Impact of Phosphate Mining on Streamflow
The impact of the phosphate mining industry on streamflow has long been in question in the State of Florida. The Florida Institute of Phosphate Research funded this study in an attempt to resolve this question. The proposal initially called for comparing land use changes to resulting streamflow changes in two basins of the Peace River watershed. These were the Payne Creek and Joshua Creek drainage basins. Close to seventy percent (70%) of the total surface area in the Payne Creek Basin has been affected by phosphate mining. No phosphate mining has taken place or is scheduled to occur in the Joshua Creek Basin, in which thirty percent (30%) of the land is used for irrigated agriculture. Initially, satellite imagery and aerial photographs were used to create a seasonal database of land use changes over the time period of 1985-2000 for use in a water budget spreadsheet model. The measured land-use elements were multiplied with several Evaporation/Evapotranspiration (ET/EV) loss factors. The range of land uses during the mining processes and subsequent reclamation yielded an element of uncertainty when trying to discern the actual runoff, ET/EV losses, drainage basin storage, and imports and exports. The modeling results of predicted seasonal and annual streamflow correlated reasonably well with the actual measured streamflow. The scatter diagram sum of least squares (R2) of model-predicted and measured streamflow values ranged from 70% to 80%. This spreadsheet model approach confirmed the general trends but did not offer a clearly defined quantitative definition of the increased contribution of mined/reclaimed phosphate lands to streamflow.
To obtain a more definitive quantitative definition, the study was significantly expanded to include an exhaustive analysis of the correlation between measured rainfall and measured streamflow. This part of the study is based on the assumption that the record of streamflow measurements at the exit of a basin provides the best indicator of the overall hydrologic conditions in that basin. For this approach, the study looked at the relationships between streamflow and rain from measured stations in the entire Southern Water Use Caution Area, using double mass curves and regression analyses for the time period of 1980-2000. Data from 1932 through 2000 was used for the Peace River Basin.
The final results of the land-use model and detailed streamflow analyses indicate that the mined basins do, in fact, increase overall streamflow. The analyses also indicate that flood-flows from mined basins are reduced by mining operations while median and base-flows are significantly increased.