Heat as a Tracer to Estimate Dissolved Organic Carbon Flux from a Restored Wetland

Abstract

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.

Organic Carbon and Disinfection Byproduct Precursor Loads from a Constructed, Non-Tidal Wetland in California's Sacramento-San Joaquin Delta

“…These inferences are consistent with the very low vertical and horizontal hydraulic conductivities measured in the deep soil compared to the shallow soil. However, these inferences conflict with the flow paths suggested by Burow et al (2005) and Gamble et al (2003) for this same study site. Based on modeling of pore-water temperature profiles, Burow et al (2005) proposed that water seeped through the shallow soil layer to the deep soil layer and flow to the adjacent ditch was primarily through the deep soil layer.…”

“…The discrepancies between the flow paths suggested in this study and those presented in Burow et al (2005) are due in large part to the difficulties in using temperature as a tracer in peat dominated systems. Burow et al (2005) provided the best possible model with the data available at the time of publication, unfortunately the study lacked appropriate considerations for heat conduction in saturated black soils exposed to long periods of direct sun.…”

“…Groundwater flow directions are primarily lateral from the pond to the adjacent drainage ditch, and upward from the sand and silt layers into the organic soils. The broader hydrogeologic framework for the study area is described in greater detail in Burow et al (2005), Gamble et al (2003), and Deverel et al (2007 …”

Processes Affecting Agricultural Drainwater Quality and Organic Carbon Loads in California's Sacramento-San Joaquin Delta

“…Our data and data from Burow et al (2005), Fleck et al (2007), and Deverel and Rojstaczer (1996) indicate that land-management practices and hydrologic conditions that result in high groundwater levels increase DOC loads 3 to over 100 times relative to late spring, summer, and fall agricultural drainage loads. Questions remain about the long-term effects of permanent flooding on DOC and THMFP loads.…”

“…Our MayNovember load estimates for the entire island are consistent with data collected at the main island drainage pump station on Twitchell Island in 1995 ( Agricultural drainage loads are generally lower than wetland drainage loads. Using groundwater chemical data and a groundwater flow model, Burow et al (2005) estimated subsurface DOC loads from a nontidal constructed wetland on Twitchell Island as 1,233 to 3,671 g/ha-day. Fleck et al (2007) reported an annual average load of 2,566 g/ha-day.…”