FRESHWATER SALINIZATION IN CARBONATE CRITICAL ZONES (Invited Presentation)

Freshwater salinization is well documented in surface waterbodies, particularly in urban and high latitude environments where road salt application and anthropogenic weathering increase major ion content in natural water. However, the mechanisms, pathways, and timescales of salinization across multiple hydrologic reservoirs in the critical zone (surface water, soil water, and groundwater) are understudied. Carbonate systems may be more vulnerable to freshwater salinization than other lithologies because congruent weathering of the bedrock leads to highly permeable and deep critical zones. We therefore interpret 25 years of Na⁺, Cl^-, base cation, and trace element data for soil, soil porewater, and groundwater and use laboratory soil experiments to understand processes that enhance salinity in carbonate critical zones. Our efforts focus on a site near Saint Louis, Missouri, that is dominated by forests (but has a few regularly salted roads), features mostly covered karst, and has a perennial spring issuing from the Plattin Limestone. Soil and soil porewater were sampled along a transect near a road, soil cores were collected for laboratory flushing experiments, and in situ and ex situ measurements of spring water chemistry were made. We observe dual retention mechanisms for Na⁺ and Cl^- in site soil. Soil porewater movement can slow ion transport to < 1.5 cm/day and is the main driver for Cl^- passage through the vadose zone. Soil cation exchange further enhances Na⁺ retention by 3 – 5 months, a process accompanied by base cation and trace element release to porewater. Deicer-related ion concentrations in soil are highest within 25 cm of the road, and ion pulses slow and attenuate with distance. Delivery of road salt contamination to the carbonate aquifer occurs on two timescales. Ions move rapidly into the karst during the winter through conduits. Over seasonal and decadal timescales, Na⁺ and Cl^- accumulate in the aquifer as water moves slowly through diffuse pathways in both the vadose and phreatic zones and when Na⁺ is retained and subsequently released via cation exchange. Our findings show that freshwater salinization in carbonate critical zones occurs both rapidly and over long timescales as contaminants like road salt move at variable rates through the system and interact with the substrate along the flow path.