Molybdenum in freshwater and marine water
Toxicant default guideline values for protecting aquatic ecosystems
Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals’ of the ANZECC & ARMCANZ (2000) guidelines.
The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.
Description of chemical
Molybdenum is a Group VIa heavy metal, in the same group in the Periodic Table as tungsten. It is used in the manufacture of specialty steel products and electronic apparatus and its salts are used in the manufacture of glass, ceramics, fertilisers and pigments (CCREM 1987). It occurs as molybdenite (MoS2) and molybdates (MoO42) in igneous or sedimentary rocks. The most common oxidation states are 4+ and 6+ (Jarrell et al. 1980).
Canadian freshwaters usually contain less than 1 mg/L of molybdenum and its concentration in seawater is usually less than 0.01 mg/L (10 µg/L) (CCREM 1987). It is an essential trace element for aquatic organisms (Eisler 1987). Abbott (1977) reported that molybdenum occurs naturally in seawater around 10 µg/L. Molybdenum (Mo) in rivers and lakes appears to be evenly partitioned between dissolved and particulate phases, whereas in marine water there is usually 10,000 times more dissolved Mo than particulate Mo (Howarth & Cole 1985). Despite this, Howarth and Cole (1985) reported that phytoplankton in open ocean contained very low Mo residues, suggesting that Mo may be less available to phytoplankton in seawater compared to freshwater, as a result of competition by high sulfate in seawater. This in turn can affect nitrogen fixation and plant growth in oxic environments with nitrate as a source. Mo can adsorb to different clay surfaces, particularly if finely divided, but generally occurs in the dissolved form at natural pH (CCREM 1987). The current analytical practical quantitation limit (PQL) for molybdenum is 0.01 µg/L in fresh water and 0.3 µg/L in marine water (NSW EPA 2000).
Around 50 chronic data points (LC/EC50, NOEC and ‘no recorded end-point’) were obtained for molybdenum. These were screened to just 9 data points on only two different taxonomic groups. Hence, acute data (in mg/L) were also included in the assessment, as follows:
Freshwater fish: two species, 96-hour LC50, 70 to 1320 mg/L (i.e. x 1000 µg/L).
Freshwater crustacean: two species, 48 to 96-h LC50/EC50, 34.4 to 3618 mg/L; Chronic, Daphnia magna, 28-day NOEC, reproduction, 670 to 2200 µg/L.
Freshwater annelid: one species, Tubifex tubifex, 48 to 96-hour LC50, 28.9 to 52.1 mg/L.
Freshwater algae: one species, Chlorella vulgaris, NOEC population growth, 10 to 15 mg/L. Although the pH range of this test was 2.2 to 8.0, it gives an indication of algal toxicity and its inclusion will only affect the size of the assessment factor (AF) used.
There were not sufficient freshwater data for molybdenum to derive either a high or moderate reliability guideline trigger value by either the AF or statistical distribution methods. Hence a freshwater low reliability trigger value of 34 µg/L was calculated for molybdenum using an AF of 20 (because the limited data were chronic) on the D. magna figure.
Marine data were only available on one species of diatom and a species of dinoflagellate. EC50 (growth) figures ranged from 4500 to 18,000 µg/L.
There were insufficient data to derive a marine trigger value for molybdenum. An environmental concern level (ECL) (refer to Section 126.96.36.199 of the ANZECC & ARMCANZ 2000 guidelines) of 23 µg/L is derived from an AF of 200 (as Mo is an essential element) and this was adopted as a low reliability trigger value. Both of these figures should only be used as indicative interim working levels.
Abbott OJ 1977. The toxicity of ammonium molybdate to marine invertebrates. Marine Pollution Bulletin 8, 204–205.
ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.
CCREM 1987. Canadian water quality guidelines. Canadian Council of Resource and Environment Ministers, Ontario.
Eisler R 1987. Mercury hazards to fish, wildlife and invertebrates: A synoptic review. US Fish and Wildlife Service, Washington DC, USA.
Howarth RW & Cole JJ 1985. Molybdenum availability, nitrogen limitation, and phytoplankton growth in natural waters. Science 229, 653–655.
Jarrell WM, Page AL & Elseewi AA 1980. Molybdenum in the environment. Residue Reviews 74, 1–43.
NSW EPA 2000. Analytical Chemistry Section, Table of Trigger Values 20 March 2000, LD33/11, Lidcombe, NSW.