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Heavy Metals

General Information: Heavy metals are elements having atomic weights between 63.546 and 200.590 (Kennish, 1992), and a specific gravity greater than 4.0 (Connell et al., 1984). Living organisms require trace amounts of some heavy metals, including cobalt, copper, iron, manganese, molybdenum, vanadium, strontium, and zinc. Excessive levels of essential metals, however, can be detrimental to the organism. Non-essential heavy metals of particular concern to surface water systems are cadmium, chromium, mercury, lead, arsenic, and antimony (Kennish, 1992).

All heavy metals exist in surface waters in colloidal, particulate, and dissolved phases, although dissolved concentrations are generally low (Kennish, 1992). The colloidal and particulate metal may be found in 1) hydroxides, oxides, silicates, or sulfides; or 2) adsorbed to clay, silica, or organic matter. The soluble forms are generally ions or unionized organometallic chelates or complexes. The solubility of trace metals in surface waters is predominately controlled by the water pH, the type and concentration of ligands on which the metal could adsorb, and the oxidation state of the mineral components and the redox environment of the system (Connell et al., 1984).

The behavior of metals in natural waters is a function of the substrate sediment composition, the suspended sediment composition, and the water chemistry. Sediment composed of fine sand and silt will generally have higher levels of adsorbed metal than will quartz, feldspar, and detrital carbonate-rich sediment. Metals also have a high affinity for humic acids, organo-clays, and oxides coated with organic matter (Connell et al., 1984).

The water chemistry of the system controls the rate of adsorption and desorbtion of metals to and from sediment. Adsorption removes the metal from the water column and stores the metal in the substrate. Desorption returns the metal to the water column, where recirculation and bioassimilation may take place. Metals may be desorbed from the sediment if the water experiences increases in salinity, decreases in redox potential, or decreases in pH.

  1. Salinity increase: Elevated salt concentrations create increased competition between cations and metals for binding sites. Often, metals will be driven off into the overlying water. (Estuaries are prone to this phenomenon because of fluctuating river flow inputs.)
  2. Redox Potential decrease: A decreased redox potential, as is often seen under oxygen deficient conditions, will change the composition of metal complexes and release the metal ions into the overlying water.
  3. pH decrease: A lower pH increases the competition between metal and hydrogen ions for binding sites. A decrease in pH may also dissolve metal-carbonate complexes, releasing free metal ions into the water column (Connell et al., 1984).
Heavy metals in surface water systems can be from natural or anthropogenic sources. Currently, anthropogenic inputs of metals exceed natural inputs. Excess metal levels in surface water may pose a health risk to humans and to the environment.

Numerical Categories:

EPA numeric aquatic life criteria are as below with the exception of the states of Alaska, Arkansas, California, Idaho, Kansas, Michigan, New Jersey, Vermont, Washington, District of Columbia, and Puerto Rico, which are subject to the revised metals criteria promulgated by EPA on May 4, 1995 (60 FR 22230). In addition, applicability of this rule may differ based on the State's compliance with Section 303(c)(2)(B) of the Clean Water Act. Guidance is in the Water Quality Standards Handbook, 2nd ed.-1993, EPA-823-B-93-002 and EPA-823-B-94-006.

Designated Use     Metal   Water Hardness (mg/l)   Limit (ug/l)
FW Aquatic Life    
                   As                                  50.0 #
                   As(III)                            190.0 *
                   As(III)                            360.0 
                   Be                                 130.0  
                                                        5.3  *
                   Cd               50                  0.66 *
                                                        1.80  
                                   150                  1.10 *
                                                        3.90  
                                   200                  2.00 *
                                                        8.60 
                   Cu               50                  6.50 *
                                                        9.20 
                                   150                 12.00 *
                                                       18.00  
                                   200                 21.00 *
                                                       34.00  
                   Hg                                   0.012 *                

                   Ni               50                 56.00 + 
                                   150                 96.00 +
                                   200                160.00 +
                   Pb               50                  1.30 *
                                                       34.00        
                                   150                  3.20 *
                                                       82.00                     
                                   200                  7.7
                                   200                  7.70 *
                                                      200.00  
                   Se                                   5.0  #
                   Zn               50                180.00 ++ 
                                   150                320.00 ++
                                   200                570.00 ++
                                                       47.00 +

*   Four-day average concentration
    One-hour average concentration
+   Twenty-four hour average concentration
++  Level not to be exceeded at any time

