Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Hillsborough County, Florida

Transcription

1 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Hillsborough County, Florida Southwest Florida Water Management District Resource Evaluation June 2012

2 Cover Photo: Photo-montage: Strawberries under frost conditions (top left); a SWFWMD hydrologist stands next to a sinkhole formed from intensive groundwater withdrawals (top right); and a Dover-area strawberry field at harvest (bottom).

3 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Hillsborough County, Florida By Robert O. Peterson and James O. Rumbaugh, III June 2012 Southwest Florida Water Management District Resource Evaluation

4 Southwest Florida Water Management District Water Resources Bureau Ken Herd, P.E., Bureau Chief Resource Evaluation Section Jerry Mallams, P.G., Manager Southwest Florida Water Management District 2379 Broad Street Brooksville, FL For ordering information: World Wide Web: Telephone: For more information on the Southwest Florida Water Management District and its mission to manage and protect water and related resources: World Wide Web: Telephone: The Southwest Florida Water Management District (District) does not discriminate upon the basis of any individual s disability status. This non-discrimination policy involves every aspect of the District s functions, including one s access to, participation, employment, or treatment in its programs or activities. Anyone requiring reasonable accommodation as provided for in the Americans With Disabilities Act should contact the Resource Data and Restoration Department, at (813) , extension 2113 or 1 (800) (Florida), extension 2113; TDD ONLY 1 (800) (Florida); FAX (813) Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the Southwest Florida Water Management District. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. Suggested citation: Peterson, R.O. and Rumbaugh III, J.O., June 2012, Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Hillsborough County, Florida

5 The hydrogeologic evaluations and interpretations contained in Hydrologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Hillsborough County, Florida have been prepared by licensed Professional Geologists in the State of Florida in accordance with Chapter 492, Florida Statutes. Robert O. Peterson Professional Geologist State of Florida License No. PG1408 Date: May 30, 2012 James O. Rumbaugh, III Professional Geologist State of Florida License No. PG Date: May 21, 2012

6 iv Acknowledgments The authors acknowledge the significant contributions and assistance provided by Leigh Vershowske, Justin Leech, Martha Butterworth, Margit Crowell, and Ken Romie in data acquisition, formatting and GIS analysis. Special recognition is given to Darrin Herbst and Michelle Maxey in the Water Use Permitting and Regulatory Support Bureaus for their assistance in providing data regarding the well mitigation program as well as editorial remarks. The Florida Geological Survey is recognized for their valuable contribution of sinkhole data recorded during the January 2010 freeze event. The authors would also like to give special recognition to Mark Barcelo, Ken Weber, Ron Basso, Angel Martin and Tamera McBride for their valuable technical contribution and constructive technical review comments.

7 v Contents Introduction... 1 Purpose and Scope... 3 History of Freeze Protection in the Dover/Plant City Study Area Previous Studies... 6 Physiographic Setting of the Dover/Plant City Study Area Hydrogeologic Framework... 6 Water Use Water-Level Fluctuation and Trends.. 16 Analysis of Historical Freezes and the 2010 Freeze Event.. 21 Water Resource Impacts Dry Wells Sinkholes Groundwater-Flow Modeling Overview of District Response to the 2010 Freeze Event Summary and Conclusions References Appendix A. Water-level hydrographs and trends for monitor sites Appendix B. Time series, box plots and correlation analysis results Appendix C. Water level drawdown maps for freeze events Appendix D. Well construction specifications for domestic/irrigation wells Appendix E. Photographs of sinkhole damage Appendix F. Calibration statistics, modeling parameters and calibration results Figures 1. Map showing the Dover/Plant City study area, Hillsborough and Polk Counties, Florida Graph showing the consecutive-day periods (35 years) with minimum temperature below 34 F In the Dover/Plant City study area Maps showing: 3. Dover/Plant City study area strawberries acreage in 1970 and Dover/Plant City study area citrus acreage in 1970 and Diagram showing the stratigraphy at Regional Observation Monitoring-well Program (ROMP) DV-1 monitoring site Map showing reported top of limestone in the Dover/Plant City study area Diagram of a cross section showing a 100-foot collapse of the Pleistocene base at site no. 2 cross section in and near the Dover/Plant City study area Map showing the potentiometric surface of the Upper Floridan aquifer measured during September 2010, in and near the Dover/Plant City study area Map showing stratigraphic cross sections in and near the Dover/Plant City study area Diagram showing geologic/hydrogeologic cross sections in the Dover/Plant City study area Graph showing a comparison of estimated water use for citrus, strawberries, and public supply in the Dover/Plant City study area,

