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.
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
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:
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).
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.
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.
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
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
Metal Wavelength Detection Limit Cd 226.50 nm > 4.0 ug/l Pb 220.35 nm > 40.0 ug/l