(Adapted from USEPA, 1987; Georgia Code, 1993; Washington Code, 1992)

# (North Carolina Code, 1994)

Designated Use       Metal   Water Hardness (mg/l)   Limit (ug/l)
Estuarine/
   Coastal 
Aquatic Life                      N/A
                     As                                50.0  #
                     Ag                                 0.10  #
                     Cd                                 8.00  *
                     Cu                                 2.90  
                     Hg                                 0.025 *
                     Ni                                 7.10  +
                     Pb                                 5.80  *
                     Se                                71.0  #
                     Zn                                76.6  *

*   Four-day average concentration
    One-hour average concentration
+   Twenty-four hour average concentration

(Adapted from USEPA, 1987; Georgia Code, 1993; Florida Code, 1993; Washington Code, 1992; Texas Code, 1991; North Carolina, 1991).

# (North Carolina Code, 1994)

Designated Use        Metal   Water Hardness (mg/l)   Limit (ug/l)
Human Consumption      As                                .05 (mg/l)
                       Ba                               1.0 (mg/l)
                       Cd                              10.0 *
                       Cr                                .05 (mg/l)
                       Cu                               1.0 ++
                       Hg                             144.0 ng/l *
                       Ni                             632.0 *
                       Pb                              50.0 *  (adults)
                       Zn                               5.0 *
  
* Ambient water criteria
  Maximum contaminant level; USEPA, 1987 
++  Level not to be exceeded at any time

(Adapted from AWWA, 1990; USEPA, 1987; Kansas Administrative Code, 1994)


Designated Use      Metal   Water Hardness (mg/l)   Limit (ug/l)
Irrigation          Cd             N/A                 10  
                    Cu                                200
                    Pb                              5,000
                    Zn                              2,000

(Adapted from Kansas Administrative Code, 1994)


Criteria for the Nine National Toxics Rule States - Dissolved Metals Criteria

   Freshwater Dissolved Metals Criteria that are not Hardness Dependent
   
Metal      Total Recoverable Metals Criteria   Dissolved Metals Criteria
   in ug/l       CMC     CCC                        CMC     CCC   

Arsenic        359.1    188.9                       360     190
Chromium (VI)   15.74    10.80                       15      10
Mercury          2.428     .0122                      2.1   N/A
CMC= Criteria Maximum Concentration
CCC= Criteria Continuous Concentration

For Hardness-Dependent Freshwater Criteria see the Federal Register
notice of May 4, 1995 (60 FR 22230).  The hardness-dependent criteria
require site specific calculations.

         Saltwater Dissolved Metals Criteria

Arsenic         68.55    36.05                       69     36
Cadmium         42.54     9.345                      42      9.3
Chromium (VI) 1079       49.86                     1100     50
Copper           2.916    2.916                       2.4    2.4
Lead           217.16     8.468                     210      8.1
Mercury          2.062     .0250                      1.8    N/A
Nickel          74.6      8.293                      74      8.2
Selenium       293.8     70.69                      290     71
Silver           2.3      N/A                         1.9    N/A
Zinc            95.1     86.14                       90     81
- Total recoverable metals criteria is from EPA National Ambient Water Quality Criteria Documents - Criteria Maximum Concentration (CMC) is the highest concentration of a pollutant to which aquatic life can be exposed for a short period of time (1 hour average) (acute); - Criteria Continuous Concentration (CCC) is the highest concentration of a pollutant to which aquatic life can be exposed for an extended period of time (4 days) without deleterious effects. - A more conservative approach to aquatic life protection may be preferred; in such cases the total recoverable metals criteria may be used.
          Human Consumption 

   Water and Organisms   Organisms Only
            ug/l 

Arsenic       0.018          0.14
Mercury       0.14           0.15
Nickel      610           4600



Health Effects: Ingestion of metals such as lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), barium (Ba), and chromium (Cr), may pose great risks to human health. Trace metals such as lead and cadmium will interfere with essential nutrients of similar appearance, such as calcium (Ca2+) and zinc (Zn2+).

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Environmental Effects: Aquatic organisms may be adversely affected by heavy metals in the environment. The toxicity is largely a function of the water chemistry and sediment composition in the surface water system (see "Environmental Fate/Mode of Transport").

