EXPLORING TEMPORAL AND SPATIAL SYMPTOMS OF THE FRESHWATER SALINIZATION SYNDROME IN A RURAL TO URBAN WATERSHED (Invited Presentation)

The freshwater salinization syndrome (FSS), a concomitant watershed-scale increase in concentrations and delivery of salinity, cations, alkalinity, and trace metals over a decadal time frame, recently has been described for urbanized areas. Yet, few studies have evaluated FSS temporal and spatial trends, as well as causal factors, at the sub-watershed scale in mixed-use areas. This study addresses knowledge gaps by examining the influence of land use land cover practices and wastewater treatment plant (WWTP) effluent on major-ion and trace-metal export from a mixed-use watershed based on monthly subwatershed sampling at 13 sites, for 1 year. Despite the low percent impervious surface cover (%ISC) in the watershed (e.g., ~8%), a positive statistical relation between %ISC, Ca²⁺, Mg²⁺, and Na⁺ during baseflow conditions suggests the occurrence of road-salt driven reverse cation exchange and weathering of the built environment. Conversely, volume of upstream WWTP effluent was positively statistically correlated with Cu and Zn concentrations during baseflow conditions suggestive of contributions from corroding drinking-water infrastructure. Additionally, a positive statistical relation between %cultivated cropland and As and Pb concentrations during baseflow conditions is likely due to the persistence of As- and Pb-containing pesticides in current or former agricultural areas. A possible mechanism may be that sorbed As oxyanions are displaced to solution by abundant Cl^-, while formation of Pb-Cl complexes could enhance mobility of this metal from such areas. Notably, exceedances of the USEPA drinking water criteria for Na and Cl were observed during winter months. Finally, chloride to sulfate mass ratios were several times greater than corrosivity thresholds (~6.3 to ~7.7), which may explain delivery of Cu and Zn by WWTP effluent. Overall, the study advances our understanding of FSS-related solute delivery in mixed-use watersheds, emphasizing the complexity of trace-metal source attribution in these systems. The findings indicate the potential impacts of historical and recent urbanizing land use and FSS-related solute delivery on human health and infrastructure in mixed use watersheds, even with relatively low percentages of ISC.