Publication Date

2016-05-09

Document Type

Honors Thesis

Department

Geosciences

Keywords

Deicing chemicals-Environmental aspects, Salt-Environmental aspects, Peatlands-Effect of salt on, Wetlands-Effect of salt on, Salt-Absorption and adsorption, Road-salt, Cation exchange capacity, Wetlands, Sodium adsorption ratio, Vanselow, Langmuir, Power-exchange

Abstract

Road-salt contamination has the potential to alter the geochemistry of calcareous wetlands. Sodium from road salt adsorbs to organic material, causing it to accumulate and be retained by wetlands. Increasing salinity of soil water from road salt contamination can change the adsorption capacity and the energetic preference for calcium vs. sodium on exchange sites of soil organic matter. The level of sodium adsorption should vary with the concentration of Na+ and other cations in groundwater, as has been demonstrated for clay minerals. However, the equilibrium constants for these cation exchange reactions on peat, which define the thresholds to which sodium can adsorb to peat surfaces by removing a previously adsorbed cation, are not constrained. Experiments using different ratios of sodium to calcium in solution can test the limits of sodium adsorption on exchange sites on peat. In this study, peat was reacted with solutions at various ionic strengths (I=1mM, 10mM, and 100mM) and with various ratios of sodium to calcium in solution (SAR= 1, 2.5, 5, 7, 10, 20, 40, 60). Values obtained from these experiments were used to calculate selectivity coefficients to determine energetic preference for sodium vs. calcium on the exchange sites. Peat for the exchange reactions was collected from a floating sphagnum moss peat mat at Hawley Bog (Hawley, MA) because it is remote from roads and should have low concentrations of sodium and divalent cations. Results inform reactions occurring in Kampoosa Bog, a calcareous fen that receives road-salt runoff from the Massachusetts Turnpike and where the dominant cation exchange reaction occurs between sodium and calcium, where sodium in solution replaces calcium on the exchange sites. The experimental results suggest that sodium preferentially exchanges with calcium at higher ionic strengths, however to a limit. Sodium appears to be the preferential cation on the exchange sites until it is 60% (I=10mM) to 80% (I=100mM) saturated, above which calcium then becomes energetically favored. The maximum sodium adsorption occurs when the SAR solutions have exchange coefficients less than 1. These findings suggest that Kampoosa Bog (I=8mM) has not reached a threshold for sodium retention, at only 6.2%, in the wetland system. Given persistent use of road salt that increases sodium in the wetland due to runoff from current salting practices, sodium would also be expected to increase on exchange sites, potentially threatening the integrity of the peat mat and the viability of wetland species.

Language

English

Comments

ii, 64 pages : color maps. Honors project, Smith College, 2016. Includes bibliographical references (pages 61-64)

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