Balancing wildlife needs and nitrate removal in constructed wetlands: The case of the Irvine Ranch Water District's San Joaquin Wildlife Sanctuary

The San Joaquin Wildlife Sanctuary (SJWS), Irvine, CA, is a 32 ha series of shallow ponds created to maximize nitrate removal rates while maintaining 90% open water and episodically exposed shoreline for avian habitat. Design elements created non-ideal denitrification conditions by diminishing an organic carbon source (emergent vegetation) and increasing sediment exposure to oxygen. SJWS aqueous nitrogen and avian data (1999–2002) were analyzed to discern whether design and operating conditions allowed for simultaneous nitrate removal and diverse, abundant avian habitat. Average TIN removal efficiency was 80% while average TN removal efficiency was 60%; the difference reflects Org-N production in the marsh. Based on Chl-a measurements, roughly 40% of Org-N produced in the system was present as algae. The highest annual nitrate removal rates occurred April–May (350–500 mg/m2/d) and September–October (250–425 mg/m2/d). First order rate constants ranged 30.7–47.5 m year−1. Seasonal plantings of barnyard grass (Echinocloa crus-galli) were intended to serve as a carbon amendment for denitrification, however, there was no difference in nitrate removal between amended and non-amended conditions, likely because data averaging obscured a small, localized enhancement signal. Average avian species richness was high, ranging between 65 and 76 species month−1, while average relative abundance was mid-range, at 65–83 birds ha−1 month−1. Birds observed included common and rare species.