Slightly elevated metal levels in natural waters may cause the following sublethal effects in aquatic organisms: 1) histological or morphological change in tissues; 2) changes in physiology, such as suppression of growth and development, poor swimming performance, changes in circulation; 3) change in biochemistry, such as enzyme activity and blood chemistry; 4) change in behavior; 5) and changes in reproduction (Connell et al., 1984).

Many organisms are able to regulate the metal concentrations in their tissues. Fish and crustacea can excrete essential metals, such as copper, zinc, and iron, that are present in excess. Some can also excrete non-essential metals, such as mercury and cadmium, although this is usually met with less success (Connell et al., 1984).

Research has shown that aquatic plants and bivalves are not able to successfully regulate metal uptake (Connell et al., 1984). Thus, bivalves tend to suffer from metal accumulation in polluted environments. In estuarine systems, bivalves often serve as biomonitor organisms in areas of suspected pollution (Kennish, 1992). Shellfishing waters are closed if metal levels make shellfish unfit for human consumption.

In comparison to freshwater fish and invertebrates, aquatic plants are equally or less sensitive to cadmium, copper, lead, mercury, nickel, and zinc. Thus, the water resource should be managed for the protection of fish and invertebrates, in order to ensure aquatic plant survivability (USEPA, 1987). Metal uptake rates will vary according to the organism and the metal in question. Phytoplankton and zooplankton often assimilate available metals quickly because of their high surface area to volume ratio. The ability of fish and invertebrates to adsorb metals is largely dependent on the physical and chemical characteristics of the metal (Kennish, 1992). With the exception of mercury, little metal bioaccumulation has been observed in aquatic organisms (Kennish, 1992).

Metals may enter the systems of aquatic organisms via three main pathways:

  1. Free metal ions that are absorbed through respiratory surface (e.g., gills) are readily diffused into the blood stream.
  2. Free metal ions that are adsorbed onto body surfaces are passively diffused into the blood stream.
  3. Metals that are sorbed onto food and particulates may be ingested, as well as free ions ingested with water (Connell et al., 1984).
Irrigation Effects: Irrigation water may transport dissolved heavy metals to agricultural fields. Although most heavy metal do not pose a threat to humans through crop consumption, cadmium may be incorporated into plant tissue. Accumulation usually occurs in plant roots, but may also occur throughout the plant (De Voogt et al., 1980).

Most irrigation systems are designed to allow for up to 30 percent of the water applied to not be absorbed and to leave the field as return flow. Return flow either joins the groundwater or runs off the field surface (tailwater). Sometimes tailwater must be rerouted into streams because of downstream water rights or a necessity to maintain streamflow. However, usually the tailwater is collected and stored until it can be reused or delivered to another field (USEPA 1993a).

Tailwater is often stored in small lakes or reservoirs, where heavy metals can accumulate as return flow is pumped in and out. These metals can adversely impact aquatic communities. An extreme example of this is the Kesterson Reservoir in the San Joaquin Valley, California, which received subsurface agricultural drainwater containing high levels of selenium and salts that had been leached from the soil during irrigation. Studies in the Kesterson Reservoir revealed elevated levels of selenium in water, sediments, terrestrial and aquatic vegetation, and aquatic insects. The elevated levels of selenium were cited as relating to the low reproductive success, high mortality, and developmental abnormalities in embryos and chicks of nesting aquatic birds (Schuler et al. 1990).

Sources:

  1. Nonpoint sources:
  2. Point sources: Domestic wastewater effluent contains metals from metabolic wastes, corrosion of water pipes, and consumer products. Industrial effluents and waste sludges may substantially contribute to metal loading (Connell et al., 1984).
Environmental Fate/Mode of Transport:

Transport in water: Water can transport metals that are bound to sediment particles. The primary route for sediment-metal transport is overland flow.

Water also transports dissolved metals. Although dissolved metals are primarily transported in overland flow, some underground transport is possible. Metals that are introduced to the unsaturated zone and the saturated zone will most likely not be transported a long distance. Dissolved metals that are carried below the land surface will readily sorb to soil particles or lithic material in the unsaturated zone and the saturated zone.

Transport in air: Metals introduced into the atmosphere may be carried to the land surface by precipitation and dry fallout. Additionally, because metals readily sorb to many sediment types, wind-borne sediment is a potential route for metal transport.