8 vi 12. Graph showing the historical trends of major public supply groundwater use in the vicinity of the Dover/Plant City study area Graph showing groundwater use at the MOSAIC Hopewell Mine, Hillsborough County, Maps showing: 14. Location of groundwater withdrawals for freeze protection in and near the Dover/Plant City study area Water-level monitor sites for the Upper Floridan aquifer in and near the Dove/Plant City study area Graph showing linear trend analysis of the 10-year recorded maximum and minimum water levels of the Upper Floridan aquifer at monitoring site Tampa 15 (Deep) Graph showing standardized water levels for the Upper Floridan aquifer at monitoring sites ROMP 60 (Ocala/Avon Park) and Tampa 15 (Deep), Graph showing water levels and hourly minimum temperature recorded at monitor site DV-1 (Suwannee), located within the Dover/Plant City study area, January 3 21, Graphs showing comparison of historical drawdown at monitor site Tampa 15 (Deep and reported impacts in the Dover Plant City study area resulting from freeze protection groundwater withdrawals Maps showing: 20. Location of dry well complaints reported between January 3, 2010, and February 5, 2010, in and near the Dover/Plant City study area Location of sinkhole events reported between January 3, 2010, and February 5, 2010, in and near the Dover/Plant City study area Graph showing reported casing depths for domestic wells in the Dove/Plant City study area Map showing minimum casing depth requirements for well construction (stipulation areas) in the Dover/Plant City study area Diagram showing an example of the Critical Operating Head for shallow water-supply wells Graph showing a comparison of well complaints and daily temperature for the January 2010 freeze event in the Dover/Plant City study area, January 1 February 5, Graph showing a comparison of sinkhole events and daily temperature records for the January 2010 freeze event in the Dover/Plant City study area January 1 February 5, Diagram showing water-level drawdown and stratigraphy at ROMP site DV-1, located within the Dover/Plant City study area Map showing the grid used for groundwater-model simulation for the January, 2010 freeze event in the Dover/Plant City study area Graph of the water-level hydrograph of monitor site DV-1 (Suwannee) showing drawdown (DD) in feet (ft) resulting from estimated pumping rates during the January 2010 freeze event in the Dover/Plant City study area Map showing water-level target sites for groundwater-flow modeling of the January 2010 freeze event in the Dover/Plant City study area Graph showing observed and simulated drawdown in the Upper Floridan aquifer (model layer 4) in the Dover/Plant City study area Maps showing: 32. Simulated drawdown in the Upper Floridan aquifer (model layer 4) on January 11, 2010 (stress period 11), in and near the Dover/Plant City study area...37

9 vii 33. Location of well complaints, Florida Geological Survey (FGS) reported sinkholes, and the Minimum Aquifer Level Protection Zone (observed 30-foot drawdown contour) in and near the Dover/Plant City study area Graph showing simulated water levels in the Upper Floridan aquifer (model layer 4) at monitor site DV-1 (Suwannee) with selected pumping reductions in the Dover/Plant City study area, January 4 19, Tables 1. Permitted water use within the Dover/Plant City study area as of January Results of Upper Floridan aquifer monitoring sites correlation analysis using the Sen-Theil Kendall test Wells used to monitor historical freeze events in the Dover/Plant City study area Results of well mitigation procedures for the January 2010 event in and near the Dover/Plant City study area...30

10 viii Conversion Factors and Datums Multiply By To obtain Length inch (in.) 2.54 centimeter (cm) inch (in.) 25.4 millimeter (mm) foot (ft) meter (m) mile (mi) kilometer (km) Area square mile (mi 2 ) 2.59 square kilometer (km 2 ) Volume gallon (gal) liter (L) gallon (gal) cubic meter (m 3 ) Flow Rate gallon per minute (gal/min) liter per second (L/s) Hydraulic Conductivity foot per day (ft/d) meter per day (m/d) Transmissivity* foot squared per day (ft 2 /d) meter squared per day (m 2 /d) Leakance foot per day per foot [(ft/d)/ft] 1 meter per day per meter (m/d/m) Temperature in degrees Celsius ( C) may be converted to degrees Fahrenheit ( F) as follows: F=(1.8x C)+32 Temperature in degrees Fahrenheit ( F) may be converted to degrees Celsius ( C) as follows: C=( F-32)/1.8 Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88) and the National Geodetic Vertical Datum of 1929 (NGVD 29). Elevation, as used in this report, refers to distance above the vertical datum. *Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft 3 /d)/ft 2 ]ft. In this report, the mathematically reduced form, foot squared per day (ft 2 /d), is used for convenience. **Leakance: The standard unit for leakance is foot per day divided foot of aquifer thickness (ft/day)/ft. In this report, the mathematically reduced form, 1 over day (1/ day), is used for convenience.

11 ix Acronyms and Abbreviations COH District DWRM FGS ft ft/day gpm IFAS L LOWESS MAL MALPZ mgd NASS ROMP S T USDA USGS WMIS WUCA Critical Operating Head Southwest Florida Water Management District District-Wide Regulation Model Florida Geological Survey feet feet per day gallons per minute Institute of Food and Agricultural Sciences leakance Locally-weighted regression Minimum Aquifer Level Minimum Aquifer Level Protection Zone million gallons per day National Agricultural Statistics Service Regional Observation and Monitor-well Program storativity transmissivity United States Department of Agriculture United States Geological Survey Water Management Information System Water Use Caution Area

