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 islands has become critical with increasing population and development in coastal 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 portion of the Florida Keys, is a heterogeneous, two-layer island with several canal networks. The Island is approximately 3 km wide and 10 km long. The upper hydrostratigraphic unit (Miami Oolite Limestone) has a hydraulic conductivity of 100 meters per day (m/day). The lower unit (Key Largo Limestone) has a hydraulic conductivity of 1200 m/day. To quantify the effects of canals on the fresh water lens of Big Pine Key, a numerical model was developed using the Dupuit and Ghyben-Herzberg assumptions. The thickness of the fresh water lens is sensitive to the location of the boundary between the upper and lower hydrostratigraphic layers. A simulation of present-day Big Pine Key, including canals, compared with predevelopment conditions shows that the total volume of the lens has decreased by 20% in response to the dredging of canals. As dredging of canals will certainly continue in the future, the numerical model was also used to investigate the types of canals that are most 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 dimensions, a canal that penetrates 2 km through the mid-section of the island reduces the volume of the fresh water lens by 7.1%. Several short canals, 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 into 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. Canals also focus ground water discharge. The three configurations of 2000 m long canals each discharge 13 to 15% of the total recharge to the island.