Northern Guam geospatial information server
Saltwater intrusion and contamination
Fresh water in the aquifer floats on top of salty groundwater. Saltwater in the ground is essentially depressed below the sea level by the weight of the overlying fresh water. If that weight is locally removed by rapid pumping, the saltwater beneath will equilibrate by moving upward into rocks previously occupied by fresh water. This is called saltwater intrusion. Essentially, extraction of water drops the level of fresh groundwater locally, reducing water pressure and allowing saltwater to flow inland and upward. The movement of saline water into freshwater aquifer leads to contamination of drinking water sources and other consequences. Contamination by saltwater can be avoided by not drilling wells that are too deep, not building them too close to each other, and not overpumping. Areas where freshwater is underlain by volcanic rock and not saltwater are far less likely to develop problems of this type.
In addition to the standard drilled vertical wells, there are horizontally drilled tunnel wells. They are more expensive to construct and maintain, but yield high quality water. Due to their unique engineering and the fact that water is not extracted from a single point in the aquifer, horizontal wells are less likely to experience saltwater intrusion problems.
The amount of saltwater contamination of the freshwater is measured by quantity of chloride (Cl-) ions (from dissolved salts). Best quality fresh water in the aquifer has <30 mg/L. Going from inland areas toward the coast and deeper into the bedrock, water in the aquifer increases in salinity. When it reaches 150 mg/L, the water is considered a mixture of fresh and salty. The limit beyond which water is not safe for drinking is 250 mg/L. Saltwater has 19,000 mg/L.
In addition to saltwater intrusion, water in the aquifer is threatened by contaminants originating from the land surface and carried by recharging water. Spills of sewage, leaks from septic tanks, industrial spills, materials washed off the land and carried by storm water, and runoff from agricultural areas all transport potentially harmful substances into groundwater that we drink. It is important to understand that because NGLA is a karst aquifer, great portion of its recharge and internal transport occurs through open fractures and conduits where water undergoes no filtering.