The Shingle Creek watershed is 44 square miles in area, and is almost entirely developed with urban and suburban land uses. Shingle Creek and its tributaries Bass, Eagle, and Ryan Creeks convey runoff from a watershed that is more than 25% impervious. Most of the lakes and streams in the watershed are listed as Impaired Waters - 13 of the 16 lakes for excess nutrients; Shingle and Bass Creek for impaired biota; Shingle Creek for low dissolved oxygen; and Shingle and Bass Creeks for excess chloride.
The United States Geological Survey (USGS) initially discovered the chloride impairment in Shingle Creek in the mid-1990s, when conducting intensive monitoring across the country as part of the National Assessment of Water Quality (NAWQA). Shingle Creek and Nine Mile Creek were selected as representative of urban streams in the Upper Mississippi River Basin. Chloride was not a parameter that at the time was routinely monitored by watershed organizations in Minnesota, which are typically more concerned with nutrients, sediment, dissolved oxygen, bacteria, and biotic integrity. However, the USGS monitoring revealed elevated chloride concentrations in Shingle Creek and to a lesser extent in Nine Mile Creek in the southern Metro area. When Shingle Creek was added to the State of Minnesota’s 303(d) list of Impaired Waters, more routine monitoring was performed on other waters across the Metro area, revealing a more extensive chloride problem. It became apparent that waters in the “Snow Belt” were more at risk of elevated chloride concentrations than waters to the south.
When the Shingle Creek Watershed Management Commission and the Minnesota Pollution Control Agency (MPCA) completed the Chloride Total Maximum Daily Load (TMDL) study for Shingle Creek, a source assessment found that over 85% of the chloride load to Shingle Creek originated in road salt – NaCl – used for winter ice control by public road authorities including the Minnesota Department of Transportation, Hennepin County, and the nine member cities. Much of the balance of the load was likely contributed by private salt use in parking lots, private roads, and sidewalks and trails. The TMDL calculated that the chloride load to Shingle Creek must be reduced by 81% to guarantee that the chloride concentration in Shingle Creek would not exceed the state’s chronic exposure limit of 230 ug/L over four days or the acute limit of 860 ug/L.
Winter ice control is necessary to protect public health and safety and to assure the free flow of essential goods and services. In the past, road authorities relied on sand or a mix of salt-sand to provide ice control and traction. However, the use of sand had its own issues: spring rains washed the sand into receiving waters, requiring dredging to maintain flow and storage capacity. When swept up, the sand could not easily be reused, requiring landfilling. And the sand accumulated on boulevards, necessitating sod cleaning and repair in the spring. Many road authorities now use straight salt for ice control.
The road authorities in the watershed have set in place an implementation program that is focused on limiting the use of salt through pre-wetting salt; using temperature sensors and GPS-guided application to use the minimum amount of salt necessary for the road conditions; better training and equipment calibration; and pre-application of salt and salt brine in advance of expected ice events. The Commission and the MPCA have offered training in proper techniques and certification to both public and private applicators. These practices are reducing the amount of road salt applied, but to nowhere near the 81% reduction that is required by the TMDL.
The Paired Intersection Study
The genesis of the Porous Pavement Paired Intersection Study was the observation that porous pavements used in parking lot applications appeared to need less wintertime plowing and salting than traditional pavements. Rather than reduce application of road salt to levels that may not be adequately protective of public safety, what if porous pavement could be used on roads to reduce the buildup of ice and thus the need to apply road salt?
The Commission was awarded a $282,000 319 research grant from MPCA and the Environmental Protection Agency (EPA) to evaluate the use of porous pavement to reduce the need to apply road salt by constructing two test porous asphalt segments on residential streets, and to monitor the pavement condition compared to adjacent, control segments. The control segments would be salted and plowed as is routine on all residential streets, while the porous pavement would be plowed but receive no road salt.
The research questions explored in this study were:
1. Estimate the effectiveness of porous asphalt in reducing the need for salt as a deicer.
2. Determine if porous asphalt can hold up to rigors of regular city street use.
3. Determine short term and likely long term maintenance requirements.
4. Measure the water quality and quantity benefits in both sandy and clay/loam subgrades.
Two project sites were selected in the City of Robbinsdale. Site 1 is located near Crystal Lake, and has a sand subgrade with a measured infiltration rate of greater than 1.5” per hour. The control segment is the north leg of 41st and Zenith Avenues North, and the test segment is the north leg of 41st and Abbott Avenues North. 41st Avenue North is a through street, and drivers southbound on both Zenith and Abbott Avenues North must stop at a stop sign. Streets in this neighborhood were being reconstructed as part of the “Victory View Neighborhood” street and utility project.
Site 2 is located just outside the Shingle Creek watershed in the Bassett Creek watershed. Site 2 was initially to be located in the City of Plymouth as part of a neighborhood street and utility reconstruction project, but late in the planning process the Plymouth City Council decided not to go forward with the neighborhood project. Robbinsdale had a mill and overlay project planned in the Site 2 area, and volunteered two intersections in that project area, which has tight clay soils with minimal measured infiltration. The control segment is the north leg of 27th and McNair Avenues North, and the test segment is the north leg of 27th and Ewing Avenues North. As with Site 1, 27th Avenue North is a through street, and McNair and Ewing both are controlled by stop signs.
Site A was constructed in September 2009, and Site B in 2010. Two adjacent residential intersections were reconstructed as part of a neighborhood-wide street and utility improvement project. The “control” intersection at Zenith and 41st Avenues North was constructed with traditional asphalt pavement. Part of the “test” intersection at Abbott and 41st Avenues North was reconstructed with porous asphalt pavement.
Because this was a research project, various instrumentation was built into the pavement of both the test and control sections. Sensors in the pavement measured pavement temperature at the surface and at various pavement depths. Another sensor monitored the amount of water being stored in the 12-inch reservoir. Equipment housed in boxes on the boulevard automatically took samples of runoff from both sections so that water quality could be compared. Finally, cameras mounted on nearby utility poles recorded the buildup and melt of snow and ice from each intersection.
The intersections were monitored for two winter seasons following construction.
The final Porous Pavement Paired Intersection Study report was submitted to the MPCA and was approved by the Commission on February 13, 2014.
Click here for pictures
Here is Wenck's presentation for 2010 Water Resources Conference
Here is Wenck's 2012 presentation for the Paired Intersection Study
Click the following links for more information on pervious pavement:http://www.sciencefriday.com/program/archives/201106246