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Keywords:

  • Copper;
  • Water quality criteria;
  • Saltwater;
  • Dissolved organic carbon

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

During the past three decades, significant advances have been made in understanding how environmental factors modify the bioavailability and the toxicity of metals such as copper in aquatic environments. Several of these advances have led to the development of guidelines to indirectly account for modifying factors, adjustment of criteria on a site-specific basis, and direct changes to the U.S. Environmental Protection Agency (U.S. EPA) freshwater quality criteria. To date, most of this effort has focused on freshwater systems, although similar modifying factors exist in marine environments as well. This paper focuses on one such modifying factor, dissolved organic carbon (DOC), and describes a method to aid in risk assessments or to refine the saltwater copper criteria on a site-specific basis. The relationship between DOC and toxicity of copper to the most sensitive saltwater genus in the U.S. EPA criteria database, Mytilus, is extensively analyzed. Dissolved copper 50% effective concentrations (EC50s) are highly correlated (r2 = 0.71, n = 54, p < 0.001) across a wide range of sample DOC concentrations (0.3−10 mg carbon [C]/L) and are explained by the equation EC50 = 11.53DOC0.54. Two equations based on DOC are proposed for consideration as a means for deriving site-specific final chronic criteria (FCC) and final acute criteria (FAC) for copper in marine and estuarine environments (Copper FCCDOC = 3.71 DOC0.54 and Copper FACDOC = 5.843DOC0.54).


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Consensus exists among regulators and dischargers that the process of developing regulations protective of aquatic life should be based on “good science” (Paquin et al. 2002). As advances in our understanding of factors that affect aquatic toxicity occur, regulations often change to reflect the new knowledge. However, converting this knowledge into regulation inevitably occurs after considerable delay due to the time needed to complete critical steps in a process that includes peer-reviewed publishing, understanding and acceptance by the scientific community, regulatory adoption, and implementation.

The evolution of freshwater quality criteria for metals is a good example of the delayed but eventual modification of regulatory criteria based on expanding knowledge. The bioavailability and thus the toxicity of metals and its implications on existing and developing water and sediment quality criteria have been recognized for years (Di Toro et al. 1991; Allen and Harsen 1996; Ankley et al. 1996). In the 1970s and early 1980s, researchers showed that copper toxicity was related primarily to the cupric ion (Cu2+) or, in some instances, its hydroxy complexes (USEPA 1984). This and other research led to the incorporation of water hardness, a factor that affects the bioavailability of copper, into the freshwater copper criteria in 1984 (USEPA 1984). It was later recognized by the U.S. Environmental Protection Agency (U.S. EPA) that unexplained factors other than water hardness further modified the toxicity of metals. Thus, in 1994, the U.S. EPA published guidelines (USEPA 1994) for site-specific studies to allow further adjustment to the criteria. In 1995, the U.S. EPA published an Interim Final Rule (60 FR 22228) promulgating that the metals criteria be applied to the more bioavailable dissolved fraction rather than total metal. Finally, the biotic ligand model (BLM) (Di Toro et al. 2001; Santore et al. 2001), which simultaneously considers many factors affecting bioavailability, is currently being considered for implementation into the freshwater criteria process and was made available by the U.S. EPA for review and public comment in December 2003 (USEPA 2003).

To date, much of the effort to investigate the environmental factors that modify the bioavailability of copper has focused on freshwater species. At this time, few studies address factors affecting bioavailability of copper and their potential application in the development of the national saltwater copper criteria. As a result, the only direct change to the saltwater copper criteria has been its application to the more bioavailable dissolved copper fraction rather than total copper. However, the U.S. EPA recognizes the need and allows for an indirect accounting of modifying environmental factors in saltwater through the development and application of water-effect ratios (WERs). In short, a WER is derived by determining the ratio of the acute toxicity of the contaminant of concern in water collected from the site of interest and in clean reference water (e.g., Site Water LC50/Reference Water LC50). A sufficient number of tests are needed to account for spatial and temporal variability at the site of interest. Site-specific criteria are calculated by multiplying the existing water quality criteria by the WER.

