EFFECTS OF SEA-WATER CANALS ON FRESH-WATER RESOURCES - AN EXAMPLE FROM BIG PINE KEY, FLORIDA

Management of the limited fresh water resources on small oceanic islan ds has become critical with increasing population and development in c oastal areas. Canals, dredged for Waterfront property and boat access, penetrate water bearing material and accelerate the natural discharge from fresh water lenses, Big Pine Key, located in the southern portio n of the Florida Keys, is a heterogeneous, two-layer island with sever al canal networks, The island is approximately 3 km wide and 10 km lon g, The upper hydrostratigraphic unit (Miami Oolite Limestone) has a hy draulic conductivity of 100 meters per day (m/day), The lower unit (Ke y Large Limestone) has a hydraulic conductivity of 1200 m/day, To quan tify the effects of canals on the fresh water lens of Big Pine Key, a numerical model was developed using the Dupuit and Ghyben-Herzberg ass umptions. The thickness of the fresh water lens is sensitive to the lo cation of the boundary between the upper and lower hydrostratigraphic layers, A simulation of present-day Big Pine Key, including canals, co mpared with predevelopment conditions shows that the total volume of t he lens has decreased by 20% in response to the dredgjng of canals. As dredging of canals will certainly continue in the future, the numeric al model was also used to investigate the types of canals that are mos t detrimental to a fresh water lens, For an island 3 km wide and 10 km long, a canal that penetrates 2 km lengthwise into the island reduces the volume of the fresh water lens by 6.5%, For the same island dimen sions, a canal that penetrates 2 km through the midsection of the isla nd reduces the volume of the fresh water lens by 7.1%. several short c anals, with a combined total length of 2 km decrease the volume of the fresh water lens by 4.0%. The deeper the penetration of the canal int o the lens, the greater the influence of the canal, Therefore, several short canals are preferred over one long canal because shorter canals have less of an effect on the total volume of the fresh water lens, C anals also focus ground water discharge. The three configurations of 2 000 m long canals each discharge 13 to 15% of the total recharge to th e island.