Organochlorine residues in shellfish from Maryland waters, 1976-1980

Shellfish samples, including the American oyster (Craessostrea virginica), the soft shell clam (Mya arenaria), the hard shell clam (Mercenaria mercenaria) and the blue crab (callinectes sapidus), were taken from the Maryland section of the Chesapeake Bay and its tributaries over a five year period (1976-80) and analyzed to determine residue levels of various organochlorine pesticides. Qualitative and quantitative information was obtained for a variety of polychlorinated hydrocarbons. The purpose of this study includes the establishment of baseline values for levels of shellfish contamination, utilization of the data as an indicator of probable levels of water and sediment contamination in the vicinity of the sampling site and ensurance that shellfish harvested for human consumption are within U.S. Food and Drug Administration guidelines for organochlorine contamination. All mean values and virtually all individual values were within FDA guidelines.     

Potential risk assessment of heavy metals by consuming shellfish collected from Xiamen, China

Concentrations of Hg, Pb, Cd, and Cr in 240 shellfish including oyster, short-necked clam, razor clam, and mud clam collected from six administrative regions in Xiamen of China were measured. The daily intakes of heavy metals through the consumption of shellfish were estimated based on both of the metal concentrations in shellfish and the consuming amounts of shellfish. In addition, the target hazard quotients (THQ) were used to evaluate the potential risk of heavy metals in shellfish on human body. Results showed that the concentrations of heavy metals in shellfish ranged at the following sequence: Cr > Cd > Pb > Hg. The concentrations of Hg and Pb in most samples were below the limits (0.3 mg?kg^?1 for Hg and 0.5 mg?kg^?1 for Pb) of national standard (GB 18406.4-2001) set in China. About 57 % of samples were found to contain more than 0.1 mg?kg^?1 of Cd, in which the highest level was found in oyster from Xiangan with a value of 1.21 mg?kg^?1. The average concentrations of Cd in oyster and mud clam samples were 0.338 and 0.369 mg?kg^?1, respectively, which were significantly higher (p?<?0.05) than those in the samples of short-necked clam and razor clam. The highest concentration of Cr was found to present in short-necked clam from Jimei with a value of 10.4 mg?kg^?1, but a mean value of 1.95 mg?kg^?1 in all the shellfish was observed, and no significant difference was found among the different sampling regions. The calculated daily intakes of Hg, Pb, Cd, and Cr through consuming the shellfish were 0.005, 0.122, 0.137, and 1.20 μg?kg^?1 day^?1, respectively, which accounted for 2.19, 3.42, 13.7, and 40.1 % of the corresponding tolerable limits suggested by the Joint FAO/WHO Expert Committee on Food Additives. The THQ values of the four metals were far below 1 for most samples, except for those of Cd and Cr in the four shellfish species with the mean values of 0.132 and 0.385, respectively. The highest THQ values of Cd were observed in the species of oyster (0.719) and mud clam (0.568). But the high THQ values of Cr observed in all the four species were derived from the applied reference dose (RfD) data of Cr(VI) due to the unavailable RfD value of total Cr. The results indicate that the intakes of heavy metals by consuming shellfish collected from Xiamen of China do not present an appreciable hazard risk on human health, but attention should be paid to consuming those with relatively high THQ values, such as oyster, mud clam, and short-necked clam.     

Trace metal residues in finfish from Maryland waters, 1978-1979

Levels of seven heavy metal residues, arsenic, cadmium, chromium, copper, lead, mercury and zinc were monitored in samples of various species of finfish harvested from the Maryland section of the Chesapeake Bay and its tributaries over a two year period (1978-79). Results of the analysis of the edible portions of these finfish are presented along with the species of finfish, date and location of harvest. A number of samples of finfish gonad and liver tissue were analyzed to study the relative level of preconcentration of heavy metals in these tissues compared to the edible (flesh) portion. Results of this study are consistent with other available data for Atlantic Coast finfish. Gonad tissue, when compared to flesh, show enrichment of copper and zinc and decreased mercury and cadmium levels. Liver tissue shows enrichment in copper, zinc and cadmium and generally lower levels of mercury compared to flesh.     