Research is currently under way to verify the BLM's applicability to saltwater species. The research shows great promise and may provide a solution to the need for a large number of WER studies nationwide; however, it is not presently being considered for application as a regulatory tool related to marine waters. Thus, the eventual acceptance and implementation of the BLM for use in marine systems is some time away.

Table Table 1.. Dissolved organic carbon and Mytilus galloprovincialis dissolved copper EC50 data used in this analysis. Data omitted from this analysis are labeled as follows: BMDL = below the detection limit of 1.5 mg DOC/L; HDOC = high dissolved organic carbon; LDOC = low dissolved organic carbon; UDQ =unacceptable data quality
 September 2000February 2001April 2001June 2001February 2003
SiteaDOC mgC/LEC50 μg Cu/LDOC mg C/LEC50 μg Cu/LDOC mgC/LEC50 μg Cu/LDOC mgC/LEC50 μg Cu/LDOC mgC/LEC50 μg Cu/L
  1. aLocations for sites are as follows: Granite Canyon reference = Monterey, CA, USA; Redwood Creek, San Bruno, Central Bay, Oyster Point, Yerba Buena Island, San Pablo Bay, Petaluma River, East San Pablo Bay, Pacheco Creek, and Grizzly Bay = San Francisco, CA, USA; Puget Sound SUZ001 = Port Susan, WA, USA; Puget Sound HCB002 = Bangor, WA, USA; Galveston Bay and West Galveston Bay = Clear Lake, TX, USA; and Narragansett Bay = Narragansett, RI, USA.

Granite Canyon referenceBMDL8.30.69.51.06.9HDOC9.4  
Granite Canyon referenceBMDL8.10.66.01.27.10.36.80.712.4
Redwood Creek2.621.73.225.02.319.1LDOC21.3  
San BrunoBMDL19.42.119.32.218.9LDOC17.0  
Central Bay (midpoint)2.020.13.227.82.617.33.516.3  
Central Bay (nearshore)4.619.43.330.92.419.6LDOC14.7  
Oyster PointBMDL20.32.220.71.916.8LDOC16.4  
Yerba Buena IslandBMDL17.81.715.31.716.9LDOC12.4  
San Pablo Bay2.118.24.924.22.113.81.814.5  
San Pablo Bay B15-B201.58.18.124.82.820.12.019.2  
Petaluma River3.022.49.050.54.023.32.322.7  
East San Pablo Bay (midpoint)1.714.24.830.32.216.52.621.1  
East San Pablo Bay (nearshore)2.314.54.523.42.518.42.219.7  
Pacheco Creek1.721.19.633.32.921.3 UDQ  
Grizzly Bay2.014.010.030.23.211.1 UDQ  
Puget Sound SUZ001        1.414.8
Puget Sound HCB002        1.113.0
Galveston Bay—Horsepen Bayou        8.771.0
West Galveston Bay        3.324.8
Narragansett Bay—U.S. EPA Dock Low Tide        1.517.1
Narragansett Bay—U.S. EPA Dock High Tide        1.416.6

The U.S. EPA has recognized the need for improved efficiency and has gone so far as to publish streamlined procedures for determining copper WERs in freshwater (USEPA 2001). Because of the numerous ongoing regulatory efforts in saltwater systems (e.g., total maximum daily loads and risk assessments), a need also exists for a simple method to aid in assessing risk or modifying the existing copper criteria in saltwater systems. This paper presents a relatively simple approach to aid in addressing this need. The approach described is similar to that used to adjust freshwater copper criteria to account for the effects of water hardness, but, in the case of saltwater, dissolved organic carbon is used as the basis for the adjustment. The method described herein represents a practical (i.e., rapid and cost effective) alternative for developing site-specific water quality criteria and shows strong correlation with WER studies where U.S. EPA procedures (USEPA 1994) were used.

METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Toxicity measurements

Chemistry and toxicity data were collected as part of a cooperative effort with regional regulatory authorities, dischargers, and nongovernmental organizations performing WER tests to establish site-specific copper criteria throughout San Francisco Bay (CA, USA) north of the Dumbarton Bridge (BACWA 2002). Samples were collected four times (September 2000 and February, April, and June 2001) at 13 sites in San Francisco Bay and at a clean-water reference site in the Pacific Ocean outside San Francisco Bay. Toxicity tests were conducted on one-half of the San Francisco Bay sites at a time, and an aliquot of reference site water was tested with each set of bay samples, yielding 47 acceptable data sets.

Another study was conducted where two samples each were collected in February 2003 from Puget Sound (WA, USA; two locations), Galveston Bay (TX, USA; two locations), Narragansett Bay (RI, USA; one location, one sample at high tide and one sample at low tide), and one sample from the ocean reference site near San Francisco Bay, yielding seven acceptable data sets.

Toxicity tests for all samples were conducted by the same laboratory (Pacific EcoRisk, Martinez, CA, USA) and using embryos of adult estuarine bivalves, Mytilus galloprovincialis (not genetically verified, referred to hereafter as Mytilus sp.), obtained from Carlsbad Aquafarms (Carlsbad, CA, USA). Various species of the genus Mytilus are commonly used as toxicity test organisms for effluents and sediment porewaters, and members of this genus are widely distributed with some species considered invasive.

The salinity of all samples was adjusted upward using reagent grade GP2 artificial sea salts or downward using reverse osmosis deionized water to a constant 30‰ as per U.S. EPA guidelines (USEPA 1995c). Samples were split into aliquots and amended with a copper stock solution to concentrations spanning the EC50 of Mytilus sp. Toxicity tests were conducted using U.S. EPA guidelines (USEPA 1995c) for the Mytilus 48-h static embryo-larval development test. The test endpoints for this test include normal shell development and mortality as measures of adverse effects. Positive, negative, and sea salt control tests were performed, all with acceptable results according to U.S. EPA guidelines (USEPA 1995c).

Water chemistry

Chemical analyses for all parameters used herein were made after the site water samples had been salinity adjusted. Sampling after salinity adjustment is important to account for modification of copper bioavailability due to changes in water chemistry of the exposure water caused by this procedure. Dissolved copper concentrations in water passing through a 0.45-μm filter were measured using Inductively coupled plasma/mass spectrometry (ICP-MS) and followed U.S. EPA Method 1640 (USEPA 1995b). Dissolved organic carbon determinations were made using a total organic carbon (TOC) analyzer and followed U.S. EPA Methods SW-846 (studies conducted in 2000–2001) and 9060 (studies conducted in 2003) (USEPA 1998, 2002).

Data quality

Holding times, analytical accuracy, analytical precision, potential contamination, and conformance to data acceptability criteria were accessed, and questionable results were investigated. Analytical chemistry accuracy and precision were monitored during the study using blanks, duplicate samples, and matrix spikes performed for each set of 20 samples. Accuracy was assessed through percent recovery analysis of external reference standards and matrix spikes. Precision was determined by calculating relative percent differences between matrix duplicate and test duplicate analyses. Limits for precision were set at 20% and accuracy at 75% minimum and 125% maximum.

Much of the data (Table 1) were of acceptable quality; however, some data were eliminated from this analysis. The minimum detection limit (MDL) was too high (1.5 mg carbon [C]/L) for proper quantification of low-level DOC in five of the September 2000 San Francisco Bay study samples. The data quality objective for the DOC MDL was subsequently lowered, and the analytical lab was able to provide better quantification on subsequent samples. These data were omitted from this analysis rather than speculating on the DOC concentration in each sample.

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Figure Figure 1.. Plot of U.S. Environmental Protection Agency saltwater copper criteria genus mean acute values illustrating the relative sensitivity of the genus Mytilus and the basis for the current water quality criteria. Each point denotes the log mean EC50 for one genus. The final acute value was calculated to be 10.39 μg Cu/L but lowered to 9.625 μg Cu/L to protect the genus Mytilus. The final acute criteria of 4.8 μg Cu/L is not shown.

