SITE-SPECIFIC APPROACH FOR SETTING WATER QUALITY CRITERIA FOR 

SELENIUM: DIFFERENCES BETWEEN LOTIC AND LENTIC SYSTEMS 

W.J. Adams, 1

J.E. Toll, 2

K.V. Brix, 2

L.M. Tear, 2

D.K. DeForest, 2 

1

 Kennecott Utah Copper 

P.O. Box 6001 

Magna, UT 84044 

2

 Parametrix, Inc. 

5808 Lake Washington Blvd. NE, Suite 200 

Kirkland, WA 

ABSTRACT 

Results of an in-depth review of the literature indicates there are significant differences in the 

bioaccumulation of selenium by fishes and invertebrates from lotic (flowing) and lentic (standing) water 

bodies and that selenate is much less bioaccumulative than selenite. Bioaccumulation in fish is a factor of 

10 or more higher in lentic systems as compared to lotic systems. These differences are a function of 

selenium’s site-specific biogeochemical cycling. Further, we observed considerable variation in bird 

accumulation of selenium from site to site. To account for differences in bioaccumulation potential of 

selenium we developed a residue-based Bayesian Monte Carlo model to derive site-specific selenium 

water quality criteria protective of fish and sensitive avian species. 

The approach uses data from a given site of interest to calibrate a model based on data from several other 

similar sites. When evaluating a specific site, the range of water and tissue concentrations is typically 

limited. This makes it difficult to use site-specific data to identify a water concentration sufficiently low 

that tissue concentrations do not exceed the tissue-effect threshold. Data from several similar sites 

provide a broader range of water and tissue residue concentrations that allow for an appropriate statistical 

extrapolation of the data to the site of interest. The Bayesian Monte Carlo model accounts for the 

significant site-to-site variability that exists in the relationship between water selenium and the mean 

tissue residue. In practice, data from similar sites are pooled to define a set of possible water and mean 

tissue residue relationships. This set of possible relationships is then used with data from the site of 

interest to determine which relationships, from the set of possibilities, best fit the specific site. Once we 

have determined which set of possible relationships fit the specific site, we extrapolate from the observed 

water concentration to a water concentration that results in a tissue residue concentration less than or 

equal to a chronic effect threshold. This value becomes the chronic water quality criterion. 

Adams, W.J., J.E. Toll, K.V. Brix, L.M. Tear and D.K. DeForest. 2000. Site-specific approach for setting water 

quality criteria for selenium: differences between lotic and lentic systems. Proceedings Mine Reclamation 

Symposium: Selenium Session; Sponsored by Ministry of Energy and Mines, Williams Lake, British Columbia, 

Canada, June 21-22, 2000. 
INTRODUCTION 

Selenium contains properties that make it unique relative to other metals and metalloids. It occurs in both 

organic and inorganic forms which are differentially toxic and is an essential element for most organisms. 

The selenium forms present in aquatic systems are controlled by the biogeochemical cycling of selenium 

which is strongly influenced by site-specific environmental factors such as redox, pH, and biological 

productivity (Lemly and Smith, 1987; Bowie and Grieb, 1991; Porcella et al., 1991). 

Reduction of inorganic selenium species tends to immobilize selenium in an aquatic system, while other 

processes, such as oxidation and biotransformation tend to make selenium bioavailable to aquatic 

organisms. Biological mechanisms such as uptake of sediment selenium by rooted plants, benthic 

invertebrates, and detritus-eating invertebrates, can act to re-mobilize selenium into the aquatic food web. 

Accordingly, lentic systems tend to bioaccumulate selenium much more than lotic systems that have 

higher flushing rates and lower productivity (Lillebo et al., 1988; Van Derveer and Canton, 1997). For 

example, Lillebo et al. (1988) demonstrated this by plotting bioaccumulation data for impounded and 

flowing waters; fish selenium residues were approximately six times greater in impounded waters than in 

flowing waters at a water selenium concentration of 10 μg/L. 