Biosorption of divalent Pb, Cd and Zn on aragonite and calcite mollusk shells

The potential of using mollusk shell powder in aragonite (razor clam shells, RCS) and calcite phase (oyster shells, OS) to remove Pb²⁺, Cd²⁺ and Zn²⁺ from contaminated water was investigated. Both biogenic sorbents displayed very high sorption capacities for the three metals except for Cd on OS. XRD, SEM and XPS results demonstrated that surface precipitation leading to crystal growth took place during sorption. Calcite OS displayed a remarkably higher sorption capacity to Pb than aragonite RCS, while the opposite was observed for Cd. However, both sorbents displayed similar sorption capacities to Zn. These could be due to the different extent of matching in crystal lattice between the metal bearing precipitate and the substrates. The initial pH of the solution, sorbent’s dosage and grain size affected the removal efficiency of the heavy meals significantly, while the organic matter in mollusk shells affected the removal efficiency to a lesser extent.     

Seasonal variation of heavy metals in ambient air and precipitation at a single site in Washington, DC

Atmospheric samples of precipitation and ambient air were collected at a single site in Washington, DC, for 7 months (for ambient air samples) and 1 year (for wet deposition samples) and analyzed for arsenic, cadmium, chromium and lead. The ranges of heavy metal concentrations for 6-day wet deposition samples collected over the 1-year period were 0.20-1.3 microg/l, 0.060-5.1 microg/l, 0.062-4.6 microg/l and 0.11-3.2 microg/l for arsenic, cadmium, chromium and lead, respectively, with a precision better than 5% for more than 95% of the measurements. The ranges of heavy metal concentrations for the 6-day ambient air samples were 0.800-15.7 ng/m(3), 1.50-30.0 ng/m(3), 16.8-112 ng/m(3), and 2.90-137 ng/m(3) for arsenic, cadmium, chromium and lead, respectively, with a precision better than 10%. The spread in the heavy metal concentration over the observation period suggests a high seasonal variability for heavy metal content in both ambient air and wet deposition samples.     

Depuration of metals from soft tissues of oysters (Crassostrea gigas) transplanted from a contaminated site to clean sites

The present study aimed at studying the transplantation of oysters from a polluted site Lau Fau Shan (existing oyster culture zone) in Deep Bay to two clean sites mariculture zones at Yung Shue O and Kat O. The hydrological data, growth of oyster shell length, mortality and metal contents in whole soft tissue were monitored from February 1993 to February 1994. The high growth phase (increase in shell length) and gametogenesis (decrease in dry weight of whole soft tissue) were noted in winter (with low temperature and high salinity), and slow growth phase and spawning in summer (with high temperature and low salinity). Significant reductions (p<0.05) of Cd, Cr and Pb concentrations (dry wt basis) in soft tissue of transplanted oysters were observed at Kat O by 29, 55 and 29%, respectively, and at Yung Shue O by 34, 44 and 34%, respectively, with respect to the baseline values for the first batch of oysters and the concentrations in samples collected from Lau Fau Shan in the same months. Maximum reductions of total metal burden in whole soft tissue (microg per individual oyster) of the first oyster batch were found for Cd, Cr, Pb and Sb to be 62, 49, 60 and 25%, respectively, at Kat O, and to be 56, 47, 32 and 49%, respectively, at Yung Shue O. For the second batch of oysters transplanted to Yung Shue O in July 1993, significant reductions (p<0.05) of Cd concentration and total burden of Cd by 60 and 21%, respectively, were observed in February 1994. The high accumulated mortality and the significant (p<0.05) lower growth rate of the transplanted oysters illustrated that the best time for transplantation of oysters should be between the end of gametogenesis and the start of spawning (i.e. January and February), and the oysters should not be kept for longer than 3 months at the transplantation sites.     

Distribution of butyltin and derivatives in oyster shells and trapped sediments of two estuaries in Cantabria (Northern Spain)

Distribution of butyltin compounds (BTs) and derivatives (monobutyltin, MBT; dibutyltin, DBT and tributyltin, TBT) was analysed in Crassostrea gigas oyster shells and the sediments trapped in the shell chambers, from two different estuaries in Cantabria (Northern Spain), with very different environmental conditions, where previous data have not been reported. Inorganic tin analysis in oyster shells was performed in order to study the degradation of BTs. Shell thickening and losses in biological growth are related with the presence of TBT, and were determined using three morphological indexes. Total BTs concentrations, expressed as Sn, ranged from 18.0 ng g(-1) to 176.7 ng g(-1) in sediments, and from 2.4 ng g(-1) to 11.1 ng g(-1) in oyster shells. Total inorganic tin concentrations ranged from 1775.0 ng g(-1) to 4781.3 ng g(-1) in oyster shells. The amount of BTs in oyster shells has been associated with the concentrations in the sediments trapped in the shell chambers. Partition coefficients between oyster sediments and oyster shells show the affinity of BTs by the sediments and the higher inorganic tin in oyster shells does not seem to be related to the BTs pollution.     

