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To address concerns about the effects of water-resource management practices and rising sea level on saltwater intrusion, the U.S. Geological Survey in cooperation with the Broward County Environmental Planning and Community Resilience Division, initiated a study to examine causes of saltwater intrusion and predict the effects of future alterations to the hydrologic system on salinity distribution in eastern Broward County, Florida. A three-dimensional, variable-density solute-transport model was calibrated to conditions from 1970 to 2012, the period for which data are most complete and reliable, and was used to simulate historical conditions from 1950 to 2012. These types of models are typically difficult to calibrate by matching to observed groundwater salinities because of spatial variability in aquifer properties that are unknown, and natural and anthropogenic processes that are complex and unknown; therefore, the primary goal was to reproduce major trends and locally generalized distributions of salinity in the Biscayne aquifer. The methods used in this study are relatively new, and results will provide transferable techniques for protecting groundwater resources and maximizing groundwater availability in coastal areas. The model was used to (1) evaluate the sensitivity of the salinity distribution in groundwater to sea-level rise and groundwater pumping, and (2) simulate the potential effects of increases in pumping, variable rates of sea-level rise, movement of a salinity control structure, and use of drainage recharge wells on the future distribution of salinity in the aquifer. Results from the simulation of historical conditions indicate that the model generally represents the observed greater westward extent of elevated salinity in the central part of the intruded area relative to the northern and southernmost parts of the intruded area. Results of sensitivity testing indicate that the extent of elevated salinity is most sensitive to pumping in areas where the source of saltwater is largely offshore, from the Atlantic Ocean, and is most sensitive to sea-level rise in areas where the source of salinity is downward leakage of brackish water from canals. Simulations of future scenarios indicate that increases in pumping near the existing interface may cause the interface to advance and decreases in pumping may cause it to retreat. Climatic effects, such as periods of prolonged drought or high precipitation, may augment or counteract long-term effects of changes in pumping on aquifer salinity at well fields. With increasing rates of sea-level rise, the freshwater-saltwater interface advances progressively inland, and flow-averaged salinities at well fields near the existing interface increase commensurately. Hypothetical southeastward (downstream) re-positioning of the existing G–54 salinity-control structure may prevent the interface from moving northwestward along and near the North New River canal, but beneficial effects are localized. Implementation of freshwater recharge wells in the city of Hallandale Beach may also have only a localized freshening effect in the aquifer and little appreciable effect on the freshwater-saltwater interface or on concentrations of salinity at well fields. Model accuracy and use are limited by uncertainty in the physical properties and boundary conditions of the system, uncertainty in historical and future conditions, and generali zations made in the mathematical relationships used to describe the physical processes of groundwater flow and transport. Because of these limitations, model results should be considered in relative rather than absolute terms. Nonetheless, model results do provide useful information on the relative scale of response of the system to changes in pumping distribution, sea-level rise, and mitigation activities. 2 Potential Effects of Alterations to the Hydrologic System on the Distribution of Salinity in the Biscayne Aquifer Introduction Saltwater intrusion of the Biscayne aquifer in Broward County, Florida (fig. 1), is a challenge for water-supply management. This issue is expected to persist, given current predictions of climate change, sea-level rise, and continued population growth. The Biscayne aquifer in Broward County is particularly susceptible to saltwater intrusion because of its high permeability, low hydraulic gradient, and proximity to an unlimited source of saltwater in the Atlantic Ocean. Additional factors that contribute to saltwater intrusion include groundwater pumping, rising sea level, anthropogenic alterations to the hydraulic system, and natural climatic variations. The Biscayne aquifer is a shallow, unconfined to semiconfined aquifer composed of highly transmissive limestone situated on top of a large, flat carbonate platform that composes the Florida peninsula. In Broward County, estimated transmissivity values in the surficial aquifer system, of which the Biscayne aquifer is the most transmissive unit, are as high as 900,000 square feet per day (ft2/d; Fish, 