SOILS AS A RESERVOIR FOR ROAD SALT STORAGE AND GRADUAL RELEASE TO GROUNDWATER

Elevated Na and Cl levels from road salt frequently persist year-round in surface waters, with high concentrations months after salting generally attributed to groundwater contributions. Indeed, two Missouri karst springs with differing land use (urban and rural) that we intensively monitored for 1.5 years show elevated Na and Cl in the shallow groundwater. Moreover, first flushing behavior as late as September reveals Cl release 6 months after salting. This behavior indicates that soils may also be a reservoir for salt storage due to non-conservative Cl behavior and cation exchange processes affecting Na. Intact soil cores collected at 1, 5, and 13 m from the road at the study sites were used to assess the Na and Cl retention capacity of the soils. Cores were irrigated 10-11 times with one of three treatments: 1) deionized water (DI), 2) 2000 ppm NaCl, or 3) two NaCl irrigations followed by DI irrigations to mimic seasonal road salt runoff. Irrigation volumes represented typical spring rainfall. Leachate was analyzed for Cl, major cations, and pH, and soils were analyzed for bulk density, water content, particle size, total C, total N, organic content, extractable Cl and major cations, and cation exchange capacity. The urban soils had higher baseline Cl: they released 57-200% more Cl than the rural soils when irrigated with DI. Rural and urban soil cores irrigated with NaCl retained 49% and 63% of Cl applied during the first irrigation, respectively, and after 10 irrigations they retained 29% and 25% of the total applied Cl. The third treatment group showed an 83% and 89% drop in leachate Cl after the first DI irrigation for rural and urban cores, then a decline of 7.6 and 11.3 ppm per cm water added for the remaining 7 irrigations. Based on their recovery rates, we estimate that the rural soils could release all the applied Cl in 15 weeks, while the urban soils would only require 11 weeks of average local spring rain. In the field, these retention times could be longer due to higher road salt concentrations in melt runoff and lower rainfall than was approximated by our experiment. Nevertheless, our results indicate the importance of soils as a reservoir for salt retention. Soils release Na and Cl during rainfall events for months following road salt application, thereby elevating surface and groundwater concentrations throughout the year.