Impact of Water Management on Coastal Hydrology in Southeastern Florida

Robert A. Renken, Joann Dixon, A. C. Lietz, Pamela Telis, Alyssa Dausman, Jeff Rogers, and Steven Memberg

U.S. Geological Survey, Miami, FL, USA

John Koehmstedt

U.S. Geological Survey, Reston, VA, USA

Scott Ishman

University of Southern Illinois, Carbondale, IL, USA

Richard Marella

U.S. Geological Survey, Tallahassee, FL, USA

Surface and ground water hydrology and natural ecosystems of southeastern Florida have been subjected to conflicting anthropogenic stresses, which are attributed to the development of a highly controlled water-management system designed to reclaim land for urban and agricultural development. During the first half of the 20th century, Everglades wetlands and coastal estuaries were viewed as a wasteland suitable only for drainage and development. Predevelopment surface- and ground-water systems were intensely transformed by construction of a complex system of canals, impoundments, control structures, levee systems, and many large well fields. These features were collectively designed to manage the competing needs of agriculture, urban users, and natural ecosystem areas. Water-management systems were used to control floods, sustain ecosystems, prevent overland flow from moving eastward and flooding urban and agricultural areas, maintain water levels to prevent saltwater intrusion, and provide an adequate water supply.

Miami-Dade, Broward, and Palm Beach Counties have experienced explosive population growth, increasing from less than 4,000 inhabitants in 1900 to more than 3.8 million in 2000. The use of ground water, the principal source of municipal supply, has increased considerably, from 3 well fields producing 65 Mgal/d (million gallons per day) in 1930 to more than 770 Mgal/d from 79 operating well fields in 1995. Agricultural water use increased from 505 Mgal/d in 1953 to almost 1,150 Mgal/d in 1988, but has since declined to 764 Mgal/d in 1995. This decline is partly due to the displacement of agriculture by urban growth. Present-day agricultural supplies are obtained largely from surface-water sources in Palm Beach County and ground-water sources in Miami-Dade County, whereas agricultural growers in Broward County have been largely displaced.

Before 1948, surface-water conveyance canals provided unregulated flow and were incapable of effectively transporting floodwaters. A lack of canal control structures exacerbated overdrainage of the aquifer during periods of low rainfall and drought. The Central and Southern Florida Flood Control project restructured the existing conveyance system through canal expansion, construction of protective levees and control structures, and greater management of ground-water levels within the surficial aquifer. Currently, gated canal control structures discharge excess surface water during the wet season and remain closed during the dry season to induce recharge.

Managed surface-water conveyance has been used successfully to increase ground-water levels near the coast to impede saltwater intrusion, minimize urban and agricultural flooding, and maintain lower inland ground-water levels. Measured canal stage at the coastal reaches increased during the latter half of the 20th century, whereas surface-water discharge to coastal bays and the Atlantic Ocean declined. Stage increase is presumably the result of efforts to prevent saltwater intrusion into major canals and aquifers. The decline in coastal surface-water dischargeis attributed to municipal ground-water withdrawals that induce recharge from the canal to the aquifer. The rerouting of surface water in major canals to secondary canals to elevate coastal ground-water levels also may have contributed to these declines. In contrast to coastal areas, long-term canal flow near the western margin of the urban area has remained relatively consistent (without a general increase or decrease in flow). Consistent long-term surface water flow along the western edge of urban areas and a decline in flow near the coast further supports municipal well withdrawals as a causative agent of declining coastal discharge.

Ground-water levels within the surficial aquifer respond quickly to rainfall; annual wet- to dry-season fluctuations within the aquifer can range from 2 to 8 ft. Wide seasonal variability in water levels can tend to obfuscate longer-term patterns. To better illustrate the effect of canal drainage on the water table, long-term “average condition” analyses were used to dampen storm and drought climatic events. During 1990-94, wet-season (October) and dry-season (April) inland water levels within the surficial aquifer averaged 1 to 4 ft lower than the “average” water levels experienced during 1940-44. However, coastal ground-water levels averaged 1 to 2 ft higher, thus reflecting efforts to minimize coastal saltwater intrusion as previously described.

A broad zone of diffusion defines the saltwater interface, and its position is largely a function of lateral movement of seawater from the ocean, seepage from tidal canals, and upconing of relict seawater. The predevelopment balance between freshwater and saltwater was altered considerably following construction of conveyance and drainage canals and municipal supply wells. Saltwater intrusion has been an issue of concern in southeastern Florida since the early 1930s; its effects weremost prominent in Miami-Dade and Broward Counties during the 1940s and 1950s, respectively. Canal drainage seems to have had the most widespread impact on saltwater intrusion, lowering water levels within the surficial aquifer and contributing to landward movement of the interface

The pre-1900 Biscayne Bay coastal ecosystem and salinity was different than it is today as recorded in sediment cores. Salinity increased in the early 1900s, remained stable until the early 1940s, and then increased above levels recorded in the early 1900s. The marine fossil record shows that coastal vegetation changes that have occurred. Sea grass became more abundant during the past century in the central bay and in the coastal Manatee Bay region. Increasing epiphytal and macro-algal habitat dwelling organisms indicate a change in substrate conditions. From the late 1980’s to present time, salinity has decreased slightly in Manatee Bay, and field observations in this region suggest the health of sea grasses is deteriorating.

Robert A. Renken, U.S. Geological Survey, 9100 N.W. 36th Street, Suite 107,

Miami, Florida, 33157, Phone: 305-717-5822, Fax 305-717-5801, , Hydrology and Hydrological Modeling