Bob Knight: Despite abundant rain, aquifer levels are trending downward
North Florida is home to a vast underground plumbing system known as the Floridan Aquifer. Comprising more than 100,000 square miles and several thousand feet in thickness, the Floridan Aquifer is like a gigantic, rock-filled bucket.
About 6 percent of the rain falling over this landscape percolates downward through the soil and into the porous limerock to replenish this hidden reservoir. The water flowing into this bucket eventually flows out through natural holes, which are Florida’s 1,000-plus artesian springs. Over the past 150 years the flow through these natural discharge points has diminished due to the installation and use of more than a million wells that extract groundwater from the same finite resource.
Springs operate by gravity. Spring flow is directly related to the difference in the height of water in the aquifer compared to the spring pool. Lower aquifer levels result in lower spring flows and vice versa.
If aquifer levels are lower than the spring outlet, then the spring reverts to a non-flowing, water-filled sinkhole. When adjacent surface water levels rise following a rainfall event, water can flow back through the spring opening into the underground aquifer.
Groundwater extraction wells consist of a metal or plastic casing, inserted in a bored hole through limerock. Water supply wells are drilled to a depth that will insure adequate water to meet a specific demand.
For example, a residential well may consist of a 2 to 6 inch well casing drilled 50 to several hundred feet into the top of the Floridan Aquifer. A well installed to supply a large agricultural center-pivot irrigation system or a public utility may have a casing diameter up to 12 inches or more, be drilled to more than 500 feet below land surface, and supply more than a million gallons per day. Water wells are generally fitted with a submersible or centrifugal pump that uses electricity or diesel fuel to pull water up to the ground surface for eventual use.
Springs and wells have similar effects on Floridan Aquifer groundwater levels. When water is removed from the aquifer, groundwater levels fall and spring flows decline. Plentiful rain raises aquifer levels due to increased recharge and less irrigation, causing spring flows to increase.
An examination of spring flows and lake levels during the past two years illustrate these hydrologic truths. Beginning in June 2017, abundant rain returned to North Florida following a moderate drought over the previous few years. By the end of 2017, after three months of high rainfall and capped by Hurricane Irma in early September, the Gainesville rain station reported a near-record annual total. In response to this abundant rainfall, aquifer levels rose, water levels in Paynes Prairie and area lakes rebounded, and spring flows increased.
However, despite these two years of above-normal rainfall, long-term aquifer levels are still trending downward and have been declining for the past 50 years. Long-term average flows in North Florida’s springs continue to be well below historic rates.
For example, flows in the Santa Fe and Ichetucknee rivers and springs remain more than 325 million gallons per day (MGD) less than historic flows during a similar historic rainfall period. This documented decline is equivalent to the combined loss of four first-magnitude springs.
In 2013, state water managers determined that the Santa Fe and Ichetucknee rivers were already being significantly harmed by a combined flow reduction of about 8 percent, or 88 MGD. When recently asked to re-evaluate their data analysis, technical staff at the Suwannee River Water Management District arrived at the same result as the Florida Springs Institute.
Over the past 20 years the combined flow reduction in these rivers is on the order of 26 percent (325 MGD), well past the point of significant harm. When challenged by Florida Springs Institute to turn this alarming trend around, District staff promised another study. In the meantime, the District’s governing board continues to issue new groundwater extraction permits.
Despite abundant rainfall, the Santa Fe and Ichetucknee rivers have lost more than one quarter of their life blood due to excessive groundwater pumping. The evidence is mounting that because of these flow reductions, these irreplaceable natural treasures are dying.
Protection and recovery of the Santa Fe and Ichetucknee rivers will only result when the public becomes adamant that state water managers face the facts and cut back on existing groundwater pumping permits.
Bob Knight is director of the Howard T. Odum Florida Springs Institute in High Springs and the author of “Silenced Springs — Moving from Tragedy to Hope.”