356-13 Evaluating Constructed Vernal Pools as Nature-Based Solutions to Urban Nutrient Pollution

Abstract:

Urbanization increases impervious surface cover, generating storm-driven pulses of nutrient-rich runoff that degrade downstream water quality. Vernal pools, seasonally inundated wetlands that function as biogeochemical hotspots, can retain and transform nutrients through microbial processes and extended water residence times. Their wet-dry cycles also exclude fish populations, providing critical amphibian breeding habitat. Despite this ecological importance, these small, isolated wetlands receive limited regulatory protection and are steadily disappearing from landscapes. Constructed vernal pools offer a promising nature-based solution for urban stormwater and habitat management, but their effectiveness in nutrient processing remains poorly understood. Our ongoing study therefore compares nutrient cycling dynamics among natural, mature constructed (>30 years old), and recently constructed (~5 years old) vernal pools at an urban restoration site near St. Louis, Missouri. Through weekly monitoring, soluble phosphorus and nitrogen concentrations are being measured in the three vernal pools’ surface water and soil porewater to capture temporally dynamic and depth-resolved nutrient cycling processes. To simplify nutrient load estimates, we calculated phosphorus as orthophosphate and nitrogen as the sum of ammonium and nitrate. Initial data for April-June 2026 (11 visits per site) show mean surface water phosphorus and nitrogen both increased with vernal pool age, with respective values of 0.06 mg/L and 0.12 mg/L in the recently constructed site, 0.07 mg/L and 0.42 mg/L in the mature constructed site, and 0.48 mg/L and 0.71 mg/L in the natural site. These results suggest that vernal pool age may help predict surface water nutrient status. Similarly, mean porewater phosphorus concentrations increased with vernal pool age (recently constructed vernal pool = 0.00 mg/L, mature constructed vernal pool = 0.01 mg/L, and natural vernal pool = 0.07 mg/L). In contrast, mean porewater nitrogen concentrations were highest in the mature constructed site (10.33 mg/L), with lower concentrations in the recently constructed site (0.78 mg/L) and natural site (4.16 mg/L). We also observed a delayed and diminished nitrate pulse in the porewater relative to the surface water at the natural vernal pool following a residential runoff event, suggesting nitrogen load attenuation during subsurface transport. As sampling continues, these initial patterns will be evaluated for seasonal consistency and used to assess whether constructed vernal pools can achieve nutrient retention capacity comparable to natural systems. 

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