THE EFFECT OF WATER FLOW ON HYDROGEOCHEMISTRY AND PLANTS IN A CONSTRUCTED FEN

A fen, 20m(W-E)x37m(N-S), was constructed in southwestern Ohio to study the effect of ground-water discharge on the development of wetland plants. The fen received artesian ground water of constant temperature (11.2 C) distributed through a perforated PVC pipe at the north end of the fen. It was observed that the temperature in the western half was consistently 1-2 degree C cooler than the eastern half during summer months. Since the density of cattail plants was higher in the western half, it was proposed that shading effect could account for the cooler temperature. However, a faster flow rate in the western half could also explain the observed temperature contrast. This study utilizes hydrogeochemical characteristics to infer the ground-water flow in this fen.

A total of thirty 12"-piezometers were installed in five rows to monitor the hydrochemistry. The results showed that the western half had lower pH and TDS, and higher redox potential and sulfate concentration than the eastern half, while pCO2 is about the same in both sides of the fen. The lower TDS and pH in the western half, coupled with a similar pCO2 level, indicate a short residence time than the eastern half for the water/soil/plant interactions to proceed. Higher redox potential in the western half indicates higher water replenish rate from the source water. Longer residence time in the eastern half would allow more intensive uptake of sulfate by plants.

In a fen environment, ground-water flow is the major mechanism that delivers nutrients for plant growth. The nitrate concentration was about 2.5 ppm without any clear spatial distribution pattern. Orthophosphate is below detection limit of 0.02 ppm. Cattail can survive in a low nutrient environment, as long as the flux is sufficient to bring in nutrients. The distribution of cattail was consistent with a higher ground-water flow rate in the western half of the fen.