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13 Introduction 1 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Hillsborough County, Florida By Robert O. Peterson 1 and James O. Rumbaugh, III 2 Introduction Periodic freezing temperatures in west-central Florida are a naturally occurring seasonal weather pattern that has the potential to cause substantial economic impact to agriculture and the residents of Florida. In January 2010, nine nights of freezing temperatures occurred over a period of 11 consecutive days in the Dover/Plant City area of Hillsborough County, Florida. This area is within the jurisdiction of the Southwest Florida Water Management District (District), which is charged with managing the area s water resources. Permitted agricultural water users growing strawberries, citrus, blueberries, nursery ornamentals, as well as tropical fish farms were at risk of experiencing physical damage and total crop loss as a result of the prolonged freeze event. Permittees with temperature sensitive crops predominantly use groundwater and surface water to provide for freeze protection during these events. Groundwater is preferred because of its relatively consistent temperature of approximately 75 F in this area and the fact that there is minimal concern regarding bacteria or pathogens exposure that would impact food safety or cause crop damage. Concentrated pumping operations are also used for row crop bed preparation and crop establishment in the early growth stage of plant development, which requires a large volume of irrigation water to be applied during the months of September and October for a period of approximately 30 days. However, these irrigation activities and associated impacts are not the primary focus of this study. Significant impacts to existing legal users (shallow wells) and property damage resulting from sinkholes is a well documented occurrence from the cumulative impacts of groundwater pumping. Concentrated, large volume, groundwater withdrawals located in the vicinity of domestic wells with shallow-set pumps have the potential to cause the Upper Floridan aquifer water levels to decline rapidly, lowering the water level below the pump intakes during crop establishment and freeze protection. Additionally, rapid water-level declines have the potential to dewater the shallow upper limestone units, resulting in reduced upward supporting pressure (force) on overlying confining units and increased flow by seepage through voids promoting the potential for sinkhole activity. One area of appreciable drawdown from groundwater withdrawal for crop establishment and freeze protection is located within a 256 mi 2 study area of Dover/ Plant City, Hillsborough County, Florida (fig. 1). Impacts have been documented in the area of Dover/Plant City since the 1970 s, coincident with crop establishment and freeze protection events. The January 2010 freeze event is considered to be the most extreme period of consecutive freezes experienced in the Dover/Plant City area in close to 35 years of record (fig. 2). 1 Southwest Florida Water Management District, Resource Evaluation Section, SWFWMD, 2379 Broad Street, Brooksville, Florida; Robert.Peterson@watermatters.org 2 Environmental Simulations, Inc., 300 Mountaintop Road, Reinholds, Pennsylvania; JRumbaugh@groundwatermodels.com

14 2 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida 82 20'0"W 82 10'0"W 82 0'0"W ZEPHYRHILLS '0"N TAMPA PASCO HILLSBOROUGH Area of Enlargment Dover/Plant City WUCA TEMPLE TERRACE '0"N PLANT CITY LAKELAND POLK PKY TAMPA 75 POLK MULBERRY UV '0"N Legend Dover/Plant City Study Area Miles Base from Southwest Florida Water Managment District State Plane Coordinates, Datum NAD 83 Figure 1. Dover/Plant City study area (256 mi 2 ), Hillsborough and Polk Counties, Florida.

15 Purpose and Scope 3 Number of Consecu8ve Days Below 34 0 F January 2010 Event Consecu8ve- Day Period 1 Figure 2. Consecutive-day periods (35 years) with minimum temperature below 34 F in the Dover/Plant City area (Source Plant City National Weather Station). Purpose and Scope The purpose of this report is to characterize the hydrogeology of the Dover/Plant City study area, document the occurrence and hydrologic consequences of the January 2010 freeze event, provide a comparison to historical documented freeze events in the same area, and discuss the District s adopted water-management strategies to minimize the future impacts from withdrawals for freeze protection. Data collected within and surrounding the area of concentrated withdrawals, along with groundwater-flow modeling were used to characterize the pumping and resulting aquifer water level fluctuations that occurred historically. Published information was reviewed and analyzed to characterize the distribution and magnitude of historical pumping and documented waterresource impacts and to compare these data with the results of the January 2010 event. Ultimately, the District has the mission to develop and manage rules for water users to minimize the potential for permitted water use to cause adverse impacts to existing legal withdrawals, environmental resources, and personal property. Following the January 2010 freeze event, the District conducted months of technical analysis culminating in the development of rules that went into effect on June 16, These rules include the establishment of the Dover/Plant City WUCA, a Minimum Aquifer Level (MAL) and associated Minimum Aquifer Level Protection Zone (MALPZ) under Chapters 40D-8, 40D-80 (SWFWMD, 2011), and Part B, Chapter 7 of the Basis of Review, Water Use Permit Information Manual (SWFWMD, 2011). History of Agricultural Irrigation for Freeze Protection in the Dover/Plant City Area The distribution of crops and methods used for freeze protection of temperature sensitive fruiting plants, nursery ornamentals, and tropical fish farms have varied substantially over the past 50 years within the Dover/Plant City area in Florida. Historically, the location of citrus and other cold sensitive crops were based on land surface slope and elevation,

