Training materials on the Biotic Ligand Model for copper 

Training materials on Copper BLM: Data Requirements 

US Environmental Protection Agency 

Office of Science and Technology 

Standards and Health Protection Division 

1200 Pennsylvania Avenue, N.W. (MC 4305T) 

Washington, D.C. 20460
Training materials on the Biotic Ligand Model for copper 

1. Data Requirements 

1.1 What water quality parameters (model inputs) are used in the 

BLM? 

The ten water quality input parameters needed to run the BLM model for copper, 

are: pH; DOC; alkalinity; temperature; major cations (calcium, magnesium, sodium, and 

potassium); and major anions (sulfate, chloride). 

1.2 Under what conditions does EPA recommend that the BLM be 

used? Are there any conditions under which EPA does not 

recommend the BLM be used? 

Currently, EPA only recommends that the BLM be used to develop copper 

criteria for freshwater systems. EPA has not yet developed recommendations regarding 

BLM-based copper criteria for saltwater systems. The ranges of water chemistry 

parameters under which the BLM was developed and calibrated for copper are defined in 

the BLM interface (HydroQual, 2005) and are shown in Table 1.1 below. These ranges 

reflect the conditions in the toxicity tests supporting the recommended copper criteria 

development. The BLM can be used when the parameters, particularly temperature, fall 

outside these ranges, as these ranges reflect data available at time of model calibration. 

Table 1.1: Ranges of Input Parameters Used to Develop the Copper BLM 

Parameter Lower Bound Upper Bound 

Temperature (°C) 10 25 

pH 4.9 9.2 

DOC (mg/L) 0.05 29.65 

Calcium (mg/L) 0.204 120.24 

Magnesium (mg/L) 0.024 51.9 

Sodium (mg/L) 0.16 236.9 

Potassium (mg/L) 0.039 156 

Sulfate (mg/L) 0.096 278.4 

Chloride (mg/L) 0.32 279.72 

Alkalinity (mg/L) 1.99 360 

 Source: HydroQual, 2005 

1.3 What information does one model run provide? 

The BLM can be run with one set of input parameters (one “data set”) to calculate 

an instantaneous criterion or it can be run with multiple data sets to calculate numerous 

instantaneous criteria. An instantaneous criterion is a “snapshot in time;” that is, it is a 

criterion that reflects the water chemistry values for each of the ten parameters at one 

specific instant in time and at one specific location. Using the BLM with one data set is 

similar to using the hardness-based equation with only one value for hardness. 
Training materials on the Biotic Ligand Model for copper 

1.4 What are the limitations of an instantaneous criterion? 

EPA does not recommend deriving a criterion based on a single ambient sample. 

An instantaneous criterion would not take into account variations in the BLM input 

parameters, some of which may vary substantially on a temporal and/or spatial scale. 

The variable nature of a BLM-derived instantaneous criterion is not a problem unique to 

the BLM or copper, as other water quality parameters (e.g., nutrients) vary spatially and 

temporally. 

1.5 How many sampling events are preferred for using the BLM to 

develop site-specific criteria? 

Because of the diversity of water bodies to which the BLM might be applied, 

EPA is not providing a specific recommendation on the preferred number of sampling 

events that should be used with the BLM. In developing a site-specific criterion, enough 

data should be collected to characterize and manage the spatial and temporal variability 

of the site. 

Instantaneous copper criteria predicted using the BLM may not reflect intrinsic 

temporal variability caused by seasonal changes in water quality parameters and other 

natural sources of variability. Because some of the BLM input parameters are known to 

vary seasonally, EPA suggests a possible starting point of at least one sampling event per 

season. EPA analyzed thirteen river and stream segments and found that BLM-predicted 

copper criteria in this study were generally higher in the spring and summer and lower in 

the fall and winter. (See “Training materials on Copper BLM: Implementation” for an 

example of how a state has addressed seasonal variability in its monitoring 

recommendations.) 

Spatial variability in the BLM input parameters caused by physical factors such as 

watershed size or the presence or absence of a point source discharge(s) to a water body 

should also be considered when determining how many sampling events should be 

collected when using the BLM to develop site-specific copper criteria. Spatial variability 

in the BLM input parameters should also be considered when determining how many 

sampling locations should be selected for development of site-specific copper criteria 

using the BLM (see question 1.6 below). 

Regardless of the number of sampling events involved, data collection should 

reflect site-specific characteristics and consider special circumstances that may affect 

copper toxicity throughout the expected range of receiving water conditions. 

EPA suggests that states develop Quality Assurance Project Plans (QAPPs) for 

sampling protocols, in order to assure that representative data are collected. Further 

information on QAPPs may be found at http://www.epa.gov/quality/qapps.html. 
Training materials on the Biotic Ligand Model for copper 

1.6 How many sampling locations are preferred within a water body 

or water segment? 

