Step Two. Screening Level Exposure Estimates and Risk Calculations

Exposure Estimates

Lead Shot Ingestion

Field sampling data from the USFWS indicated that wetland substrates in the shot fall zone contained an average of 236.7 (standard deviation = 225.3, n = 8) and a maximum of 470 lead shot per square foot. The USFWS sampling method used an Ekman dredge to collect surfical substrate samples from the high lead shot density zone. Information provided in the Nahant Marsh integrated risk assessment indicated that an average of 26.8 (standard deviation = 87.3, n = 48) and a maximum of 533 lead shot were observed per square foot (E & E 1997). The E & E sampling method used a coring tool to collect substrate column samples along line transects throughout the lead shot fall zone. The different sampling methods and strategies used by the two organizations likely explain the disagreement in the results.

There are no shot density data available for the soil cover in the grassy portion of the shot fall zone.

Surface Water Toxicity

One surface water sample collected in the shot fall zone indicated a concentration of dissolved lead at 0.80 mg/L (E & E 1997).

Wetland Sediment/Soil Toxicity

The concentration of lead in two wetland sediment samples collected from within the shot fall zone were 9.4 and 27.3 mg/kg. The samples contained lead shot, but the shot was extracted by sieving prior to chemical analysis.

Ten different PAH compounds were detected in the two wetland substrate samples near the shoreline of the marsh in front of the club house (see Table 5 page 22).

Terrestrial Soil Toxicity

The concentration of lead in eighteen surficial soil samples collected from between the club house/driveway and marsh shoreline ranged 15.1 to 35,900 mg/kg determined by chemical analysis (E & E 1997). The mean and 95% upper confidence limit for these data are 7525 and 4724 mg/kg, respectively. Many of the samples contained lead shot when collected in the field, but the shot was extracted by sieving prior to chemical analysis.

Contaminated Food Chain Pathway

The food chain exposure estimates for the woodcock and robin feeding entirely on earthworms that are living in the lead contaminated soil area are outlined in

Table 2.

Table 2. Exposure estimates (mg/kg-d) for selected soil lead concentrations for the woodcock and robin food chain pathways.

The following formula was used to calculate the screening level exposure estimates:

Dose = soil lead concentration X average bioaccumulation factor X
average food ingestion rate X dry weight to wet weight conversion factor.

The maximum soil lead concentration at Nahant Marsh was 35,900 mg/kg, the mean was 7525 mg/kg, and the 95% upper confidence limit was 4724 mg/kg (all in dry weight).

Bioaccumulation factors (BAF) are reported in the literature for earthworms (Beyer 1990, Morgan and Morgan 1991, Wei-chun Ma 1987). The literature provides a range of BAFs for lead in earthworms of 0.07 to 9.15 (average 1.24).

The average body weight for woodcock is 0.196 kilograms (Joyce 1990). Woodcock species ingest an average of 0.042 kilograms of food daily (Joyce 1990). Normalized for body weight, this equals 0.21 grams food per gram body weight per day. We assume for this model that the woodcock’s diet consisted solely of earthworms from the contaminated soil.

The average body weight for robin is 0.0773 kilograms (Dunning 1984). The robin’s estimated average food ingestion rate is 0.1175 kilograms of food daily (Hazelton, et al. 1984). Normalized for body weight, this equals 1.52 grams food per gram body weight per day. We assume for this model that the robin’s diet consisted solely of earthworms from the contaminated soil.

The dry weight to wet weight conversion factor of 0.2987 was based on an average of two values for percent moisture reported in the literature of 70 and 84 percent (average = 77) (USEPA 1994 and Tyler 1973).

Incidental Soil Ingestion Pathway

The exposure estimates for the woodcock and robin incidental soil ingestion while foraging for earthworms that are living in the lead contaminated soil area are outlined in Table 3.

Table 3. Exposure estimates (mg/kg-d) for selected soil lead concentrations for the woodcock and robin incidental soil ingestion pathways.

The following formula was used to calculate the screening level estimate for soil ingestion:

Dose = soil lead concentration X average food ingestion rate X
soil ingestion rate factor X dry weight to wet weight conversion factor.

The soil ingestion rate factor for woodcock is 0.1 and 0.094 for the robin (Beyer et al 1994).

Risk Calculations

The screening level hazard quotients (HQ) were calculated by dividing the exposure estimate by the no effect or low effect benchmark ecotoxicity value.

The HQs exceeded one in the waterfowl lead shot ingestion pathway, terrestrial soil toxicity and avian food chain/soil ingestion exposure pathways (Table 4).

The HQ did not exceed one for surface water and for wetland sediment toxicity. These media are not predicted to be contaminated to the level to inhibit the production of waterfowl food resources (Table 4).

