DETERMINING THE INFLUENCE OF GROUNDWATER AND GROUNDWATER- SURFACE WATER INTERACTION ON PHOSPHOROUS LEVELS AND CYANOBACTERIA BLOOMS IN A SMALL EUTROPHIC LAKE USING A “CONTACT TRACING” APPROACH: NORTHERN VERMONT

Lake Carmi is a small (568 hectare) and shallow (<11m) eutrophic lake in northern Vermont that has had severe cyanobacteria blooms over decades from P input. Beaches are often closed during summer months. A 2008 TMDL report indicated that P is primarily delivered to the lake by tributaries and dominantly has an agricultural source. The Lake Carmi TMDL targeted P reduction from surface water sources, primarily through agricultural constraints. Since cyanobacteria blooms have persisted in spite of P mitigation measures, this study seeks to determine the extent to which groundwater contributes P to Lake Carmi.

Our methodology is to first build a 3D framework for the lake basin in GIS from surficial and bedrock mapping, and the spatial analysis of private well logs (isopach, bedrock surface contour, and static water level (SWL) maps), and then to integrate the chemical analyses of groundwater and surface water samples into this framework. Nine shallow (<6 m) monitoring wells were completed around the lake perimeter to access the surficial aquifer. P loading to Lake Carmi can be derived from external (agriculture, shoreline development, and geogenic (bedrock and surficial materials)) and internal (lake sediments) sources. To discriminate between the external sources, we used a “contact tracing” approach where we examine specific chemical tracers that may accompany P in groundwater and surface water in each sample. Tracers useful for anthropogenic activity include NO_X, NaCl, and herbicides, whereas geogenic tracers comprise major and trace elements derived from bedrock formations and surficial deposits. Stable isotope (²H and ¹⁸O) analysis of all water samples and recharge-ages of selected groundwater samples were also completed.

Preliminary results show that: 1) significant amounts of P are available in the vadose and saturated zone sediments for mobilization in groundwater; 2) By comparing monitoring well SWLs with lake level elevations, regions of inflow and outflow can be defined; 3) Trace element analysis of surface- and groundwater suggests a geogenic P source for some of the mobile P in groundwater; 4) Currently available P (total) data ranges are: 6-46 ppb for private water wells and springs, 18-418 ppb for monitoring wells, 60-127 ppb for tributaries, and 31-78 ppb for lake samples.