Because BLM input parameters may vary spatially within a water segment or 

water body, multiple sampling locations may be appropriate. The unique characteristics 

of each site should be considered, including variability in BLM input parameters. For 

example, relatively homogenous systems may require fewer sampling locations as 

compared with more heterogeneous water bodies. If necessary, larger water segments 

could be divided into smaller segments. 

1.7 What are the options for calculating a single numeric site-specific 

criterion from multiple BLM-derived instantaneous criteria? 

The BLM calculates a copper criterion value for each set of input parameters 

(e.g., each ‘data set’). For example, if a state has 10 data sets or sampling events for a 

particular site, then the BLM will calculate 10 unique instantaneous copper criteria 

values. A state has several options for developing a single numeric site-specific criterion 

from the BLM output. The site-specific criterion should protect the water body, i.e., its 

designated use for aquatic life, under a variety of circumstances (e.g., seasonal 

conditions, high and low flows) and should not be exceeded more than the time allowed 

by the state standard (e.g., once every three years, on average). Site-specific conditions 

may influence the selection of an appropriate statistical metric for calculating a numeric 

criterion for copper. 

If the water quality parameters and BLM-derived copper criteria are relatively 

constant over a range of seasonal and flow conditions, (i.e., there is little variation in the 

input parameters and instantaneous criteria), then using the geometric mean of all 

instantaneous criteria may be appropriate. A geometric mean is a measure of central 

tendency and is less likely to be affected by outliers than an arithmetic mean. 

If a water body exhibits significant seasonal variations in the BLM input 

parameters and BLM-derived instantaneous copper criteria, then it may be best to 

develop seasonal criteria using seasonal geometric means. In such water bodies, 

averaging on an annual basis could result in a criterion value that is potentially underprotective during parts of the year (e.g., fall and winter). 

If the BLM-derived copper criteria vary significantly for reasons that cannot be 

easily explained (e.g., are not seasonal), then a lower percentile value (e.g., 5th) may be 

best to ensure that the water body is sufficiently protected and the criterion is not 

exceeded more than the state standard allows. 

If there are significant spatial differences in the instantaneous BLM-derived 

criteria for a water segment, then dividing the segment into smaller sections may be 

appropriate, as described in Question 1.6. 
Training materials on the Biotic Ligand Model for copper 

1.8 To which of the water quality parameters is the BLM most 

sensitive? 

The sensitivity of various input parameters can vary depending on site-specific 

characteristics. Generally, the BLM model, when applied to copper, is most sensitive to 

pH and DOC (EPA, 2002a). 

1.9 How do variations in DOC and pH affect model output? 

Although the effects of variations of DOC and pH will vary by water body, 

generally higher concentrations of DOC and higher pH values will lead to higher (less 

stringent) criteria values for copper. 

1.10 How much do pH and DOC typically vary within a water body? 

EPA conducted a preliminary analysis of thirteen river and stream segments over 

ten years with data obtained from the United States Geological Survey (USGS). EPA 

selected the sites, which included large rivers and smaller streams, based on their rich 

data records containing monthly measurements of all BLM input parameters. At these 

sites, pH typically varied temporally by one unit. Most of the sites had DOC values that 

varied by a factor of two. However, the variability of these and other parameters may be 

different for different water bodies. EPA’s Metals Translator Guidance notes that “pH 

may vary over several units as a result of acidic precipitation in the watershed, 

photosynthetic activity in the water body (lowest pH at dawn and highest pH in early 

afternoon coincident with peak photosynthetic activity of phytoplankton and other 

aquatic vegetation), or effluent discharge to the water body” (EPA, 1996b). 

1.11 What are the preferred analytical methods to measure the water 

quality parameters required by the BLM? What methods are 

available for DOC? 

EPA provides Guidelines Establishing Test Procedures for the Analysis of 

Pollutants at 40 CFR Part 136. Information on these analytical methods is available at 

http://www.epa.gov/waterscience/methods/ . Although DOC is not regulated as a 

contaminant, there are several scientifically-defensible methods available to measure 

DOC, such as EPA Method 415.3 (Dissolved and Total Organic Carbon and UV 

Absorbance at 254 nm in Source Water and Drinking Water), as well as methods 

developed by ASTM International and Standard Methods for the Examination of Water 

and Wastewater. 
Training materials on the Biotic Ligand Model for copper 

1.12 Are there ways to estimate parameter values where the needed 

measurements are missing? 

EPA is currently assessing the feasibility of developing parameter estimation 

techniques. Such values may be based on geographical location (e.g., water hardness and 

alkalinity exhibit noticeable geographic trends) or correlations between parameters (e.g., 

alkalinity can be readily predicted from hardness). In the meantime, EPA suggests using 

the BLM only where data are available, as is further explained in Question 1.1 of 

“Training materials on Copper BLM: Implementation.”