Intermediate-Timescale Vertical Exchange in a Peatland and Implications for Landscape Patterning

Stream tracer tests typically provide estimates of hyporheic exchange on timescales of minutes to days, relevant for addressing questions about rapid biogeochemical cycling and microbial uptake. Estimates of longer-timescale surface-subsurface exchange are also useful, particularly for assessing potential legacy effects of water contamination or nutrient enrichment, effects of seasonal forcing on macrophyte and geomorphic dynamics, and processes such as mineral dissolution or evaporative enrichment that occur over long flow paths. Increasingly, naturally occurring tracers such as ^{3}H, ^{3}He, ^{223}Ra and ^{224}Ra, D, and ^{18}O are being combined with inverse modeling approaches to quantify these exchange processes. In the Everglades, use of 3H/3He and Ra have revealed decadal and weekly to yearly timescales of vertical exchange between surface water and the subsurface limestone aquifer, suggesting that legacy effects of phosphorus contamination due to slow exchange between the aquifer and surface-water are likely, and that shorter-timescale mixing results from precipitation and water management activities. Here we add to the picture by using profiles of Cl^{-} to quantify monthly-timescale exchange between peat porewater and surface water in geomorphically distinct zones of slightly different elevation. Resulting quantification of vertical exchange fluxes allows a better assessment of how minor spatial differences in topography in an otherwise nearly flat landscape drives fluxes in the biogeochemically reactive peat layer that may impact freshwater storage, nutrient and vegetation community dynamics, and ultimately, the geomorphic patterning of vegetation and microtopography that underlies the highly valued biodiversity and connectivity of the Everglades ridge and slough landscape. Hyporheic flow patterns suggested a significant ridge-to-slough exchange of water and reactive nutrients during seasonal rewetting but—unlike in many boreal peatlands—did not evidence a subsurface biogeochemical control on landscape patterning.