Chapter 2. Main soil contaminants and their fate in the soil environment

Introduction

Lead authors: Bernd M. Bussian, Natalia Rodríguez Eugenio (GSP/FAO), Susan C. Wilson (University of New England, Australia)

Contributing authors: Andrea Ceci (Sapienza University, Italy), Carolina Parelho (cE3c, Centre for Ecology, Evolution and Environmental Changes, and Azorean Biodiversity Group, Azores, Portugal), Dmytro Semenov (National Scientific Center «Institute for Soil Science and Agrochemistry Research named after O.N. Sokolovsky» (NSC ISSAR), Ukraine), Mojtaba Yahyaabadi (Isfahan Agricultural and Natural Resources Research and Education Center, AREEO, Iran)

Thousands of naturally-occurring and synthetic elements and compounds are released to the environment that can have a negative impact on human health and the environment. The fate in the soil, including retention or mobility to other environmental compartments and effects on living organisms is determined by the intrinsic characteristics of the contaminant and by the local soil properties.

Trace elements occur naturally at very low (or trace) concentrations in the environment but are mostly toxic to organisms when soil concentrations are increased by human activities or by natural processes. Globally, the most common trace metal contaminants are arsenic, lead, and mercury. Localized pollution of other trace metals such as cadmium and chromium can also pose a serious health risk.

Radionuclides are contaminants that produce ionizing radiation during the decay of active atoms. Anthropogenic sources of nuclear pollution include the global fallout from atmospheric nuclear weapons testing during the middle decades of the last century, operations of nuclear facilities, and nuclear accidents. Considerable radionuclide contamination also occurs at sites where uranium is mined, and legacy contamination in formerly mined areas is a major issue globally.

Compared with pollution by trace elements and radionuclides, knowledge about the overall footprint of soils that are considered polluted by organic contaminants is much lower, although the diversity of contaminants is enormous. Organic contaminant-polluted soils are mainly localized around industrial or urban centres, although some organic contaminants have a ubiquitous distribution due to their potential for long-range transport and their persistence in the environment. Organic contaminants can easily enter the food chain and pose a serious threat to human health. Contamination occurs often through water re-use on land and as a legacy of the use or disposal of persistent and bioaccumulating organic chemicals. Organic contaminants that have been detected at numerous sites across the globe in recent years with impact on human health and the environment even at very low concentrations are perfluoroalkyl chemicals (PFAS).

Considerable societal concern exists about the use of pesticides. Broad groups of pesticides have been designated as persistent organic pollutants (POPs) under the Stockholm Convention. These organic contaminants often show long-range transport potential and cause adverse effects on the environment and on human health. Although the Stockholm Convention regulates use, a continuing problem in many countries is spills and leakage from depots of obsolete pesticides. Many studies have highlighted concerns about the unknown effects of pesticide mixtures in soils on ecosystems and human health; however, given the great number of pesticides in use it is virtually impossible to study and assign toxicity risks to all the possible combinations.

Emerging contaminants are a broad group of mainly synthetic chemicals and their transformation products that have only recently been monitored in the environment, but are generating concern among the scientific community and policy makers due to the potential toxicity or risk to human and environmental health. Pharmaceutical and personal care products are a class of emerging chemical contaminants that have been used extensively for decades. Antibiotics, fungicides and other drugs are taken daily by humans and are widely given to livestock to promote growth and to reduce or prevent diseases. Plastics present a range of complex global challenges including microplastics and plasticizers that are found in all environmental compartments including soils. The manufactured nanomaterials (MNMs) present many societal benefits but soils are a sink for which these materials pose unique nanoscale aspects to fate, behaviour, bioavailability and toxicity.

Soils have the ability to filter, buffer, attenuate and degrade contaminants, which is determined by both soil properties and the characteristics, concentration and form of contaminant. Soil properties also control the fate and transport of soil contaminants, but understanding the relationship between specific soil properties and the fate of contaminants varies significantly with contaminant. Sustainable soil management focuses, in part, on the retention of contaminants at the point of contamination so that they can be either naturally attenuated or (in the case of more harmful contaminants) rendered harmless through remediation techniques.

This chapter briefly describes the main soil contaminants and their characteristics, and provides some information on the origin and main sources. The chapter also describes the soil properties that control the fate, mobility and interactions of the contaminants in the soil matrix with other environmental compartments.