Sanibel's present drainage system has had a pronounced effect on the water-table aquifer. Since the canals and ditches are dug through very permeable sand and shell, water in the water-table aquifer flows rapidly out of the aquifer and into the adjacent canal where there is a positive gradient. This discharge from the groundwater system has increased the rate of recession during the dry season, and has caused temporary depletion of storage in the aquifer. Salt Intrusion Salt intrusion into Sanibel's ground and surface waters is a major problem - and it has worsened as development has increased. The problem is caused principally by lowering the water level in the interior basin.
Saline intrusion has important ramifications for the island's ecosystems.
Vegetation not adapted to salt water is destroyed by increases in salinity. Many freshwater wildlife species cannot tolerate higher salinities. Finally, saline intrusion into the water-table aquifer causes problems for many residents who use this water for household needs. In a barrier island environment such as Sanibel, sufficient supply of fresh water is a major problem. Under normal conditions, a shallow, unconfined aquifer, which contains fresh water, exists just below the land surface.