Butyltins in water, biofilm, animals and sediments of the west coast of India

Biofilm, fish, oyster, mussel, clam, surface seawater, suspended particulate matter (SPM), and sediment samples were collected from marine and/or estuarine waters of the west coast of India. These samples were analysed for butyltin derivatives such as dibutyltin (DBT) and tributyltin (TBT). The concentrations DBT plus TBT varied between 2.4 and 8.3, 163 and 363 ng/l, 5 and 2853 ng/g dry wt in the SPM, seawater and sediment samples, respectively, of the Marmugao harbour. The values of DBT plus TBT ranged between 0.60 and 29, 123 and 242 ng/l and 1.4 and 65 ng/g dry wt in SPM, water and sediment samples, respectively, collected from the Mandovi estuary. In the Dona Paula Bay the DBT plus TBT varied from 10 to 89 ng/l in surface seawater, and TBT from 10 to 513 ng/g in biofilm samples. For the coastal sediment samples the concentration of DBT plus TBT ranged between 36 and 133 ng/g dry wt of sediment. For the animal samples the DBT plus TBT ranged between 58 and 825 ng/g dry wt of the tissue. Mussel tissues contained the highest amount of DBT plus TBT (825 ng/g dry wt tissue), whereas highest TBT concentration was recorded in the oyster (732 ng/g dry wt). TBT was generally the most abundant butyltin compound in most of the samples suggesting fresh inputs and/or less degradation of TBT. A wide range of the observed butyltin concentrations suggests the presence of localized areas of contamination. Leaching of tributyltin-containing antifouling paints from the ocean going ships, fishing and recreational boats, barges, and the inputs of TBT from the Goa shipyard and dry dock facility situated in the harbour are the probable sources of the DBT and TBT in the samples of the west coast of India. Higher levels of TBT were observed in biofilm relative to that in the surrounding seawater. When fed on TBT contaminated biofilm of the diatom Navicula subinflata, butyltin concentrations in the clam Paphia malabarica increased over the period of feeding suggesting the importance of biofilm in the transfer of butyltins to higher group of organisms.     

Effect of shore position and environmental metal levels on body metal burdens in the barnacle, Elminius modestus

Body (thorax and prosoma) weight, shell growth, and Zn, Cu, Fe, Mn and Ca levels were measured in artificially settled Elminius modestus over a 14-month period following transfer to high, mid and low tide positions in the uncontaminated Menai Strait and two mid tide sites within a Zn and Cu polluted bay on the North West coast of Anglesey. Barnacles showed higher mortality and slower growth at the upper shore position than at any other site. There was no evidence of Zn and Cu regulation by the bodies or shells, and both seemed to reflect environmental levels. Body metal levels were lowest in early winter and the highest second year values were found in January (Zn), March (Cu, Fe) or May (Ca). Higher shore barnacles contained larger concentrations of Zn than mid or low tide animals. Differences in feeding activity and the amount of testes may account for these variations. Reciprocal transplants between polluted and 'clean' sites indicate that body Cu and Zn accumulation and loss was very rapid. Spatial and temporal changes in shell metal levels closely mirror that of the body.     

Rapid detection of toxic metals in non-crushed oyster shells by portable X-ray fluorescence spectrometry

The aim of this study was the multi-elemental detection of toxic metals such as lead (Pb) in non-crushed oyster shells by using a portable X-ray fluorescence (XRF) spectrometer. A rapid, simultaneous multi-element analytical methodology for non-crushed oyster shells has been developed using a portable XRF which provides a quick, quantitative, non-destructive, and cost-effective mean for assessment of oyster shell contamination from Pb. Pb contamination in oyster shells was further confirmed by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The results indicated that Pb is distributed in-homogeneously in contaminated shells. Oyster shells have a lamellar structure that could contribute to the high accumulation of Pb on oyster shells.     