16 4 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida and the orientation of crops to large bodies of water (ocean, estuaries, lakes, and canals) (Bill et. al. 1979; Johnson, 1963). Locating adjacent to water bodies is important because they provide a source of heat to regulate air temperature in the surrounding area. As heat rises, low lying areas and slopes in topography will confine and convey colder temperatures. Two types of freezes occur in Florida: advective freezes and radiation freezes. Advective freezes result when cold air masses move rapidly down from the north with cold fronts causing rapid temperature drops, and typically, windy conditions. Radiation freezes and/or frost occur on clear, calm nights where latent heat is rapidly radiated to the atmosphere from surfaces and objects. Temperatures during advective freezes can be much lower than radiation freezes, last longer, and are more difficult to address in terms of crop protection, primarily due to windy conditions. Both advective and radiation freezes may result during different periods of the same cold front and require different freeze protection techniques for varying landscapes, crop types, and crop orientations. From the late 1800s through the early 20th century, citrus growers used small fires to protect their trees and fruit from freezing while strawberries were protected with straw and pine needles. In the mid 20th century, prior to the 1970s, methods of freeze protection used by growers included oil burning heaters (stack/cone smug pots kerosene/diesel burners), tire burning, windmills of various sizes and distributions, green houses, and, eventually, large volume overplant irrigation. More recently, these methods have been replaced by lower volume irrigation coupled with tail-water recovery (Casey, Regalado, and Lynne, 1993). Growers may adjust the volume of water applied throughout the growing season by changing out emitters. This is done to address variable crop water needs including crop establishment, heat stress, and freeze protection. Freeze protection using relatively warm groundwater for strawberries and other cold sensitive crops will typically be implemented when the temperature falls below 34 F and continue until the temperature rises above 34 F. Many plants and fruits are affected by the physical property when water is converted from a liquid to solid ice on or within fruit, bark 82 10'0"W 82 10'0"W 28 10'0"N PASCO HILLSBOROUGH POLK PASCO HILLSBOROUGH POLK 28 10'0"N '0"N 28 0'0"N UV 60 UV '0"N Total of approx. 1,648 acres Total of approx. 8,508 acres Miles Miles Legend Dover/Plant City study area 27 50'0"N Base from Southwest Florida Water Managment District State Plane Coordinates, Datum NAD 83 Figure 3. Dover/Plant City study area strawberries acreage in 1970 and Interpreted from 1970s black and white photography and 2010 Natural Color DOQQs (SWFWMD, 2010). Acreages outside the study area are not shown.

17 History of Agricultural Irrigation for Freeze Protection in the Dover/Plant City Area 5 and leaves. The Institute of Food and Agricultural Sciences (IFAS) recommendations for freeze protection irrigation application rates are 3,000 gallons per hour per acre for citrus and about 6,800 gallons per hour per acre for other crops (strawberries, blueberries, ornamentals) (Parsons and Bowman, 2009). Many factors go into this recommendation including costs, plant hardiness, method of freeze protection (for trees, shrubs, fruit, flowers or buds), and the capacity of the crop to utilize the heat contained in irrigation water (heat of fusion and vaporization). Similar to crops, tropical fish farms have unique water-temperature requirements for freeze protection that are addressed mostly with the use of groundwater circulation and various forms of covers over the ponds. In the Dover/Plant City area, citrus was originally the predominant crop type. However, strawberries began production in the area around 1878 (Albregts and Howard, 1984) and quickly became a significant water use. Movement of the citrus industry southward over the years to avoid freezing temperatures has been well documented (Martsolf, 1990). More recently, the Florida citrus industry has suffered from hurricane damage, disease, urban-development and competition from foreign markets, which resulted in substantial crop acreage reduction and land use transition (personal communication, Michael Sparks and the Florida Department of Agriculture, Florida s Annual Citrus Conference, 2008). Unlike citrus, the acreage of strawberries has not been displaced over time, and has grown within the Dover/Plant City study area. It is estimated that the total strawberry acreage has increased from less than 1,000 acres in 1958 to just over 8,500 acres in 2010 and that citrus has decreased by approximately 12,000 acres over the same time frame ( Source: USDA Florida Agricultural Statistics Service, Source: Hillsborough County Cooperative Extension Service and Hillsborough County Economic Development; Source: District estimate). The fluctuation of strawberry and citrus acreages from 1970 to 2010 is shown in figures 3 and '0"W 82 10'0"W PASCO POLK PASCO POLK 28 10'0"N HILLSBOROUGH HILLSBOROUGH 28 10'0"N '0"N 28 0'0"N UV 60 UV '0"N Miles Total of approx. 15,854 acres Total of approx. 4,062 acres Miles Legend Dover/Plant City study area 27 50'0"N Base from Southwest Florida Water Managment District State Plane Coordinates, Datum NAD 83 Figure 4. Dover/Plant City study area citrus acreage in 1970 and Interpreted from 1970s black and white photography and 2010 Natural Color DOQQs (SWFWMD, 2010). Acreages outside the study area not shown.

