Geosyntec was contracted by Brice Environmental Services to conduct a hydraulic study at a contaminated site under investigation for migration of petroleum hydrocarbons to a sensitive water body. Non-aqueous phase liquids remain in the subsurface in a 500-foot long plume which is approaching the Buskin River, a managed subsistence harvest river for salmon, and the coastline tidal zone which provides a habitat for a variety of shellfish, marine birds, and marine mammals.  Previous water level data have shown variation in groundwater flow direction at the Site ranging from north (directly to the Buskin River) to the northeast (directly to the ocean).  The variation was attributed to tidal effects.  Geosyntec conducted a hydraulic study to: 1) isolate major diurnal and semi-diurnal lunar and solar frequencies from water levels and calculate an average groundwater flow direction and gradient; and 2) estimate hydraulic conductivity using nearby tidal station data.

Seventeen monitoring wells and three surface water stilling pipes were used to measure water levels. Data was collected every 1 minute for 88 hours (3.5 days) using pressure transducers during one of the largest tidal swings of the year. Tidal influence was isolated using the Serfes (1991) and Godin (1966) compound averaging method and the resulting average was used to calculate site-wide flow direction and gradient.  Preliminary results suggest that there are two primary flow directions at the Site: easterly flow on the bluff within the fenced area parallel to the Buskin River and northerly flow in the estuary toward the Buskin River. Without tidal influence, it appears that there may be little interaction between the two flow regimes but, additional data may be needed to verify the presence of a groundwater divide.  Hydraulic conductivity was estimated by comparing tidal gauge data and measured water levels in each well (lag time) and the dampening of the signal in each well (tidal efficiency). The relative hydraulic conductivities show a spatial variation with the highest values located in the estuary adjacent to the Buskin River and intermediate values located east-west along a bluff. Spatial variation of hydraulic conductivity is consistent with known site geology and previous hydraulic conductivity data. Spatial variation was mapped to identify preferential flow paths for contaminant fate and transport. A robust monitoring well network, data analysis and modeling can be used to recognize spatial and temporal variations and comprehend the influence of tides and rivers on groundwater flow and, thus contaminant fate and transport in coastal zones.