THE RELATIONSHIP OF HYDRAULIC GRADIENT AND GROUNDWATER FLOW IN CONSTRUCTED WETLANDS

Constructed wetlands have been used to remove nutrients from agricultural tile drainage. Nutrient removal processes in the surface flow has been extensively studied, however. Less focus has been placed on the effect of groundwater flow on the nutrient cycling. The wetlands of the study area are divided into three cells located at a demonstration farm in Lexington, Illinois. The objective of the study was to determine if there is a relationship between groundwater gradient and net groundwater flow through the wetlands. To find this relationship, data was collected for both the water levels for 14 wells around the wetlands and the net groundwater flow in each wetland cell. Water levels of the wells were measured both manually and automatically through the use of pressure transducers. The groundwater tile inflow and surface outflow were continuously monitored using a weir. The resulting data shows the net flow of the groundwater ranges from -8.22 to 1 L/s, but remains relatively constant with an average flow of -0.36 L/s, and the hydraulic gradient adjacent to the wetlands ranges from .1 to .01. It was expected that expect there to be a proportional relationship where as the gradient increases so does the net flow. Analysis of the data shows that the net flow of the groundwater remains constant even as the gradient changes, and is therefore independent of gradient. This indicates that there is another factor(s) controlling groundwater flow. To account for the net flow remaining constant despite the changing gradient, more data must be collected. Seepage meters placed along a cross section in the three cells of the wetland will provide data on the location and rat of groundwater inflow and outflow. Wells placed beneath the wetland will provide data on the hydraulic gradient of the groundwater exiting the wetland. In addition, calculations using data from a tracer test conducted at the wetlands provides information about the quantity of groundwater flow into and out of each of the three cells. This data should determine if the steady net flow is the result of a change in the cross sectional area for groundwater inflow and outflow, or if the input and output of groundwater itself changes proportionately.