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Data from the last set (June 2001) of the San Francisco Bay samples were suspect, with much of the suspicion centered on the DOC analyses. Five sites well inside San Francisco Bay had DOC concentrations reported to be below detection limits of 0.3 mg C/L. A review of DOC data from the San Francisco Bay Regional Monitoring Program (SFBRMP) (see www.sfei.org) showed that the DOC concentrations historically range between 0.7 and 10.8 mg C/L within San Francisco Bay based on 595 DOC analyses conducted throughout the bay between 1993 and 2001. Consistently, the lowest DOC concentrations in the bay are measured near the Golden Gate Bridge, where marine water enters the bay on the incoming tide. Additionally, a DOC concentration of 11 mg C/L reported for the reference site outside the bay was unusually high. In fact, it was higher than any historically measured concentration in the SFBRMP and any other site reported in this analysis. This datum is also suspect since it is an open-ocean clean water reference site, and the average DOC in all other samples from this site in this study was 0.8 mg C/L. Two toxicity tests of the June 2001 samples were invalid because of data inconsistencies with U.S. EPA WER guidance acceptability criteria (USEPA 1994). Thus, these data were eliminated from this analysis. By omitting 13 data sets of unacceptable quality, 54 data sets of acceptable quality were used for this analysis (Table 1).

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Mytilus was chosen to assess toxicity because of its sensitivity to copper. To illustrate this, a probability distribution of genus mean acute values (GMAVs) for dissolved copper (USEPA 1995a) is shown in Figure 1. Organism sensitivity to copper varies by more than three orders of magnitude (GMAV <10 to>1,000 μg copper [Cu]/L). The acute and chronic water quality criteria (WQC) are designed to be protective of most organisms most of the time. The policy of the U.S. EPA is to protect 95% of the genera; thus, the WQC are based on the 5% GMAV, except if these values are not low enough to protect commercially important or threatened and endangered species. The 5% concentration is known as the final acute value (FAV).

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Figure Figure 2.. Plot of dissolved copper EC50s for embryos of Mytilus galloprovincialis as a function of dissolved organic carbon (DOC) in the salinity-adjusted water samples (r2 = 0.71, n = 54, p < 0.001).

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In the development of the saltwater copper criteria, the Mytilus embryo-larval development test was used by the U.S. EPA as an acute test. Though some suggest it should be considered a critical life stage estimate of chronic toxicity, this issue has yet to be resolved. Consequently, the FAV was initially calculated by the U.S. EPA to be 10.39 μg Cu/L but was lowered to 9.625 μg Cu/L to protect Mytilus, a commercially important bivalve (USEPA 1995a). The chronic criterion was calculated by dividing the FAV by the copper final acute-chronic ratio (ACR) of 3.127. The final ACR is the log mean of individual ACRs for several species for which complimentary acute and chronic tests have been performed. The acute criterion was calculated by dividing the FAV by 2 and rounding to two significant figures. For copper, this process yielded a dissolved final chronic criterion (FCC) of 3.1 μg Cu/L and a dissolved final acute criterion (FAC) of 4.8 μg Cu/L. Since the copper saltwater criteria are based on Mytilus, present efforts to develop a simple method to aid in assessing risk or to adjust the criteria focus only on this genus.

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Figure Figure 3.. Plot of predicted versus measured dissolved copper EC50s for embryos of Mytilus galloprovincialis using the equation EC50 =11.53DOC0.54. The solid line represents unity of predicted and measured EC50s. The dotted lines represent a factor of ±2 from the line of unity.

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Figure Figure 4.. Cumulative frequency plot of copper EC50s (nominal) for Mytilus galloprovincialis reference toxicity tests performed by the laboratory used in this study.

