TESTING FOR EVIDENCE OF SUBSURFACE PHOSPHORUS TRANSPORT TO A SMALL LAKE (LAKE CARMI) IN NORTHWESTERN VERMONT, USA

Excess phosphorous (P) and nitrogen derived from agricultural and residential landscapes are considered drivers of lake eutrophication, especially in rural areas lacking industrial or municipal wastewater inputs. For several decades, Lake Carmi -- which is surrounded by agricultural land and dense shoreline development -- has experienced severe eutrophication. It is identified as impaired by the Vermont DEC under Section 303(d) of the Clean Water Act due to high P concentrations, which led to the preparation of a TMDL to limit P transport to the lake. Despite implementation of remediation strategies targeting surface water transport of P (mainly reduced fertilizer application and transition from crops to hay fields), eutrophication continues to plague Lake Carmi. Prior research elsewhere has shown that groundwater can be a significant deliverer of P to lakes, raising the possibility that groundwater flow is a source of P to Lake Carmi. The goal of this project is to test the hypothesis that groundwater plays a significant role in P transport to Lake Carmi. When sorption sites on mineral surfaces become saturated with P species, any additional P may not be sorbed, enabling P to move more freely as a dissolved or colloidal mobile phase. To assess the potential role of groundwater in P transport, this report focuses on P concentrations in shallow aquifer sediments obtained from monitoring wells installed into the saturated zone surrounding Lake Carmi.

Analysis of extractable P in vadose and saturated zone sediments indicates high amounts of environmentally-available P (651 ± 93 mg/kg, modified Mehlich extraction). The extractable P displays a negative correlation with mean grain size, reflecting the higher adsorption potential of P to clays, and implying that low sorption capacity of sandy layers in the subsurface may foster P transport. While the source of this available P is unknown, the apparent abundance of available P in the subsurface indicates the potential for shallow groundwater flow as an ongoing mechanism of P transport into Lake Carmi. To further test this possibility, mobile P will be quantified from groundwater samples of shallow (sediment) and deep (fractured rock) wells surrounding the lake, and P flux to the lake via subsurface flow will be considered within a hydrogeological model for the watershed.