1988). The groundwater and surface-water systems are closely connected, and the groundwater system reacts quickly and markedly to precipitation. Average annual precipitation between 1981 and 2010 was about 58 inches (in.), and generally ranged from about 2 inches per month (in/mo) in December to almost 9 in/mo in June. Land surface elevations are typically less than 20 feet (ft) in eastern Broward County (appendix fig. 1–3), and topographic relief is minimal. The Biscayne aquifer is in contact with the Atlantic Ocean offshore. The onshore potentiometric head gradient drives fresh groundwater toward the Atlantic, whereas differences in fluid density force the denser saltwater from the Atlantic Ocean into the deeper parts of the aquifer, beneath overlying freshwater-bearing parts of the aquifer (fig. 2). The Biscayne aquifer is the primary source of water supply in Broward County, and groundwater withdrawals from the aquifer have steadily increased since the early 1900s. Groundwater extraction via pumping lowers groundwater levels in the aquifer, increasingly allowing saltwater to intrude the aquifer from the east. Groundwater pumping has been cited as a substantial cause of saltwater intrusion at several locations along the Atlantic coast (Lacombe and Carleton, 2002; Monti and others, 2009; Payne, 2010; Langevin and Zygnerski, 2013). Rising sea level in the Atlantic Ocean reduces the eastward potentiometric gradient in the Biscayne aquifer, thereby elevating groundwater levels between the recharge areas and the coast. The reduced onshore potentiometric gradient allows saltwater to enter the aquifer more readily. Local mean sea level has risen approximately 0.5 ft since 1950 (fig. 3). Numerous studies indicate that sea level in the North Atlantic has been rising for thousands of years; for example, Fairbanks (1989) indicates it has risen hundreds of feet in the past 18,000 years, and Kemp and others (2011) indicates it has risen nearly 10 ft in the past 2,500 years. The local hydrologic system has been substantially altered to allow urban development in southeastern Florida, including the eastern part of Broward County. An extensive network of canals has been constructed over decades to promote drainage and route excess water to the Atlantic Ocean during major precipitation events. Drainage tends to lower the water table and reduces the seaward movement of fresh groundwater. During the dry season (October to May), the canals are used to control saltwater intrusion by providing a source of freshwater that leaks into the Biscayne aquifer and maintains water-table elevations in the aquifer. Broward County faces the possibility of increased saltwater intrusion. Population growth in the county is expected to continue, potentially increasing groundwater pumping (fig. 4), reducing groundwater levels, and increasing saltwater inflow to the aquifer. Sea-level rise has been predicted to continue, potentially reducing the eastward potentiometric gradient in the Biscayne aquifer and in tidal canals, and driving additional saltwater into the aquifer. In response to rising sea level, alterations to surface-water management are being considered to adapt to increased coastal flooding during major storm events. Such alterations are likely to affect salinity distribution in the aquifer, given the strong connection between the surface-water and groundwater systems. To address these concerns, the U.S. Geological Survey (USGS) in cooperation with the Broward County Environmental Planning and Community Resilience Division, initiated a study to examine causes of saltwater intrusion and predict the potential effects of future alterations to the hydrologic system on salinity distribution in eastern Broward County. Purpose and Scope The purpose of this report is to (1) evaluate controls on the salinity distribution from 1950 to 2012, and (2) to simulate potential effects of possible future alterations to the hydrologic system on salinity distribution in the Biscayne aquifer in the southern and central parts of eastern Broward County. The simulated area extends from the area just east of the Atlantic coastline to the area just west of the major canals separating the Everglades from the urbanized part of the county, and from the C–14 basin in the north to the C–9 east and west basins in the south (fig. 1). Historical and future hydrologic conditions affecting saltwater intrusion in the northeastern part of the county were studied previously (Langevin and Zygnerski, 2013). The study period represents the timeframe during which the greatest alterations to the hydrologic system were made and for which records are available to quantify the alterations. The study period also extends 50 years into the future, during which time proposed hypothetical changes to management of the hydrologic system might occur. Of particular interest are production well-field areas that potentially will be most affected by

Potential effects of alterations to the hydrologic system on the distribution of salinity in the Biscayne aquifer in Broward County, Florida