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

2.4.2 Potassium and Trend Analysis of Selected Chemical
Constituents
One of the more interesting and unanticipated finding of the analysis of gage site
water quality data on the Peace River (SWFWMD 2002) was an apparent
increasing trend in dissolved potassium. Statistical analysis revealed that the
trend was significant and unrelated to increases or decreases in flow, indicating
an increasing rate of loading from the watershed. It was speculated that the
trend was most likely attributable to increasing fertilizer application within the
watershed and thus may have some value as an indicator of increasing
agricultural activity within the watershed.
An increasing trend in dissolved potassium is also clearly evident for the Alafia
River (Figure 2-38). To determine if the trend is statistically significant and
unrelated to flow, potassium concentration was regressed against flow and the
residuals were tested against time using the Kendall’s tau test. This nonparametric
test revealed a positive and highly significant trend of increasing
concentration and hence load (p value = 0.0000, slope = 0.00012). Other
parameters, including conductance, pH, nitrate, phosphorus, calcium, potassium,
sulfate, fluoride were also regressed against flow and their residuals determined.
Specific conductance, phosphorus, fluoride, and other parameters believed
associated with groundwater all showed statistically significant declining trends
(Table 2-9). These trends are consistent with the reduction or elimination of
discharges associated with mining when groundwater use by the industry was
especially high. As the discharge of pumped ground water is reduced, one can
expect reductions in those parameters typically associated with an increased
ground water contribution. In addition, since the withdrawn ground water is used
in the processing of the mined ore, its phosphorus concentration would be
expected to increase. When the release of this process water is reduced or
eliminated, P concentrations should be expected to decline, as should
concentrations of those parameters associated with phosphate such as fluoride.
As can be seen from Table 2-9, the pH of the river has increased. It is believed
that this is attributable to the reduction or elimination of discharged phosphate
mining related process water with its lowered pH. Results of Kendall’s tau
analysis on selected parameters from the Withlacoochee and Myakka Rivers are
included for comparison (Tables 2-10, 2-11). Very few parameters (silica on the
Withlacoochee, at Holder and pH and fluoride in the Myakka near Sarasota)
showed statistically significant (p < 0.05) declines in loading; but numerous
parameters (conductance, calcium, chloride, hardness, magnesium, potassium,
sodium and sulfate) in the Myakka River have shown significant increased
loading. This increased loading is most likely related to a significant increase in
agricultural irrigation with groundwater. The Myakka River (Table 2-11 and
related Appendix figures) offers an interesting contrast to the Alafia River where
decreased loading of a number of parameters is likely due to the elimination or
curtailment of mining related groundwater discharges.
2-76