22-4 Hydrologic Budget for a Eutrophic Lake, Lake Carmi, Northern Vermont

Session: Advances in characterizing groundwater, surface water, and their interactions (Posters)

Presenting Author:

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Abstract:

Lake Carmi, in northern Vermont, has experienced algae blooms related to phosphorus loading.  To better understand the phosphorus budget, it is necessary to determine the hydrologic budget of the lake.  We developed a hydrologic budget that was verified using the calcium and magnesium budget for the lake.

Lake Carmi is situated in a bedrock basin that extends south of the lake towards the Missisquoi River and northward toward the Pike River.  Glacial till and glaciolacustrine deposits make up much of the surficial deposits.  A network of nine shallow groundwater-monitoring wells were installed in the surficial aquifer along the perimeter of the lake. 

Using data from nearby meteorological stations, annual direct precipitation and evaporation on the lake and its watershed were calculated using Thiessen polygons and the Thornthwaite method.  The shallow groundwater budget was determined by measuring groundwater elevation relative to the lake at each of the monitoring wells.  Hydraulic conductivity was calculated at each well using slug [JK1] tests.  The Dupuit equation was used to calculate groundwater flow to and from the lake.  Lake storage is relatively constant, thus net outflow from the lake is equal to net inflow.

Calcium and magnesium concentrations are relatively stable in the lake, so loading to the lake must be equal to the export from the lake.  A Piper Plot of surface water, groundwater, and lake water showed that lake chemistry is a mixture of 80-85% surface water and 15-20% groundwater.  Based on the Piper Plot analysis, surface water inflow to the lake is 4-5.6 times groundwater inflow.  Assuming water flowing from the lake has the same concentration of calcium and magnesium as the lake, we calculated the water outflow necessary to export calcium and magnesium at a rate sufficient to maintain the mass balance in the lake.  The net water outflow from the lake is 0.2-0.3 m³/s.

The one surface outflow at the northern end of the lake is insufficient to account for this outflow.  The majority of the outflow is therefore likely occurring as groundwater flowing from the southern end of the lake through surficial deposits filling the bedrock basin to the Missisquoi River.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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