Chapter 1
Geochemical Prediction of Metal Dispersion in Surface and Groundwater Systems

Martin Mkandawire

Chemistry Department, Cape Breton University, Sydney, Nova Scotia, Canada

Email: martin_mkandawire@cbu.ca

Abstract

Dispersion of contaminants in an aquatic system refers to a process by which dissolved phase concentrations are reduced by the spreading of the plume and hydrodynamic through mixing with cleaner, surrounding water. Thus, metal dispersion in the environment is very distinct and environment dependent but dictated by physical-chemical processes that influence sorption and redox reactions. The reduction in plume concentrations of metal contaminants by dispersion, for instance in groundwater flowing in aquifer sands and gravel, is a very weak process compared to the turbulent mixing processes that occur in the open channel flow of surface water systems like streams. In a nutshell, effective geochemical predictions of metal dispersion cannot ignore the geochemical, geotechnical background including the bedrock and material composition and properties, physical features like topography and the hydrological regime, as well as contaminant transport dynamics. Further, the climatic factors play a crucial role in the overall predictions. Prediction of metal dispersion is necessary in pollution-prone environments, like mining sites, for establishing early warning systems when the contaminants would reach receiving environments of interest, as well as designing preventive and remediation strategies. This chapter discusses fundamentals in designing effective predictions of metal dispersion in both surface and groundwater.

Keywords: Phreatic process, geochemical modelling, plume migration, advective transport, oxygen ingress, stored acidity, finite element

1.1 Introduction

The dwindling availability of potable water resources has resulted in increased measures to protect water sources, especially from pollution. However, the legacy of industrial development has negatively manifested through the increased content of metal in the environment. The mere presence of high levels of metals in the environment is not a concern unless there is a reasonable risk of exposure to humans or degradation of the environment where the metals are located is likely to occur. Therefore, the dissolution of metals from contaminant sources and dispersion into receiving water sources, where they become exposure pathways is an issue. Consequently, monitoring and predicting metal contaminant dispersion into the environment is an important component in both water management and pollution control.

Classic procedures for monitoring and predicting metal contamination dispersion are conducted through regular sampling for water quality analysis. For surface water, it involves setting representative sampling stations, usually in a gradient, on the water bodies of interest; it is more complex for groundwater because it also involves the monitoring of wells. The data from hydrochemical monitoring can indicate the potential enrichment of metals into a system from the metals' sources. However, this information is temporal and spatial since it does not indicate metal dispersion in real time. Pertaining to the issues of complexity and time involved, it is often not possible to conduct sufficiently realistic sampling and laboratory experiments to predict the long-term behavior of the metal, especially of dispersion into the water systems [1, 2]. Geochemical models can be used to interpret and predict processes that may take place over timescales that are not directly achievable in sample analysis and experiments. Consequently, there has been a growth in the development of geochemical models which predict pollutant and metal dispersions into the environment. This development has generally gone hand-in-hand with advances in numerical techniques for solving complex mathematical problems as well as improvements in calculation speed and capability, and the general accessibility of computers [3, 4].

The accuracy in predicting metal dispersion into surface and groundwater systems has significant implications in water resource management, pollution control, and it can greatly impact the reasonability and scientific significance of pollution control strategies [5]. Developing effective geochemical prediction tools for metal dispersion requires a sound understanding of the prevailing hydrogeochemical processes and the behavior of the metal in the aquatic system. Metal dispersion depends solely on their inherent chemical properties vis-a-vis chemical form and speciation in the water, as facilitated by the prevailing hydrogeochemical processes. For instance, the complex nature of sediment-soil-water interactions in different hydrodynamic zones can produce a manifold of effects, including mobilization, concentration, and dispersion of metals at both short and long timescales [6]. Chemical erosion due to variations in natural climate and hydrodynamic conditions significantly influences the concentration of metals dissolved in water. Depending on their dissolved form and redox status, some metals form free or complex cations when dissolved in water while others, including some metalloids, are present as anions in their dissolved form [7]. Differences between groups of metals have important consequences for the partitioning of the metals among several dissolved and particulate phases, which are generally described by sorption of particulates, precipitation in minerals, and complexation in solutions.

The partitioning is significant for the dispersion of metals and divides the total amount of a pollutant into a 'dissolved' fraction and several "adsorbed" fractions. The fraction of a metal which is adsorbed onto the geological material in the ground is influenced by every hydrogeological process. Similarly, the fraction of metal adsorbed on particulate matter is subjected to any process which affects particulates, such as settling and resuspension in surface waters. In groundwater systems, metals are adsorbed by initial fast reactions, followed by slow adsorption reactions and are then redistributed into different chemical forms with varying mobility [8]. This distribution is controlled by reactions of metals such as (i) mineral precipitation and dissolution, (ii) ion exchange, adsorption, and desorption, (iii) aqueous complexation, and (iv) biological immobilization and mobilization.

With the number of parameters to be considered in the development of model theory, more and more geochemical prediction models have been developed with various model algorithms [9]. Consequently, there are often notable differences between modeling results (i.e., predicting metal dispersion) due to different theories and algorithms for which these models are based upon; different models lead to inconsistent prediction results. Therefore, harmonization of geochemical predictions tools for metal dispersion models is important, this can be achieved by having a set of basic, "must-be-included" parameters that each modeling code should include. This chapter describes the major processes involved in metal dispersion in surface and groundwater and discusses how these can be incorporated in developing effective geochemical prediction tools for metal dispersion into surface and groundwater system. To put things into perspective, this chapter presents how sources of metal pollutants dispersed into the environment cause environmental issues, showing how predictions are influenced by the metal sources and chemistry. Generalized processes involved in metal dispersion are later discussed. This is followed by the discussion of the selected process and how they can be incorporated in the development of effective geochemical prediction tools for metal dispersion into surface and groundwater systems.

1.2 Metal Sources and Contamination

1.2.1 Fundamental Processes

All metals are always in a dynamic cycling. Natural systems keep the metal geochemical cycle at equilibrium, which controls the accumulation of metals in the environment or in a single ecological range. However, the geochemical cycle may sometimes be either disturbed by natural processes or altered or accelerated by human activities (i.e., anthropogenic).

Natural processes can either be pedogenic, lithogenic, or geogenic, or even a combination of all. Examples of natural processes that bring about the occurrence of metals in the environment are comets, erosion, volcanic eruptions, and the weathering of minerals. Generally, metals occurring naturally have a great adsorption capacity in soil and geological material and thus are not readily soluble. The bonding energy between naturally occurring metals and soil material is very high compared to that with anthropogenic sources. In most cases, the threat is low except during a natural disaster. In addition to a number of uncertainties, this makes the development of prediction models for natural systems less a bit more difficult to develop than in anthropogenic sources.

Metals from anthropogenic sources tend to be more mobile due to their soluble and mobile reactive forms. There are a variety of anthropogenic sources, which include mine tailings, high metal wastes in landfills, land application of fertilizer, animal manures, sewage, compost, pesticides, fuel combustion residues (e.g., from coal and petrochemicals), and atmospheric deposition [10]. The most common metal(loid) pollutants are arsenic, cadmium, chromium, copper, nickel, lead, and mercury.

Mining and processing of metal ores, coupled with the industrial disposal of metals,...