Total lead concentrations in surface soil (the surface to 2 feet) ranged from 15.1 (background sample 109) to 40,100 milligrams per kilogram (mg/kg) (sample 118). A lead concentration above three times the background concentration was found in the southernmost sample collected (sample 112 containing lead at 26,400 mg/kg), indicating that contamination extended further south. Older shooting platforms were found in the wooded area south of the existing platforms. Remnants of those older platforms were identified southward, to near Wapello Avenue. The two northernmost samples collected (101 and 104) had much lower concentrations (at 164 and 126 mg/kg, respectively) than all other on-site surface soil samples, although these concentrations still exceeded three times the background concentration. Those data, however, provide some evidence that lead-contaminated soil may not extend beyond the existing northernmost shooting platforms (no older platforms were found north of the existing gradient location.
The ground water samples were analyzed for total lead. Analytical results indicated that the on-site well contained lead at 4.5 micrograms per liter (µg/L). Lead was not detected in the other two wells or in the field blank. The benchmark for lead is the Safe Drinking Water Act action level of 15 µg/L.
The results of the surface water samples are depicted on Table 1. The EPA freshwater (chronic) Ambient Water Quality Criteria (AWQC) for many metals (including lead) are dependent on water hardness (USEPA, 1996a). The AWQC for lead (AWQCs for metals are for dissolved concentrations), adjusted for total hardness, for the sample collected above the shot outfall is 13.5 milligrams per liter (mg/L), which was not exceeded by the dissolved lead concentration in the sample. (Because of the nature of the pre-printed field sheets and tags, each sample was given three sample numbers; one each for total metals, dissolved metals and total hardness-EPA sample numbers 0051 and 010). Both the total and dissolved concentrations for lead were below the detection limits at the downgradient sample location (002/-006/-011). No upgradient sample was collected because the marsh does not appear to have ?????
sediment samples were collected away from the shot outfall area during the PA/SI in July 1997. Those samples were collected from the marsh upgradient of the shot outfall area (ASUYC-M), from the marsh near 1-280 (007) and immediately upstream of where the drainage enters the Mississippi River (008). The samples were analyzed for total lead. Lead concentrations ranged from 16.3 to 73.2 ??????
4.4 CONCLUSIONS
Based on the information and analytical results of waste soil samples collected during previous investigations and the START F-SIJRA, the surface and subsurface soils and sediments at the Nahant Marsh site are contaminated with heavy metals and PAHS. Two contaminated waste source areas (the terrestrial and marsh components) have been delineated by field screening, laboratory results, and land-based surveys for removal assessment purposes (those areas have been outlined on Figures 4-1 and 4-2, respectively).
The terrestrial component of the area found to be contaminated encompasses an area of roughly 900 feet by 300 feet. The waste source that is conprised by marsh habitat extends approximately 300 feet out from the shoreline along approximately 1,800 feet of shoreline. Based on the sediment profiles, shot were generally confined to the upper 6 inches. The boundaries of the waste source areas did extend during the 1999 removal and the final sediment and soil removed consisted of 49,162 yds of sediment from the marsh component and 10,416 yds of soil from the terrestrial component (including the clay fragments containing PAHS).
Studies at other lead sites (E & E, 1993; EA Engineering, Science, and Technology, 1994 and 1996) that guided this effort suggested that the primary ecological risk posed by lead shot in sediments is to waterfowl through direct ingestion. Those studies provided guidelines for assessing potential risk in units of shot per square feet (ff) - low risk for shot densities 1= than I per ff , moderate for I to 19.5 shot per ff and high for shot densities greater than 19.5 shot per ff - In addition, a Screening
Level aological Risk Assessment was conducted by USFWS in June 1998 for the Nabant Marsh site. USFWS determined that a lead shot density of 10 per square foot in wetlands had a very low toxicity for birds.