ASSESSING THE EFFECT OF FLUCTUATING RIVER STAGE ON AN MTBE GROUND-WATER PLUME DISCHARGING TO SURFACE WATER, PASCOAG, RI

A field study was conducted to determine short-term and seasonal impacts of major river stage fluctuations on a contaminant plume discharging to a river. Hydraulic head and volatile organic contaminants were monitored in a well transect over eight months. The hypothesis is that the hydraulic gradient changes in the near-river zone due to changing relative head relationships which then affects the distribution and concentration of contaminants discharging to the river. The results of this study have importance to the understanding of groundwater/surface water interactions and to sites where contaminated groundwater discharges to surface water bodies.

The study site is located in Pascoag, Rhode Island, where a major gasoline groundwater contamination problem occurred in Fall 2001. Since then, a contaminant plume has developed between the source zone (a former gas station) and a river 220 m down-gradient. Field data suggests that the plume, predominantly methyl tert-butyl ether (MTBE), discharges along a 520 m section of the river. The rectangular stone-lined river channel owes its origin to a former mill and is approximately 3.5 m wide and 2 m deep. Flow in the channel is regulated at a reservoir 1100 m upstream with sustained discharge ranging from nearly zero to over 90 m³/min. The channel is hydraulically connected to the aquifer and is predominantly gaining. Due to the adjustments at the reservoir and precipitation, river stage varies in the short-term by as much as 0.40 m in an hour and seasonally by 1.0 m. These river stage fluctuations are reflected within minutes in monitoring wells located 30 m from the river channel.

Hydraulic head data indicate that head relationships and gradients changed over the entire study period and on a daily basis. River stage varied over a range of 1.12 m while monitoring well head varied over a range of 2.14 m at 30 m from the channel. Contaminant concentrations and distributions also appear to be influenced by the changing relative head relationships. It appears that when river stage is higher the plume discharges closer to the source zone. When river stage is lower, discharge is diminished at this location and it is suspected that it may occur at a different, currently unidentified location.