EXPERIMENTS INVESTIGATING NA-CA EXCHANGE ON PEAT TO UNDERSTAND ROAD SALT RETENTION IN A CALCAREOUS FEN

Kampoosa Bog (Stockbridge, MA) is a calcareous fen bordered by an interstate highway that receives high loads of road-salt runoff (NaCl) annually. Sodium from salt adsorbs on peat, occupying up to 15% of cation exchange sites. Episodes of sodium desorption have been observed during more dilute conditions associated with snowmelt, illustrating that cation exchange can be a dynamic field reaction. Therefore, a series of laboratory experiments were conducted using salt solutions to test the energetic preference for Na and Ca adsorption on peat. These tests also constrain the equilibrium constants for this cation exchange reaction. Similar experiments have been performed on different clay minerals, but little information on exchange coefficients exists for soil organic matter, particularly peat.

Peat from a remote bog (Hawley Bog, Hawley, MA, peat pH= 3.8–4.5) unaffected by road salt pollution was reacted with solutions at various ionic strengths (I = 1, 5, 10, 20, 50, and 100mM) and with various ratios of sodium and calcium in solution, as defined by the sodium adsorption ratio (SAR) where SAR=[Na][Ca]^-0.5 and [Na] and [Ca] represent concentrations of sodium and calcium in mmol L^-1. At each ionic strength, SAR values were 1, 2.5, 5, 7, 10, 20, 40, 60 (mmol L^-1)^0.5. Following equilibration between peat and SAR solutions, peat was reacted with 1M NH₄AcO at pH = 7. Experiments with SAR solutions at ionic strengths > 20mM were washed with three, 40ml aliquots of ultrapure water prior to reacting with NH₄AcO to remove entrained SAR solution. Filtered solutions were analyzed by ICP-OES for Ca, Na, Mg, K, and Al.

Selectivity coefficients calculated from the experimental results suggest that sodium preferentially exchanges with calcium at higher ionic strengths (I > 1mM), however to a limit. Sodium is the preferential cation on exchange sites until it is 60% (I = 10mM) to 80% (I = 20mM) saturated, above which calcium is energetically favored. This correlates with SAR values above 20-40 (mmol L^-1)^0.5. These findings suggest that the upper meter of peat at Kampoosa Bog, where the ionic strength of groundwater is 8mM and SAR ≈ 1, has not reached a threshold for sodium adsorption. Additional sodium adsorption would be expected if groundwater chemistry was further altered to higher ionic strength and SAR value due to accumulation of road salt.