Extractable soil heavy metals following the cessation of biosolids application to agricultural soil

Changes in soil heavy metal extractability following the cessation of biosolids applications were studied in a long-term field experiment. Two anaerobically digested biosolids from wastewater treatment plants in Madrid (Sur and Viveros) were applied to cropland from 1983 to 1990. Soil samples were collected in the 1st, 5th and 9th year after the last biosolids application. Soil pH did not vary significantly after biosolids applications. Organic matter and total heavy metals (Zn, Pb, Cd, Ni, Cr and Cu) concentrations initially increased but then declined over time, mostly after the first 5 years following biosolids application. Metal extracted with DPTA increased in Sur treatments during the 1st year and diminished thereafter. However, in Viveros treatments, heavy metals extracted increased during the 1st year, declined in 1995, and showed a slight increase in 1999. These changes in heavy metal extractability were widely observed in the percentage of extractable metal recovery (EMR). The differences observed in the pattern of the two sources of biosolids applied could be due to the different rates of decomposition of their organic matter.     

Decomposition of heavy metal contaminated nettles (Urtica dioica L.) in soils subjected to heavy metal pollution by river sediments

Two incubation experiments were conducted to evaluate differences in the microbial use of non-contaminated and heavy metal contaminated nettle (Urtica dioica L.) shoot residues in three soils subjected to heavy metal pollution (Zn, Pb, Cu, and Cd) by river sediments. The microbial use of shoot residues was monitored by changes in microbial biomass C, biomass N, biomass P, ergosterol, N mineralisation, CO(2) production and O(2) consumption rates. Microbial biomass C, N, and P were estimated by fumigation extraction. In the non-amended soils, the mean microbial biomass C to soil organic C ratio decreased from 2.3% in the low metal soil to 1.1% in the high metal soils. In the 42-d incubation experiment, the addition of 2% nettle residues resulted in markedly increased contents of microbial biomass P (+240%), biomass C (+270%), biomass N (+310%), and ergosterol (+360%). The relative increase in the four microbial properties was similar for the three soils and did not show any clear heavy metal effect. The contents of microbial biomass C, N and P and ergosterol contents declined approximately by 30% during the incubation as in the non-amended soils. The ratios microbial biomass C to N, microbial biomass C to P, and ergosterol to microbial biomass C remained constant at 5.2, 26, and 0.5%, respectively. In the 6-d incubation experiment, the respiratory quotient CO(2)/O(2) increased from 0.74 in the low metal soil to 1.58 in the high metal soil in the non-amended soils. In the treatments amended with 4% nettle residues, the respiratory quotient was constant at 1.13, without any effects of the three soils or the two nettle treatments. Contaminated nettle residues led generally to significantly lower N mineralisation, CO(2) production and O(2) consumption rates than non-contaminated nettle residues. However, the absolute differences were small.     

The Akoya pearl oyster shell as an archival monitor of lead exposure

The Akoya pearl oyster (Pinctada imbricata) was experimentally exposed to (a) constant levels of lead (Pb) at 180 microg L(-1) for nine weeks, or (b) two short term (pulse) exposures of Pb at 180 microg L(-1) (three weeks each) with an intervening depuration period (three weeks), to assess its utility as an (i) accumulative monitor of Pb contamination and an (ii) archival monitor for discriminating constant versus pulsed Pb exposure events. P. imbricata showed similar reductions in growth (based on shell morphology and wet weight) and Pb accumulation patterns for whole tissue and shell in response to both Pb exposure regimes. Thus the whole oyster was deemed an inappropriate accumulative monitor for assessing short-term temporal variation of Pb exposure and effect. However, using secondary ion mass spectrometry, Pb was shown to accumulate in the successively deposited nacreous layers of the shell of P. imbricata, documenting the exposure history of constant versus pulsed Pb events. Patterns of Pb deposition not only reflected the frequency of Pb exposure events but also their relative durations. Thus, the shell of P. imbricata may be employed as a suitable biological archive of Pb exposure.     

