Access past research done by the Colorado Stormwater Center

Roadway Deicing Operations in the City of Fort Collins and Impacts to an Urban Stream

The Urban Stream Chloride Analysis of Winter 2016-2017 was a study that evaluated the chloride loadings from roadway deicing operations (RDO) delivered to an urban stream within Fort Collins and to asses the potential impacts of the chlorides on water quality. RDO is a critical component of insuring safe roads throughout the winter, but can also result in contributing substantial amounts of chloride to urban waterways.  Chlorides delivered at high concentrations in winter runoff due to the use of deicing materials can be lethal to certain aquatic species.  From the study, it was found that though RDO is a significant source of chlorides in Spring Creek, but it is not the only source. Private property owners also apply chlorides for deicing purposes that are another source of chlorides in winter runoff.

A mass balance approach was used to estimate the portion of chlorides that can be attributed to RDO.  Total in-stream mass of chlorides were determined by continuously monitoring four locations along Spring Creek for specific conductivity.  Specific conductivity was related to chloride concentrations through a linear regression relationship determined by collecting grab samples at each of the four sites and measuring chloride concentrations for multiple events during the winters of 2016 and 2017. Using chlorides concentrations with stream discharge, loads were calculated at each of the four monitored locations for each storm event that occurred in the winters of 2016 and 2017. In stream loads of chloride were compared to the mass of chloride applied by RDO. The application amounts were determined by the Automated Vehicle Location (AVL) system on snowplows and were provided by the City of Fort Collins Streets department. Relationships between the amount of chlorides applied and delivered with characteristics of winter storm events (i.e. precipitation depth, air temperature, pavement temperature) were created.  These relationships could be used to help inform the Streets department about which applications amounts and storms may be delivering the most chlorides to Spring Creek.

It was found that more chlorides applied by RDO would typically lead to more chlorides delivered to the stream which is displayed in Figure 6 below.

The 2017 study found that not all the applied chloride load from RDO was delivered to the stream system, which would mean that the environmental system has some capacity to store chlorides. An increase for in stream baseline chloride levels was found between the 2012 and 2017 study. This could indicate that chlorides are being stored by the system and are being released into the stream. For the first storm event in 2017 there was 305,000 lbs of chlorides applied in total for all four subwatersheds, and at the most downstream monitoring point in the stream only 25,000 lbs of chloride were delivered. This could mean that the chlorides are being stored in the system and since chlorides are a conservative ion, the chlorides persist in the system and begin to leach into the stream causing higher concentrations of chlorides in the baseflow over time.

The figure below is an example of the chloride loading from RDO and the chlorides delivered in-stream during and after the application. The figure also displays the air temperature, pavement temperature, and the precipitation recorded. This type of figure was created from every site, and every relevant month for the study.

Bioretention Media Mixtures – A Literature Review

The research conducted on bioretention media was aimed to address phosphorus removal in bioretention cells. The study reviewed potential amendments to bioretention media mixtures that could remove particulate and dissolved phosphorus from stormwater runoff. The study reviewed the use of water treatment residuals (WTRs), biochar, coconut coir, other industrial byproducts (e.g. Fly Ash), and general sand/compost/gravel mixes.

It was found that certain industrial byproducts are capable of efficient phosphorus removal and have a high sorption capacity for phosphorus. However, there has not been enough research on the practical use of the industry byproducts of interest (e.g. blast furnace slag). When using an amendment for bioretention media, there needs to be an adequate amount of research that displays the ability of the material to remove phosphorus while not remobilizing any harmful pollutants (like Al3+) in order to ensure public and environmental safety. The use of WTR’s to remove phosphorus from wastewater or stormwater has been a popular research topic for past few decades and some of that research has been focused on the potential negative human and environmental effects of WTR application. The research focused on the human and environmental impacts of WTRs found that there can be an increase in exported aluminium and natural organic matter, but the levels exported were within surface body water standards.

The use of coconut coir as a bioretention media amendment has been shown to remove phosphorus from stormwater, but not as efficiently as WTRs. The use of biochar to remove phosphorus from stormwater has produced mixed results. Some studies actually show a net export of phosphorus from biochar amended sands, and some studies show a reduction in phosphorus. However, the particular biochar mixes vary greatly based on the components the biochar. This study concluded that biochar would not be a useful amendment for phosphorus removal in bioretention cells until further research can show an effective use of biochar for phosphorus removal. The study found that certain types of general sand mixes can be more effective at removing phosphorus from stormwater. The general mixes of interest include: zeolite and silica sand, activated bauxite, maerl, peat, wollostonite, laterite, vegitated sand with loam, and gravel and cobbles with cattails. The issue with most of the general mixes is that the phosphorus sorption capacity is variable and can be difficult to quantify. It was concluded that using one or more of the general mixes along with WTRs would promote long term phosphorus removal in bioretention cells.

The research conducted also found that the use of compost in bioretention cells can cause an export of phosphorus and nitrogen. However, compost is effective at reducing heavy metals in stormwater runoff. It is suggested that if compost is to be used in a bioretention cell that it should be placed above a layer that is amended with phosphorus removing materials.The Colorado Stormwater Center concluded that the use of WTRs as an amendment for bioretention cells could reduce the levels of phosphorus in stormwater runoff for many years (depending on how much WTR is applied) while maintaining an adequate hydraulic retention time and not causing a human or environmental health issue.

