU’s (almost clear water) since the windmills have been installed on 10/15/2006. The summers of 2005 and 2006 had very green water in the large pond from algae. In 2007, the water was much clearer and most of the algae had disappeared as a result of the aeration. Emergent and submergent vegetation increased in the large pond as well. We did not get a chance to measure nutrients prior to the windmills so no nutrients sampling was completed. However, the measured lower turbidity is an indicator of better water quality.

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Large Pond Dissolved Oxygen results: This chart illustrates the DO levels measured in the pond before and after the installation of the windmills on 10/15/06. Aeration definitely improved the DO levels within the pond and overall. During the winter, DO levels homogenized throughout the pond under the ice at levels above 6 ppm with aeration. During the summer after installation, DO levels at 9’ did not drop to 0 ppm as previously experienced without aeration.

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Large Pond Water Temperature results: Water temperatures were more consolidated with the mixing due to aeration. Summer temps in 2006 at depths of 9-12’ were cooler than the same temps a year later. Bottom temperatures approached nearly 70 degrees F in August 2007 due to mixing. However, discharge temperatures were generally lower in 2007 than in 2006 as illustrated below. With the mixing due to aeration, overall surface temperatures did not get as warm in 2007 as the previous year with aeration. The large pond discharge water was measured several degrees cooler than its surface temperatures.

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Outlet Temperatures

Dissolved Oxygen in Pond Outlets & Ditch & Stream

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DO Results (ponds, ditch, &stream): There were some interesting results when we looked at the dissolved oxygen levels of the two ponds, different points in the road ditch and the stream. We did not think to measure these sample points before the windmills were installed, so we have no unaerated data for these points.

1. Clearly, the large pond discharge (1 - purple line) had better DO results than the small pond discharge (2 - green line). So the aeration is leaving a better foot print on the environment in DO.

2. The stream measured between 6ppm DO and 7.25 ppm DO until a beaver moved in to dam the stream around the end of August. Consequently DO levels dropped significantly in the stream and the ditch.

3. DO levels picked back up in the ditch after the beaver dam was removed.

4. In the fall as wind speeds picked up, we saw increased DO levels rise in both ponds and the ditch while the stream continued to decline.

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Temperature Results (ponds, ditch, & stream): We did not have any pre-stormwater pond temperature data. So it was difficult to assess whether there was a cause and effect from the stormwater pond system on the stream. We do know that ponds will heat up with summer sunshine. However we did observe that the temperature of the stream increased over the summer and even surpassed the ditch and small pond temperatures about the time that the beaver dam was installed. Cost: The price of 2 windmills $ 3,360 The price of DO meter $ 500 (Hanna 9146-04) The price of a NTU meter $ 425 (Hanna 93703-11) Assembly time for two windmills 2 men @ 12 hours each.

Installation time 1 man with backhoe @ 4 hours; 2 men installing towers and fan assemblies @ 3 hours. 1 diver to place aerator stones @ 1 hour. Sample time 1 man hour per sample event.

Implementation: The County will continue to aerate the stormwater pond as it appears to have an overall beneficial effect. In addition to treating stormwater, the ponds have attracted wildlife, such as Canadian geese, wood ducks, mergansers, teal, mallards, herons, osprey, and even a mink or two. Status: Completed