Background/Objectives: Identifying and quantifying groundwater discharge (upwelling) to surface water, especially in a marine, tidally influenced, and heavily urbanized setting can be quite challenging.
The interaction between groundwater and surface water often has important implications for sediment projects where identification of groundwater discharge locations and quantification of rates are needed for successful remedial design. An initial screening assessment of available methodologies to identify and quantify discharge was conducted. This presentation focuses on the tools, techniques and analytical methods ultimately used on the Gowanus Canal Superfund Site in Brooklyn, New York to investigate groundwater discharge to the Canal for remedial design.
Approach/Activities: The approach first utilized Distributed Temperature Sensing (DTS) and Trident Probe surveys to identify potential groundwater discharge areas. Following these surveys, a quantitative approach was used to characterize groundwater flow into the Canal, focusing on higher potential areas. Methods included ultrasonic seepage meter surveys to provide high resolution specific discharge measurements across tidal cycles; vibrating wire piezometer nests for evaluating long-term vertical hydraulic gradients beneath the Canal and estimating long-term specific discharge rates; pressure transducers to monitor Canal stage and groundwater levels in the uplands; and barge-mounted sonic drilling rigs for sediment core collection and temporary well installation to obtain hydraulic conductivity data through flexible wall permeameter testing and slug testing.
Results/Lessons Learned: Investigation data were compiled and integrated to form a comprehensive understanding of groundwater discharge to the Canal. Results include: a canal-wide assessment of potential discharge locations from the DTS and Trident Probe surveys and quantified short-term discharge from seepage meter data, compilation of sediment hydraulic conductivity values from lab and field derived measurements; vertical hydraulic gradients in sediments and glacial deposits beneath the Canal; estimation of long-term specific discharge for each measurement station; estimation of discharge velocity; and canal-wide interpolation of long-term specific discharge. The compilation of methods proved to be a satisfactory suite of tools to identify and quantify discharge. Moving forward, results will be used to assist calibration of a groundwater flow model to simulate remedial design scenarios, NAPL mobility evaluation and sediment cap design, and refinement of the overall conceptual site model.