18 6 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida Previous Studies The historical impacts resulting from groundwater withdrawals in the Dover/Plant City area for freeze protection have been well documented. The earliest publication found addressing freeze protection impacts was Hall and Metcalfe (1977). This article summarizes the occurrence and impacts associated with the consecutive freezes occurring between January 17 and 22, In addition to describing the hydrogeologic setting and the groundwater drawdown impacts in the area, references are provided that address prior studies conducted for a proposed City of Tampa wellfield and other work performed by Menke, Meredith, and Wetterhall (1961). These were some of the early publications describing geology in the Dover/Plant City study area. Conclusions presented by Hall and Metcalfe state that the large volume of groundwater withdrawals associated with this freeze were found to have caused at least 22 sinkholes, substantial property damage (well failures) and inconvenience (interrupted water supply) to nearby residents. Following Hall and Metcalfe (1977), a study was conducted using remote sensing techniques (fracture trace lineament analysis) to assess the potential for sinkhole occurrences in the Dover/Plant City area (Cooper, Byron, and Degner, 1981). The focus of the report was the same event reported by Hall and Metcalfe and included additional work with the analysis of aerial photography and LANDSAT high-altitude, color-infrared imagery. Fracture-traces and lineaments were defined in the area on the basis of soil and vegetation tonal alignments as well as the alignments of known historical sinkholes. It was concluded that many of the new sinkholes documented for the 1977 freeze event appeared to be located in close proximity to mapped fracture-traces or lineaments and the intersections between these features. Additionally, it was concluded that the intersections of traces and lineaments corresponded to high risk moisture conditions in wet depressions and areas of lower topographic elevation and are prone to be at risk for sinkhole development. A study of a freeze event occurring in January 1985 was prepared by Bengtsson, Downing, and Geurink (1986). This report provides analysis of the cause and effects of the 1985 event, compares this event to prior events using water level data analysis, LANDSAT lineament analysis, and numerical groundwater-flow modeling, and presents solutions to address the impacts. The stated conclusions of this study indicate that both frost/freeze and crop establishment pumping have had appreciable impacts on the Dover/Plant City area including numerous well failures and sinkhole formation. Recommendations presented in the report to minimize future impacts that were implemented prior to the 2010 event include adoption of new well construction standards, implementation/expansion of tailwater recovery systems, and implementation of a public information campaign for notification of impending drawdown impact conditions. Physiographic Setting of the Dover/Plant City Area The majority of the 256 mi 2 Dover/Plant City study area lies within the Polk Upland physiographic province, with small portions extending into the Gulf Coastal Lowland and Zephyrhills Gap/Western Valley provinces (White, 1970). The Polk Upland is separated from the Gulf Coast Lowland by a 25 ft scarp, generally oriented north south along the west and southwestern boundary of the study area (Arthur and Rupert, 1989 and Arthur et. al., 2008). The lowlands to the south are generally associated with the North Prong of the Alafia River Basin. The portion of the Zephyrhills gap within the northern portion of the study area appears to include the upper tributaries of the Hillsborough River Basin including Pemberton Creek, Campbell Branch, Itchepackesessa Creek, East Canal, and Blackwater Creek. Land-surface elevations across the study area range from 40 and 140 ft above the National Geodetic Vertical Datum (NGVD). Lower elevations are located in the flowing surface-water flood basins, main channels and tributaries. Higher elevations are located south of Interstate 4 in the Dover and Plant City area. Hydrogeologic Framework The hydrogeology of the study area was previously described by Applin and Applin (1944), Cathcart and Peterson (1958), Carr and Alverson (1959), Menke, Meredith and Wetterhall (1961), Cathcart (1963), Hall and Medcalfe (1977), Cooper and Degner (1981), Gilboy (1985), Bengtsson et al. (1986) and Clayton (1991). Regional hydrogeology is also described by Miller (1986) and more recently by Scott et al and Arthur et al. (2008). In general, the stratigraphy of the study area from land surface starts with a thin veneer of undifferentiated quartz sand, clayey sand, sandy clay with organics and peat of Holocene or Pleistocene origin, ranging from 10 to over 150 ft in thickness in sand filled paleo-sinkholes. Below this unconsolidated material is the Pliocene and Miocene age Peace River Formation of the upper Hawthorn Group. The Bone-Valley Member of the Hawthorn Group is the first mappable geologic unit in the study area within 20 ft of land surface (Scott et. al, 2001). The Bone Valley Member consists of thicker units of waxy and dense colloidal clay and sandy clay with phosphate nodules (Bengtsson, downing and Geurink, 1986). The thickness of this unit as described by Metcalfe (1977) and Cooper et. al. (1981) can range from 23 to 50 ft in the area. However, in the areas of active karst and sinkhole activity, the thickness of clay confining material can be displaced by several tens of feet of sand into the upper portion of the underlying limestone unit. Within the lower Hawthorn Group is the Arcadia Formation, which is found between 50 and 83 feet below land surface (Clayton, 1991). This formation is interbedded with limestone, fine-grained dolostone,

19 Hydrogeologic Framework 7 ROMP DV-1 Figure 5. Stratigraphy at Regional Observation Monitoring-well Program (ROMP) DV-1 monitoring site (SWFWMD 2010).

