Trace Metals Characterization in Glauconitic Coastal Plain Soil and Associated Groundwater

Summary: Soils containing the mineral, glauconite, are common throughout the New Jersey Coastal Plain. Glauconite and associated clay minerals naturally contain elevated concentrations of arsenic and several other hazardous trace metals, along with abundant aluminum, iron and manganese. Even with “low-flow” methods, collection of groundwater samples by pumping inevitably disturbs the soil matrix surrounding a monitoring well. This often results in elevated sample turbidity and trace metal concentrations that are not representative of actual groundwater quality. During this investigation, specialized techniques were used to obtain representative groundwater samples with trace metal concentrations significantly lower than reported initially based on low-flow sampling. Statistical evaluations supplemented these efforts, linking trace metals in groundwater to natural sources in the surrounding soil.

Princeton Geoscience performed an extensive soil and groundwater remedial investigation at a site located in the New Jersey Coastal Plain. During the early stages of the project, a background study identified natural arsenic concentrations in soil at concentrations as high as 100 mg/kg. Research indicated that the arsenic was associated with glauconite, a mineral constituent of the soil matrix, and NJDEP accepted that the soil concentrations of arsenic were of natural origin. Later in the project, arsenic and several other trace metals, including lead, chromium and beryllium, plus aluminum, iron, and manganese were detected in groundwater samples, generally with the highest concentrations noted at downgradient locations.

Because the soil of the aquifer matrix was known to contain trace metals, it was evident to us that the elevated metals concentrations in groundwater samples resulted mainly from turbidity introduced into the wells during purging. When traditional techniques (i.e., low-flow purging, redevelopment of the wells) failed to produce low-turbidity samples, Princeton Geoscience developed other specialized techniques to obtain a representative characterization of natural vs. facility-related groundwater impacts. Based on results of these activities and a statistical comparison to published ranges of metal concentrations in New Jersey glauconitic soils, NJDEP conceded that concentrations of aluminum, iron and manganese were likely of natural origin, but maintained that the trace metal detections were of indeterminate or discharge related origin.

Princeton Geoscience then compared groundwater concentrations of the trace metals at the site to concentrations of the metals NJDEP had acknowledged were of natural origin and found a moderate to strong correlation for each of the trace metals in question. Although NJDEP has not revised its statement with respect to the trace metals in groundwater, no remediation has been required to address this issue and Princeton Geoscience has indicated an intention not to include metals as groundwater contaminants in the Classification Exception Area information to be submitted for this site.