Nutrient Sources in Urban Areas – A Literature Review

Research shows that leaf litter, atmospheric deposition, fertilizers and (to a lesser extent) pet waste are all potential sources of nutrients in stormwater.  When the above sources are deposited onto impervious surfaces, they accumulate as street solids and become “directly-connected” to the stormwater system.  Rough estimates of average annual nutrient loads show that the nutrients measured in street solids are on the same order of magnitude as those measured in urban stormwater runoff.

Although leaf litter is a “natural” source of nutrients, education and outreach programs could focus on keeping fallen leaves out of the stormwater system through proper collection and recycling.  Activities such as blowing leaves and other vegetative debris into roadways should be discouraged.  A side benefit of this type of program would be reduced maintenance to the stormwater system due to leaves clogging pipes and inlets.

The control of nutrients contributed by atmospheric deposition through a local education and outreach program would likely be limited as many of the sources are non-local and/or naturally-occurring.  However, several studies did identify vehicle emissions as a significant localized source of nitrogen.  This suggests that any education and outreach campaign that focuses on reducing vehicle usage may also be contribute to a reduction of nitrogen in stormwater.

Fertilizer use is the most often targeted source of nutrients in stormwater management campaigns.  While it may seem most intuitive that a reduction or elimination of the use of fertilizers may be the most effective education and outreach approach, in fact research shows that poorly maintained lawns (those with low/no fertilizer applied) actually export more nutrients to runoff compared to fertilized lawns.  Research also shows that the highest export of nutrients from lawns occurs in during the late fall through early spring months when the ground is frozen and/or vegetation growth is limited.  Therefore, an education and outreach program targeting fertilizer use should focus on applying the proper amounts of fertilizer (according to manufacturer and/or Extension recommendation) during the proper seasons.

Research on the contribution of pet waste to stormwater nutrients is rather limited, but few studies have identified it as a potential source.  Many cities already have education and outreach programs targeting the proper collection and disposal of pet waste for bacteria reduction, however the same programs may also contribute to nutrient reduction.

Lastly, a review of research on street solids was included because they represent an aggregate measure of nutrients accumulating in the urban environment.  Street sweeping programs operated by local utilities are an obvious activity that can significantly reduce nutrients in stormwater, however such operations may be limited to certain streets and do not specifically target private citizens.  Education and outreach programs that aim to “keep the streets clean” could educate citizens that what appears to be relatively innocuous “dirt” is actually akin to trash and litter collecting on the streets.  Engaging citizens to prevent materials such as leaves, grass clippings, fertilizer from reaching streets and potentially even remove dirt and other accumulated materials as they would trash and litter would be an interesting (and potentially highly effective) program.

700 Wood Street Bioretention Cell Performance Monitoring Results


The City of Fort Collins has required that water quality best management practices (BMPs) be implemented on all new developments but the performance on one particular BMP, the bioretention cell, has not been studied extensively in Fort Collins. Research on bioretention cell performance in other parts of the United States have shown that they can be an effective water quality BMP. This has led the City of Fort Collins to believe that the use of bioretention cells as a “low impact development” technology (LID) will be used extensively to meet regulations for new or re-development. This study aimed to assess the performance of the newly constructed bioretention cell at 700 Wood St in Fort Collins, Colorado. This BMP performance monitoring will help revise the BMP design criteria for bioretention cells in Fort Collins.

Volume Reduction Results

Under the current design criteria the bioretention cell achieved a 20% volume reduction. However, in subsequent years a modification was added to the bioretention cell. The modification included an underdrain riser that would encourage infiltration in the basin rather than having the water conveyed through the stormwater system. The riser effectively raised the depth of the underdrain system by 12 inches. Volume reduction with the modification was 80%. This led to the recommendation that the City of Fort Collins should change their design criteria for bioretention cells to include and underdrain raiser to promote infiltration in bioretention cells.

Water Quality Results

The bioretention cell was able to improve upon many water quality parameters. Total suspended solids (TSS) concentrations entering the cell typically ranged from 80-120 mg/L and the average effluent TSS concentrations were typically between 10-20 mg/L. It was not possible to determine if the modification increased TSS removal performance. However, the stormwater system received less TSS loading with the modification due to improved volume reduction. Total nitrogen was removed at about a 25% removal rate. Influent concentrations of total nitrogen were 4 mg/L on average, and effluent concentrations were 3 mg/L on average. The bioretention cell was exporting phosphorous throughout the monitoring period, which was determined to be most likely due to the addition of compost in the bioretention media and potential fertilizer use on the vegetation covering the cell. The average influent concentration of total phosphorus (TP) and dissolved phosphorus (DP) are about 0.4 mg/L and 0.2 mg/L, respectively. The average effluent concentrations of TP and DP were found to range from 0.6-1.1 mg/L and 0.5-0.8 mg/L, respectively. When comparing the annual average load of phosporus to the storm sewer system it was determined that, through volume reduction, the annual average load was reduced with the modification to the cell.

Due to the issue of the bioretention cells exporting phosphorus the City of Fort Collins has requested a study of bioretention media mixtures to determine if there is a way to use bioretention cells to remove phosphorus rather than export it. This bioretention media mixture study was conducted by the Colorado Stormwater Center and can be found at – Bioretention-Media-Mixtures-Literature-Review