20 8 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida carbonate clay, chert and abundant phosphate nodules. Below the Miocene age Hawthorn Group is approximately 3,300 ft of limestone, dolostone, and anhydrites ranging in age from the Oligocene to Paleocene Epoch of the Tertiary Period. The stratigraphy logged at the Regional Observation Monitor-well Program (ROMP) DV-1 well site (refer to figure 5) consist of the following units in ascending order; upper portion of the Avon Park Formation; Ocala limestone; Suwannee limestone; and Tampa Member of the Arcadia Formation. This log shows the geology of the study area near the maximum drawdown recorded during the January 2010 freeze event. Carr and Alverson (1959), Cathcart and Peterson (1958) Menke, Meredith and Wetterhall (1961) and Cathcart (1963) have reported that the hydrogeology of the study area is indicative of a paleo-karst region with the presence of numerous sediment-filled sinkholes that appear to date from the late Miocene to present. Drilling, well installation/pump activation and testing of aquifer hydraulic properties in the study area have been difficult because sand filled cavities are frequently encountered. As depicted in figure 5, the upper most limestone unit is the Tampa Member of the Arcadia Formation. This unit is generally the first competent geologic unit within which many of the existing shallow domestic well casings are completed. However, the top of the competent limestone is substantially variable throughout the area, ranging from -139 ft to 86 ft, NGVD. Geologic logs describing stratigraphy at ROMP sites located within the study area were used in combination with well completion logs (278 individual well completion logs) to define the relative elevation of the upper-most limestone. Results are presented in figure 6. The distribution of the top of limestone shows the large degree of variability of this surface. Corresponding well completion records for casing depths of shallow domestic wells were found to vary from less than 10 ft to over 325 ft below land surface. A large sediment filled sinkhole that was discovered along I-4, near the Tampa 15 Deep well site, was observed to contain Pleistocene aged sediments at a depth of more than 110 feet below land surface (Cathcart and Peterson, 1958, Cathcart et. al., 1959). This site was originally characterized and mapped from geologic logs collected by the US Geological Survey to define the structure contours extending down to the top of limestone or calcareous clay. A corresponding geologic cross section generated by Cathcart and Peterson, including four boring locations in and near the Dover study area, was constructed using this data (fig. 7). Boring site no. 2 on the figure shows the geology at the sinkhole location. This sinkhole is a prominent and well known feature that has reactivated on a number of occasions historically and was documented to have reactivated during the January 2010 freeze event. The Florida Department of Transportation has been required to performed site mitigation of the sinkhole area on a number of occasions, including the 2010 event, to stabilize the road foundations. It is likely that the geologic confine- ment in the area of the sinkhole provides minimal hydrologic separation between the surficial aquifer and underlying Upper Floridan aquifer (Cathcart and Peterson, 1958, Cathcart et. al., 1959). The saturated thickness of the surficial aquifer ranges between 15 and 50 ft (Bengtsson et. al., 1986) in the Dover/ Plant City study area. The principal withdrawals from the surficial aquifer are for residential irrigation and for dewatering projects related to mining and infrastructure installation. To the southeast of the study area, the intermediate aquifer system may exist locally in small lenses or permeable intervals defined principally in the Miocene age material. These permeable zones are separated by varying thicknesses of low permeability confining units. For the study area, the intermediate aquifer permeable intervals are not present and the hydrostratigraphic unit is referred to as the intermediate confining unit. The approximate northern boundary of the intermediate aquifer extends to the central portions of Polk and Hillsborough counties and does not extend into the study area (Basso, and Hood, 2005, Arthur et. al., 2008, and Knochenmus, 2006). The Floridan aquifer system within the study area consists primarily of vertically continuous sequences of carbonate rocks of generally high permeability, and is separated into two principal hydrostratigraphic units consisting of the fresh potable water of the Upper Floridan aquifer and the highly mineralized water of the Lower Floridan aquifer. The hydrostrigraphic units of the Upper Floridan aquifer from top to bottom consist of the Miocene-age Tampa Member of the Arcadia Formation, the Oligocene-age Suwannee Limestone, the Eocene-age Ocala Limestone and upper portion of the Eocene-age Avon Park Formation. Middle confining unit II, located within the middle Avon Park Formation, is composed of limestone, dolomite, and lenses or nodules of gypsum. This unit forms a tight bottom boundary of the Upper Floridan aquifer in the Dover area. Highly mineralized or saline water is found below Middle confining unit II in the Lower Floridan Aquifer. Total thickness of the Upper Floridan aquifer averages about 1,100 ft in the Dover area (Miller, 1986). Average potentiometric-surface elevations across the study area in 2010 ranged from about 10 feet NGVD near the southwestern corner to over 100 feet NGVD in the northeastern corner (Ortiz, 2011). A persistent hydrologic divide crosses through the study area with a northeast to southwest orientation of declining water-level elevation (see figure 8). This divide generally marks the boundary between the Central West-Central Florida Groundwater Basin and the Southern West-Central Florida Groundwater Basin. Local groundwater flow is generally to the northwest and south away from the divide to areas of discharge along the Hillsborough River and Alafia River surface-water basins. However, it should be noted that withdrawals on one side of the divide can affect the

21 Hydrogeologic Framework '0"W 82 10'0"W 82 0'0"W 98 PASCO '0"N HILLSBOROUGH Dover/Plant City WUCA '0"N POLK PKY 75 POLK UV 60 Legend Dover/Plant City study area Limestone Top -- ROMP Sites Feet NGVD < '0"N >-50 to -25 >-25 to 0 >0 to 25 >25 to 50 >50 to 75 > Miles Limestone Top -- Well Construction Sites Feet NGVD <-75 >-75 to -50 >-50 to -25 >-25 to 0 >0 to 25 >25 to 50 >50 to 75 >75 Base from Southwest Florida Water Managment District State Plane Coordinates, Datum NAD 83 Figure 6. Reported top of limestone in the Dover/Plant City study area (SWFWMD, 2010).

22 10 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida PASCO POLK HILLSBOROUGH Dover/Plant City Study Area Site No. 4 Site No. 3 Site No. 2 4 Site No Site No. 2 Site No scale in miles 80 Site No. 3 Site No Intermediate Elevation in feet Upper Floridan Aquifer Figure 7. Cross section showing a 100-foot collapse of the Pleistocene base at site no. 2 cross section in and near the Dover/Plant City study area. -60

23 Hydrogeologic Framework '0"W 82 10'0"W 82 0'0"W PASCO '0"N HILLSBOROUGH 60 Dover/Plant City WUCA '0"N POLK PKY POLK 20 UV '0"N Legend Dover/Plant City study area Contour - September Interval 10 feet Miles Base from Southwest Florida Water Managment District State Plane Coordinates, Datum NAD 83 Figure 8. Potentiometric surface of the Upper Flordan aquifer measured during September 2010, in and near the Dover/Plant City study area. Datum is NGVD 29. Potentiometric contours from Ortiz,U.S. Geological Survey, 2011.

