Precision ground water sampling in coastal aquifers using a direct-push, shielded-screen well-point system

Conventional ground water sampling methods are often expensive and lack the sampling resolution required to categorize geochemical processes in surficial aquifers. Here, we describe the application of a commercially available gas vapor sampling probe to ground water sampling in shallow coastal aquifers. The system involves a small-diameter, shielded-screen well-point sampler that can provide high resolution (similar to 10 cm) to depths of 10 m or greater depending on the aquifer matrix. Examples of the system's utility are presented for two geologically contrasting environments (Waquoit Bay, Massachusetts, USA; Ubatuba, Brazil). In Waquoit Bay, the sampler allowed us to resolve a sharp saline transition zone (similar to 2 m) and the associated changes in nutrients and trace metals that were associated with biogeochemical reactions at this interface. The system also proved useful in Ubatuba, Brazil, where small-scale gradients in sediment composition and permeability controlled the ground water salinity profile.