This document summarizes a study of the Floridan Aquifer/Chipola River System funded by the USGS and FDEP. The objectives are to identify nutrient sources to the aquifer, characterize hydrologic transport processes using modeling, and match nitrate concentrations in springs using the model. The study area includes the Dougherty Karst Plain where the Floridan Aquifer is recharged through sinkholes and rivers. A MODFLOW model is being used to simulate nitrate tracking from recharge areas and calibrate to spring discharge concentrations. Local grid refinement is being added to improve flow path and travel time estimates in key springs.
1. The Floridan Aquifer/Chipola River
System Study
Christy Crandall
U.S. Geological Survey
Tallahassee, Florida
850 942-9500 ext. 3030
crandall@usgs.gov
Funded by: U.S. Geological Survey
National Water Quality Assessment Program (NAWQA)
and
Florida Department of Environmental Protection (FDEP)
2. STUDY OBJECTIVES
• Identify significant sources of nutrients to the Floridan aquifer
system in the lower ACF basin and in the Chipola River basin.
• Characterize hydrologic and transport processes occurring
along flowpaths from areas contributing recharge to discharge
points of interest using a ground-water flow and particle
tracking model.
• Use flow and tracking model to match nitrate concentrations in
ground water from areas contributing recharge to 6 NAWQA
trend wells, Jackson Blue Spring, Baltzell spring group, and
Sandbag Spring—springs that flow into the Chipola River.
• Use the ground-water flow and tracking model to test
hypothetical scenarios changing management practices in
using the flow and tracking model.
3. Contaminant occurrence in the Upper
Floridan aquifer and recharging Rivers
Purpose of study is to determine:
Factors affecting nitrate occurrence and distribution in the Upper
Floridan aquifer in the Dougherty Karst Plain
• Distribution of travel times from recharge to discharge
• Land use effects on nitrate concentrations
• Transport processes in ground water
• Effects of Withdrawals on flowpaths and travel times
5. • Vertically contiguous sequence of limestone and
dolostone of late Paleocene to early Miocene age
ranging from 0 to 1250 feet thick in the study area
• Sand overlying clay and limestone
• Clay lenses in places between the sand and
Limestone
• Highly potable
• Contains numerous springs, sinks and other karst
features—highly vulnerable.
Floridan Aquifer System
7. Floridan Aquifer System in the
Dougherty Karst Plain
• High rates of direct recharge through sinkholes and
indirect recharge through overburden—mostly sand and
silty sand
• High rates of discharge to large incised streams through
springs.
10. Northern Extent of
Floridan Aquifer System
•Confinement--Recharge occurs mainly in unconfined and semi-confined areas
•Potentiometric surface—flows southward to rivers from northern extent
11. • Ground water makes up the
majority of discharge during
low-flow conditions in the
Dougherty Karst Plain.
•For example at least 63
springs identified and sampled
along the Chipola River.
•(Barrios and Chellette, 2004)
12. Existing Models 2006
Models from Elliott Jones and Lynn
Torak 1996 and 2006
• MODFE Finite element transient
2-D model developed to simulate
the effects of 4000 irrigation wells
on baseflow conditiotns in the Flint
river.
14. Comparison of Model Features
MODFE Developed to simulate the
effects of irrigation on the
Flint/Apalachicola Rivers baseflow
• Steady State (Torak and
others, 1996) and then 1-
year transient (1999-
2000) (Jones and Torak,
2006)
• Variable Element Size
• 1 layer 2-D model
• 4000 Wells simulated
MODFLOW Developed to simulate
nitrate tracking and concentrations
recharging rivers
• Steady State
• Uniform cell-size (1000
m)
• 2 layer surficial/residuum,
UFA fully 3-D model
• Over 4000 Wells
simulated
•
15. MODLFOW model derived the following
starting parameters where available
from Torak and Jones
• Hydraulic parameters
• Aquifer tops and bottoms
• Pumping data
• Recharge
• Boundary conditions
• River and drain stage and conductance
• Starting heads
24. Regional Model
• UFA broken into 3 layers
• Local Grid Refinement in areas of interest
• Karst Features added throughout—
– Sinkhole
– Conduit layer
28. Local Grid Refinement
• 12 layers—
– 3 in the surficial
– 9 in the Floridan
Improves flow path accuracy and travel time
estimates
Better areas contributing recharge definition
31. Summary
• Add local grids at Balztell and Sandbag
Spring Group as well
• Finalize nitrate travel time estimates and
area contributing recharge with these
models
• Finish report