24 12 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida other side as this is not a physically-based hydrologic divide. The divide can be shifted as a result of significant groundwater withdrawals, affecting the potentiometric surface orientation. The Upper Floridan aquifer is the principal source of water in the SWFWMD and is used for industrial, mining, public supply, domestic use, irrigation, and brackish water desalination in coastal communities. The locations of lithostratigraphic data collection sites in the Dover/Plant City area that are summarized and reported by the Florida Geological Survey in Bulletin No. 68 (Arthur et.al., 2008), and which are used for this report to characterize the geology and hydrogeology of the study area are shown in figure 9. Two cross sections (A A and B B ) extending through the study area in an orthogonal orientation passing near the center of maximum observed drawdown at DV-1 are shown in figure 10. The Brandon Karst Terrain is a geomorphologic region in east-central Hillsborough County, west of the Dover/Plant City area consisting of thinly confined limestone units that have been heavily weathered by chemical dissolution to form karst topography (refer to figure 9 showing the Brandon Karst Terrain). This area has been characterized as having a high density of paleo-sinkholes and modern sinkholes, well defined internal drainage patterns, springs and regionally high transmissive limestone units with leaky confined aquifer conditions (Jones and Upchurch, 1993). This area of higher transmissivity and conduit flow conditions contributes to a flat orientation of the potentiometric surface observed in the Upper Floridan aquifer and has the effect of attenuating potentiometric surface drawdown resulting from pumping wells within and surrounding the area. The characteristics of this area contribute to some groundwater flow modeling difficulties to be discussed later in the report. Water Use Water use within the study area has substantially increased from the early 1970s to current. Estimated water used on an annual average basis for strawberries, citrus and metered public supply is summarized in figure 11. Data used to generate this figure are contained in the District s database and estimated water use reports ( , 2009) as well as published sources from IFAS, the Hillsborough County Cooperative Extension Service and Hillsborough County Economic Development, USGS Estimated Water Use Reports, District Estimated Water Use Reports, and the USDA/NASS Census of Agriculture. Both citrus and strawberry water-use quantities were calculated using current Agricultural Irrigation Model (AGMOD) application rates for the respective crops and the estimated historical acreage data. Figure 11 illustrates how water used for citrus has decreased to some extent in recent years whereas water use for strawberries has steadily increased along with public supply. In review of this figure, it is important to note that citrus acreage and associated water use calculations prior to 1979 were not available. However, the general trend of citrus between 1935 and 1971 was estimated by the U.S. Geological Survey (Robertson and Mills, 1974) using the assumption that the percentage of acreage irrigated increased from 10% in 1935 to close to 75% in Using this approach and the assumption that citrus irrigated acreage increased from 75% in 1971 to 100% in 1979, water use was estimated for the period extending back to Land use transition from agriculture to residential development within and surrounding the Dover/Plant City area since the early 1960s has required the development and expansion of public supply wellfield production in eastern Hillsborough County and western Polk County. The distribution and trend of major public supply use within and near the Dover/Plant City study area, including Tampa Bay Water s South Central Hillsborough Regional wellfield, the Brandon Urban Disbursed wellfield, the City of Plant City wellfield and the City of Lakeland wellfields, is shown in figure 12. Permitted average annual water use in the Dover/Plant City study area at the beginning of 2010 was 157 mgd, with 28% having been permitted for agricultural use, 33% for mining and dewatering, 34% for public supply and the remaining (5%) being divided up among industrial/commercial and recreational aesthetic uses. The detailed water-use quantities by permitted water use within the study area as of January 2010 are listed in table 1. The annual average water use for mining and dewatering uses is allocated to a single permit that contains multiple mines and processing facilities (MOSAIC Fertilizer). This permit has a large footprint covering much of western Polk County, as well as portions of Manatee, Hardee and De Soto Counties, and areas generally south of State Road 60 in Hillsborough County. Groundwater withdrawals used for mining by MOSAIC Fertilizer s nearest mine to the Study Area, the former Hopewell mine, has been reduced in recent years (fig. 13). Period of record groundwater use for phosphate beneficiation at the Hopewell mine has averaged less than 3 mgd on an annual average basis, or less than 6% of the 2010 annual average estimated water used by MOSAIC. Groundwater withdrawal quantities for agricultural freeze protection permitted at the time of the 2010 freeze event were approximately 891 mgd (504 Water Use Permits with 707 wells) with 54% allocated to strawberries, 33% to citrus, 8% to nurseries and the remaining 5% permitted to blueberries, fish farms and other uses. A map illustrating the January 2010 magnitude and distribution of permitted freeze protection withdrawal quantities located within the study area and peripheral area extending out 10 miles is provided in figure 14. Freeze protection withdrawals distributed within the study area will be used as the basis for groundwater-flow modeling to be discussed later in this report.

