Retrofitting agricultural detention systems can economically enhance nitrogen treatment with payment for services approach.

Despite their global presence and multiple services including water storage and treatment, limited field studies have focused on quantifying water quality effectiveness of agricultural stormwater detention areas (SDAs; farm ponds). Limited data available comes from unverified modeling studies. Economics is another neglected aspect of SDAs. An SDA in the Everglades basin of Florida was monitored to quantify its baseline nitrogen (N) treatment. Design flaws of underutilized surface storage in variably ponded areas and short-circuiting were rectified by retrofitting the SDA through compartmentalization, increase in outflow structure control elevation, and installing channel plugs. Post the retrofits, total N retentions doubled (7500 from 3700 kg), a similar trend was observed with respect to surface water volume indicating that water retention drives N retention. Targeting enhanced water retention through retrofits can achieve reductions in downstream nutrient flows. The cost of additional N treatment from retrofits was $12/kg, an order of magnitude less than the published costs for other detention systems including newly constructed wetlands ($180/kg) and tailwater recovery systems ($214/kg). A scale-up showed that retrofitting SDAs could reduce the N discharge to an estuary in the Everglades basin by over 50 %. A payoff to the farmers for SDA retrofits through a payment for environmental services program can achieve higher N treatment economically and facilitate large-scale adoption.