Based on increased awareness of the ecotoxicological effects of selenium, a number of water quality 

monitoring programs have been implemented to evaluate potential selenium contamination at freshwater 

sites. In order to interpret the significance of the selenium concentrations measured under these 

programs, several authors have proposed selenium guidelines for various environmental compartments 

(e.g., Lemly, 1993a; Skorupa et al., 1996). Specifically, guidelines have been proposed for surface water, 

sediment and various tissues, including ovaries, whole body, diet, liver, eggs and testes based on the 

authors’ reviews of published and unpublished literature. 

These guidelines include recommended toxicity thresholds for abiotic (water, sediment) and biotic 

(various fish tissues and diet) compartments. Given the site-specific factors that influence selenium 

bioavailability, bioaccumulation, and toxicity in aquatic systems, we believe that the use of a single 

guideline value for selenium in surface waters is inappropriate. Different sites will require different 

selenium water concentrations to ensure that concentrations in tissues such as fish and bird ovaries do not 

exceed a toxic threshold. Site-to-site variability has been demonstrated for fish by Van Derveer and 

Canton (1997) and for birds by Adams et al. (1998). Van Derveer and Canton (1997) used a sedimentbased bioaccumulation model to demonstrate that fish in a lotic system in Colorado were not at risk at 
water selenium concentrations of approximately 30 μg/L, three times higher than concentrations at which 

effects were observed in Belews Lake. 

Adams et al. (1998) evaluated the differences in bioaccumulation in shorebirds (Figure 1). They used 

probabilistic regression models to assess water, food chain and bird egg residues from fifteen lentic sites 

in the western United States. Uncertainty analysis of the regression models provided a probability 

distribution of waterborne selenium concentrations associated with bird egg tissue residues. The 90th and 

10th percentile water selenium concentrations associated with a selenium concentration of 20 mg/kg dw 

in bird eggs ranged from 6.8 to 318 μg/L. These studies support the concept of the need for site-specific 

water quality guidelines for selenium based on a bioaccumulation model and measurement of selenium 

concentrations in critical tissues. The Adams et al. model is represented in this paper (Figure 2) using a 

revised selenium concentration of 16 mg/kg dw in mallard duck eggs as a threshold for chronic effects, 

i.e., EC10 for duckling hatchability and survival (Fairbrother et al. 1999). The 90th and 10th percentile 

water selenium concentrations associated with a selenium concentration of 16 mg/kg dw in bird eggs 

ranges from 4.6 and 213 μg/L, respectively. These data indicate a large degree of variability in selenium 

concentrations that are potentially protective for sensitive bird species at different sites depending upon 

site-specific biogeochemistry, bioaccumulation and bird feeding behavior. 

SITE-SPECIFIC BIOACCUMULATION EVALUATION 

The concept that site-specific differences in the biogeochemistry of selenium can significantly alter the 

potential for toxicological effects is best demonstrated by site-specific differences in bioaccumulation of 

selenium in sensitive species. We have previously examined this for birds (Adams et al., 1998). To 

evaluate differences in selenium bioaccumulation in fish we evaluated published fish residue data for 

essentially the same sites evaluated for birds by Adams et al. (1998). We separated the data into lotic 

sites (flowing water; short hydraulic retention times, i.e., minutes to days) and lentic sites (standing water; 

long hydraulic retention times, i.e., weeks to years). The concept of lotic versus lentic is one that is used 

to typify extremes in biogeochemistry (oxygen content, redox, hydraulic retention time, carbon content) 

that is important in terms of controlling the formation of reduced selenium forms including organoselenium compounds. In lotic environments, selenium in the water column is most often found in the 

form of selenate and migration to sediments is limited. In lentic environments, selenate is less prevalent, 

selenite is more common, and both forms are biologically and chemically reduced to elemental and 

organo-selenium forms. These reduced forms are prevalent in lentic sediments and form the basis for 

