USING SHALLOW MONITORING WELLS TO EVALUATE PHOSPHORUS INFLUX TO A SMALL EUTROPHIC LAKE IN NORTHERN VERMONT

Lake Carmi is a small (5.6 km²), shallow (<10 m) eutrophic lake in Vermont that has had persistent Cyanobacteria blooms over decades caused by P influx from external (agricultural, shoreline development, and natural) and internal (lake bottom sediments) sources. A preliminary one-year study in 2020-21 analyzed P (total) levels from groundwater from deep bedrock wells (avg =10 ppb), shallow surficial springs (avg = 20 ppb), surface water tributaries (avg =87 ppb), and Lake Carmi (avg = 50 ppb). In order to evaluate the fate and transport of P in the surficial aquifer, a parallel study was initiated in 2020 that included the installation of 9 monitoring wells along the perimeter of Lake Carmi, which have been monitored for P (total), P (dissolved), P (ortho), NO_x, NO₃^-, NO₂^-, and NH₃ and multiple other analytes on an annual basis from 2021-23. Hydraulic conductivities and gradients were also calculated for these wells using slug tests and water table-lake level surveys on an annual basis.

The monitoring wells were installed to a maximum depth of 6 meters (or refusal) by a Geoprobe hydraulic punch, screened over a 1.5 – 3.0 m interval, and developed using a submersible pump. Sediment cores were taken with a split spoon and described in detail. Groundwater samples were procured using low-flow techniques that included a peristaltic pump, plexiglass flow-through cell, and Hanna multimeter for field parameters (pH, conductivity, ORP, and temperature).

Very low N concentrations (NO_x, NO₃^-2, NO₂^-, and NH₃) near or below detection limits for 8 of the 9 wells indicate minimal anthropogenic signature, although average Cl values span from 0.5 – 119 ppm, indicating some anthropogenic input. In comparison with the shallow surficial springs from the earlier study, average P (total) concentrations in the monitoring wells were considerably higher, ranging from 42-213 ppb. P (dissolved) and P (ortho) averages varied between 22 – 97 ppb and 8 – 70 ppb, respectively. Total Carbonate Hardness (TCH) concentrations fluctuated between 129 – 501 ppm.

Preliminary analysis suggests different chemical and hydrogeological settings between wells in the northern and southern parts of Lake Carmi. We will continue to study the complex spatial and temporal trends in the surface water quality of Lake Carmi and the surrounding aquifers (groundwater).