Sarasota County receives about 54 inches of rainfall annually, ranging from around 35 inches in drought years to about 75 inches in wet years. This rainfall deposits nutrients from the atmosphere on the landscape and water bodies (see Chapter 5). Some of the rainfall is intercepted by tree canopy, evaporates, or filters into soil. In areas where natural pervious ground has been converted to impervious rooftops, roads, and parking lots, rainfall cannot seep into the ground and instead accumulates to create surface or subsurface runoff. The construction of manmade drainage ditches and canals has increased the volume of runoff that can transport excess nutrients downstream in the watershed.
The type and concentration of nutrients varies with the amount and intensity of rainfall and the type of land cover — the ratio of impervious and pervious coverage, soil types, and vegetation. Typical urban stormwater runoff contains about 2.0-2.4 mg/l total nitrogen and 0.3-0.5 mg/l total phosphorous, about twice the state threshold for ambient water quality in estuaries (Sarasota County 2015).
Florida law enacted in the 1980s requires use of stormwater best management practices (BMPs) in all new developments to intercept and “treat” runoff before it is discharged to receiving water bodies. Once built, these systems require appropriate operation and maintenance to continue removing pollutants. Older neighborhoods built prior to state stormwater rules provide little or no stormwater treatment.
Functional and regulatory differences exist between surface and subsurface runoff flowing into natural water bodies (see Chapter 6.1) and runoff captured and held in stormwater ponds (see Chapter 6.2). Stormwater BMPs like ponds and swales intercept polluted runoff to protect the ecology and recreational uses of natural water bodies downstream, and thus are not subject to the water quality standards of natural waters. The best performing BMPs collect and treat polluted water while also providing their own beneficial ecological, recreational, and aesthetic values.
Estimate Annual Nutrient Loads from Stormwater
Robust long-term monitoring and modeling programs that take into account surface and subsurface runoff, stream flows, and organic and inorganic nitrogen concentrations are essential for accurately estimating and managing nutrients in watersheds.
Quantify Costs and Effectiveness of Stormwater Best Management Practices
Stormwater Best Management Practices (BMPs) can capture and treat nutrient-laden runoff before discharge to water bodies, some with greater efficiency than others. Where conventional methods, such as building wetlands or retention ponds, are not feasible or too costly, smaller green infrastructure solutions should be encouraged, especially in dense urban settings. Examples include green roofs, rain gardens, cisterns, pervious pavement and multi-use “stormwater parks.” Additional research is needed to quantify the costs and effectiveness of all stormwater BMPs.
Update State Stormwater Treatment Rules to Require Use of Best Science for Nutrient Reduction
Florida’s current stormwater regulations were adopted in the 1980s, and research has shown that conventional stormwater treatments do not meet nutrient removal standards. Comprehensive updates, incorporating new data and new treatment technologies including green infrastructure, have been completed but not adopted. Enactment of new stormwater treatment rules is urgently needed to address nutrient-impaired waterways.
Adopt or Update Local Ordinances to Provide Guidelines for Stormwater Pond Management
Stormwater ponds and canals must be maintained to retain their nutrient removal efficiency, yet are generally lacking in long-term maintenance. Local ordinances would provide a mechanism for outreach, inspection and certification to ensure that stormwater ponds and canals are managed and maintained to maximize nutrient removal.