LEAD CONCENTRATIONS IN SOIL PROFILES OF A TRANSECT NEAR AN INTERSTATE HIGHWAY IN NEW JERSEY

Concentrations of Pb from the 7 soil profiles along an approximately 170-meter transect perpendicular to the interstate highway 95 at Lawrenceville, New Jersey were studied by digestion of pulverized soil samples with 10% of H₃PO₄ and 75% of HNO₃. The loamy soil samples were collected from A and B horizons at a depth between zero and 100 cm. The digestions were conducted by dissolving 0.5 grams of pulverized samples into 20 ml of respective acids. Elemental concentrations of the dissolution were then measured by an ICP. The concentrations of lead (Pb) in soil samples calculated from the incomplete acid digestion range from 15 mg/g to 10.44 mg/g depending on the distance from the highway and depth within the soil profiles. Pb concentration decreases down the profile and away from the highway. The average Pb concentrations digested by HNO₃ are 1.5 times higher than that by H₃PO₄, while the concentration of As has the opposite preference for the digesting acid. In addition, the modal abundance of minerals in the soil samples was measured by quantitative X-ray powder diffraction. The main minerals in the soil samples are quartz (>40% on the surface), feldspar (~5%), and smectite and kaolinite clay minerals with small amounts of hematite and goethite. Lack of lead minerals in the soil samples and the high concentrations of Pb near the soil surface indicate that the main source of the Pb is likely the dust remnants from leaded gasoline used in the past. Because Pb concentrations also decrease with increasing distance from the highway, the main transport mechanism of the lead is likely the soil dust. Pb concentrations in a nearby creek seem to be influenced more by the winter deicing salt through chloride complexation than by the soil dust. High concentration of Pb near the soil surface adjacent to the highway indicates that Pb dust can still be a health issue for motorists in a dry period.