Sources of salt (chloride) in groundwater include atmospheric deposition, road salt, saltwater intrusion, fertilizers (potassium chloride), backflushing of water softening systems, and human waste (septic systems). Because concentrations of chloride in precipitation are low, and there are no near-surface naturally-occurring salt deposits in Maryland, elevated chloride levels in shallow groundwater in non- coastal areas usually indicate the effects of land-based human activity. Chloride concentrations greater than about 250 mg/L give drinking water a salty taste.
Chloride is a relatively conservative ion; it does not enter into oxidation-reduction reactions, does not form complexes under most groundwater conditions, is not sorbed onto mineral surfaces, and is not involved in many significant biochemical reactions. As a result, chloride is affected little by chemical reactions once it enters groundwater, and can be indicative of land use.
Road salt can have a major impact on groundwater quality. In Maryland, state-maintained roads are treated with sodium chloride, calcium chloride, or magnesium chloride, depending upon the air temperature. Historically, sodium chloride has been used most widely as a deicing agent. Adverse effects of sodium chloride include damage to roadside vegetation and aquatic life, soil compaction due to ion exchange, contamination of surface and groundwater supplies, and damage to concrete and metal structures. Calcium magnesium acetate has been promoted recently as a more environmentally benign alternative to traditional deicing salts; however, it is more expensive than inorganic deicing salts.
In some coastal areas, pumping of aquifers can cause brackish or saltwater to enter an aquifer. This can be caused by either regional pumpage or by localized conditions. Examples of known saltwater intrusion in Maryland include Kent Island (Queen Annes County), Ocean City (Worcester County), Annapolis Neck and Mayo Peninsulas (Anne Arundel County), Baltimore Harbor area, and Indian Head (Charles County). Sea-level rise associated with climate change is also a growing concern for saltwater intrusion in coastal aquifers.