Analytical Techniques:

A. Total Metals: Includes all metals, organically and inorganically bound, both dissolved and particulate (APHA, 1992). Most samples will require digestion before analysis to reduce organic matter interference and to convert metal to a form that can be analyzed by Atomic Absorption Spectroscopy or Inductively Coupled Plasma Spectroscopy.

General methods:

  1. Direct Atomic Absorption Spectroscopy or Inductively Coupled Plasma Spectroscopy: Sample must be colorless, transparent, odorless, single phase, and have turbidity < 1 Nephelometric Turbidity Unit. Otherwise, sample must first be digested.
Digestion methods:
  1. Nitric Acid Digestion: Digestion is complete when solution is clear or light-colored.
  2. Nitric Acid - Hydrochloric Acid Digestion: Digestion is complete when digestate is light in color.
  3. Nitric Acid - Sulfuric Acid Digestion: Digestion is complete when solution is clear.
  4. Nitric Acid - Perchloric Acid Digestion: Digestion is complete when solution is clear and white HClO4 fumes appear.
  5. Nitric Acid - Perchloric Acid - Hydrofluoric Acid Digestion: Digestion is complete when soultion is clear and white HClO4 fumes appear.
B. Individual Metals: Requirements vary with the metal and the concentration range to be determined (APHA, 1992).
  1. Lead, Cadmium
    1. Flame Atomic Absorption Method: Sample is aspirated into a flame and atomized. The amount of light emitted is measured.
      • Detection Limits: Detection range may be extended 1)downward by scale expansion or by integrating the absorption signal over a long time, and 2)upward by dilution of sample, using a less-sensitive wavelength, rotating the burner head, or by linearizing the calibration curve at high concentrations.
        Metal    Wavelength   Detection Limit     Optimum Range
         Cd       228.8 nm      0.002 mg/l        0.5 - 2.0 mg/l
         Pb       283.3 nm      0.05 mg/l         1.0 - 20 mg/l
      • Interferences: Chemical interference by a lack of absorption by atoms that are bound in molecular combination by the flame.

    2. Electrothermal Atomic Adsorption Spectrometry: The high heat of a graphite furnace atomizes the element being determined.
      • Detection Limits: Use a larger sample volume or reduce the flow rate of the purge gas to increase sensitivity.
        Metal    Wavelength   Detection Limit      Optimum Range
         Cd       228.8 nm      0.1 ug/l           0.5 - 10.0 ug/l
         Pb       283.3 nm      1.0 ug/l           5.0 - 100  ug/l
      • Interferences: Interferences by broadband molecular absorption; and chemical (formation of refractory carbides) and matrix effects.
    3. Inductively Coupled Plasma (ICP) Method: Ionization of an argon gas stream by an oscillating radio frequency. High temperature dissociates molecules, creating an ion emission spectra.
      • Detection Limits:
        Metal    Wavelength     Detection Limit    
         Cd       226.50 nm      >  4.0 ug/l        
         Pb       220.35 nm      > 40.0 ug/l         
      • Interferences: Spectral interference from light emissions originating elsewhere (other than the source). Physical interference from changes in sample viscosity and surface tension.

    4. Cd Dithizone Method: Cadmium ions react with dithizone to form a pink/red color that can be extracted with chloroform. Extracts are measured photometrically.
      • Detection Limits: Detection limit is 0.5 ug/l Cd in a 15 ml final volume with a 15 cm light path.
      • Interferences: Interference if [Pb]>6 mg/l, [Zn]>3 mg/l, or [Cu]>1 mg/l.

    5. Pb Dithizone Method: Lead is mixed with an ammoniacal citrate-cyanide solution and extracted with dithizone in chloroform to form a cherry-red Pb dithozate.
      • Detection Limits: Detection limits are > 1.0 ug/l Pb in a 10 ml dithizone solution.
      • Interferences: At pH 8.5 to 9.5, dithizone complexes with bismuth, stannous tin, and monovalent thallium.

  2. Mercury
    1. Cold Vapor Atomic Adsorption Method:
      • Detection Limits: Choice method for all samples with [Hg] < 2ug/l.
      • Interferences: None.

    2. Dithizone Method: Mercury ions react with dithizone solution to form an orange solution that is measured in the spectrophotometer.
      • Detection Limits: Most accurate for samples with [Hg] > 2ug/l.
      • Interferences: Copper, gold, palladium, divalent platinum, and silver react with dithizone in acid solution.