Effects of lowering interior canal stages on salt-water intrusion into the shallow aquifer in southeast Palm Beach County, Florida

Abstract

Land in southeast Palm Beach County is undergoing a large-scale change in use, from agricultural to residential. To accommodate residential use, a proposal has been made by developers to the Board of the Lake Worth Drainage District to lower the canal stages in the interior part of the area undergoing change. This report documents one of the possible effects of such lowering. Of particular interest to the Board was whether the lower canal stages would cause an increase in salt-water intrusion into the shallow aquifer along the coast. The two main tools used in the investigation were a digital model for aquifer evaluation and an analytical technique for predicting the movement of the salt-water front in response to a change of ground-water flow into the ocean. The method of investigation consisted of developing a digital ground-water flow model for three east-west test strips. They pass through the northern half of municipal well fields in Lake Worth, Delray Beach, and Boca Raton. The strips were first modeled with no change in interior canal stages. Then they were modeled with a change in canal stages of 2 to 4 feet (0.6 to 1.6 metres). Also, two land development schemes were tested. One was for a continuation of the present level of land development, simulated by continuing the present pumpage rates. The second scheme was for land development to continue until the maximum allowable densities were reached, simulated by increasing the pumping rates. The results of the test runs for an east-west strip through Lake Worth show that lowering part of the interior canal water levels 3 feet (1.0 metre), as done in 1961, does not affect the aquifer head or salt-water intrusion along the coastal area of Lake Worth. As a result, no effect in the coastal area would be expected as a result of canal stage lowering in other, interior parts of the study area. Results from the other test runs show that lowering interior canal water levels by as much as 4 feet (1.2 metres) would result in some salt-water intrusion for either land development scheme. Salt-water intrusion is dependent on the location, and amount of water withdrawn, from well fields.