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In aqueous solutions, copper occurs either bound to particulate and colloidal matter or as a variety of dissolved chemical species. It has been shown that copper ions in the dissolved fraction form complexes with DOC and, consequently, reduce their bioavailability to aquatic organisms (Santore et al. 2001). It has also long been recognized by the U.S. EPA that freshwater criteria should be adjusted upward in surface waters where total organic carbon is significantly above 2 to 3 mg C/L (USEPA 1984). However, until the recent development of the freshwater BLM, the U.S. EPA (USEPA 1984, 1995a) has not developed or implemented a means to adjust either the freshwater or the saltwater criteria based on the knowledge that DOC affects bioavailability. It stands to reason that DOC may reduce the bioavailability and, thus, lower the toxicity of dissolved copper in saltwater in a predictable manner similar to that in freshwater. If the effect of DOC on copper bioavailability were quantified, then a means would exist to more easily and effectively assess the risk of copper in saltwater. The relation between DOC and toxicity to the most sensitive saltwater genus in the U.S. EPA criteria database, Mytilus sp., is now known and presented in this paper.

Dissolved copper EC50s for embryos of Mytilus sp. determined using U.S. EPA chronic estimator test methods (USEPA 1995c) are highly correlated (r2 = 0.71, n = 54, p < 0.001) across a wide range of sample DOC concentrations (Figure 2). This relation is explained by the equation EC50 = 11.53DOC0.54 (equation with ±95% confidence intervals: EC50 = 101.062 ± 0.046 DOC0.542 ± 0.097) between DOC concentrations of 0.3 to 10 mg C/L. This range of DOC concentrations is sufficient to apply to most ambient estuarine and marine waters. Additional testing may be needed for application to sediment porewaters and some effluents where DOC concentrations are higher than 10 mg C/L.

The previous equation was used as a predictive model with DOC concentrations measured in the test samples and compared to EC50s based on measured copper concentrations. The results are illustrated in Figure 3. All the data fall within a factor of ±2, a range used in the assessment of predictability of the BLM (Di Toro et al. 2001; Santore et al. 2001; Paquin et al. 2002) and considered an acceptable level of agreement. The range of ±2 appears to be a reasonable estimate to use for Mytilus sp. as well. Laboratory reference toxicity test EC50s (nominal concentrations, n = 92) using copper, reference site water, and Mytilus sp. conducted by the laboratory that performed the tests used in this study ranged from 4.7 to 19.4 μg Cu/L, a factor of 4.1 (Figure 4). Assuming that interlaboratory variability is greater than intralaboratory variability, using a factor of ±2 is likely a conservative evaluation range.

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Figure Figure 5.. Plot of the predicted site-specific dissolved copper water-effects ratios (Copper WERDOC) based on dissolved organic carbon (DOC) concentrations of the sample and the equation Copper WERDOC = 1.20DOC0.54.

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The relationship between DOC concentrations and EC50s can be further used to estimate site-specific WERs. Because of the large range of DOC used to develop the equation EC50 = 11.53DOC0.54, DOC can be used to estimate the toxicity of most samples without extrapolating. This includes samples collected from sites of interest as well as from reference stations or synthetic laboratory water (if used as a reference for WER development), both of which commonly have low DOC concentrations. Consequently, WER estimates can be derived using the same method as having actual toxicity test data. That is, EC50s can be estimated for the sites of interest and divided by estimates of EC50s for the reference or synthetic laboratory seawater.

If reference site or synthetic laboratory water data are not available, as may be the case in a retrospective risk assessment, then an alternative procedure could be used. If each predicted EC50 (Figure 3) is divided by the U.S. EPA FAV (9.625 μg Cu/L), a DOC-based WER estimate for each location and sample using site-specific sample DOC concentrations is generated. The relationship between the WER and DOC concentrations is explained by the equation Copper WERDOC = 1.20DOC0.54 (Figure 5). This model can be used easily and cost effectively to develop WER estimates for sites of interest and assess risks in ambient waters. Furthermore, but with additional substantiation, it may also prove useful in estimating toxicity of copper in sediment porewaters and effluents. It accounts for the need for either upward or downward adjustment of water quality criteria for site assessments. However, DOC concentrations would have to be extremely low, approximately 0.71 μg C/L, to achieve WER estimates of <1.

