Heat as a tracer to estimate dissolved organic carbon flux from a restored wetland.

Heat was used as a natural tracer to characterize shallow ground water flow beneath a complex wetland system. Hydrogeologic data were combined with measured vertical temperature profiles to constrain a series of two-dimensional, transient simulations of ground water flow and heat transport using the model code SUTRA (Voss 1990). The measured seasonal temperature signal reached depths of 2.7 m beneath the pond. Hydraulic conductivity was varied in each of the layers in the model in a systematic manual calibration of the two-dimensional model to obtain the best fit to the measured temperature and hydraulic head. Results of a series of representative best-fit simulations represent a range in hydraulic conductivity values that had the best agreement between simulated and observed temperatures and that resulted in simulated pond seepage values within 1 order of magnitude of pond seepage estimated from the water budget. Resulting estimates of ground water discharge to an adjacent agricultural drainage ditch were used to estimate potential dissolved organic carbon (DOC) loads resulting from the restored wetland. Estimated DOC loads ranged from 45 to 1340 g C/(m2 year), which is higher than estimated DOC loads from surface water. In spite of the complexity in characterizing ground water flow in peat soils, using heat as a tracer provided a constrained estimate of subsurface flow from the pond to the agricultural drainage ditch.

Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon.

Specific UV absorbance (SUVA) is defined as the UV absorbance of a water sample at a given wavelength normalized for dissolved organic carbon (DOC) concentration. Our data indicate that SUVA, determined at 254 nm, is strongly correlated with percent aromaticity as determined by 13C NMR for 13 organic matter isolates obtained from a variety of aquatic environments. SUVA, therefore, is shown to be a useful parameter for estimating the dissolved aromatic carbon content in aquatic systems. Experiments involving the reactivity of DOC with chlorine and tetramethylammonium hydroxide (TMAH), however, show a wide range of reactivity for samples with similar SUVA values. These results indicate that, while SUVA measurements are good predictors of general chemical characteristics of DOC, they do not provide information about reactivity of DOC derived from different types of source materials. Sample pH, nitrate, and iron were found to influence SUVA measurements.