Table 4. Risk hazard quotients for the screening level contaminant exposure pathways at Nahant Marsh, Davenport, Iowa.

Lead Shot Ingestion

The averages from two studies on lead shot distribution for the wetland portion of the shot fall zone (26.8 to 236.7) exceeded the LOAEL value of ten. The lead shot ingestion pathway HQ was much greater than one.

Surface Water Toxicity

The exposure estimate for surface water lead (0.80 µg/L) did not exceed the lowest site specific screening value (3.7 µg/L). The HQ for direct toxicity to aquatic life from exposure to lead contaminated surface water is less than one.

Wetland Sediment/Soil Toxicity

The maximum exposure estimate for lead in wetland sediments (27.3 mg/kg) did not exceed the screening value (31 mg/kg). The HQ for direct toxicity to benthic macroinvertebrates from exposure to lead in sediments is less than one.

The maximum concentrations for seven of the ten PAHs compounds detected at the marsh exceeded the respective chemical screening values (Table 5). The HQ for direct toxicity to benthic macroinvertebrates from exposure to PAH contaminated substrate is greater than one for the PAH compounds.

Table 5. Screening values (mg/kg), exposure estimates (mg/kg) and risk hazard quotients for the ten detected polycyclic hydrocarbon compounds that were detected in wetland soils and lowland fill sediment at Nahant Marsh, Davenport, Iowa.

Terrestrial Soil Toxicity

The maximum soil lead concentration of 35,900 mg/kg exceeded the screening value for soils of 45 mg/kg. Thirteen out of 18 (72 percent) of the soil test values exceeded this screening level value. The HQ for direct toxicity to terrestrial invertebrates and plants from exposure to lead contaminated soil exceeds one.

Contaminated Food and Soil Ingestion Pathway

The predicted dose of lead for the woodcock from feeding on contaminated food items (375 mg lead/kg-d) and incidental soil ingestion (30 mg lead/kg-d) were evaluated independently and compared with the NOAEL value of (3.85 mg lead/kg-d).

The calculated HQs for the woodcock food chain and soil ingestion pathways each exceeded one. Adverse effects are predicted for the woodcock from feeding in the wet meadow and terrestrial portions of the lead shot fall zone. This calculation is based on the 95% upper confidence limit value for soil lead contamination.

The predicted dose of lead for the robin from feeding on contaminated food items (2660 mg lead/kg-d) and incidental soil ingestion (202 mg lead/kg-d) were evaluated independently and compared with the NOAEL value of (3.85 mg lead/kg-d).

The calculated HQs for the robin food chain and soil ingestion pathways each exceeded one. Adverse effects are predicted for the robin from feeding in the terrestrial portion of the lead shot fall zone. This calculation is based on the 95% upper confidence limit value for soil lead contamination.

Uncertainty Analysis

Surface Water Toxicity

The single measurement exposure estimate for surface water lead may underestimate actual concentrations. Lower pH conditions related to high respiration rates of phytoplankton during blooms that occur later in the summer may increase the dissolved lead concentration. The water retention or flushing rates for the wetland during late summer may be longer and help maintain dissolved lead concentrations. These conditions could result in an HQ greater than one.

Lead concentrations in the surface water from within the shot fall zones at six former trap and skeet ranges from across the country were between 1.4 and 838 mg/L (Stansley et al 1992). Only one surface water sample was available from the site for evaluating this exposure pathway.

Wetland Sediment/Soil Toxicity

The exposure estimate for lead and PAH contamination to wetland sediments is based on only two samples. Additional samples are needed to adequately characterize the wetland substrate because of their potential heterogeneity.

Terrestrial Soil Toxicity

The HQ for soil toxicity is much greater than one. However, the leaves or flowers of plants growing in the terrestrial portion of lead shot fall zone do not appear to show symptoms of chemical stress. There does not appear to be any bare ground areas, even in the spots with a layer of clay target debris. However, the nearshore and shoreline zone adjacent to the shooting platforms contain cattail plants with deformed flowers. This screening level assessment has not fully characterized the bioavailability and ecotoxicity to cattail species from lead residues in the wetland soils.

Contaminated Food and Soil Ingestion Pathway

The food chain transfer models used to estimate the levels of lead in earthworms and exposure to the avian receptors are based on conservative desk top calculations and not site-specific data from the marsh. There are uncertainties as to the potential of bioaccumulation of lead in earthworms living in the soil at the site.

Site specific soil parameters such as pH, grain size, organic carbon content may regulate the bioaccumulation of lead in soil dwelling invertebrates. The state in which the lead is partitioned in the soil column (fragments of elemental lead or sorbed and dissolved ions) may regulate the bioaccumulation of lead in soil dwelling invertebrates. These factors may result in a different estimate of exposure.