The Relationship of Four Substrates to N, P Removal and Biomass Establishment in a Greenhouse Natural Swimming Pool Experiment

Natural swimming pools, (NSPs) are constructed bodies of water, with an impermeable liner between the soil and water, a designed hydraulic and skimmer system and a complex ecological community as a filter. The NSPs rely on biological filtration, the interaction of a balanced system including bog vegetation, bacteria and substrate to reduce nitrogen and phosphorus levels in the NSPs water column to less than 30 mg/l^-1 NO₃ and 0.01 mg/l^-1 P. These are the levels recommended by the 2007 German FLL Guidelines for NSP Design and Operation. While many studies have quantified the nutrient removal mechanisms of treatment wetlands and stormwater bioretention systems, no studies have quantified the nutrient removal capabilities of substrates and vegetation in the low nutrient environment of an NSP designed as a landscape amenity. In this study, a mass balance analysis of vegetated NSP greenhouse mesocosms estimated phosphorus and nitrogen inputs, outputs and storage for substrate treatments of 1) haydite/clay, 2) norlite, 3) growstone and 4) river gravel. All treatments contained Saururus cernuus, lizard’s tail, and Iris versicolor, blue flag iris, and all aboveground biomass was harvested, weighed and analyzed for P and N after 15 weeks. System outputs consisted of nutrients contained in harvested biomass while root biomass and substrate were considered nutrient storage compartments. The haydite/clay substrate treatment was the only substrate able to reduce phosphorus levels of the mesocosm water to 0.01mg/l^-1. Plant direct uptake proved insufficient to remove all nutrient inputs, and substrate type was statistically significant in nutrient storage. All substrate treatments reduced mesocosm water NO₃ to the target levels of 30 mg/l^-1. Iris produced significantly higher biomass than the lizards tail, and plant biomass was positively correlated with NO₃ removal. Two substrates demonstrated potential as substrate for bogs in natural swimming pools. Norlite and haydite/clay reduced NO₃ nutrients to 26.01and 5.83 mg/l^-1, and .02 and 0.01 mg/l^-1 for P, respectively. It seems probable that the addition of calcined clay to norlite might also improve P removal from the 0.02 mg/l^-1 measured result to the recommended levels of 0.01mg/l^-1.