Groundwater-surface water interactions in a wetland environment

During the summer of 2002, as a part of the University of Minnesota Geology Department Summer Internship Program, I had the opportunity to work with Rebecca Clotts in a research project characterizing the wetlands of the Cottonwood Lake Area (CWLA) in Stuttsman County, central North Dakota.  For my part of the study, I considered the seasonal chemical differences in the wetlands, as well as compared the chemistry of two permanent wetlands (P1& P8) located roughly 100 ft apart.  These two wetlands are virtually identical in terms of groundwater support, geology (glacial till), and climate.  On July 11, 12, and 26, 2002, I collected samples in the field at the Cottonwood Lake Area.  Water samples were collected from various wetlands and wells to be later analyzed for dissolved inorganic carbon (DIC), total inorganic carbon (TIC), alkalinity, and trace element anions and cations.  All of the water samples were filtered through a 0.45 micron filter.  At each of the sites where water samples were collected the temperature, pH, salinity, conductivity, and total dissolved solids (TDS) were recorded.  Mud samples were also taken from each of the wetlands sampled using a Bola jar, to be picked for ostracods at a later date.  The day of field collection, alkalinity was measured though titration of 1 N HCl with a pH indicator.  At the Stable Isotope Laboratory of the University of Minnesota, DIC data was collected by extracting the DIC in the form of CO2 gas.  The CO2 was then analyzed in a mass spectrometer to determine the ratio of 13C to 12C.  Major and trace element water samples were analyzed at the Geochemistry Lab in Pillsbury Hall.  TDS was calculated from the sum of the major anions and cations.  TIC data was collected using a coulometer.  My data was added to a year of similarly collected samples.  Through the analysis of water samples collected this summer and comparison to data collected during the spring and winter, several seasonal changes in water chemistry were noticeable.  Dissolved inorganic carbon (DIC) is a good indicator of productivity, specifically photosynthesis.  The lighter isotope of carbon, 12C, is preferred for uptake during photosynthesis.  A higher ratio of 13C to 12C (higher d13C) was found in the summer than in the winter or spring.  As 12C is used during the photosynthetic processes, the d13C value increases (becomes more positive).  The data also showed seasonal differences in total dissolved solids (TDS).  TDS values are lower in the spring than in the summer.  This trend may be explained by the influx of large amounts of H2O in the form of snowmelt, diluting the wetlands.  Chemical differences between the wetlands P1 and P8 were also noticeable.  P1 has higher DIC values (in July P1 had an average d13C value of ?3.34, whereas the average P8 value was ?8.52) and therefore seems to be more productive than P8.  DIC values are also lower in water from the wells than water from the wetlands.  This is intuitive because there is no photosynthetic activity taking place in the groundwater.  While there must be some groundwater ? surfacewater interaction, as there are dissolved solids in the wetlands which would not have come from precipitation, the TDS data suggest that they are significantly different bodies of water.  For July the average TDS value for the wetlands was 1477 mg/L, and the average value for the wells was 5661 mg/L.  This reaffirms the hypothesis that the groundwater contributes very little to the wetlands.