Winter Nitrogen Treatment of Three Experimental Bioretention Gardens in Salt Lake City, UT

Nitrogen removal in stormwater bioretention systems during times of plant dormancy is of particular interest in the Wasatch Front where most of the limited annual precipitation falls in winter and spring. Three bioretention cells were established in 2010 to test the nitrogen treatment performance of different vegetation communities during times when the majority of the plants are dormant: 1) a no-plant control, 2) an upland native community that does not require irrigation in semi-arid climates, and 3) a wetland community that requires irrigation. To test the performance of these three treatments during winter and before plants re-activate in the spring, runoff events were simulated to represent an average precipitation regime using a synthetic stormwater blend created by mixing tap water with commercially available mulch soil amender. Mass load reductions were measured for total nitrogen (TN), total Kjeldahl nitrogen (TKN), ammonia (NH4), and nitrate and nitrite (NOx) from one synthetic storm each month from November to March. The cells were said to “retain” nitrogen if the mass of nitrogen in the effluent was less than the total mass of nitrogen in the influent; cells are said to “leach” nitrogen if the mass of nitrogen in the effluent was greater than the influent. Preliminary findings show that minimum nutrient treatment performance occurred in January. For the month of January, the control cell leached 25.5 grams of TN, the upland cell leached 3.42 grams of TN, and the wetland cell retained 17.96 grams of TN. All cells retained TKN and all cells leached NOx in all trials to date. Testing is still in progress, and the data presented at the conference will include results for all three treatment systems through the winter of 2011-2012