Chemical and isotopic evaluation of water sources to the fens of South Park, Colorado

Understanding the source of water supplying high-altitude wetlands in Colorado has gained importance as water supply demands increase in the region. The fens of South Park are rare in their very high water nutrient concentrations and they support a valuable ecosystem dependent on groundwater discharge. Chemical and stable isotopic techniques are applied to determine the proportion of surface water and groundwater discharging at Tarryall Mire and Link Ditch Fen. The major ion chemistry of the fens can be evolved from either surface water or groundwater using a reasonable sequence of reactions driven by decomposition of organic matter, reduction of pH and dissolution of carbonate minerals, resulting in large increases in calcium and bicarbonate concentrations in the fens. The water chemistry alone is a poor indicator of fen-water origin because biologically mediated reactions alter the water chemistry. By contrast, the absence of geothermal exchange reactions and evaporative effects allow the stable isotopic composition of the water to be an excellent tracer in this environment. The stable isotopic data indicate that local groundwater contributes significantly to the fens, providing over three-quarters of the water. A high degree of variability in the contribution of groundwater to the fens is observed both spatially and temporally. By late summer, streams in the wetland appear to be gaining and exhibit characteristics of discharged groundwater.