A REVIEW OF THE ARSENIC CYCLE

Water Research Pergamon Press 1972. Vol. 6, pp. 1259-1274. Printed in Great Britain

A REVIEW OF THE ARSENIC CYCLE IN

NATURAL WATERS*

JOHN F. FERGUSON and JEROME GAV1S

Department of Geography and Environmental Engineering, The Johns Hopkins University,

Baltimore, Maryland 21218, U.S.A.

(Received 16 November 1971)

Ahstract--A review of the occurrence and cycling of arsenic in fresh waters is presented. The

fate of arsenic in natural waters has received little attention in past years, in spite of the fact

that arsenic is toxic and probably carcinogenic through exposure by drinking water.

The chemistry of arsenic in aqueous systems is reviewed. Thermodynamic information is

summarized in an Eh-pH diagram for a system including sulfur. Mechanisms for removal of

arsenic from the solution phase to the sediments are discussed. The possible microbially-

mediated reactions of arsenic, including oxidation of arsenite, methylation of arsenic species,

and reduction of arsenate, are discussed with reference to the locale of the reaction in the

water column or in the sediments and to the toxicological significance of the reaction products

and the rates of reaction.

A cycle of reactions for arsenic in a stratified lake is proposed and evidence is summarized

relating to the occurrence and importance of particular reactions.

The potential pollutional hazard of arsenic is from ingestion of drinking water with high

concentrations of arsenic, rather than consuming arsenic containing aquatic organisms.

Although arsenic is greatly concentrated in aquatic organisms, it is evidently not progressively

concentrated along a food chain. In addition, arsenic when consumed as an organically-bound

species in flesh evidently has low toxicity.

The global cycle of arsenic is discussed. While volcanic activity is the original source of much

of the arsenic in sedimentary rocks, in recent times weathering of arsenic has been approxi-

mately in balance with deposition of arsenic in sediments. Human activities, including the use

of arsenic, the burning of fossil fuels, increased erosion of land and the mining and processing

of sulfide minerals, have increased the amount of arsenic entering the oceans by at least a factor

of 3. This increase will have no effect on the concentration in the oceans for many hundreds of

years.

However, these cultural contributions are the source of high localized concentrations in many

fresh waters. Careful surveillance and increased knowledge of the fate or arsenic in the aquatic

environment are needed to insure that there will be no public health hazard.

THE DISCOVERY that mercury is readily cycled from the sediments to solution and

concentrated by organisms through the food chain has led to speculation that similar

cycles may occur with other trace elements. Much of this speculation has centered on

arsenic. Arsenic, like mercury, has been widely used and misused for its toxic proper-

ties, and large accumulations due to human activities now exists in soils and sediments.

This paper is intended as a companion to the previous paper (GAvIS and FERGUSON,

1972) that discussed the cycle of mercury in natural water systems. The same considera-

tions apply. The same approach will be used. But the central question, whether arsenic

pollution is an important threat to aquatic organisms or human health, must be

examined in a slightly different way. Although there have been instances of arsenical

poisoning to humans through water ingestion, there are no current examples of toxic

accumulations in food organisms.

Arsenic has a long history of use for its toxic and medicinal properties. Arsenical

compounds were used in medicine 2000-3000 yr ago in the Orient. Therapeutic

applications have continued through the ages. Arsenic also gained the reputation of a

* Presented at the XV General Assembly of the IUGG, Moscow, U.S.S.R., August, 1971.

WATER 6[ 11--n 1259

1260 JOHN F. FERGUSON and JEROME GAVIS

dangerous poison in the Middle Ages, and until the nineteenth century was the

preferred poison of most homocidal practitioners. In the nineteenth century, the

therapeutic use of arsenicals against trypanosomal or spirochaetal organisms became

classical examples of chemotherapy. These demonstrated the widely differing toxicity

and effectiveness of arsenic in different oxidation states and combined with different

organic ligands.

In