Capabilities and Limitations of Stormwater Treatment Areas for Reducing
Phosphorus Loads to the Water Conservation Areas: A Biogeochemical Perspective
Thomas A. DeBusk and Forrest Dierberg
DB Environmental, Inc., Rockledge, Florida, USA.
Alterations in natural drainage patterns in south Florida during the past century have
resulted in pronounced adverse impacts to the Everglades ecosystem. These hydrologic
perturbations have been exacerbated by phosphorus (P) loadings from agricultural and
urban runoff. To attenuate P loadings in runoff from the agricultural areas located in the
northern portion of the watershed, the State of Florida has undertaken a program to build
six, large (920– 6670 ha) treatment wetlands, termed Stormwater Treatment Area
(STAs).
The four STAs currently in operation are removing a substantial load of P that otherwise
would have entered the Water Conservation Areas (WCAs). Research dedicated towards
further improving the performance of STAs has revealed several key biogeochemical
factors that influence P removal. Three of the most important factors identified to date
include water column P speciation, vegetation type, and sediment P stability.
Water column P speciation, or the partitioning of P between particulate, dissolved organic
and dissolved inorganic fractions, is particularly important with respect to P sequestration
and transport in the watershed. During 1998-1999, a period during which the western
flow path of STA-1W provided an outflow total P concentration of 14 ug/L, the
sequential wetlands (Cells 2-4) within this flow path removed 95% of the inflow soluble
reactive P (SRP), 90% of the inflow particulate P (PP), but only 50% of the inflow
dissolved organic P (DOP). The presence of DOP, which is not readily available to biota
and is readily transported through the conveyance system, probably places a lower limit
on STA outflow concentrations.
Improved understanding of P biogeochemistry in south Florida wetlands recently has led
to management practices that likely will enhance STA performance. Submerged aquatic
vegetation (SAV) communities are now recognized to be particularly effective at
enhancing STA P removal. Submerged macrophyte photosynthesis promotes
precipitation of CaCO3 in the moderately “hard” agricultural drainage waters (ADWs)
that enter the STAs. Phosphorus removal is enhanced under such conditions due to the
adsorption or co-precipitation of P with CaCO3. It is unclear, however, whether STA
performance will be comparable during periods when treating Lake Okeechobee waters,
due to their typical two-fold lower calcium concentrations than the ADWs.
Sediments are the ultimate P sink for the STAs. Because of water column precipitation of
CaCO3, some SAV-dominated wetlands (e.g., STA-1W Cell 4) have removed 250 – 400
grams calcium for every gram of P removed from the water column. Phosphorus
fractionations of sediments have revealed that much of the sediment P is associated with
calcium, which contributes in part to the stability of the sediment P.
The inflow, highly loaded regions of the STAs contain high concentrations of P in
sediment porewaters, which may contribute to a substantial internal loading of P to the
water column under certain environmental conditions. Such a reflux of sediment P also is
likely to occur in the impacted areas of the WCAs once they receive STA discharges,
rather than direct discharges of ADWs. Sediment porewater P concentrations in outflow
regions of STA wetlands are substantially lower than inflow concentrations. Whereas the
“highly loaded” regions of an STA at times may export sediment P, the flux of P from
sediment porewaters in the outflow region of STA-1W Cell 4 was found to be negligible,
which probably has contributed to the low total P outflow concentrations provided by this
wetland.
Continued research towards understanding these key biogeochemical aspects (i.e., water
column P speciation, aquatic plant ecology/processes, and sediment P stability) will lead
to improved design and operational strategies for the STAs.
Thomas DeBusk, DB Environmental, 414 Richard Rd., Suite 1, Rockledge, FL, 32955,
Phone: 321-639-4896, Fax: 321-631-3169, dbenvlabs@aol.com