Stormwater Nutrient Reduction in Bioretention Cells.

Nutrients like nitrogen and phosphate are ubiquitous in stormwater runoff and stormwater is often introduced into surface waters without treatment. Thus, receiving waters are impacted, with serious consequences for aquatic organisms and the food web.

Bioretention systems are suitable elements to reduce the nutrient load of stormwater and manage the amount of stormwater introduced to receiving waters. But most effective compositions of bioretention systems need yet to be determined.

We built 16 mesocosms with different porous media to study the contaminant retention capacities. We used four media (mix 1: 80% sand, 20% compost; mix 2: 60% sand, 40% compost; mix 3: 60% sand, 15% compost, 15% shredded cedar bark, 10% water treatment residuals; mix 4: 60% sand, 30% compost, 10% water treatment residuals) that had been replicated four times. We continuously acquired outflow data since 2011 and measured the contaminant transport for several storms.

We monitored 8 storms between 2012 and 2014 at our research facility on the Washington State University Research and Extension Center campus in Puyallup, Washington. Effluent ammonia concentrations for the first two storms where higher than the influent concentrations resulting in an export of ammonia. However, the effluent concentrations of the six following storms where less than the influent concentrations resulting in a reduction of ammonia. Total Kjeldahl nitrogen, total phosphorous and ortho-phosphorous concentrations show the same pattern of exporting nutrients for the first two storms and reducing nutrients for the following six storms and this pattern was observed in all four of the tested bioretention soil mixes.

In average over the eight sampled storms, ammonia was reduced by 88 to 92%, Total Kjeldahl nitrogen by 13 to 17%, total phosphorous by 5 to 40%, and ortho-phosphorous by -9 to 18%. Eutrophication in urban waters can be reduced by treating stormwater in bioretention cells.