PHOSPHORUS AND HYDROLOGIC BUDGETS FOR LAKE CARMI, VERMONT

Lake Carmi is a shallow eutrophic lake in northern Vermont. Algal blooms, including cyanobacteria, have become increasingly common in the lake. The lake has a surface area of 5.6 km² with an average depth of 6 m. It is in a 31 km² basin. Lake elevation is relatively consistent, indicating no change in storage. As part of an effort to better understand the source of nutrient loading to the lake, we studied the phosphorus and hydrologic budgets of the lake during the summers of 2021 and 2023.

Nine monitoring wells were installed along the perimeter of the lake for groundwater sampling and to measure water table elevation. Hydraulic conductivity at each well was calculated using slug tests. A total station was used to level in the top of each well relative to the top of water in the lake to determine the hydraulic gradient. Hydraulic gradients indicate that the lake receives groundwater from the south. Groundwater exchange with the lake varied elsewhere along the lake perimeter. Specific discharge at each well was calculated. These data were extrapolated along the perimeter of the lake to determine that about 1050 m³/day of shallow groundwater flows into the lake. Evaporation rates are one-third to one half precipitation rates. Stream water flow to the lake is intermittent. The lake discharges to the north. Based on evaporation and precipitation rates and reasonable assumptions about surface runoff to the lake, the lake must have a net loss of water at a rate of 0.3 – 0.4 m³/sec. The nature of this output is uncertain, perhaps a deeper groundwater flow system or through the marsh at the south end to the Missisquoi River.

Phosphorus concentrations in the lake water vary seasonally, 25 µg/L in June to 50 µg/L in September. The average total phosphorus concentration of water samples collected from the monitoring wells in 2021 was 39 µg/L. This is a net flux total phosphorus of 28 gr/day PT or about 10 kg/yr. The increase in concentration over the summer requires a net phosphorus loading of 8 kg/day. This is well below estimates of phosphorus loading from shallow groundwater flow, suggesting other sources. The decrease in phosphorus concentration between September and June requires a net loss of 3 kg/day phosphorus.

We are continuing to investigate the hydrologic and phosphorus budgets of Lake Carmi to better understand the source of the phosphorus loading.