Stochastic analyses to identify wellfield withdrawal effects on surface-water and groundwater in Miami-Dade County, Florida

Several stochastic analyses were conducted in Miami-Dade County, Florida, to evaluate the effects of
wellfield withdrawal on aquifer water levels, canal stage, and canal flow. Multiyear data for withdrawals
at four water-supply wellfields, water levels at the S-121 canal control structure and groundwater head at
a nearby monitoring well were used to determine the interrelation between wellfield withdrawals and
water levels in the canal and aquifer. A spectral analysis was performed first on the wellfield withdrawals,
showing similar patterns of fluctuations, but no well-defined seasonality. In order to compare
water-level response with withdrawals at each wellfield, the intercorrelation effects between wellfields
was removed through a ‘causal chain’ approach where the inter-wellfield correlation is used to isolate
the wellfield/water-level correlation.

Most computed correlations have magnitudes less than 5 percent, but with statistical significance
above 90 percent. Results indicate that withdrawals from the wellfields most distant from the canal had
no significant correlation to the canal levels. However the highest correlation was not at the wellfield
closest to the canal, but at the two wellfields at the intermediate distance that have higher withdrawal
rates. The hydraulic interconnectivity of the canal with the rest of the canal network, covering the study
area, allows the canal equalizes with all connected canals. This explains why proximity to a particular
canal location does not appear to be as important a factor as the withdrawal rate. Groundwater levels are
more highly correlated to a wellfield on the same side of the canal, and to pumping wells in the same
wellfield on the same side of the canal. This indicates that canals are an effective barrier and source/sink
for the groundwater. Further nonlinear correlation analysis indicates that high withdrawal rates disproportionally
affect water levels and are the predominant effect on the canal.