DISSOLVED ORGANIC MATTER CHARACTERIZATION IN HYDROLOGICALLY-CONNECTED PRAIRIE POTHOLE WETLANDS IN NORTH DAKOTA

The U.S. Geological Survey Cottonwood Lake Study Area (CWLSA) lies within the Prairie Potholes region (PPR) in the northern Great Plains of south-central Canada and north-central United States. Pleistocene glacial till dominates the landscape, which includes millions of internally drained wetlands in depressions left behind during glacial retreat. Low-permeability groundwater flow paths interconnect the wetlands within small (km scale) sub-basins. Topographically elevated wetlands receive most of their water as direct precipitation, whereas topographically lower wetlands receive a combination of direct precipitation, overland runoff, and groundwater discharge. Transpiration and evaporation are the primary mechanisms for water loss, resulting in locally high solute concentrations, particularly in the topographically low groundwater discharge wetlands. The PPR supports significant fluxes of the greenhouse gasses CO₂, CH₄ and N₂O, but the net carbon balance is poorly quantified. The purpose of this study is to understand a portion of the carbon cycle involving the nature of dissolved organic matter. Dissolved organic carbon (DOC) concentrations in CWLSA surface waters are elevated, ranging from 18 to 55 mg C L^-1, whereas groundwater values are lower, ranging from 2 to 13 mg C L^-1. An exception is groundwater and pore water immediately adjacent to wetlands. These pore waters have DOC concentrations ranging from 20 to 42 mg C L^-1. The chemistry of the DOC varies with pH. Greater specific UV absorbance, or SUVA, values indicative of a greater aromatic content are associated with lower pH upland wetlands. Organic fractionation data show that 29 percent of the DOC pool in the discharge wetland is hydrophobic organic acids and 25 percent transphilic organic acids. These charged components of the DOM pool make a contribution to Gran titration alkalinity. Typically quantified as HCO₃^- equivalents, as much as 15 percent of the titration alkalinity is attributable to DOC in some higher DOC PPR wetlands. Thus, characterizing DOC transformations is important for understanding the overall C balance of the PPR.