Combining Chemical and Physical Data to Differentiate Dissolved-Phase Groundwater Discharge to Surface Water from Local Soil/Sediment Influence


Combining Chemical and Physical Data to Differentiate Dissolved-Phase Groundwater Discharge to Surface Water from Local Soil/Sediment Influence

Laura McWilliams, Ph.D. and Helder Costa (Haley & Aldrich, Inc.)



Laura McWilliams, Ph.D.

Haley & Aldrich, Inc.

Speaker Bio:

Dr. Laura McWilliams has a B.A. in Geophysics and a B.S. in Chemistry, both from the University of California at Berkeley, and a Ph.D. in Oceanography from the MIT-Woods Hole Oceanographic Institution Joint Program. She has over 20 years of investigative experience. Her focus is on contaminated sediment sites and the adjacent uplands, which may be potential sources of historical and/or ongoing contamination. She specializes in planning and managing multidisciplinary investigations for complex upland and sediment sites and also offers experience in developing multi-component remediation and dredged material management strategies for these sites. A particular strength is the development of clear and concise conceptual site models (CSMs) to facilitate communication with agencies and other stakeholders and build consensus.


Groundwater discharge to surface water is a pathway of growing concern in evaluating the potential impacts of polycyclic aromatic hydrocarbons (PAHs) from upland sources to adjacent surface water. This presentation describes how the use of innovative investigation techniques helped alleviate concerns related to groundwater discharge from a former MGP to the adjacent San Francisco Bay. Field measurements and chemical compositional analysis were combined to 1) map and quantify the magnitude of groundwater discharge, and 2) differentiate between transport and discharge of upland PAHs via groundwater migration vs. elevated PAH concentrations in pore water resulting from contact with contaminated sediments. Two-dimensional numerical modeling was used to focus the field study on a predicted groundwater discharge zone, just below the low tide line. A Trident Probe survey followed by deployment of UltraSeep meters were successful at mapping and quantifying the zone of groundwater-driven flux through the sediments. However, chemical data (from upland groundwater, pore water, and co-located sediment) provided the crucial information required to identify the source of the PAHs measured in the discharging pore water. PAH compositional features (i.e., ratios of individual PAHs or sums of PAHs) were used to distinguish dissolved-phase PAHs from PAHs sorbed to particles and/or from DNAPL, both of which can be entrained in a groundwater or pore water sample. In addition, predictive phase partitioning was used to identify PAH sources in groundwater and in pore water samples collected along a field-delineated groundwater discharge flowpath. Pore water PAH concentrations and compositions indicated that the PAHs in pore water were primarily influenced by the contamination in the sediments with which the pore water was in direct contact, rather than a dissolved-phase plume. Regulators agreed there was no need for upland groundwater treatment or remediation, and pore water impacts would be addressed via a reactive cap design for contaminated nearshore sediments.


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