Characterization of Municipal Solid Waste Incineration Residues From Facilities with Different Air Pollution Control Systems

Abstract

Municipal Solid Waste incinerator residues produced in two types of facilities were exhaustively characterized: granulometry, mineralogy, chemical composition, leaching behavior, and elemental distribution as a function of particle size. Air Pollution Control (APC) residues coming from a semi-dry scrubber have shown higher solubility than fly ashes originating in an Electrostatic Precipitator (ESP), as well as higher contents in volatile metals (Cd, Hg). Different metal speciation and distribution as a function of particle size have been found in fly ashes (ESP residues) and APC residues. In APC residues, heavy metals (with the exception of Hg) show a parabolic distribution with maxima in the smallest and largest particles, following the same profile as soluble salts. Metal distribution for APC residues exhibits that metals generally are not associated with silicate aluminate matrix. Results show the effect of adding lime to APC residues in metal speciation and distribution.     

Shell deformations in the Baltic clam Macoma balthica from southern Baltic Sea (the Gulf of Gdansk): hypotheses on environmental effects

High-resolution digital photography and graphical image analyses systems have been used to define external morphometric characters of shell deformations in four populations of the Baltic clam Macoma balthica from the Gulf of Gdansk (southern Baltic Sea). The proposed shell deformation indices (SDI), which were based on the relationship of selected dimensions in the posterior and the anterior part of the shell, showed at least three morphological features that provide a distinctive diagnosis of "regular" and "deformed" clams: the presence of flexure on the posterior side (SDI1), elongated posterior region (SDI2), and shell growth (SDI3). The degree and prevalence of deformed clams varied locally over space. Increase in percentage contribution of aberrated shells with depth, corresponding to oxygen depletion profile in the Gulf, suggests low oxygen concentrations as the main agent exerting a deforming influence. The observed morphological aberrations developed with age (size) of a bivalve, suggesting a long-term effect of causal factors, and were accompanied by lightening shell weight, possibly due to decalcification of previously deposited calcareous material during anaerobic metabolism. It is hypothesized that hypoxic/anoxic conditions and a subsequent presence of hydrogen sulfide on a deep organic-rich sea bottom induce shell form alterations that enable the pumping of oxygenated water from above the anoxic layer. Such a morphological modification highlights the functional significance of shell deformations in protective response to the ambient low-oxygen concentrations. Sediment organotin concentrations fall within moderate to high contamination range and, therefore, may also have an adverse impact on the shell form. DNA analyses of the fragment of mitochondrial gene cytochrome oxidase I (COI, 393 base pairs) showed homogenous genetic structure of regular and deformed clams, indicating that shell deformations in M. balthica are primarily driven by acclimatization to the ambient environmental conditions.     

牡蛎壳粉末投加UASB反应器的运行特性

采用牡蛎壳粉末作为UASB反应器的辅助介质,探讨了牡蛎壳钙盐的溶出特性,系统考察了运行期中牡蛎壳粉末添加方式、进水有机负荷、水力停留时间等因素对反应器出水COD、碱度与pH的影响,分析了污泥比产甲烷活性变化。结果表明,当初始pH从5到9变化时,牡蛎壳粉末溶出Ca2+浓度为40~65 mg/L,平衡pH稳定在7.7~8.0;当进水COD负荷从3.4 kg/(m3·d)逐渐增至7.0 kg/(m3·d)、牡蛎壳粉末投加量从1.5 g/d逐渐增至3.2 g/d时,与未投加的反应器相比,投加牡蛎壳粉末反应器的启动周期缩短了10%左右,COD去除率与比产甲烷活性分别提高了13.3%和22%。投加牡蛎壳粉末可有效提供碱度,加快污泥的颗粒化进程。     

Accumulation of lead and cadmium in the marine prosobranch Nerita saxtilis, chemical analysis, light and electron microscopy.

