I was recently asked to provide an initial estimate of the sustainable or safe yield of groundwater from the Floridan Aquifer System in the Suwannee River Water Management District (WMD). I discuss an approach to estimating safe yield in my book Silenced Springs in Chapters 3 and 9. This email reiterates that discussion and provides an estimated range of maximum average safe yield that protects natural systems first and then identifies a reasonable amount of groundwater that is available for human uses in each WMD.
This analysis is based on the paper Ron Clarke and I prepared on the water budget for the entire Floridan Aquifer System. The United States Geological Survey (USGS) prepared a pre-development Floridan Aquifer water budget by using the earliest possible maps of aquifer levels and stream flow data. Their work estimated that on average, the FAS historically discharged about 14 billion gallons per day (BGD) through springs and diffuse leakage to surface waters in Georgia, Florida, and Alabama. About 11 BGD of this discharge was to Florida’s surface waters. The USGS also estimated that this average groundwater discharge to natural systems had declined to about 13 BGD by 1980 due to pumping, with Florida’s share of this total at about 10 BGD.
The Florida Springs Institute (FSI) assembled all readily-available spring flow records and found that by the 1930s, spring and groundwater inflows to Florida’s rivers was about 10.5 BGD with the following approximate breakdown by WMD:
- Northwest Florida WMD 2.4 BGD
· St. Johns River WMD 1.3 BGD
· Suwannee River WMD 4.7 BGD
· Southwest Florida WMD 2.1 BGD
These estimates can be used, in combination with millions of dollars’ worth of WMD-funded science, to estimate safe groundwater yields. The WMDs, in their efforts to implement spring and river minimum flows and levels as required by the Water Resources Protection Act of 1973, have consistently determined that a flow reduction between 3 and 15 percent will be significantly harmful to the human use and water resource values protected under the Act. The U.S. Environmental Protection Agency staff have indicated that they believe springs are especially sensitive to flow reductions and that a safe MFL should be around a 5% reduction of average flows. Taking the existing research as a whole into consideration, the FSI has concluded that a 5% average spring flow reduction is a protective goal and that the maximum average flow reduction should be limited to less than 10%.
These percentage flow reductions, in combination with the historic spring flows by WMD listed above can be used to come up with a first approximation of groundwater safe yield by WMD as follows:
- Northwest Florida WMD 120 to 240 million gallons per day (MGD)
· St. Johns River WMD 64 to 128 MGD
· Suwannee River WMD 237 to 475 MGD
· Southwest Florida WMD 104 to 207 MGD
For a total Floridan Aquifer safe yield between 524 and 1,049 MGD. This is an estimated range for the average safe yield of groundwater pumping , not only in North Florida, but also in South Georgia where a few billion gallons per day of the recharge to the Floridan Aquifer is occurring. Current USGS groundwater pumping estimates in this area are roughly 3.5 BGD. This analysis indicates that we are pumping between 2.5 to 3 BGD too much water out of the springs and rivers than what is needed to protect their health. The effects of these excessive withdrawals are readily apparent to all who are watching the declining health of Florida’s springs.
In conclusion, the Suwannee River WMD (2010 USGS pumping estimate of about 219 MGD) is the only WMD pumping within a reasonable safe yield. The Northwest Florida WMD (about 253 MGD) is in second place, while St. Johns (979 MGD) and Southwest Florida (965 MGD) are far and away the worst offenders. This inequity between WMDs is possible because of the fact that groundwater in the Floridan Aquifer does not respect WMD boundaries.
I hope this information will help you focus your efforts to inform the public and their leaders.
Robert L. Knight, Ph.D.
Howard T. Odum Florida Springs Institute
5302 NW 156 Avenue
Gainesville, Florida 32653