The Effects of Soil Properties on Rain-garden Bioretention and Soil Processes in a Semi-arid City Melissa Mckinley 1, Jordan Brown 2, and Mitch Pavao-Zuckerman 2 1 Northern Arizona University, 2 University of Arizona IntroductionMethodsResults Conclusions Acknowledgments: Funding was provided by NSF-REU and the Paul Galvin trust. We thank Ryan Schaefer for laboratory assistance, Marcus Tuller, Mercer Meding, and Aditya Verma for providing soil columns, Kitty Lohse and Erika Gallo for assistance in locating sampling sites and textural data, the Arizona Lab for Emerging Contaminants for chemical analysis, and the Watershed Management Group for conceptual input.  Rain-gardens are vegetated depressions that are filled with permeable substrates and designed to provide points of infiltration for stormwater in areas with high ratios of impervious surface (Figure 1)  Rain-gardens can ameliorate urban stormwater management by capturing runoff and sequestering runoff pollutants  Theoretically, soil in rain-gardens can modify the chemistry of the runoff they intercept and retain pollutants via several biogeochemical mechanisms  Rain gardens can significantly relieve stress on municipal water filtration plants  It is unclear how soils in semi-arid cities function to sequester pollutants and how specific soil properties related to bioretention  rain-gardens are used in city of Tucson, AZ, but it is not know which local soil type or which soil properties are best suited for basin functioning  Soils were collected from three ephemeral washes during June 2012 in Tucson, AZ (Figure 2). Each of the three sites were chosen based on soil texture  Soil chemical properties mostly corresponded with texture (Table 1)  Loam soil had the finest texture and the highest SOM, C, and N content  Sandy Loam was the textural intermediate between Sand and Loam, but was also intermediate with respect to bulk density, total C, and SOM  Pollutant removal efficiencies are high compared to other studies (e.g., Hsieh and Davis 2005) suggesting that Tucson's native soils have potential for use in bioretention  Mulch does not change the removal rate of lead in coarse soils, but decreases these rates in the finer soils. Such decreases might relate to simultaneous decreases in infiltration For nitrate removal in Sand, slowing the infiltration rate increases the removal efficiency so mulch may impede N retention.  The net effects of mulch on infiltration rate and bioretention in soil columns are not consistent among soil types, but these relations may manifest differently in real basins  It was common in finer soils for pollution removal rates to be positively correlated with infiltration, so we presume bioretention to be donor-controlled; less flow-through resulting in less removal  Money from the U of A Green Fund has been secured to test bioretention in experimental rain-gardens at the Biosphere 2 campus and to continue monitoring of current installations in Tucson FIGURE 1. A typical rain-garden in Tucson, AZ positioned along a street curb to capture stormwater runoff as it flows down paved streets. FIGURE 3. Soil columns were set-up to maintain a 5 cm head, Effluent was collected from drip spouts at regular time intervals.  To calculate pollution removal efficiency of three local soil types (with and without mulch)  Make recommendations for bioretention basin design with respect to soil types Objectives Loam Sandy Loam Sandy FIGURE 2. Four soil samples from each wash were extracted at a uniform depth. Each wash was known to contain soil with different textural characteristics. TABLE 1. Biophysicochemcial properties of soils collected July 2012 from three sites in Tucson, AZ. Reported are means (SE) from four subsamples. TABLE 2. Pollution removal efficiencies (%) of six soil mixtures after 60 min. Efficiencies of 0% indicate a relatively higher [pollutant] in soil column effluent.  Infiltration rates closely followed what is expected for the given textures with Sand having the highest rate of flow-through and mulch only slowed infiltration rates of finer textured soils (Table 2.)  All soil mixtures had high metal removal efficiencies (> 65%)  Mulch increased cadmium retention in Sandy Loam soils, but decreased retention in Loam soil  Mulch decreased lead retention in all soils  A mulch layer only enhanced nitrate retention in Sand.  The Sandy Loam failed to retain and actually exported nitrate  To characterize soils from each site, we measured soil physical properties, pH, total carbon (C) and nitrogen (N), and soil organic matter (SOM)  Soil mixtures were loaded into columns (15 cm depth, 7 cm diameter) and flushed with synthetic runoff solution to which we added 1.5 mg/L of nitrate (NO 3 -N; as NaNO 3 ) along with 0.01 mg/L of lead (Pb) and cadmium (Cd). Effluent was collected in 15 min. intervals for 60 min. then chemically analyzed  We calculated pollution removal efficiency (%) by comparing the concentration of pollutants in the effluent vs. the concentration entering the soil column References: Hsief, C., and A.P. Davis Evaluation and optimization of bioretention media for treatment of urban storm water runoff. Journal of Environmental Engineering 131(11):