uptake by benthic invertebrates and subsequent food chain bioaccumulation. 
The results of our bioaccumulation data analysis indicates that a clear and distinct difference between the 

patterns of bioaccumulation by fish in lotic and lentic environments (Figure 3). The data presented here 

are predominately for warm water fish species (e.g., centrarchids, cyprinids, and ictalurids). Selenium 

bioaccumulation factors for fish from lentic environments are typically a factor of 10 or more higher than 

those from comparable water concentrations in lotic environments. Additionally, these data indicate that 

bioaccumulation factors for fish and selenium are inversely related to exposure concentration. This is 

consistent with data reported for several other metals (Brix and Deforest, 2000). Recognition of this 

relationship provides insight into the variability that exists in reported bioaccumulation factors (BAFs) for 

selenium, i.e., highest BAFs occur at the lowest exposure levels. 

Further analysis of the selenium fish residue data from lotic systems indicates that tissue selenium 

concentrations remain fairly constant across a range of water concentrations up to about 13 μg/L. In 

contrast, in lentic systems, selenium tissue levels begin to increase as selenium water concentrations 

increase above 1.0 μg/L. The hockey-stick regressions presented in Figures 4 and 5 demonstrate distinct 

differences in bioaccumulation between lotic and lentic systems. While the slopes of the upper parts of 

the regressions are similar, the inflection points are more than a factor of ten different. Recognizing that 

selenium is an essential element for fish, the lower part of each regression is thought to represent a range 

of concentrations across which fish can actively regulate selenium uptake. Recognition of these 

differences in bioaccumulation provides the basis for developing tools to assess site-specific 

bioaccumulation. 

SITE-SPECIFIC WATER QUALITY CRITERIA METHODOLOGY 

Bioaccumulation data leave little doubt that water selenium concentrations protective of aquatic life and 

wildlife differ from site to site as a function of selenium’s site-specific biogeochemical cycling. 

Consequently, from a regulatory perspective to avoid over-regulation with associated costs, there is a 

need for developing a site-specific water quality criteria methodology for selenium. Existing 

methodologies for deriving site-specific water quality criteria such as water effect ratios are not applicable 

to selenium because unlike most contaminants, for selenium, the diet is the critical exposure pathway. 

Therefore, approaches for deriving site-specific water quality criteria must be based on the dietary 

exposure pathway to be appropriately protective for both birds and aquatic life. Given this need, 

identification and agreement on tissue toxicity thresholds for use in site-specific bioaccumulation models 
are essential. To that end, thresholds for birds and aquatic organisms was recently reviewed and 

summarized by DeForest et al. (1999) and Fairbrother et al. (1999). 

Overall, Adams et al. (1998) found a high correlation between water and mean egg selenium 

concentrations that is strongly influenced by site-specific factors. In light of the observed site variability, 

the following was concluded: first, the numerical water quality criterion for selenium is best used as a 

screening tool. Second, when waterborne selenium approaches or exceeds the criterion, a site-specific 

assessment should be used to determine whether existing water concentrations pose risk, and if so, 

identify a safe selenium water concentration for the site. 

The conclusions of Adams et al. (1998) led us to develop a methodology for developing site-specific 

water quality criteria using tissue residue concentration and effects threshold data. We have applied the 

methodology to selenium, but it is applicable to any constituent for which tissue residue-based endpoints 

are of concern. In general, setting water quality criteria protective of tissue residue-based endpoints for 

metals including selenium poses problems because the bioaccumulation factor (BAF) is not constant and 

is inversely related to the exposure concentration (Brix and DeForest, 2000). This must be taken into 

account in the model. Factors influencing BAF include: site-specific water and sediment chemistry, 

trophic relationships, and the degree of spatial and temporal co-occurrence of habitat, stressor and the 

endpoint of concern. 

