Subsurface water chemistry

In spite of these major limitations, considerable progress has been made in understanding many of the important factors that influence subsurface water chemistry. Na+, Ca2+ and Cl- account for the major portion of dissolved components in most brines (Figure 8.6). Ca2+, which can comprise up to 40% of the cations, usually increases relative to Na+ with depth (Figure 8.7). Br and organic acids are commonly found at concentrations of 1 to 2 g L1 (Land, 1987). The bicarbonate concentration is largely limited by carbonate mineral solubility, and sulfate is generally found in low concentrations as a result of bacterial and thermal reduction processes. 

Independent of the molecular properties of contaminants, the subsurface solid phase constituents are a major factor that control the adsorption process. Both the mineral and organic components of the solid phases interact differentially with ionic and nonionic pollutants, and in all cases, environmental factors, such as temperature, subsurface water content, and chemistry, affect the mechanism, extent, and rate of contaminant adsorption. 

The solubility of contaminants in subsurface water is controlled by (1) the molecular properties of the contaminant, (2) the porous media solid phase composition, and (3) the chemistry of the aqueous solution. The presence of potential cosolvents or other chemicals in water also affects contaminant solubility. A number of relevant examples selected from the literature are presented here to illustrate various solubility and dissolution processes. 

We will discuss the chemistry of subsurface waters further in subsequent sections of this chapter as it relates to specific carbonate mineral diagenetic processes such as secondary porosity formation. 

Groundwater. The uppermost part of the earth s rocks constitutes a porous medium in which water is stored and through which it moves. Up to a certain level, these rocks are saturated with water that is free to flow laterally under the influence of gravity. Subsurface water in this saturated zone is groundwater, and the uppermost part of the zone is the water table. The chemistry of the groundwater is influenced by the composition of the aquifer and by the chemical and biological events occurriog in the infiltration. 

O Melia was elected to the National Academy of Engineering in 1989. He has received many awards, including the 1982 Distinguished Lecturer of the Association of Environmental Engineering Professors. He is a member of many societies and organizations and has served as Director, Vice President, and President of the Association of Environmental Engineering Professors. O Melias research interests are in aquatic colloid chemistry, water and waste-water treatment, and modeling of natural surface and subsurface waters. 

A chief goal of this book is to help the reader understand controls on the chemical quality of surface-and subsurface-waters, both pristine and polluted. The focus is on inorganic processes and on the chemistry of soil and groundwaters, with less said about the chemistry of precipitation, surface-waters, or the ocean. The book leans heavily on the principles of chemical thermodynamics and the concept of chemical equilibrium. Chemical equilibrium, whether attainable or not, represents the reference state for purposes of explaining the concentrations of aqueous species in the hydrosphere. Concepts of chemical kinetics are introduced when they are known and seem applicable. 

Chave, K.E., 1960. Evidence on history of sea water from chemistry of deeper subsurface waters of ancient basins. Am. Assoc. Pet. Geol. Bull., 44 357—370. 

Subsurface physical, chemical and biological conditions are quite variable in space and time. These factors can be the source of considerable "natural" variability in ground-water chemistry at both clean and contaminated sites. Sampling and analytical errors or variability can be controlled if these activities are planned and documented as protocols which take into account the unique characteristics of individual sampling points and conditions. It is critical that a sound hydrogeologic basis exists for the steps in the sampling protocol which precede actual sample collection. 

WHI] Whiting, K. S., The thermodynamics and geochemistry of arsenic with application to subsurface waters at the Sharon Steel Superfiind site at Midvale, Utah, Ph. D. Thesis, Colorado School of Mines, (1992), Dept, of Chemistry and Geochemistry. Cited on page 213. 

Williams, P.M., 1967. Sea surface chemistry organic carbon, nitrogen and phosphorus in surface films and subsurface waters. Deep-Sea Res., 14 791—800. 

---