25 Hydrogeologic Framework '0"W 82 10'0"W 82 0'0"W 98 PASCO '0"N A HILLSBOROUGH W W-4254 B' Dover/Plant City WUCA TAMPA 15 DEEP 4 W '0"N 75 ROMP DV-1 AVPK POLK PKY W B W POLK W A' UV '0"N Legend Dover/Plant City study area Brandon Karst Terrain Cross Section Stratigraphic boring Miles Base from Southwest Florida Water Managment District State Plane Coordinates, Datum NAD 83 Figure 9. Stratigraphic cross sections in and near the Dover/Plant City study area.

26 14 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida ALTITUDE, IN FEET RELATIVE TO NGVD A Northwest 200 W TAMPA15 W W Peace River Arcadia Suwannee Ocala Avon Park Arcadia A' Southeast LEGEND Intermediate Intermediate Aquifer Upper Floridan Aquifer W Extends to 5,000 feet ALTITUDE, IN FEET RELATIVE TO NGVD B B' Southwest Northeast 200 W W W W-30 W ,000 (5) 40,000 (10) 60,000 (15) 80, ,000 (20) Peace River Arcadia Suwannee Ocala Avon Park 0 20,000 (5) 40,000 (10) 60,000 (15) 80, ,000 (20) Figure 10. Geologic/hydrogeologic cross sections in the Dover/Plant City study area (SWFWMD, 2011). See figure 9 for locations of sections Water Use, In Million Gallons Per Day Citrus - Es*mated Citrus Strawberries Public Supply Figure 11. Comparison of estimated water use for citrus, strawberries, and public supply in the Dover/Plant City study area, (SWFWMD, 2011).

27 Hydrogeologic Framework 15 Groundwater Use, In Million Gallons Per Day Tampa Bay Water Brandon Urban Dispersed Wellfield Lakeland Wellfield Tampa Bay Water South Central Hillsborough Regional Wellfield Plant City Wellfield Figure 12. Historical trends of major public supply groundwater use in the vicinity of the Dover/Plant City study area, 1964 to Table 1. Permitted water use in gallons per day within the Dover/Plant City study area as of January Includes mining and dewater uses located closest to the study area. Agricultural Industrial & Commercial Mining & Dewatering Public Supply Recreational Aesthetic Mean 75, ,848 17,432,667 1,614,461 71,648 Median 41,300 10, ,000 17,000 17,700 Minimum , ,700 Maximum 1,591,000 3,050,000 51,475,000 24,100, ,500 Count Total Perm. Q 44,365,250 5,015,800 52,298,000 53,277,200 1,791,200 Total Perm. Q* NA NA 3,584,000 NA NA % of Total % of Total* NOTE: Total Perm. Q* = Permitted quantity for MOSAIC s Hopewell Mine Only % of Total* = Percentage of total, accounting for MOSAIC s Hopewell Mine Only NA = Not applicable

28 16 Hydrogeologic Impacts Observed During the January 2010 Freeze Event in Dover/Plant City, Florida 4.5 Groundwater Use, In Million Gallons Per Day Monthly Use Annual Average Use Figure 13. Groundwater use at the MOSAIC Hopewell Mine, Hillsborough County, Water-Level Fluctuations and Trends The history of potentiometric-surface level monitoring in the study area spans the timeframe from 1959 to present at Tampa 15 Deep. This well, along with others installed in the area are used to define historic water-level fluctuations over a period of 50 years. Water-levels at wells Tampa 15 Deep, ROMP DV-1 (Suwannee), ROMP DV-2 (Lower Hawthorn / Tampa), ROMP 61 (Suwannee/Avon Park), Turner Floridan, McIntosh Floridan, GG-D (TBW s Upper Floridan aquifer well), Sanlon Ranch (Upper Floridan aquifer) and (Suwannee/ Avon Park) are recorded on an hourly or 15-minute interval. These wells are important as they have the appropriate monitoring frequency to be useful for both long-term and short term resource evaluation. A map showing these well locations is provided in figure 15. Historical water-level observations for these wells are shown in figures A-1 through A-9, contained in Appendix A. Surficial aquifer water-level monitoring in the Dover/Plant City area at DV-1s and DV-2s extend from 1990 to current. Hydrographs of surficial aquifer water-level data are presented as figures A-10 and A-11, in Appendix A. Water-level data for the referenced surficial aquifer monitor sites show no apparent trends over the short period of record. Additionally, comparing the average surficial aquifer water level and the average Upper Floridan aquifer potentiometric-surface level recorded at ROMP Sites DV-1 and DV-2 (figures A-10, A-2 and A-11, A-3, respectively), there is over 50 feet of water-level difference between the two aquifers at these locations suggesting a reasonably well confined Upper Floridan aquifer. Locally-weighted regression (LOWESS) smoothed trend analyses of the aggregated daily data for all well sites are included on the graphs in Appendix A. These graphs confirm that well Tampa 15 Deep has shown a consistent declining trend for the period of record, whereas Sanlon Ranch and ROMP 60 have shown a consistent increasing trend since the early 1980s. Long-term monitoring data for the Upper Floridan aquifer at Tampa 15 Deep shows that the 10-year average potentiometric-surface level has dropped from a mean elevation of approximately 65 feet NGVD in the 1960s to approximately 57 feet from 2000 to Additionally, linear trend analyses of the 10-year recorded maximum and minimum water-level values recorded at Tampa 15 Deep indicates a declining trend (figure 16). These data and trend analyses demonstrate that there is a clear decline of long-term average potentiometric-surface levels recorded at Tampa 15