Alafia River Minimum Flows and Levels - Southwest Florida Water ...

While elevated phosphorus concentrations in streams can potentially be ascribed
to numerous sources (e.g., waste water treatment plant discharges, some
industrial discharges, fertilizer applications by agriculture or from residential
areas), there can be little doubt that the elevated concentrations seen in the
Alafia River from approximately 1960 (when routine water quality analysis began)
to the early to mid 1980's are directly associated with phosphate mining activities
in the watershed. Most of this mining (89%) has occurred in the sub-basins of
the North and South Prongs. Supporting data are seen in elevated
concentrations of a number of other chemical constituents, for example fluoride
(an element commonly found in association with phosphate). Beginning in the
mid-1970's, there is a rather sudden decline in phosphorus and other chemical
constituents found in association with phosphate ore (e.g., fluoride, silica). This
decline is graphically apparent (see Figure 2-34). Concomitant declines in
fluoride and phosphorus are evidence of a change in mining practices that lead
to dramatic reductions in phosphorus (and other constituent) loading to the Alafia
River system around 1975-1980.
Unfortunately, there are no long-term records of in-stream phosphorus
concentrations at the three gage sites prior to phosphate mining in the watershed
(this is true of the upper Peace River also – see SWFWMD 2002). It is therefore
difficult to determine if current in-stream concentrations approach those that
would have been expected absent mining impacts. While there has been a
considerable decrease in loading (perhaps an order of magnitude or more),
concentrations of phosphorus (the majority of which is in the most biologically
available form, phosphate) are still high when compared to most natural stream
systems. Friedemann and Hand (1989) determined the typical ranges of various
constituents found in Florida lakes, streams and estuaries. Based on their
finding, 95% of all Florida streams exhibited total phosphorus concentrations less
than 1.99 mg/l P. The 1990 to 1999 decade mean for the Alafia River at Lithia is
2.54 mg/l P (the median is 2.40) which despite large decreases in concentration
still places it among the rivers with the highest P concentrations in the state.
Similar trends in instream phosphorus concentrations are also evident in the
Peace River (SWFWMD 2002). As with the Alafia River, these elevated levels
were also attributable to past mining practices and discharges from related
chemical processing plants. Similarly to the Alafia River, dramatic decreases in
P concentration also occurred in the Peace River (Figure 2-35, although they
occurred a few years later (around the mid-1980s) than at Lithia. Like the Alafia
River, other constituents found in association with phosphate also followed
similar trends in concentration (e.g., fluoride – see Appendix) and similar declines
as phosphate mining practices changed.
2-70