The statistical technique we use in our methodology is Bayesian Monte Carlo analysis (BMC). Monte 

Carlo methods are numerical techniques for generating a representative sample from a probability 

distribution function (PDF). BMC evolved from earlier procedures used to ensure that the PDF of a 

model’s predictions was consistent with observed data. These earlier acceptance/rejection procedures 

involved deleting predictions that were inconsistent with observations (Hornberger and Spear 1980, Beck 

1987, Woodruff et al. 1992). The difference between BMC and earlier procedures is that BMC is more 

statistically rigorous. BMC defines the acceptance/ rejection procedure using the axioms of probability 

theory, as expressed in Bayes’ theorem (Bayes, 1763). Additional details on the site-specific 

methodology is reported by Toll et al. (1999). Only a brief synopsis of the method is presented here. 

The model approach is one that uses a generic bioaccumulation model for fish or birds, such as that 

developed by Adams et al. (1998). The purpose of the generic model is to describe bioaccumulation as 

reported in the literature for a wide variety of sites. This model provides an a priori estimate of 

bioaccumulation potential for a given site in the absence of any site-specific data. The generic model’s 
prediction interval (Figure 6) is based on data from all sites in the data set. It describes the range of 

possible site-specific relationships. The data set for the generic model can be updated as additional site 

data are obtained. Tissue residue data from a given site can then be compared to the model’s estimated 

value. A determination is then made as to whether or not the site-specific bioaccumulation is greater or 

less than what would be predicted by the generic model. If the site specific-tissue residue, at a given 

water concentration is less than predicted by the generic model, then the model calculates a water quality 

criterion that is higher for the site. This becomes the site-specific concentration in water that is protective 

of a given tissue threshold concentration. Likewise, if the tissue residue at the site is greater than the 

generic model would predict, the site-specific water quality criterion would be revised downward to 

insure that the tissue threshold is not exceeded. 

CONCLUSIONS 

(1) Literature data indicate that selenium bioaccumulation varies from site to site for both birds and 

aquatic organisms. 

(2) There is considerable evidence supporting that the conclusion that bioaccumulation of selenium is less 

at lotic sites than at lentic site. 

(3) There is a need for a methodology to derive site-specific water quality criteria for selenium. 

(4) Using a residue-based Bayesian Monte Carlo model, site-specific selenium water quality criteria can 

be calculated for sensitive avian and aquatic species. 

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Bayes, Rev. T. 1763. An essay toward solving a problem in the doctrine of chances. Philos. Trans. R. 

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Bowie, G. L. and T. M. Grieb, 1991. A model framework for assessing the effects of selenium in aquatic 

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DeForest, D.K., K.V. Brix, and W.J. Adams. 1999. Critical review of proposed residue-based selenium 

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Irrigation Water Quality Program, Sacramento Field Office, March 1996. 74 pp. 

Toll, J. E., L. M. Tear, K. V. Brix, D. K. DeForest, and W. J. Adams. 1999. A method for determining 

site-specific water quality criteria protective of tissue residue-based endpoints, Society of Toxicology and 

Chemistry presentation (manuscript in preparation). 

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Figure 1. Relationship between water-selenium and mean bird-egg-selenium.
Figure 2. Percentiles of Log water selenium distribution that would

produce a mean avian egg selenium value of 16 mg/kg dw.

-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

W ater Selenium (ug/L)

5%

10%

25% 75%

95%

50%

90%

 0.1 0.3 1 3 10 30 100 300 1,000 3,000 10,000

Figure 3. Log bioaccumualtion factor (BAF) vs. water selenium (WS) for lotic

and lentic aquatic systems 

0

1

2

3

4

5

-10 10 30 50 70 90 110 130 150

WS

log FISH / log WS (BAF)

BAF of log Lotic data BAF of log Lentic data

Log. (BAF of log Lotic data) Log. (BAF of log Lentic data)
Figure 4. Regression of water selenium versus whole fish selenium in lentic

environments

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

3.00

-1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00

Log Water Selenium (ppb)

Figure 5. Water selenium versus whole body fish selenium in lotic environments

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

3.00

-0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00

Log Water Selenium (ppb)