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Figure Figure 6.. Comparison of water-effect ratios (WERs) based on actual toxicity tests and WERs based on estimations using dissolved organic carbon and the equation Copper WERDOC = 1.20DOC0.54. The solid line represents unity of toxicity test and DOC-based WERs. The dotted lines represent a factor of ±2 from the line of unity.

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Figure Figure 7.. Plot of the saltwater chronic and acute copper criteria (FCC and FAC, respectively) estimated as a function of dissolved organic carbon (DOC) concentrations by the equations Copper FCCDOC = 3.71 DOC0.54 and Copper FACDOC = 5.77DOC0.54, respectively.

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Increases in saltwater copper criteria in estuaries should be expected when using the WER development process. Water quality criteria data are commonly developed using reference or laboratory control waters having low organic carbon concentrations. In several instances, test guidelines specifically restrict organic carbon to ≤2 mg C/L in waters used to determine the toxicity of chemicals (USEPA 1995a, 1996a, 1996b; ASTM 1999; OECD 2000). Estuaries commonly have higher DOC concentrations than laboratory water used to develop the saltwater criteria and open marine sites. Since there is such a strong inverse relationship between copper toxicity and sample DOC concentration and because the copper saltwater criteria are based on data developed in low-DOC waters (e.g., highly polished natural seawater or synthetic laboratory seawater), it is only reasonable to expect WERs to often be>1.

A comparison of WERs developed at each site from actual test data and WERs estimated from DOC concentrations and the U.S. EPA FAV (the alternative procedure described previously) show reasonable agreement between the two methods (Figure 6). Most of the data fall within a factor of ±2. However, as WER values increase, the DOC-based WER estimate method tends to predict lower WERs than the method of deriving WERs from actual test data. This overall bias may be acceptable if it is understood that the bias exists when important risk management decisions are made and in instances where cost of developing toxicity test based WERs are prohibitive.

Other equations can be derived as a screening level estimate of site-specific copper criteria directly from DOC concentrations. Multiplying the Copper WERDOC by the U.S. EPA copper FCC or FAC across a range of DOC concentrations from 0.3 to 10 mg C/L produces estimates of a DOC-corrected saltwater FCC or FAC for copper (Figure 7). The equations for these relations are Copper FCCDOC = 3.71DOC0.54 and Copper FACDOC = 5.77DOC0.54. These final equations could be considered for modifying the existing saltwater copper criteria and are recommended on an interim basis until a mechanistic based method can be more fully developed, such as a marine BLM.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Over the past three decades, our understanding that a variety of environmental factors affect the bioavailability and the toxicity of copper in water has grown significantly. This advancement in science has led to a better understanding of how to quantify the bioavailability of copper and to better estimate concentrations that are safe to aquatic organisms. Albeit a slow process, incorporating new methods to account for factors that modify metal bioavailability continues to be used primarily to adjust freshwater criteria. Evidence now exists showing that a similar adjustment to the saltwater copper criteria may be possible using a relatively simple and straightforward approach. It has been shown here that the toxicity of copper to the genus on which the U.S. EPA saltwater criteria is based is affected by DOC in a predictable manner (EC50 = 11.53DOC0.54). This relationship is consistent over a wide range of DOC concentrations and is highly predictable within a factor of ±2. Equations have been derived to demonstrate the effect of adjusting the copper saltwater criteria on a site-specific basis and to demonstrate how site-specific conditions might necessitate either an upward or a downward adjustment of the criteria. Therefore, it appears reasonable that this method be considered a simple means to account for the effects of DOC and for the possible adjustment of the existing saltwater copper criteria on a site-specific basis.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Acknowledgement—We are grateful to Kay Ho of the U.S. EPA, Narragansett, Rhode Island; Cindy Howard of the University of Houston at Clear Lake, Clear Lake, Texas; and Bill Peters of Parametrix Inc., Bellingham, Washington, for supplying samples for this study; and to Jeff Cotsifas of Pacific EcoRisk, Martinez, California, for coordinating the sample collection, toxicity testing, and chemical characterization. We are also grateful to the San Francisco Bay Wide Stakeholder Group Coordinating Committee for their cooperation and collaboration in this study.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES
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