The potential value of the marine prosobranch Nerita saxtilis as an efficient biological monitor to heavy metal pollution in the Red Sea was investigated. Storage ability of lead and cadmium was compared in shell, headfoot and digestive gland of the marine snail N. saxtilis collected from Al-Hamrauin area at El-kuseir (lead, 300.35 +/- 28.53 microg/l, 1,716 +/- 16.14. cadmium 20.01 +/- 1.8 microg/l, 161.72 +/- 21.4 mean +/- S.D. for water and sediment, respectively) relative to that of inhabiting marine water and sediment employing atomic absorption spectrometry to determine the organ with highest capability of heavy metal accumulation. The influence of metal storage on light microscopic structure of that organ was investigated. Also, the ultrastructure localization of storage sites in the same organ was determined employing transmission electron microscopy. The digestive gland was shown to accumulate both metals at conccntrations that are several orders of magnitude higher than those in the surrounding marine water. The bioaccumulation capability of lead and cadmium was ranked in the following order; digestive gland > headfoot > shell for lead and digestive gland > shell > headfoot for cadmium. In spite of its evident highest metal storage capability, no histopathological changes were observed in the digestive gland of that marine prosobranch. Enlarged electron dense vesicles and many granules were observed in ultrathin sections in digestive cells of these snails and are suggested to be the sites of storage of detoxified metals. The results of that finding indicate the possibility of using the marine prosobranch N. saxtilis as biomonitor for heavy metal contaminants in the Red Sea.     

Trade-offs between elimination and detoxification in rainbow trout and common bivalve molluscs exposed to metal stressors

The purpose of this paper was to examine trade-offs between elimination and detoxification in rainbow trout and three common bivalve molluscs (clam, oyster, and scallop) exposed to cadmium (Cd), copper (Cu), and zinc (Zn) based on recent reported experimental data. We incorporated metal influx threshold with subcellular partitioning to estimate rate constants of detoxification (k_d) and elimination (k₂). We found that the relationships between k₂ and k_d were negative for rainbow trout and positive for bivalve molluscs. However, the relationships between k_d and % metal in metabolically detoxified pool were found positive for rainbow trout and negative for bivalve molluscs. Our results also indicated that rainbow trout had higher accumulation (∼60-90%) in metabolically active pool when exposed to essential metals of Cu and Zn and had only 10-50% accumulation in response to non-essential metal of Cd. Based on a cluster analysis, this study indicated that similarity of physiological regulations among study species was found between Cd and Zn. Our study suggested that detoxification can be predicted by an elimination-detoxification scheme with the known elimination rate constant. We concluded that quantification of trade-offs between subcellular partitioning and detoxification provides valuable insights into the ecotoxicology of aquatic organisms and enhances our understanding of the subcellular biology of trace metals.     

Three decades of tributyltin in the coastal environment with emphasis on Arcachon Bay, France

This account briefly reviews the fate, bioconcentration and effects of tributyltin (TBT) three decades after its introduction into the coastal environment. Data on TBT-related events that occurred in Arcachon Bay (gastropod imposex, oyster recruitment failure, oyster shell malformations, concentrations in water and sediments) are revised and reinterpreted. Historically, TBT concentrations in the waters of the open bay seldom exceeded 1 ng Sn l(-1) in the 1960s, likely surpassed the range of 100 ng Sn l(-1) from 1977 to 1981 (years when no oyster spat were collected and the local shellfish industry collapsed), and decreased to about 1 ng Sn l(-1) in the late 1980s and early 1990s following legislation in January 1982 banning TBT use. TBT pollution is discussed in relation to the green tides that have developed in Arcachon Bay since 1982.     

Bioaccumulation of arsenic compounds in aquacultural clams (Meretrix lusoria) and assessment of potential carcinogenic risks to human health by ingestion

This study surveyed the total arsenic (As) and As species contents in clams (Meretrix lusoria) farmed in areas of hyperendemic blackfoot disease (BFD) in southwestern Taiwan. Total As and As species in sediment and pond water were also analyzed to examine the bioaccumulation of As in clams in their exposure environment. Moreover, potential carcinogenic risks associated with the ingestion of As in aquacultural clams were evaluated probabilistically. The average total As contents in medium-sized and small clams were 7.62 and 10.71 microg/g (dry wt), respectively. The content of the As species in this study was approximately 61% of the total As content. The other unquantified As species are possibly arsenocholine, arsenosugar and arsenolipid. The average ratios of inorganic As contents to total As contents in clams ranged from 12.3% to 14.0% which are much higher than that found in the farmed oyster (Crassostrea gigas), indicating that humans may expose to larger quantities of inorganic As by ingesting the same amount of clam as oyster. Using different ingestion rates derived by the average consumption method and the questionnaire method, the estimated risks to human health associated with consuming clams from the BFD area ranging from slightly to largely exceed the standard target risk. Based on the estimation of the TR model, a 0.18g/day-person of the safe ingestion rate of clams in the BFD region is recommended.